Causes and Types of Cracks

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Causes and Types of Cracks

Introduction

It is fundamental that hardened reinforced concrete cracks in the tensile zone when subjected to externally imposed structural loads. By means of appropriate design and detailing techniques, these cracks can be limited to acceptable levels in terms of structural integrity and aesthetics.

Concrete is also liable to crack in both the plastic and hardened states due to stresses which it intrinsically (naturally) sustains by the nature of its constituent materials. Cracks of this nature while it does not impede the structural integrity, may be unsightly and gives a picture that the structure is somewhat compromised structurally. This is often not the case. The reasons causing the cracks should be determined through some inspection and assessment, so that effective repairs can be carried out. Effective repairs here means the use of suitable (compatible) materials (to ensure the repair materials and substrate concrete have close similarity in performance characteristics) and correct method of repair (to give a quality and durable repair).

Cracks in concrete can form soon after the concrete is placed or after it has hardened. Cracks that form soon after placing are often due to the following factors such as poor rheological properties of the concrete, harsh weather and en\/ironmental conditions during concrete and poor site practices when it comes to compaction and curing the concrete. On the other hand, cracks that form years after hardening may be due to corrosion of reinforcement (initially cracks and later leading to spalling) during the change of pH in the concrete cover zone. The time frame when intrinsic cracks are likely to form is now better known and to limit them to acceptable levels by means of design, selecting of materials and/or construction techniques are also achievable.

Intrinsic strains can be cumulative; thus, although one particular mechanism may not be sufficient to initiate cracks, it may enlarge previously formed cracks. This can make diagnosis difficult.

The intrinsic cracks formed due to the above mentioned reasons and scenarios are the main focus of this course. The flow in any assessment and repair project would generally comprise of the following steps after cracks form:

Step Task Actions Discussed mainly

in

1 Process of assessment Defects and their

classification and causes

Sections 1, 2 & 3
2 General planning Consider:

Options

–      Principles

–      Methods

Sections 3 & 4
3 Design of repair work Definitions of the intended

use of product requirements:

–          Substrate

–          Products

–          Work

–          Specification

Sections 3 & 4
–     Drawings
4 Repair work Choice and use of products

and equipment.

Tests of quality control Health and safety

Section 3
Acceptance of repair work Acceptance testing

Remedial works documentation

Section 4

 

The lessons will provide some knowledge on how to identify and assess the impact of cracks that form on a concrete structure. This should enable Engineers and Contractors to take measures which will either minimise, prevent or control intrinsic cracks. It will also give some knowledge on how assessment is done to diagnose existing cracks, which is essential if effective remedial measures are to be specified.

 

Types of Intrinsic Cracks

A comprehensive “family tree” of crack types is shown in Figure 1. It highlights the simple ! classification of the four main types cracks (highlighted in red in Figure 1) considered here:

  • plastic
  • early thermal contraction
  • long-term drying shrinkage
  • chemical oi change //v c/Mem/sf/y

 

The factors affecting each type of crack categorised above, the recommendations and remedial measures are given in greater detail in section 2. In order to classify the type of cracks, it is, needless to say, necessary for an inspection be carried out. In some cases, the pattern of cracks is clear enough to identify the type of cracks. If not, some form of non- invasive and/or invasive testing may be needed to establish the cause(s) of cracking. Both cases are discussed further in sections 3 and 4.

These terms are important as they help to distinguish between types of crack. They also provide an important initial guide in any diagnosis because in very broad terms the periods in which these types of crack appear are:

  • plastic – first few hours;
  • thermal contraction – one day to two/three weeks;
  • drying shrinkage – after several weeks or even months.
  • chemical – years later

 

It is possible that cracks may form due to a particular reason and may worsen due to other reasons. As such, some cracking may involve combinations of the various types. Furthermore, recommendations made to avoid one type of crack may increase the risk of another type

To illustrate the cracks discussed, Figure 2 shows a hypothetical concrete structure with various types of intrinsic crack shown in typical situations.

The primary and secondary causes of the cracks do provide information on how such cracks can be minimised. eg. if the cause is excessive bleeding, then address the amount of bleed water. If the cause is early rapid drying, then effective curing should be carried out.

Principles of Crack Formation and Control

This section outlines the principles which govern the formation of cracks in both plastic and hardened concrete, and the way in which reinforcement controls cracks in hardened concrete.

Formation of Cracks

The mechanism of the formation of cracks in concrete is somewhat complex, largely because of time-dependent factors. There are two basic philosophies, either of which may be used as convenient or appropriate.

Tensile Strenqth

Consider an unstressed element of concrete under certain conditions of moisture and temperature (Figure 3). If the concrete is either cooled or dried and is free from restraint, it will reduce in length and no cracks will develop.

If, howe\/er, the ends of the element is fixed in such a way that the original length is maintained, then a tensile stress will develop which is equal to the stress that would be required to pull the free element back to its original length. In practice the restraint may not be external; in large elements the core of the concrete may be warmer and wetter than the external surfaces and thus cause internal restraints.

In the most simple terms, the concrete will crack when the tensile stress exceeds the tensile strength. However, several time-dependent factors must be considered. First, assuming that hydration continues with time (ie. the concrete matures), the “mechanical” properties of the concrete change. In particular the modulus of elasticity increases and thus the stresses induced by a given strain also increases. It is reasonable to assume that the tensile strength increases at not dissimilar rate, thus rendering these factors almost academic in this context.

Another factor is that of creep (creep, in simple terms, is the shrinkage of concrete over long term that is caused by the drying of the pore water in concrete), which causes the stress induced by a sustained strain to reduce with time as shown in Figure 4. It is only when the net tensile stress exceeds tensile strain — sometimes called “relaxation” — has a major effect on reducing the incidence of cracking in concrete.

Tensile Strain Capacity

An alternative concept is that cracks form when the tensile strain exceeds the tensile strain capscity of the concrete. The strain capacity of hardened concrete develops with time in a manner not dissimilar to strength and has been studied by many researchers, notably Houk et aI*^* and Houghton***.

Initially the concrete is in a quasi-liquid phase and capable of large deformation, but the strain capacity rapidly drops to a minimum as the cement paste stiffens, and then slowly rises again as hydration proceeds. The stress “relaxing* effect of creep is very significant at early stages and reduces with time. An example of this effect is shown in Figure 5.

This accounts for the difference between the strain capaicity de ermined by an instantaneous test in the laboratory, and that known to exist from observation of cracks in the field.

From the above it can be seen that:

  • cracks will not form unless there is some form of restraint;
  • creep is beneficial in that it reduces the risk of cracking

 

Influence of Reinforcement (Shrinkage and Thermal Effects)

Because it acts as a form of internal restraint, steel reinforcement governs the spacing and widths of cracks in hardened concrete. It does not have the same effect in plastic concrete, nor do these principles apply to fibre reinforcement.

Crack Spacing

Consider an element of reinforced concrete subjected to conditions which produce uniform tensile stress in the concrete. A crack may form at some random point. At the crack the stress in the concrete must be zero. Away from the crack it will increase until, at some distance away (S+), the stress will be unaffected by the crack. Because the crack has caused the concrete stress to be reduced to less than the tensile strength of the concrete within +Sn of the crack, the next crack to form must be more than S< away from the first (Figure 6). Thus the minimum spacing of cracks is S+.

If a second crack forms at a distance way from the first greater than 2S+, then there will be an area between the cracks which is not affected by either crack and in which another crack could form (Figure 6a). However, if the second crack forms at less than 2S+ from the first, there will be no “unaffected” area and another crack cannot form between them (Figure 6b).

Thus, the maximum crack spacing when all cracks have formed will be 2S<. The average crack spacing S is clearly between So and 2S+ and has generally been assumed to be 1.55, but some theoretical calculations suggest it could be 1.33Sm. At a crack, any tensile force is carried by the reinforcement. Away from the cracks, bond will cause a transfer of stress from the steel to concrete so that, as described above, at some distance the concrete stress will reach its tensile strength and a further crack can form. The distance S+ is thus defined by the efficiency with which force can be transferred from steel to concrete. A variety of theories and formulae have been devised to predict how S+ will vary as a function of the quantity and arrangement of the reinforcement.

For members subjected predominantly to tension, studies*3) suggest the following general relationship between So and these variables:

So — k1C + k20/p

Where:

k1 = a constant

k2 = a constant which will depend on bond strength C = cover

0 = bar diameter

P = steel ratio of concrete section

 

Each of these factors is discussed in detail below.

  • Bar diameter. Small diameter bars have a greater perimeter for a given cross- sectional area than large diameter bars; thus they will transfer stress
  • Steel percentage. For a gi\/en bar size, the perimeter will be increased by increasing the percentage of steel
  • Type of steel. Deformed bars transfer stress more efficiently than plain ones
  • Large cover tends to produce large crack spacing with commensurately large crack widths at the surface. Therefore crack control should be placed as near to the concrete surface as durability and other considerations follow, and preferably outside the main structural reinforcement.\

 

In many walls and slabs which have little crack control steel (compared with beams containing heavy structural reinforcement), the term kz0/p is much larger than k‹C. The relationship presupposes that the reinforcement is stronger than the immature concrete and that it will not yield at the crack position. To ensure this the amount of reinforcement must f« be less than pc, where p – fct/fy Act – tensile strength of concrete at relevant age and f/ = characteristic yield strength of the reinforcement.

The above is largely based on the research work of Hughes, first published in 1968 4) much of which has now been incorporated in the British Standard fOr water-retaining structures BS 8007*^*, previously BS 5337*^*.

Crack Width

As long as p (defined above) is provided, the steel will not yield and the maximum crack width will be given simply by W = 2Sn x strain. At this stage, it is the crack width at the concrete surface which is being considered. Internal variations in crack width and their significance are discussed further at later sections.

In Singapore, reinforced concrete design is carried out according to Eurocode 2 — Design of Concrete Structures. There are 3 parts:

a. Eurocode 2 — Design of Concrete Structures, Part 1-1, General Rules and Rules for Buildings (SS EN 1992-1-1)
b. Eurocode 2 — Design of Concrete Structures, Part 2, Concrete Bridges, Design and Detailing Rules (SS EN 1992-2)
c. Eurocode 2 — Design of Concrete Structures, Part 3, Liquid Retaining and Containment Structures (SS EN 1992-3)

 

Eurocode 2 — Desiqn of Concrete Structures, Part 1-1. General Rules and Rules for Buildinqs (SS EN 1992-1 1)

Crack control requirements under serviceability limit state (SLS) is given under section 7 of SS EN 1992-1 1. The limiting value, W ., for the calculated cracl‹ width, Wi‹, taking into account the proposed function and nature of the structure and the costs of limiting cracking, is given in Table 7.1N of the code (reprocluced below):

Table 7.1N : Recommended values of Wmax (fT)fTI)

Exposure class Reinforced members and

prestressed members with unbonded tendons

Prestressed members with bonded

tendons

Quasi-permanent load

combination

Frequent load combination
4 1 0.2
XC2, XC3, XC4 0.3 0.2*
XD1, XD2,

XS1, XS2, XS3

Decompression
Note 1 For X0, XC1 exposure classes, crack width has no influence on durability and this

limit state is set to give generally acceptable appearance. In the absence of appearance conditions this limit state may be relaxed

Note 2 For these exposure classes, in addition, decompression should be checked under

the quasi-permanent combination of loads

 

Eurocode 2 — Desiqn of Concrete Structures, Part 2. Concrete Bi-idqes, Desiqn and Detailinq Rules (SS EN 1992-2)

Crack control requirements under serviceability limit state (SLS) is given under section 7.3 of SS EN 1992-2. The limiting value, Wmax, for the calculated crack width, Wx, taking into account the proposed function and nature of the structure and the costs of limiting cracking, is given in Table 7.101N of the code (reproduced below):

Table 7.101N : Recommended values of Wmax (mm)

Exposure class Reinforced members and

prestressed members with unbonded tendons

Prestressed members with bonded

tendons

Quasi-permanent load

combination

Frequent load combination
X0, XC1 0.§a 0.2
XC2, XC3, XC4 0.3 0 .2b
XD1, XD2,

XS1, XS2, XS3

Decompression
A For X0, XC1 exposure classes, crack width has no influence on durability and this

limit state is set to guarantee acceptable appearance. In the absence of appearance conditions this limit state may be relaxed

B For these exposure classes, in addition, decompression should be checked under

the quasi-permanent combination of loads

Eurocode 2 — Design of Concrète Structures. Part 3. Liquid Retaining and Containment Structures CSS EN 1992-3)

It is convenient to classify liquid retaining structures in relation to the degree of pi otection against leakage required. Table 7.105 gives the classification. It should be noted that all concrete will pei mit passage of small quantities of liquids and gases by diffusion

Table 7.105 : Classification of tiqhtness

Tightness Class Requirements of leakage
0 Some degree of leakage acceptable, or leakage of liquids

irrelevant

1 Leakage to be limited to a small amount. Some surface staining or

damp patches acceptable

2 Leakage to be minimal. Appearance not to be impaired by

staininq                  

3 No leakage permitted

Appropriate limits to cracking depending on the classification of the element considered should be selected, paying due regards to the required function of the structure. In the absence of more specific requirements, the following may be adopted.

Tightness Class 0 – The provisions of 7.3.1 of EN 1992-1-1 may be adopted
Tightness Class 1 – Any cracks which can expected to pass through the full

thickness of the section should be limited to wk1. The provisions in 7.3.1 of EN 1992-1-1 apply where the full thickness of the section is not cracked and where the conditions in (112) and (113) below are fulfilled

Tightness Class 2 – Cracks which may be expected to pass through the full

thickness of the section should generally be avoided unless appropriate measures (e.g. liners or water bars) have been incorporated.

Tightness Class 3 – Generally, special measures (e.g. liners or prestress) will he

required to ensure watertightness

To provide adequate assurance for structures of classes 2 or 3 that cracks do not pass through the full width of a section, the design value of the depth of the compression zone should be at least xmin calculated for the quasi-permanent combination of actions. Where a section is subjected to alternate actions, cracks should not be considered to pass through the full thickness of the section unless it can be shown that some part of the section thickness will always remain in compression. This thickness of concrete in compression should normally be a least xmin under all appropriate combinations of actions. The action effects may be calculated on the assumption of linear elastic material behaviour. The resulting stresses in a section should be assuming that the concrete in tension is neglected.

If the provisions of 7.3.J (111) for tightness class 1 are met then cracks through which water flows may be expected to heal in members which are not subjected to significant changes of loading or temperature during service. In the absence of more reliable information, healing may be assumed where the expected range of strain at a section under service conditions is less than 150 x 10-^

If self-healing is untimely to occur, any crack which passes through the full thickness of the section may lead to leakage, regardless of the cracl‹ wiclth

Silos holding dry materials may generally be designed as Class 0 however it may be appropriate for Class 1, 2 or 3 to be used where the stored material is particularly sensitive to moisture

Special care should be taken where members are subject to tensile stresses due the restraint of shrinkage or thermal movements

Acceptance criteria for liquid retaining structures may include maximum level of leakage.

Restraint and Provision of Joints

If sufficient movement joints (ie. contraction and expansion joints, not construction joints) are provided at close enough centres, then restraint will be reduced.

The location of construction joint should ideally be proposed by the contractor for approval by the designer. However, there may be designs in which the locations of construction joints are restricted (eg. featured joints in fair faced concrete) in which case this should be clearly indicated in the contract document.

Curing of Concrete

The preceding sections presupposes that the concrete is properly cured. In this context curing means protecting the concrete from both rapid drying out and rapid cooling. These aspects are dealt with in greater detail in the later sections.

If the concrete is not properly cured, then uncontrolled cracks may form, even when carefully detailed reinforcement and joints are provided.

Recommendation

The current most popular method of controlling cracks is to provide sufficient reinforcement, if necessary in conjunction with movement joints, to produce a lot of fine cracks rather than a few wide cracks.

1.5 Structural Cracks and Cracks arising from Constructional Movement

Accidental load, creep, design overload (Structural) and formwork settlement, subgrade movement (Constructional Movement) may also lead to structural cracks (see Figure 1). While cracks of such nature are not discussed in this course, it is mentioned here as accidents arising from such cracks are present.

The Tampines flyover viaduct collapse during construction in 2017 comes to mind. The Straits Times online of date 14a July 2017 stated ”Preliminary investigations showed that corbels – which had been put up to support the weight of pre-cast beams between two piers – had collapsed”.

The Business Times online on 15’^ July 2017 reported ”In most construction failures, human error ranks high as a probable cause. Yesterday’s work site collapse in Upper Changi Road East is unlikely to be an exception”.

  • Design of the corbel. Given that signs of cracking at the corbel at other column heads, the adequacy of the design needs to be looked into.
  • Strength of the concrete. The compressive strength of the concrete must be sufficient to take on the weight of the girder and deck at the time of casting. The corbel acts like a cantilever and tal‹es a high bending moment and shear, the question would be if the concrete strength was adequate at that point of time. Movement of formwork support. If the formwork support was not properly erected, any sudden movement of the formwork base would create a sudden load that could cause cracking to the corbel. Was the formwork founded on solid ground to evenly distribute tile weight of the wet concrete?

 

The above causes could lead to cracking in concrete that are of structural nature and consequences.

Cracks on the corbels of column heads at some MRT viaducts can also be seen on the photographs shown on next page. The cracks appear to be repairecl from the physical appearance.

 

Photographs below show cracked corbel supporting the beam of a building

Photographs below show the temporary steel support needed before the structural replacement of new corbel could be carried out.

Summary

  • It is fundamental that hardened reinforced concrete cracks in the tensile zone when subjected to externally imposed structural loads. This happens when the tensile strength of the concrete is exceeded. It can also occur when the tensile strain exceeds the tensile strain capacity of the concrete.
  • By means of appropriate design and detailing techniques, these cracks can be limited to acceptable levels in terms of structural integrity and aesthetics.
  • Concrete is also liable to crack in both the plastic and hardened states due to stresses which it intrinsically (naturally) sustains by the nature of its constituent materials.
  • Fresh concrete is cast either into a metal formwork, wooden formwork, or onto contact with lean concrete or a previously cast concrete member. Such contact surfaces will create an external restraint, which may affect the tendency of increased cracking.
  • When large mass concrete is cast, the entire casting operation may take hours or days. The eadier cast concrete undergoes heating up and expansion while fresh concrete is still being cast onto the “hardened* concrete. Internal restraint is formed in such case which may affect the tendency of cracking.
  • The four main types of intrinsic cracks are:
    – plastic cracks
    – early thermal contraction cracks
    – long-term drying shrinkage cracks
    – chemical or change in chemistry
  • The standard primary and secondary causes of such cracks are given and therefore remedies are also gi\/en. However, it is possible that cracks may form due to a particular reason and may worsen due to other reasons. As such, some cracking may involve combinations of the various types. When in doubt, seek adUice.
  • Intrinsic cracks usually lead to durability issues such as quickening of carbonation and subsequent corrosion of reinforcement. However, cracks ai ising from accidental load, creep, design overload, formwork movement or subgrade settlement may lead to cracking which have slructural significance. Study of such causes and reasons are very specific.

 

Prevention and Control

Introduction

This section describes more about plastic cracks, early age thermal cracks and long term drying shrinkage cracks and cracks due to chemical reactions — it’s causes and some considerations on the prevention and control.

Plastic Cracks

As the name implies, plastic cracks occur in concrete before it hardens, say 1 to 8 hours after placing — although they are often not noticed until the following day. Generally they can be identified as one of two types, namely plastic settlement or plastic shrinkage. Most plastic settlement cracks appear in deep sections, but plastic shrinkage cracks are most common in flat slabs exposed to high rates of evaporation. However, areas of apparent overlap sometimes occur, as discussed later. Both types are governed in contradictory ways by the phenomenon of bleeding.

Bleeding

Bleeding is the name given to the action of water arising to the top of concrete shortly after compaction. This water is forced upwards when the heavier solid particles gravitate downwards.

Bleeding is not a result of poor compaction, nor can it be eliminated by “improved” compaction. All concrete is subject to settlement, but bleed water is seen at the surface only if the rate of evaporation is less than the rate of bleeding. When the rate of evaporation is low, bleeding will be seen as a layer of lean water on the surface of the concrete — it should not be confused with laitance, which is a mixture of water, cement and very fine aggregate. Figure 7 illustrates the pattern of bleeding of two typical concrete mixes. After compaction, there is a short dormant period followed by a period during which the rate of bleeding is sensibly uniform. This ends when either the setting of the cement prevents further movement or the solid particles come effecti\/eIy into contact and thus cannot settle any more.

General factors affecting bleeding

  • Air entrainment. The use of air entrainment significantly reduces
  • “Ultra-fines” Mixes which are cohesi\ie because they ha\ie a high ultra- dines content (ie. cement, very fine sand, silt etc.) will bleed less than harsh mixes.
  • Rate of Bleeding is increased by a high rate of evaporation during the period immediately after placing and compacting concrete.
  • Water content. Wet mixes bleed more than dry
  • Mixes containing retarders usually bleed more than normal.
  • Mixes placed at low temperatures usually bleed more than normal.
  • Depth of section. For a given concrete mix, a deep section bleeds for a longer time than a shallow section.

 

It is important to note that the effect of changes to concrete mixes on plastic cracking is not straightforward and must be considered in detail.

In a local project, it is shown that the existing M-Sand is not graded and the coarse aggregate is rather coarse. Figure 8 shows the combined grading curve based on original mix with 739 kg/m3 of M-Sand and 995 kg/m3 of coarse aggregate. It is obvious that the aggregate is gap graded and can easily cause bleeding.

However, if we reduce the coarse aggregate content and increase the natural sand to 1:1 ratio, the aggregate grading will improve as shown in Figure 10. However, it will increase the fine content and we may have to change the superplasticizer to achieve the required workability.

The mix is then adjusted with natural sand 770kg/m3 and aggregate at 1000kg/m3 with adjustment to the admixture to overcome the bleeding problem.

 

Effect of Blended Cements

In very general terms for most OPCs in the UK, using GGBS as a partial replacement will increase the rate and amount of bleed. This is most noticeable when high replacement levels are used (ie. 60%+); at low replacement levels the bleed properties of the OPC are liable to dominate.

In contrast, most sources of selected PFA used in UK will produce concretes with a slower rate of bleed than the equivalent OPC mix, especially when the constant volume method of mix design is used.

However, at normal temperatures both PFA and GGBS tend to slow the rate of setting. Therefore, in deep sections concrete made from these materials can bleed longer than their OPC counterparts; see Figure 7.

 

Effect of Fibres

The pro\/ision of secondary reinforcement, in the form of fibres (common polypropelene or steel) will usually reduce the rate of bleed of a given concrete. This is because the addition to the mix of a fine material with high specific surface impedes the free exchange of solids and water. As fibres have no direct effect on cement setting times, they will reduce the amount of bleed also.

 

Plastic Settlement Cracks

Plastic settlement cracks occur only when there is a high amount of bleeding and settlement, and there is some form of restraint to the settlement.

 

Types of Plastic Settlement Crack

Restraint governs the type of plastic settlement cracks as follows:

  • Cracks directly over formwork tie bolts or reinforcement which is fixed (as opposed to a “floating” mesh) near the top of a section (Figures 11a and 11b).
  • Cracks in narrow columns and walls where settlement is restrained either by simple wedging or arching of the concrete or by a mushroom head to a The restraint may be aggravated by the presence of horizontal links (Figures 11d).
  • Cracks at change of depth of section, particularly in trough or waffle floors (Figure 11e). A photograph shown in Figure 11(e) shows a trough or waffle door with repaired crack lines that formed as the two precast T-beam butt-jointed each other, the crack line formed at the change of depth of section.
  • In a project, plastic settlement cracks formed within 12 hours after the casting of the reactor base was completed. The pattern of cracks were mapped onto drawing (shown below). It was noted that the direction of the cracks were in plane with the top layer reinforcement. Cores were extracted at the crack lines and it was confirmed from the visual observation of the cores that the crack had formed directly in plane with the top layer reinforcement.

 

Prevention of Plastic Settlement Cracks

There are three ways of dealing with plastic settlement cracks:

  • Reduce the bleeding and settlement,
  • Reduce the restraint;
  • Adopt the technique of

 

Reduction of Bleedinq/Settlement

The provision of even small quantities of fibres will reduce the amount of bleed and settlement and will increase the resistance of the concrete to both types of plastic crack.

Air-entraining admixtures also significantly reduce bleeding and can be a most effective way of reducing bleeding and settlement.

 

Reduction of Restraint

Many of the restraints described earlier are fundamental features of a structure‘s design. However, cracks over tie bolts can be eliminated by the simple expedient of rearranging the tie bolts. Cracks over steel fixed near the top of a deep section may be reduced simply by increasing the cover.

 

Re-vibration

Plastic settlement cracks and their consequences can be completely eliminated (as opposed to prevented) by re-vibration of the concrete after they have formed (Figure 12). It is often thought that re-vibration is harmful to concrete; this is not true.

 

Re-vibration should not be applied too soon as a second phase of bleeding can still cause settlement cracks. The correct time can easily be determined by simple site trials; it will be the latest time that a vibrating poker can be inserted into the concrete and removed without leaving a significant trace.

It is essential that such re-vibration extends below the top layer of reinforcement.

It should be noted that, when blended cements are used, the time at which correct re- vibration should take place can be prolonged in cold weather. However, in hot climates, it is sensible to carry out trials to establish the latest time in which re-vibration can be carried out.

 

Plastic Settlement Cracks over Reinforcement Steel

Of the various forms of plastic settlement cracks shown in Figure 11, probably one of the most common is that caused when the concrete “breaks its back” over the steel reinforcement in the top of the deep sectons (Figure 11c).

 

If the cover to the top steel is minimal, say 20mm, this type of cracking can occur as early as 10 to 20 minutes after compacting the concrete. If the cover is increased, the time to cracking is increased and the risk significantly decreased.

 

Coring often reveals the presence of some internal cracking (Figure 13) or, in extreme cases, horizontal delamination. Furthermore, it follows that if the concrete has settled around the steel bar there is likely to be a crescent-shaped void under the steel which, initially at least, will be filled with bleed water. The effea of this void is to reduce the area of bond between steel and concrete. Although not directly relevant to cracking, the reduction in bond is so likely that it should always be considered when there is a risk of occurrence of plastic seklement cracks over steel.

 

Plastic Shrlnkaqe Cracks

Plastic shrinkage cracks occur within a few hours of placing concrete, although they are often not noticed until at least the next day. They should not be confused with long-term drying shrinkage cracks. Plastic shrinkage cracks are most common in slabs, but they also occur in the top faces of wall etc.

Concrete slabs which are correctly power trowelled should not exhibit plastic shrinkage cracks because the action of power floating and trowelling is a form of recompaction and tends to dose the cracks as fast as they form.

 

Types of Plastic Shrinkage Crack

Plastic shrinkage cracks usually take one of three forms:

  • Diagonal cracks at approximately 45′ to the edges of the slab (Figures 14a and 16), the cracks being 0.2 to 2m apart.
  • A large random map pattern (Figures 14b and 17). Cracks following the pattern of the reinforcement (Figure 18) or other physical features such as change of section.

 

Although the cracI‹s can be wide at the top (up to 2 or 3mm), they rapidly diminish with depth (Figure 19). Nevertheless, in all but minor cases they will usually pass through the full depth of a slab, in contrast with most types of plastic settlement crack.

They will probably not be noticed in the soffit of suspended slabs unless the top of the slab is very wet, and they cannot be seen underneath ground slabs unless cores are taken. These limitations may explain why it was not appreciated till the late 1970s that plastic shrinkage cracks can pass right through slabs (as opposed to stopping at the steel).

 

Plastic shrinkage cracks rarely reach the free edges of the slab because these are free to move (eg. the edge of a road slab). This is a very important way of differentiating them from long-term drying shrinkage cracks if the time of formation is unknown.

However, plastic shrinkage cracks will form up to the edge of a slab which has been cast against a previous pour, especially if there is continuity of steel, because this acts as a restraint (Figures 14c and 17).

If cracks follow the pattern of the top reinforcement, it may be difficult at first to determine whether they are due to plastic shrinkage or plastic settlement. If it can be shown that the cracks pass through the slab and follow the pattern of the steel then they are almost certainly plastic shrinkage cracks which have been orientated by the steel.

Another difficult case to diagnose with certainty is in trough or waffle floor construction. Generally, plastic shrinkage cracks will tend to form diagonal or random patterns but, because the formwork acts as a restraint, they may align themselves to follow the layout of the troughs. If, however, the cracks form only along the line of the troughs, then the mechanism is most likely to be plastic settlement.

 

Causes of Plastic Shrinkage Cracks

After concrete is placed, it experiences a multitude of intrinsic forces, caused by physical, electrostatic and “chemical” effects. Some of these cause tensile stresses and strains in the concrete; compressive mechanisms are basically of no consequence in this instance. Of these forces, the most significant is that caused by drying out of the surfaces of the concrete.

 

Primary Cause — Rapid Drying Out

If the surface of the concrete dries out, menisci forms between the solid particles and capillary forces are set up* *. This occurs when the rate of evaporation exceeds the rate of bleeding. As early as 1956, Lerch described this as the basic cause of plastic shrinkage cracking* *. The ACI Report on Hot weather concreting 10) states that drying out and cracking are likely if the rate of evaporation approaches 1kg/mh; this figure should not be treated rigorously.

The observed plastic shrinkage of concrete is shown in Figure 20 and it can be seen how shrinkage follows the end of bleeding, confirming the above explanation of plastic shrinkage cracking.

 

By comparing the time base of Figure 5 and Figure 20, it can be seen that massive shrinkage due to early drying typically coincides with the time of minimum strain capacity.

The use of retarders will generally extend the time during which the concrete has a very low tensile strain capacity and this usually increase the risk of plastic shrinkage cracks*^ ^*. The use of blended cements will also prolong the time during which concrete is vulnerable to plastic shrinkage. However, the secondary effects on water demand and blend of the concrete when blended cements are used are so complicated that it is difficult to predict whether they will increase or decrease the risk of plastic shrinkage cracks in practice.

 

Secondary Causes

Tensile strains can also be caused by a sudden fall in ambient temperatures and by the autogenous shrinkage which is caused by the early reaction of cement and water. It is generally accepted that these two mechanisms are not sufficient to initiate deep cracks but they may well increase the width of plastic shrinkage cracks caused by primarily rapid drying.

Research by Schaeles and Hover 12 on mortars suggests that the rate of screeding affects the formation of plastic shrinkage cracks, a slow rate being beneficial. Their work also shows that the cracks tend to run parallel to the screed board, ie. at right-angles to the direction of screeding. Nevertheless, un-finished mortars showed a much higher tendency to crack than those which were finished by trowelling. It is probable that the trowelling operation was reworking the surface of the mortar at the time when cracks would normally form. This bears out the field observation that plastic shrinkage cracks are rarely seen in industrial floor slabs that ha’ve been correctly power floated and trowelled. However, the use of a trowel to close plastic shrinkage cracks should not be encouraged as a remedial measure as it is unlikely to close deeper into the concrete.

Finally, movement of formwork at early age can cause tensile strain; it is not possible to quantify this effect and it is beyond the scope of this course.

 

Prevention of Plastic Shrinkage Cracks

It would be possible, theoretically at least, to reduce the risk of plastic shrinkage cracking by increasing the rate of bleeding, so that it exceeds the rate of evaporation. This is a risky philosophy to adopt in practice as changes to mix design may have unknown adverse effects on early strain capacity. Furthermore, it would increase the risk of plastic settlement cracks in deep sections.

Consequently, the only sure way to avoid shrinkage cracks is to reduce the rate of evaporation by means of early curing.

 

Curing

The importance of curing, even under supposedly mild conditions, is not widely appreciated. The rate of evaporation of water from concrete is a function of the dryness of the air and wind speed. The dryness of air (expressed as relative humidity, r.h.) is affected by temperature; the effect of all three factors on evaporation is shown in the nomograph in Figure 21.

 

It can be seen from the nomograph that the effect of wind speed on rate of evaporation is very significant. For example, the rate of evaporation in air on a warm summer day (say air temperature 220C, concrete temperature 270C and r.h. 40%) will be only 0.2kg/mh.

Even a light wind of 8km/h will double this rate of evaporation, an in a 32km/h wind (not a high speed in a multi-storey structure) the rate will exceed 1 kg/mh. The effect of wind on crack formation has been confirmed by observation both in the Iaboratory *^^**1 4* and in the fieId(1 .

The temperature difference between concrete and the air may also have a significant effect on evaporation. Large differences may occur in winter, particularly when prolonged agitation of ready-mixed concrete is carried out. At an air temperature of SOC and a concrete temperature of 200C, with r.h. of 80% in a 32km/h wind, the rate of evaporation becomes a surprising 1.1kg/mh. Thus the phenomenon of plastic shrinkage cracking is not limited to hot or windy days. It was explained earlier that it is essential to limit the rate of evaporation of water from the concrete during the critical time when the strain capacity is near its minimum, namely from about two to six hours after placing, perhaps earlier in hot conditions. This means that curing must begin earlier than usual. In the UK, curing trends to be carried out when all the finishing operations haven been completed or when there is a break in other concreting functions.

 

Mix Design

If it is not possible to eliminate the risk of plastic shrinkage cracks by improved early curing, then changes to the concrete mix must be considered. First, check that the concrete does not contain an admixture with retarding effects. If it does, try to replace it with one that does not retard rather than to counter it by adding a compensating accelerator.

Second, consider the use of air entrainment. Limited field evidence suggests that air- entrained concrete exhibits plastic shrinkage cracks less often than plain concrete. At first, this might seem illogical because, air entrainment reduces the rate of bleeding, it should increase the risk of plastic shrinkage cracks at a given rate of evaporation. However, most commercially available air-entraining agents are detergents or surfactants, and therefore reduce the surface tension across the menisci formed upon drying. Surprisingly, there has been little or no research on this subject.

Finally, it can argued that, because plastic shrinkage cracks occur only when numerous factors combine in a particular but generally unpredictable manner, almost any change to the concrete mix will reduce the risk of cracking. Although this may not be a very elegant engineering solution, it may be perfectly acceptable in practice.

 

Use of Fibres

It is well known that the use of most types of fibre significantly reduce the risk of plastic shrinkage cracks. This may seem surprising because the effect of reducing the rate and amount of bleed would appear to increase the risk that a given rate of evaporation would exceed the rate of bleeding. However, test have shown that the reason for the benefit is that the tensile strain capacity of concrete at the critical period of 2 — 4 hours after placing is so low that the addition of fibres will increase the tensile strain capacity by a factor of 2 or 3; this is usually sufficient to restrict the formation of plastic shrinkage cracks. In this respect polyproplene fibres, because they have a very high specific area, are more effective than steel; as little as 0.1% b volume was Shown to be sufficient*16)

Although there is considerable experience in the use of fibres, quantitative field data is limited. Users are therefore recommended to consider specialist or manufacturer’s recommendations on this matter.

Remedial measures

Plastic cracks often form in the top face of sections — eg. plastic shrinkage cracks in slabs, plastic settlement cracks on top of deep beams and walls. Thus they can be in exposed conditions, and this, coupled with the fact that they may form so early in the concrete’s life, means that they may widen as thermal contraction and drying shrinkage takes place. Consequently it may not be wise to fill plastic cracks with rigid materials until it is certain that these long-term effects have dissipated. The repair of plastic shrinkage cracks in road slabs is particularly critical. This is because the cracks are wide at the top and can rapidly take in detritus which may cause subsequent spalling and prevent the later satisfactory application of sealing materials. Clearly, wide cracks in slabs are not likely to be self- healing at the top and are likely to spall following freeze/thaw cycles unless efficiently sealed.

Plastic cracks by their nature pass through the cement matrix and around aggregate particles; therefore they are ragged and capable of transferring shear providing there is sufficient reinforcement to maintain aggregate interlock. Consequently full structural repairs (eg. using epoxy or polyester resin) are rarely necessary.

In many instances, the simple expedient of brushing cement grout into cracks will be satisfactory. This should be done as soon as possible after the concrete has hardened. However, it may be necessary to supplement this later with a flexible penetrating sealant if thermal contraction or drying shrinkage causes further movement.

To eliminate the risk of corrosion, plastic shrinkage cracks over steel must be efficiently sealed if the concrete is in an exposed state. Reduced bond strengths due to voids under top steel must be considered by the designer and suitable measure taken.

Early Thermal Contraction Cracks

The reaction of cement with water, known as hydration, is a chemical reaction which produces heat. If an element of concrete is big enough and is insulated by adjacent materials including formwork, then the rate of heat development in the first 24 hours is likely to exceed the rate of heat loss to the atmosphere, and the concrete temperature will rise.

After a few days, the rate of heat development falls below the rate of heat loss and the concrete will cool. As with nearly all materials, this cooling will cause contraction of the element. Theoretically there will be no cracking if this contraction is unrestrained. In practice, however, there is bound to be some restraint and this can be considered as being divided into two components:

– External restraint. If the concrete is cast onto a previously hardened base, or if it is cast adjacent to or between similar elements, without the provision of a movement joint, then it will be externally restrained. External restraint can be reduced between \/ertical lifts of concrete by limiting the time interval between pours so that the temperature history of adjacent pours is not too dissimilar.

– Internal restraint. The surface of an element of concrete is bound to cool quicker than the core; it will also respond to daily temperature variations more than the core. Therefore, there will be differential strains across the section and, where this differential is large, such as in thick sections, cracks may develop at the surface at least. This internal restraint cannot be avoided but the risk of cracking can be reduced, as discussed later.

In practice the balance of these two components will depend upon several factors, the most important being the geometry of the element, the nature of formwork and its striking time. The benefits, or otherwise, of early striking are examined in detail by Harrison*17)

The temperature recorded for a local project is given below. The slab is 22.8m x 20.5m x 1.6m (thick). The mix design used is 400kg/m 3 of PBFC cement (BS4246), water is 145l/m3, fine and coarse aggregate are 832 and 990kg/m 3 respectively, Tamseal admixture is 3.2kg/m 3, Adva 181 superplasticizer/retarder is 3.831/m^, resulting in a water/cement ratio of 0.36. The specification calls for a maximum temperature differential of around 25^C.

Fi om the temperature data, the peak core temperature is around 560C at A2 and the lowest temperature is recorded at the topmost corner at around 43*C at A1. In this case the specification requirement is met (Temperature difference is 56 — 43 = 13°C), lower than the specified maximum temperature differential of around 25O C

FIGURE SHOWING THE LOCATION OF SENSORS INSTALLED

Frequently, in the concrete specifications used for mass concrete structures, there are stipulations concerning:

– Peak temperature of the concrete; e.g. no greater than 700C

– Maximum temperature differential of the concrete, e.g. no greater that 270C

Where does the above requirement come from and how does it relate to early age thermal concrete cracking?

F’eak temperature of concrete

Delayecl ettringite formation (DEF) is expansion and cracking of concrete associated with the delayed formation of the mineral ettringite which is a normal product of early cement hydration. DEF is a result of high early temperatures (above 70°C — 80°C) in the concrete which prevents the normal formation of ettringite. There is also concern that cracking may have occurred in larger in-situ concrete structures resulting from the build up of heat from the heat of hydration in the early life of the structure. DEF can be prevented by limiting the internal concrete temperature to 70°C — 800C during its very early Iife*40*.

Maximum temperature differential of concrete

Cracking resulting from restraint to thermal movement most commonly occurs in walls cast into rigid bases as described in BS 5337. During the temperature rise period, tl e concrete has a relatively low elastic modulus and the compressive stresses due to restrained expansion are easily relieved by creep. During cooling, the concrete matures and, when the thermal contraction is restrained, the tensile stresses generated are less easily relieved. These can be of sufficient magnitude to cause cracking which commonly occurs at the half or one-third points along a bay.

Estimating earlv thermal crack widths

The maximum acceptable temperature reductions given in Table 3.2 apply to pours that are subjected to a well defined form of thermal restraint. In practice, however, restraints result in differential thermal strains which depend on the nature of the temperature distribution and the ratio of the “hot’ and “cold” areas. Experience has shown that by limiting temperature differentials to 20 °C in graVel aggregate concrete, cracking can be avoided. This represents an equivalent restraint factor R of 0.36 and the corresponding values for concrete with other aggregate types are given in Table 3.2 of BS 8110-2:

 

In Singapore, only granite aggregate is used. Therefore, the limiting temperature differential to prevent cracking of concrete using granite aggregate is estimated using the formula:

Applying the above values to the formula gives a limiting temperature to prevent early age thermal cracking as t = 27.70 C. This is the basis of specifying a maximum temperature differential of not more than 27OC in the concrete specifications in Singapore.

It is to be noted that the external restraint factor varies under different conditions, shown in table 3.3 of BS 8110 : Part 2, reproduced below:

BS 8007 uses a sti ain based approach and this has been maintained in EN 1992-3 (SS EN 1992-3)*41 *. It is genmrally assumed that compressive stresses induced during heating are relieved by creep. I-lence the i-estrained contraction c,that may lead to cracking, is related to the drop from the peak temperature in the section to the mean ambient temperature Ti. the coefficient of thermal expansion of concrete a , the restraint 4 and the creep coefficient according to the equation:

Applying the above values to the formula gives a limiting temperature to prevent early age thermal cracking as T —- 24.60 C.

Again, it is to be noted that the external restraint factor ( varies under different conditions, shown in annex L of SS EN 1992-3, reproduced below:

Thus, one of the most important factors which assist in diagnosing whether the crack is due to early thermal contraction as opposed to long-term drying shrinkage is a knowledge of when the crack first forms. A crack which forms in the first two weeks is unlikely to be drying shrinkage unless the element is a thin slab subjected to extreme drying conditions. Conversely, cracks which form after a period of several weeks or months cannot be early thermal contraction cracks.

Early thermal cracking and its differentiation from long-term drying shrinkage was highlighted in the 1970s and the work of Hughes and others culminated in the philosophy of “limit state of cracking” in BS 5337 and later in BS 8007. Anson and Rowlinson *1 **^ examined the restraint factors used in BS 8007 and suggested some modifications. Other researchers have produced papers on the factors which affect temperature build-up and subsequent cracking and CIRIA have produced a comprehensive treatise on the subject(20)

Therefore, the following aide-memoire should suffice to enable certain key symptoms to be recognised and suitable measures taken.

Factors affecting Temperature Rise

Initial temperature of materials. Warm materials lead to warm concrete, but in practice the temperature of the aggregates and water are more significant than that of the cement.

Ambient temperature. During hot weather, concrete will develop a high peak temperature but there may be a greater differential between peak and ambient temperature in the colder seasons ie. greater cooling and contraction.

Dimensions. Larger sections produce more heat but there are limits above which the problems do not get worse**1

Curing. After the formwork has been removed, concrete should not be cured by spraying with water (presumably cold) as this will increase temperature differences — known as thermal shock. Instead, the concrete should be kept warm for a suitable period.

Formwork striking time. Early striking may reduce peak temperatures but will also increase temperature differences.

Type of formwork. Timber formwork will produce warmer concrete than steel or grp but the temperature differences will be reduced. Additional insulation will have a similar effect.

Admixtures. Accelerators will inevitably produce heat more quickly; retarders may simply delay the onset of the hydration: they will not reduce the total heat generated.

Cement content. More cement means more heat.

Cement type. In simplistic terms there will be an increasing order of total heat production as follows:

  • Low heat PBFC (BS 4246, BS EN 197)
  • PBFC (BS 146, BS 1370)
  • Low heat PC (BS 1370)
  • SRPC (BS 4027, BS EN 1370)
  • OPC (BS 12, BS EN 197)
  • RHPC (BS 12, BS EN 197)
  • HAC (BS 915, BS 1370)

 

Blended cements. Blended cements usually reduce heat but their use may require other changes eg. formwork striking time.

Factors affectinq Early Thermal Contraction Cracks

Types of aggregates. Limestone and, to a lesser extent, granite aggregate concretes have lower coefficient of thermal expansion than other dense aggregate concretes and should therefore crack less.

Tensile strain capacity. The advantage of using blended cements to reduce the temperature rise (and therefore subsequent fall) of the concrete may be partially offset by a reduction in creep and tensile strain capacity. Lightweight aggregate concretes have a high tensile strain capacity and their use can be beneficial.

Reinforcement. Crack widths can be reduced by increasing the amount of reinforcement. Crack widths are reduced by (a) the use of small diameter bars (b) the use of deformed rather than plain reinforcement, and (c) reduction of over to the minimum allowable.

Stress raisers. Cracks are often initiated by sudden changes in section, box-outs and even formwork tie bolt holes. They may also start where high concentrations of reinforcements are curtailed.

External restraint. This can usually be dealt with by providing movement joints. However, when two pours of concrete cannot be separated (eg. a wall cast onto a hardened base), the effect can be minimised by reducing the delay between the two pours.

Internal restraint. This occurs when the outside face of a concrete section cools quicker than the core. It is commonest in thick sections and can be minimised by delaying the removal of formwork, the use of insulation, or both.

Of the above factors, some result from fundamental features of the structure, eg. dimensions; some can be determined by the designer, eg. reinforcement, and some are in the hands of the contractor eg. curing. However, factors may be inter-related, such as the provision of crack control steel and the spacing of joints, and liaison is necessary between the designer and the contractor if uncontrolled racking is to be avoided.

 

Use of Blended Cements

GGBS and PFA usually reduce the heat developed, and therefore the rise in temperature of the concrete, generally in proportion to the amount of OPC replaced. For further guidance see CIRIA report No. 91. However it follows that this also means that the properties of hardened concrete at any given time will be affected. In particular, the tensile strength and strain capacity at the important early age of 2 — 4 days can be reduced by an amount which is more than the benefit gained from the reduced temperature.

This effect was first noticed by Bamforth who concluded that, in order to gain a net benefit from the use of blended cements, certain minimum amounts need to be used depended on section thickness. These recommendations are summarised in Table 2 and have been incorporated in the Concrete Society Working Party Technical Report No. 40 *The use of GGBS and PFA in concrete** *

 

ExamgolesI Early Thermal Contraction Cracks

Cantilever Walls

Probably the most common form of easy thermal contraction cracking is on cantilever walls in, for example, service reservoirs, retaining walls, bridge abutments and basements. Figure 24 shows a case of early thermal contraction cracks in a water-retaining structure designed to CP 2007**^*.

Figure 24: Early thermal contraction cracks of the order of 0.1 to 0.tmm width in a settlement tank wall. The photo was taken a week after filling the adjacent tank and shows water seeping through the cracks, which sealed autogenously within a few days.

The requirements of oack control reinforcement given in BS 8007 as well as the impact to cost to meet the crack widths stated are not discussed here.

With cantilever walls, an alternative approach to the control of thermal contraction cracks is to incorporate them in crack-inducing joints****. This can be a very economical solution and can also be adopted on a contract where it is not possible to alter the basic design parameters.

 

Larqe Pours

The traditional method of limiting early thermal contraction cracks in large sections is to reduce cement content substantially or impose strict limitations on thickness or depth of pour.

Nowadays, many large and deep foundations exists and the philosophy is based on continuous pours using the theory of insulation rather than cooling. This relies on a knowledge of the likely peak core temperature and the assumption that, if the maximum temperature difference between the core and the concrete surface is limited to 200C, cracks will not form. The 200C figure is acknowledged to be o\/er simplified and conservative; if relevant data can be obtained it can be suitably increased 21)

The temperature difference is limited not by keeping the concrete cool but by keeping the surfaces warm by insulation. Insulation will have little effect on the core temperature, but will significantly raise the surface temperatures and hence reduce temperature differences. It is usually necessary to monitor these temperatures by means of thermocouples or other means until sufficient experience has been gained to know when the insulation can be removed.

Some contracts ha\ie utilised the technique of cooling the concrete by injection of liquid nitrogen*2^*. This technique has the advantage that it requires low capital outlay and can be (literally) switched on when extremely hot weather occurs.

 

Case Study of a Mass Goncrete Foundation Base

The concrete foundation base had the following data and requirements:

  • Maximum thickness of foundation 2.693m
  • Specification peals temperature ñ 650C (Use 750C to avoid delayed ettringite formation)
  • Specification wax allowable temperature differential s 20*C (Use s 270C for granite aggregate)
  • Intent of the specification is to limit and minimize crack formation

A mock-up block (2.km x 2.5m x 2.693m) was carried out to simulate as-close to actual conditions to study the temperature profile.

To reduce the peak temperature of the concrete, the following actions were taken:

  • Use low heat cement type PBFC
  • Reduce the cement content
  • Keep ambient temperature low by casting at night
  • Keep aggregate and sand temperature lower by casting at night
  • Use chilled water for mixing concrete

 

The temperature of mock-up was monitored using thermocouples. From the results of the temperature monitoring, improvement to the assumptions from the mock-up were taken into consideration for the actual casting. This included the when to remove the side formwork and when to insulate the top of the element. This allowed the actual casting to be accomplished with a hiph chance of success.

The peak temperature profile for the mocl‹-up and actual casting is shown below:

The actual casting was a success. The temperature data were:

  1. Peak temperature = 730C (below the maximum allowable of 750C)
  2. Max temp differential was = 28*C

The concrete casting was a success, no cracks were observed!

 

Warping of Thin Slabs

Thin external paving slabs tend to be subjected to severe changes in temperature differentials as the bottom of the concrete heats up and subsequently slowly cools, but the top surface rapidly responds to the daily changes in the ambient temperature. This is more prevalent in countries with cooling climates.

 

Use of Fibres

Currently there is little evidence of the use of synthetic or steel fibres to control early thermal contraction cracks. However, the use of steel fibres in industrial floors may help to control cracks caused by warping of thin slabs.

 

Recommendations

There are no simple recommendations for minimising early thermal contraction cracks because they can be controlled only by carefully co-ordinated planning by both the designer and the contractor.

The fundamental factors which have to be considered and specified are:

Design and specification

  • Restraint (size of pour, spacing of movement joints)
  • Distribution steel (designed in relation to all other factors, particularly restraint)
  • Heat development (section thickness, cement type and content) Aggregate type

Construction

  • Restraint (sequence and timing of pours, additional mo\/ement joints)
  • Heat development (choice of concrete materials and formwork type)
  • Cooling (striking of formwork, curing, insulation)

 

Remedial Measures

Thermal contraction cracks in thin slabs, as described in the above, will normally penetrate through the full concrete section. If sufficient steel is provided to limit the crack width at the surface to 0.2mm after the initial thermal cycle (ie. one to two weeks), it is unlikely that the cracks will open up significantly during subsequent drying shrinkage. Conversely, if the initial cracks are wide and not closely spaced, then they are unlikely to be controlled by the steel. They should be regarded as ‘live’ cracks and should be repaired accordingly. Clearly the same applies to thermal contraction cracks in mass concrete. Cracks in large pours do not usually extend through the concrete section and repairs may be concerned more with durability than with water retention.

Warping cracks should be treated in exactly the same way as long-term drying shrinkage cracks.

 

Lonq-term Dryinq Shrinkaqe Cracks

Cracks that form for reasons outlined in previous sections of this course are often mistakenly diagnosed as long-term drying shrinkage cracks. Drying shrinkage may be defined as the reduction in volume in concrete caused by chemical and physical loss of water during the hardening process and subsequent exposure to unsaturated air. The resulting reduction in volume, as already stated at the beginning of this course, can cause cracks only if the concrete is restrained in some way.

 

Factors affecting shrinkaqe

All constituents of concrete influence drying shrinkage either indiVidually or as a result of their interaction. The shrinkage of a particular concrete mix is also affected by additional factors such as temperature history, curing methods, relative humidity and ratio of volume to exposed surface.

Whether or not the drying shrinkage is sufficient to cause cracks depends on the properties of the concrete, the degree of restraint and the detailing of any reinforcement.

 

Concrete constituents

Water

The more water that is available to evaporate from the concrete, the higher the tendency to shrink on drying and the lower the capacity to resist tensile stress. Consequently, the water content of a concrete has the most significant effect on its long-term drying shrinkage.

Cement

As a general rule, it can be assumed that the influence due to cement types is minimal and that cement per se is of importance only so far as the amount of cement used affects the water content of a particular mix.

Blended Cements

The use of blended cements and their properties have no large impact that governs the long-term drying shrinkage cracking in concrete.

Aqqreqate

Sound, normal weight aggregates for concrete have low shrinkage compared to cement paste. Apart from the obvious economic advantage of using aggregate, the more that can be included in the mix, the more pronounced will be the reduction in the effect of the cement paste’s high drying shrinkage.

Admixtures

An admixture affects the drying shrinkage because it either changes the water requirement of a mix or influences in some other way the behaviour of the cement paste. The materials present in admixtures are diverse and advice on the effect on a mix of a particular product should always be obtained from the manufacturer.

 

Relative Humidity

Considered as a factor that is contributes to the loss of moisture from the concrete that leads to shrinkage, the relative humidity of the air surrounding the concrete plays an important part in determining the amount of shrinkage. In practice, little control can be rs 57 exercised over long-term relative humidity and fortunately in our local climate of average 70-80% rh, the effect of moisture loss is a gradual one.

 

Curinq

In this context, the term “curing” refers to measures taken to pre\lent the rapid or too early loss of water from exposed concrete. Curing may sometimes play a dual role by providing insulation against heat loss, thus reducing thermal gradients in large units.

Size and Shape of Unit

The rate of moisture loss decreases rapidly as the distance from the surface of the concrete increases and a large thick section will retain more water for a longer period than a thin slab. This can be accounted for by using the concept of effective section thickness, which is defined as twice the volume divided by the area of any surfaces exposed to drying; see reference 26.

Factors affectinq Lonq-Term Dryinq Shrinkaqe Cracks

These factors, in addition to those referred to in the preceding section, are similar to those covered in section of thermal cracking. They are restraint, tensile strain capacity and the provision of reinforcement and joints. Their particular effect on drying shrinkage is covered here.

Restraint

The effect of external restraint can generally be reduced or eliminated by careful design measures such as provision of slip layers, suitable bearings or free movement joints, and is not dealt here.

Internal restraint can rarely be avoided, but its effects can be reduced by careful design. The core of a large element of concrete takes a long time to dry out, but any exposed faces tend to shrink if they are subjected to a drying environment. This differential drying shrinkage will tend to cause cracks at the surface but they will not necessarily penetrate deep into a massive or thick section. The most obvious examples of internal restraint occur in (a) mass concrete bridge abutments or dams and (b) road and floor slabs which cannot loose water through their base.

Although cracks in slabs caused by cycles of relative humidity (and indeed aggravated by daily thermal cycles) will initially be only on the surface, they may later penetrate right through the slab if the design and construction of reinforcement or joints is not correct.

 

Tensile Strain Capacity

The concept of cracking occurring when the restrained tensile strain exceeds the tensile strain capacity of the concrete has already been discussed earlier.

 

Provision of Reinforcement and Joints

The effect of conventional steel reinforcement on cracks has been also been described earlier but it is worth recalling that such reinforcement does not prevent crack formation although it does control their widths.

Use of Fibres

Fibres have insignificant effects on unrestrained long-term drying shrinkage of concrete. Furthermore, there is usually a similar reduction in creep and so the benefit on cracking in practice could be doubtful.

Some specialist suppliers offer steel fibre-reinforced concrete for industrial floors which, by giving savings in slab thickness and/or the elimination of joints, offsets some of the extra cost of fibres.

Curinq

Curing is essential to improving the hydration process, it will also improve all the mechanical properties of the concrete, notably tensile strain capacity. It is for this reason alone that curing helps to reduce cracks due to drying shrinkage. However, even this improvement may not be maintained with prolonged curing because the benefit of creep reduces with time.

Recommedations

To Reduce Drying Shrinkaqe

  • the highest possible aggregate content
  • the largest possible maximum aggregate size
  • concrete with a workability as low as is compatible with ease of placing and achieving full compaction
  • suitable admixtures, but consult the manufacturer regarding possible side effects

 

To Reduce Drying Shrinkaqe Crackinq

In addition to the above

  • provide adequate curing to exposed surfaces particularly on large flat areas
  • eliminate external restraints as much as possible particularly by providing movement joints where applicable
  • providing sufficient crack control or distribution steel

 

Remedial Measures

To specify the type of repair to an unacceptable long-term drying shrinkage crack, it is necessary to determine the basic cause and the likelihood of further movement.

The basic principle that rigid materials may be used to repair “dead” cracks while flexible materials may be used to repair “li\ie” craCks will apply.

 

Crazinq

Crazing is the cracking of the surface layer of concrete into smaller irregularly shaped contiguous areas (see Figure 25). The cracks are shallow that they do not affect the structural integrity of concrete and in themselves should not lead to subsequent deterioration of the concrete. Crazing generally occurs in

  • the floated or trowelled surface layers of concrete slabs and
  • the formed surfaces of cast stone and concrete

 

The crazing pattern IS often an hexa ona/for with a of 5 to 75mm across each “map”. The small dimensional pattern is typical of wet-cast vibrated concretes, especially concretes cast against smooth formwork or moulds. The surface crazing cracks are typically between 0.05 and 0.50mm wide and down to 2 or 3mm depth at the most. Autogenous healing through calcite formation is common, particularly for the smaller pattern.

Crazing may be present but not noticeable until the cracks are filled with dirt. All factors being equal, concrete made with white cement will not craze more than normal; it may appear to do so simply because the cracks are more readily seen on a white surface. Many observers believe that the incidence of crazing in power-trowelled industrial floors is increasing and causing some concern. However, as long as the crazing “cracks” have no perceptible depth, they should be of no consequence other than aesthetically.

 

Causes of Crazinq

Crazing is generally accepted as being as being the result of surface tensile stress caused by shrinkage, differential moisture movement, of the surface relative to the mass. Thus craving is not necessarily an ageing phenomenon but occurs whenever critical conditions are exceeded. The duration of exposure is therefore irrelevant although prolonged exposure will increase the probability of critical conditions.

Crazing is caused by stresses resulting from differential moisture movement due to:

  • a high moisture concentration gradient; and/or to
  • a discontinuity in composition near the exposed

It follows from (a) that neither a porous material incapable of supporting a moisture concentration gradient no an impermeable materials is likely to craze.

 

It follows from (b) that a material whose surface is different from the underlying material (eg. because laitance has been brought to the surface as a result of over-trowelling) is likely to craze.

Climatic Conditions

The most important climatic factors governing crazing are the relative humidity and the air temperature during a drying period. The lower the relative humidity and temperature, the more severe the conditions.

Type of Formwork

The type of formwork affects the permeability of the formed concrete surface and is a most important factor.

Mixes

In general, rich or wet mixes have a greater risk of crazing.

Recommendations to Reduce the Incidence of Crazinq

Floated or Trowelled Surfaces

  • Avoid over-rich or excessively wet mixes. Mixes with high cement contents may result in surface laitance which will allow differential moisture movement and hence be prone to crazing.
  • Compaction should remo\le trapped air and leave a concrète surface ready for the surface finish intended. If \/ibration continues for too long, the surface layer will be too rich and too wet and crazing may result.
  • Avoid excessive finishing or any procedure which will depress coarse aggregate and produce a concentration of cement paste and fines at the surface. Remove bleed water before performing finishing operations. Do not dust fine materials on to the flat surface to absorb bleed water. Delay steel trowelling until the water sheen has disappeared from the slab surface.
  • Applying adequate and continuous curing as soon as finishing has been completed. Curing should be a continuous process, ‘very often, the curing is not done diligently. Wetting and drying often does more damage (resulting in crazing) than compared to continuous curing.

 

Formed Concrète Surfaces

  • Avoid over-rich or excessively wet mixes.
  • Avoid the use of formwork with a face of low permeability. Such formwork is much more likely to produce a concrete surface which has a different porosity from the body of the concrete.
  • Curing should be adequate for the formwork system employed and should be continuous after formwork is struck, if this is less than three days from casting. Again, wetting and drying cycles should b avoided.
  • Curing agents. If concrete is treated with curing agents, then it will not allow the moisture movements necessary to cause crazing. Seek manufacturer’s advice on the appropriate curing agents.

 

Remedial Measures

Although doing nothing does not sound like a remedial measure, autogenous natural weather healing has been mentioned earlier and consideration should be given to allowing this to occur, especially for the smaller crazing pattern associated with vibrated concretes.

 

Other Types of Crack

Chlorides / Carbonation and Corrosion of Reinforcement

Carbonation

When reinforcing steel is embedded in conwete, it does not normally corrode because, in the inherently alkaline environment, a protective passive layer forms on its surface. However, if the depth of cover is insufficient or the concrete is highly permeable, then the concrete may carbonate as deep as the reinforcement and the protective layer may be at risk. The process of carbonation is variable and is fully discussed by Roberts****.

New concrete is alkaline and has a pH of 12-13 and it acts as a passive film to protect the rebar. Carbonation of the concrete takes place over time over the life of the structure. As the carbonated concrete front propagates through the concrete cover to reach the steel, depassivation of the area (changes from alkaline to acidic, pH 9.5-11.5) around the steel occurs leading to the initiation of corrosion.

• COC + Ca(OH)z e + CaCO + He0 (pH 8-9)

 

Chlorides

When exposed to chlorides over extended periods and when the chlorides in the pore solution exceeds the threshold level, chloride ions activate the surface of the steel to form an anode. The passive layer will also be broken down in the presence of excessive amounts of chloride ions. The chlorides can originate from sodium chloride (sea-water) in the local context.

When the protective passive layer breaks down, the steel is liable to rust or corrode and, this is an expansive process, it can cause the concrete to crack and spall. Cracking and spalling are particularly noticeable at corners of beams and columns over the main steel, although the pattern of links and stirrups can often be seen also. Such cracks will usually show signs of rust stains.

 

Dissimilar Metals

When two different metals are contact with each other, the metal of a lesser galvanic series (or electropotential series) will corrode.

When two metals are submerged in an electrolyte, while also electrically connected by some external conductor, the less noble (base) will experience qalvanic corrosion. The rate of corrosion is determined by the electrolyte, the difference in nobility, and the relative areas of the anode and cathode exposed to the electrolyte. The difference can be measured as a difference in voltage potential. the less noble metal is the one with a lower (that is, more negative) electrode potential than the nobler one, and will function as the anode (electron or anion attractor) within the electrolyte device functioning as described above (a qalvanic cell). Galvanic reaction is the principle upon which batteries are based.

Typically, zinc (less noble) will corrode in preference to steel (more noble)

Rate of Corrosion

The corrosion rate is influenced by:

  • Chloride ion content
  • pH level (carbonation)
  • Goncrete permeability
  • Availability of moisture to conduct ions in tl e concrete
  • Cover to the reinforcement

 

Likelihood of Corrosion using Measurements

The technique of half-cell potential measurement *2 to assess the condition of reinforcement is now well established. It is widely used to determine the likelihood of reinforcement corrosion in structures, particularly for structures near the coast or at sea.

The permeability of concrete will be reduced by the use of blended cements and GGBS is recognised as being particularly efficient at chlorides penetrating to the reinforcement 2

The different phases and rate of damage to the concrete by corrosion-induced-cracking can be gi aphically presented by the following time-damage graph:

to — is the time taken for the de passivation of the layer around the steel to taI‹e place. You can delay the onset the agents of carbonation oi chlorides by applying a coating on the concrete

Ti — is the time thereafter TO whereby by corrosion of the steel takes place. Applying coating on the concrete at this stage does not help much as the corrosion has already started.

A corrosion cell is formed when reinforcement starts to corrode, liI‹e a small battery, and a small voltage can be measured. Four elements are needed to complete a corrosion cell:

  • An anode
  • A cathode
  • tonic continuity through an electrolyte (i.e. concrete)
  • Metallic / electrical connection (i.e. rebar) between ihe anode and cathode

 

Remedial Measures

For a detailed consideration of remedial measures to corrosion-damaged concrete, see Concrete Society Technical Report No. 26*^^* and BS EN 1504 34)

Alkali-Silica Reaction Cracks

One form of alkali-aggregate reaction, namely alkali-silica reaction (ASR), is known to have caused cracking and expansion in concrete structures. The parts of structures affected are of high alkali content and have been subject to ground-water, rain or heavy condensation.

Where ASR has caused cracking, there is often persistent dampness along the edges of cracks, discolouration and signs of expansion of the concrete, but absence of spalling. ASR is a reaction between hydroxyl ions in the pore fluid within a concrete and certain forms of silica occasionally present in significant quantities in the aggregate. The production of the reaction is a gelatinous silicate hydrate containing sodium, potassium, calcium and water. Its formation and growth can occasionally produce internal stresses within the concrete which are large to induce micro-cracking, expansion and visible micro-cracking.

Diagnosis of cracking due to ASR is not straightforward because ASR gel can be found infilling micro-cracks in concretes unaffected by ASR. To establish that ASR is probably the primary cause of cracking, it is necessary to rule out other causes of cracking and expansion, to show that substantial quantities of ASR gel are present in the concrete and to show that the crack distribution is typical of that induced by ASR.

Photo below shows a concrete barrier affected by ASR cracking

To minimise the risk of cracking due to ASR, a number of approaches are possible:

  • use an aggregate with a long history of satisfactory performance
  • test the aggregate for reactive silica
  • test the cement aggregate combination for reactive silica
  • place an alkali limit on the concrete or the cement
  • use a blended cement

In Singapore, tile Building Control Authority (BCA) Act Part VIA Supply of Essential Construction Materials (‘the Act’) & BCA (Importer’s Licensing) (Amendment) Regulations 2016 (‘the Regulations’) have imposed a testing regime on imported aggregates to determine the quality of aggregates imported from overseas sources. A licensed importer (‘licensee’) is subject to the testing regime and is required to obtain an ‘Import Permit’ each time he imports aggregates from his source.

 

 

 

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