Crack Inspection and Assessment

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Inspection and Assessment

Introduction

An inspection and assessment is normally undertaken to determine the cause(s) of the cracks in order to determine the effective remedial strategies. If the assessment on why the crack has formed is incorrect, the selected method of repair may not work. Hence, it is important to have the cause diagnosed correctly.

Guides and References

BS EN 1504 — Products and Systems for the Protection and Repair of Concrete Structures 3 4) is mainly a comprehensive set of standards for

material requirement for various concrete repair techniques, it also covers the selection of repair methods based on principle(s) ie. the purpose to which the repair method will address the deterioration issue, right through the quality control during the repairs and maintenance thereafter.

Therefore, even BS EN 1504 calls for the proper assessment of defects before deciding on the appropriate materials and techniques used for the repairs. BS EN 1504 is broken down to the following parts:

  1. Definitions
  2. Surface protection systems for concrete
  3. Structural and non-structural repair
  4. Structural bonding
  5. Concrete injection
  6. Anchoring of reinforcing bars
  7. Reinforcement corrosion protection
  8. Quality control and evaluation of conformity
  9. General principles for the use of the products and systems
  10. Site application of products and systems, and quality control of the works

 

In the context of cracks in concrete, perhaps Parts 2, 3 and 5 would be more relevant in terms of material characteristics and requirements to be used. The challenge is to produce a range of products that satisfy the repair principles and performance. Part 9 is the key document for the Engineer as it provides a structured approach to the selection of repair method after the cause of the cracks is determined.

The various steps in the process of assessment, choosing repair option, selection of product and systems, site execution and monitoring of structures described in BS EN 1504 Part 9 are illustrated below:

Steps:

  1. Assess structure.
    Possible reasons for the damage, the present condition and durability of the structure, its operating en\/ironment and impact on its future durability.
  2. Choose options
    These range from doing nothing to partial or complete
    demolition and replacement. Consider aspects such as the likely long-term performance of protection or repair works, the acceptable number and cost of future repair cycles and the cost of future repair cycles and the costs of alternative protection or repair options, including future maintenance and access.
  3. Select repair principles
    Select appropriate principles, such as concrete restoration, structural strengthening and cathodic
    protection as described in more detail below.
  4. Choose repair methods
    Choose methods appropriate to selected repair principles in the light of available products and systems.
  5. Specify material performance.
    Select materials with the performance characteristics required for the chosen application.
  6. Carry out repair
  7. Set out ongoing requirements
    Develop instructions on inspection and maintenance to be undertaken during the remaining life of the structure.

Site application and the associated quality control are covered in Part 10 of the Standard which gives general guidance for the preparation, application, and quality control of the selected system. Ultimately, Engineers, Material Suppliers and Contractors will benefit from the cross consultation with each other using the Standard as a reference to ensure success in a project.

 

Carry out Inspection and Assessment

We will take a detailed look into Steps 1, 2 and 3 here. Steps 4 to 7 will be discussed in the next Section as the topics fall appropriately under materials, repair methods and quality control during repairs.

Step 1 — Assess Structure

The objective here is to determine what has caused the cracking. It is likely that the following background information would be useful in any assessment:

Historical Information on the Cracks Lines.

Information such as when the structure was completed and when the cracks were first noticed. Environmental conditions such as weather (rain or hot weather). External influences such as premature dismantling of formwork and loading of a newly cast slab may lead to the cracks. It is important to keep records such as sequence of casting, time of casting, duration of casting and when curing commenced and stopped.

Mappinq of Crack Lines.

The pattern of the crack lines should be drawn (known as mapping) for record keeping. Often, the mapping of the crack lines may be able to provide preliminary information as to the type and nature of cracks that have been formed. Moreover, the mapping may be used later as records highlighting the quantities and locations of the repairs. It is useful at this stage to measure the widths of the crack lines and indicate it on the mapping. The UK codes which limit widths make no attempt to define the measuring techniques to be used. The most common methods currently available use graduated magnifying devices, templates or feeler gauges. These can be accurate for wide cracks, but when two people measure the width of a crack of the order of 0.1mm to 0.2mm (ie. BS 8007 limits) it is unlikely that they will obtain similar or even repeatable results. This is probably because cracks are not simple, parallel sided “canyons”. The width of a crack line would usually be wider on the surface of the concrete compared to width within the concrete and cracks may:

  • taper quickly from the surface (eg. Plastic shrinkage cracks)
  • remain approximately parallel throughout the section (eg. Thermal contraction cracks in thin walls)
  • widen within the section (eg. Thermal gradient cracks in deep foundations. These may be quite wide and yet not extend to the surface)

 

Cracks may be regarded as unacceptable if they

  • are aesthetically unacceptable (eg. in off-form fair face concrete); exceed requirements of concrete specifications (SS EN 1992 specifications of limiting crack widths to 0.2miv, 0.3mm or 0.4mm depending on the applicable clauses)
  • make the structure non-watertight (BS 8110*^5* suggests that cracks up to 0.3mm wide are generally aesthetically acceptable. BS 8007 36 recommends crack width limits of 0.1mm in location of “critical aesthetic appearance” and 0.2mm elsewhere, and, by implication, expects therefore that all cracks less than 0.2mm will prove to be watertight in all circumstances. When water percolates through cracks, it dissolves calcium hydroxide salts from the cement matrix and then, on contact with carbon dioxide in the atmosphere, deposits crystals of calcium carbonate. This action of autogenous healing, can be effective at sealing cracks, although the process is likely to produce unsightly stains on the surface. However, the likelihood of a crack being sealed by this mechanism depends on:
    – the width of the crack
    – the head of water
    – whether the crack is tapered or parallel
    – whether the crack is liable to further movement when the structure is commissioned.

 

However, in the case of a structure such as an elevated water tower, it is probable that no leakage would be acceptable because the stains produced by water or calcium carbonate deposits are bound to be unsightly on the outside of the tank. In order to satisfy this onerous requirement — which is a combination of watertightness and aesthetics — it will be necessary to design the tank in one of two ways. The first would be to ensure that the tank has no racks and that all contraction and shrinkage are accommodated at movement joints fully protected by waterstops. The second and more secure solution would be to line the tank internally so that water cannot reach the cracks or joints;

 

Affect the durability of the structure (If cracks allow the ingress of carbon dioxide (and chlorides if present)) and this will break down the passive state of the steel surface, an electrolytic cell will be established at or near the crack and thus corrosion of the steel will take place. Various studies(3 3 (38a have revealed different views of the impact of crack width and the direction of crack on the impact of corrosion of the reinforcement. Still, the fact remains when favourable conditions exist, ie. the presence of supply of oxygen and water, cracks that intersect to the reinforcement (whether cracks which follow the line of reinforcement (coincident cracks) or cracks which cross the line of reinforcement (intersecting cracks) will initiate reinforcement corrosion.

 

Are structurally significant (not considered here)

 

Photographs of Crack Lines

Photographs of the crack lines, both general and close-ups, should be taken as much as possible for record keeping, also, the pattern of the crack lines may provide information of the possible causes.

In the above photograph, slanted crack lines were noted to on the column and wall. Core samples were extracted on the cracks itself. The cracks were found to have penetrated the plaster layer into the surface of the concrete as shown in the photograph below.

Extraction of Samples

Quite often, core samples are taken at pre-determined locations of the crack lines after reviewing the historical information and results of the mapping survey. Extraction is usually in the form of taking core samples. The diameter and depth of the core will vary depending on the physical dimensions of the structure such as thickness of the component (beam or slab) and the width of crack. One of the most useful information that can be obtained from the core samples is the depth of the crack from the surface of the oonwete.

 

Testing

Testing would provide information such as depth of concrete cover to reinforcement, depth of carbonated concrete from the concrete surface, amount of chloride content in concrete, likelihood of corrosion of reinforcement using half-cell potential measurement, petrographic examination to determine alkali-silica reaction.

Measuring depth of carbonation

The depth of carbonated concrete can be measured by spraying a phenolphalein solution on the freshly extracted core sample. The carbonated zone will remain colourless while the non-carbonated zone will remain fuscia in colour. This test is covered by BS EN 14630, Products ance systems for the protection and repair of concrete structures, Test IVlethods, Determination of carbonation depth in hardened concrete.

Where the depth of carbonated concrete has exceeded the depth of concrete cover, the passive oxide layer surrouncling the steel bar is de- passivated, allowing the initiation of corrosion of the steel bar to take place The application of an anti-carbonation coating will not be of use at this stage.

Measurinq chloride content

The chloride content in the concrete sample can be measured using the titration method in the laboratory covered in BS 1881-124: 2015 Testing concrete, Methods of analysis of harclened concrete.

In chloride-free content concrete the steel is protected by a passive oxide film that is formed in the highly alkaline environment of cement paste (> pH 13). This passive film is being continuously corroded, but also continuously reinstated by the hydroxyl ions in the pore water. However, when chloi ides reach the steel, they act as a catalyst to accelerate the breakdown of the passive layers. This process begins at very low chloride contents, but it is only when the chloride ions reach a critical level in relation to the hydroxyl ions in the pore water that the rate of breakdown of the passive film exceeds the rate of replenishment, and significant corrosion begins. The chloride content at which this occurs is known as the ‘threshold’ level.

A threshold value of 0.4% chloride (by binder weight) has evolved in many countries, but it is believed to err on the side of safety. Reported threshold levels vary from 0.25% to 2.5% (a tenfold difference) indicating that there is unlikely to be a fixed value for a given concrete. Experts have suggested that the chloride threshold level is best considered in terms of corrosion risk. This approach was suggested by Browne, who proposed the risk classification in the below table:

Chloride (% by weiqht of binder) Risk of corrosion
< 0.4 Neqligible
0.4 to 1.0 Possible
1.0 to 20 Probable
Greate than 20 Certain

 

The concrete coi e is typically cut in equal increment of thickness starting front the surface reaching the depth of concrete cover. Hence, a concrete with a concrete cover of 50mm may but cut in 20miv increments. Each 20mm increment would then be tested for the chloride content.

 

A chloride content exceeding the threshold value at the steel location would suggest the increased risk of corrosion.

When excessive chlorides are present, it must be remembered that the corrosive process cannot be simply chemically arrested by, for example, sealing the surface; chloride attack can continue slowly with the presence of oxygen and moisture. All the concrete adjacent to the cracks and surrounding the affected steel must be removed. The steel must be thoroughly de-rusted and protected from further attack, and concrete replaced with layers of repair mortar. However, it is quite likely that some further areas of the concrete will subsequently crack due to the incipient anode effect; thus a decision will have to be made to determine how much apparently yet unaffected concrete should also be removed. This decision is usually governed by financial restraints, and pei iodic monitoring and repair is sometimes more economical than complete removal.

 

Measuring concrete cover and diameter of steel bar

The instrument shown in the abo\ie photograph is a covermeter. It measures the depth of the concrete cover as well as the diameter of the rebar. A more advanced type of covermeter is shown below.

Measurinq the chance of corrosion of steel bars

The instrument shown in the below photograph is a half-cell potential meter with the probe. It is used to measure the degree of reinforcement corrosion (readout in mV with reference to Cu/CuSO‹ electrode). ASTM C876-09 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete gives the test method.

The criteria for determining degree of corrosion is shown on the table below:

 

mV (Cu/CuSO4) Chance of Corrosion
Greater than – 200 5%
–     200 tO – 350 500/«
–     350 to – 500 95’/o
Lesser than – 500 Visible evidence

 

The photograph below shows a screenshot taken from the software used in Canin+ corrosion analysis manufactured by Proceq.

Measurement of uniformity / strenqth / depth of crack of concrete

The instrument shown below is the ultrasonic pulse velocity (UPV) tester. It is used to measure the uniformity of concrete and the applications can also be used to measure the strength of concrete, and in some cases the depth of cracks in concrete.

From the historical information , mapping of crack lines and photographs, extraction of samples and testing, an assessment is made on the nature and possible cause of the crack line. Other useful information if a’vailabIe, would assist in the assessment and deciding the repair options. These include:

  • original design approach
  • condition during construction
  • the environment during its service life, including exposure to contamination
  • history of the defects
  • clients current requirements and any proposed future change of use
  • approximate extent and likely rate of increase of defects (without repair)
  • importance of whole-life costing of the repairs, which is strongly recommended as the basis for selecting the final repair option, looking at the value over the intended remaining life of the structure, rather than just the capital cost of the repairs

 

Step 2 — Choose Options

The following options shall be taken into account in deciding the appropriate action to meet the future requirements for the life of the structure:

  • do nothing, but monitor
  • reanalyse the structural capacity of the weakened element
  • prevent or reduce further deterioration
  • improve, strengthen or refurbish all or part of the structure
  • replace all or part of the structure demolish, completely or partially

 

Step 3 — Select Repair Principles

In BS EN 1504 Part 9, the Principles governing repair of deteriorated concrete structures are set out below. For each type of deterioration discussed below, a suitable repair principle (or principles) from the list is suggested.

Principles related to defects in concrete
Principle 1 Protection against ingress
Principle 2 Moisture control
Principle 3 Concrete restoration
Principle 4 Structural strengthening
Principle 5 Increasing physical resistance
Principle 6 Increasing resistance to chemicals
Principles related to reinforcement corrosion
Principle 7 Preserving or restoring passivity
Principle 8 Cathodic control
Principle 9 Cathodic protection
Principle

10

Control of anodic areas

Again, in the context of cracks in concrete, perhaps Principles 1, 3, 4, 7 and 10 would be more relevant in terms of choosing the appropriate method of spam based on the principle. The methods based on the latter Principles are discussed further in the next section “Rectification of Cracks”.

 

Residual Structural Capacit’y

When the residual structural capacity is in doubt, always consult Engineers experienced in the strengthening design. Weakening at the point of repair (eg. due to the loss of concrete section in a compression member, or loss in cross-section of reinforcement bar due to corrosion) can be calculated through a structural design check. (Note — In a structure suffering from cracks, spalling that has been damaged by fire, the strength of both the concrete and the reinforcement bars can be significantly reduced, refer to CSTR 68, Assessment, design and repair of fire-damaged concrete structures).

 

Summary

An assessment to determine the cause(s) of cracks will typically involve an inspection, non-invasive (or invasive) testing and review of the defect history.

Testing may include measuring of crack widths, sampling by coring, testing of physical properties of concrete such as strength and cover of reinforcement and chemical testing of hardened concrete such as depth of carbonation and chloride content.

It is important to identify the cause(s) of the crack lines or defects. The principle (purpose) will guide the selection of the repair method as in the case of BS EN 1504 — Part 9. For CSTR No. 22, guidance to the selection of the repair method is given based on the type of crack ie. li\ie, fine, wide, fracture or multiple cracks (Table 3).

Other considerations that would influence the selection of approach and repair method are:

  • clients current requirements and any proposed future change of use
  • approximate extent and likely rate of increase of defects (without repair)
  • importance of whole-life costing of the repairs, which is strongly recommended as the basis for selecting the final repair option, looking at the value over the intended remaining life of the structure, rather than just the capital cost of the repairs

 

Rectification of Cracks

Introduction

This section covers the different types of repair methods, selection of materials and monitoring of site repairs after an assessment on the cause of cracking is determined, making reference to both CSTR No. 22 and BS EN 1504. There is also a brief section discussing how the repair works carried out should be checked for effectiveness and on- going maintenance after the repair works are completed.

Guidance and Reference

Re ai s sin CSTR No 22 as a Reference Guide

GSTR No. 22 separates dormant cracks and live cracks when determining the type of repair. The following is suggested:

Dormant cracks which are unlikely to widen, close or extend further. These cracks are subdivided as follows:

  • Fine cracks — up to 1mm wide
  • Wide cracks — from 1 to 6mm wide
  • Fractures — over 6mm wide

 

Li\ie cracks which may be subject to further movement

Materials

The traditional approach to cracks considered to be structurally significant was to cut out and repair with fresh mortar or concrete. These cracks may now be injected with various materials either to rebond the substrate or to act as void fillers.

Dormant Cracks

Materials used may be:

  • Cement grouts
  • Epoxy resin
  • Polyester resin
  • Synthetic latex

Live Cracks

Materials used may be:

  • Polyurethane resin
  • Acrylic gels
  • Flexible epoxy resins

 

Riqid Fillers (for Dormant Cracks)

To close a crack against surface water penetration, it may be sufficient to brush in a cementitious slurry. The crack should be three to four times wider than the largest aggregate particle. The grout should be modified with a styrene butadiene or styrene acrylate polymer to increase adhesion and reduce permeability.

Cement grouts can be injected to seal cracks such as construction joints in mass concrete structures. For fine cracks, from about 0.5mm to 1.0mm wide, a specially ground, air- separated cement (all finer than 40 micron) can be used.

Sodium silicate is suitable for brushing into fine cracks on horizontal surfaces. It has also been used for injecting into fine cracks but has now been superseded by synthetic resin materials.

During the past three decades, epoxy resins have become the most used materials for injecting dormant cracks and it is claimed that some formulations can penetrate as fine as 0.01mm*3 *. Epoxies are preferred materials because:

  • Formulations are available which will harden in wet conditions and adhere to moist concrete.
  • Excellent adhesion to fresh and hardened concrete can be
  • They have low curing shrinkage
  • They have good mechanical strength in the presence of water and are resistant to a wide range of chemicals, including alkalis and aggressive ground

 

Polyester resins are also used for injection repairs though lesser used nowadays. These materials generally cost less than epoxies and can have lower viscosity, thus achieving better penetrations. Care should be exercised when using these reins in wet conditions as bond strength can be reduced.

Synthetic latexes, such as styrene butadiene, acrylic, polyvinyl acetate, or co-polymers of these, are cheaper still than polyesters, but have considerably less strength than the resins. Care should be taken in continuous wet conditions as these latexes rely on air curing to harden. Polyvinyl acetate (PVA) should not be used under damp conditions as it is soluble. These should only be considered perhaps mainly to fill gaps or voids and where adhesion strength is not critical.

 

Flexible Sealants (for Live Cracks)

Plasto-elastic resin fillers are now available for sealing cracks subject to differential movements and standing water pressures.

A wide range of sealants for movement joints are available. The most commonly used are bitumen-based compounds, polysulphides and polyurethanes. There is a much wider choice of coatings and membrane materials which can be used for surface sealing of fine cracks. These and the sealant types are reviewed in under the section Repairs to live cracks.

Repairs to Dormant Cracks

Fine Cracks

Fine cracks will most commonly be repaired by injecting with an epoxy resin or other suitable synthetic resin.  If the crack is structurally significant, it should always be repaired in this manner. Cracks which extend through a structure are treated by fixing injection points at intervals along the crack at the surface. The distance between the injection points is generally about two-thirds of the depth of penetration required.

Between these 2 points, the surface of the crack is sealed on both sides of the structure; epoxy and polyester resins have been used for the surface sealing. Care must be taken when fixing the injection points to ensure that they are not blocked by detritus.

Several devices are used to inject resins into cracks; the common type is are modified grease guns or sealant guns, pressure pots actuated by elastic bands to give the pumping pressure and special-purpose twin-line metering pumps. The guns and enclosed pressure pots have the limitation that the resin will begin to increase in viscosity soon after mixing and will soon harden so that it cannot be injected and may block and damage the equipment. This method is wasteful, as a proportion of the mixed resin is often unused because the equipment must be cleaned out before the hardening is too far advanced.

Twin-line metering pumps deliver resin and hardener, in correct proportions, through delivery lines to a point close to the injection nipple. The materials are thoroughly mixed continuously in a mixing head, and pumped directly into the crack. Only a small amount of the resin is retained in the mixing head and, when injection stops, this is flushed out with solVent from a tank which is part of the equipment.

Wide Cracks

Repair of a wide crack on a vertical surface will often be most practical by the injection method. Cracks on a horizontal surface can be repaired by injection but simpler methods can also be used. The crack can be treated by sealing the underside, where this is accessible. The repair material is then poured into the crack, either starting in the middle and working towards the ends, or starting at one end.

The materials used can be epoxy resins or cement grouts, the latter with or without a synthetic latex added to the mix. Cracks with a surface width of about 1 to 2mm will probably be easier to fill using an epoxy material because of the lower viscosity. Ordinary Portland cement grout added with hydraulic binders are available and hardening time reduced.

Fractures

Fractures can be repaired using epoxy resin mortar or proprietary grout, noting that epoxy resin mortar are more costlier than grouts. The ad’vantages are the epoxy resin mortar are better bond to the substrate and cure more quickly.

Repairs to Live Cracks

Cracks subjected to movement even when after repairs should be treated carefully. Suffice to say, the repaired cracks will still be expanding and contracting after the repairs. CSTR No. 22 suggests the following options:

  • Injection of cracks using injection using flexible resins, such resins are relatively new in the construction industry
  • Creating movement joints to cater to wider range of movement that cannot be treated under injection. Movement joint alone in itself is a topic and justifiably cannot be taught in this course. Broadly, this involves creating a joint usually by saw cutting a groove and gunning a sealant into the groove line. Various types of sealants are a\/aiIabIe and can only be selected after considering the following:
    – Range of movement expected
    – Is the joint exposed or embedded
    – Is the joint subjected to external loading
  • Larger movements may require the installation of a flexible membrane (such as Hypalon PVC membrane) over the groove. Such joints usually offer the widest movement required in an expansion joint.
  • Surface sealing. Where multiple fine cracks are evident, the cracks may be sealed with flexible coatings to bridge over the cracks. There are typically cementitious based coatings or polymer based coatings in the market that are suitable for such use. They are suitable to bridge over fine cracks below 0.3mm when applied at sufficient dry-film-thickness.

 

Repairs Using BS EN 1504 as a Reference

As mentioned earlier in the section under “Inspection and Assessmen1”, Principles 1, 3, 4 and 7    would be appropriate when dealing with repairs of cracks in concrete. The table below is taken from Part 9 of BS EN 1504, Tables 1& 2.

 

Principle No. Principle and its

definition

Methods based on the

principle

Principle 1 [PI] Protection against

ingress

Reducing or preventing the ingress of adverse agents, eg. Water, other liquids, vapour, gas, chemicals and biological agents

1.1 lmpregnation
Applying liquid products

which penetrate the concrete and block the pore system

1.2 Surface coating with

and without crack bridqinq ability

1.3 Locally bandaged

cracks*1*

1.4 Filling cracks
1.5 Transferring cracks into joints*1
1.6 Erecting external

panels 1 (2)

1.7 Applying

membranes( 1*

Principle 2 [MC] Moisture Control

Adjusting and maintaining the moisture content in the concrete within a specified range of values

2.1 Hydrophobic

impregnation         

2.2 Surface coating
2.3 Sheltering or overcIadding *1 (2)
2.4 Electrochemical

treatment*1)(2)

Applying a potential

difference across parts of the concrete to assist or resist the passage of water through the concrete. (Not for reinforced concrete without assessment of the risk of inducing

corrosion).

Principle 3 [CR] Concrete Restoration

Restoring the original concrete of an element of the structure to the originally specified shape and function.

Restoring the concrete structure by replacing part of it

3.1 Applying mortar by

hand

3.2 Recasting with

concrete

3.3 Spraying concrete or

mortar

3.4 Replacing elements
Principle 4 [SS] Structural Strengthening

Increasing or restoring the structural load bearing capacity of an element of the concrete structure

4.1 Adding or replacing

embedded or external reinforcing steel bars

4.2 Installing bonded

rebars in preformed or drilled holes in the concrete

4.3 Plate bonding
4.4 Adding mortar to

concrete

4.5 Injecting cracks,

voids or interstices

4.6 Filling cracks, voids

or interstiGes

4.7 Prestressing — (post-

tensioning) 1*

Principle 5 [PR] Physical Resistance

Increasing resistance to physical or mechanical

attack

5.1 Overlays or coatings
5.2 Impregnation
Principle 6 [RC] Resistance to

Chemicals

Increasing resistance to the concrete surface to deteriorations by chemical attack

6.1 Overlays or coatings
6.2 lmpregnation
Principle 7 [RP] Preserving or restoring

passivity

Creating chemical conditions in which the surface of the reinforcement is

7.1 Increasing cover to

reinforcement with additional cementitious

mortar or concrete

7.2 Replacing

contaminated or

maintained in or is

returned to a passive condition

carbonated concrete
7.3 Electrochemical

realkalisation of

carbonated concrete*^)

7.4 Realkalisation of

carbonated concrete by

diffusion

7.5 Electrochemical

chloride extraction*1

Principle 8 [IR] Increasing Resistivity

Increasing the electrical resistivity of the concrete

8.1 Limiting moisture

content by surface treatments, coatings or sheltering

Principle 9 [CC] Cathodic Control

Creating conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction.

9.1 Limiting oxygen

content (at the cathode) by saturation or surface coating)*2*

Principle 10 [CP] Cathodic Protection 10.1 Applying electrical

potentia(\ 1*

Principle 11 [CA] Control of Anodic Areas

Creating conditions in which potentially anodic areas of reinforcement are unable to take part in the corrosion reaction

11.1 Painting

reinforcement with coatings containing active pigments

11.2 Painting

reinforcement with

barrier coatings

11.3 Applying inhibitors to the concrete*1) 2)
1     These methods may make use of products and systems not covered

by the EN 1504 series

*2*   Inclusion of methods in this Prestandard does not imply their approval.

 

Example on the use of BS EN 1504 on the condition of reinforced concrete exposed to chlorides

Reinforced concrete exposed to chlorides in sea water

How to use of BS EN 1504 to determine the proposed treatment to the steel and concrete

  • Step 1 – Assess structure – Investigation indicate:
    – Depth of concrete cover = 50mm
    – Chloride content = 0.010% m/m (threshold = 0.035%)
    – As the actual chloride content has not exceeded the threshold, =» i.e. initiation period
  • Step 2 – Choose options – Options are:
    – Do nothing(and let chlorides enter)
    – Apply a coating (to apply a barrier to chloride penetration and cause a delay of the onset of corrosion)
  • Step 3 – Select repair principles –
    – If application of a barrier coating is the desired option, refer to Principle 1 (Plj for the next awion
  • Step 4 – Choose repair methods
    – Choose methods appropriate to selected repair principles in the light of available products and systems.
  • Step 5 – Specify material performance – Select materials with the performance characteristics required for the chosen application.
    – Chosen application is apply coating
    – Use Part 2 — Products and Systems for the Protection and Repair of Concrete Structures
    – For Protection against Ingress, choose from 3 different types:
    – Hydrophobic impregnation (H)
    – Impregnation (I)
    – Coating (C)
  • Step 6 – Carry out repair. Apply coating following the requirements of Part 10 — Site application of products and systems and quality control of the works
    – Surface preparation requirements to concrete surface
    – Application method e.g. by spray or roller
    – Monitoring the film thickness by pre or post application

Photo below showing effect of water on surfaces treated with impregnation hydrophobic coating (method 2.1)

Sketch below showing crack induced in a slab by creating a cut or by installing a pressed metal prior to casting (method 1.5)

Photo below showing applying of mortar onto concrete by hand (method 3.1)

Photo below showing spraying concrete (method 3.3)

Photo below showing installed bonded rebars in drilled holes in the concrete (method 4.2)

Photo below showing structural strengthening using plate bonding (method 4.3)

Photo below showing injecting of cracks (method 4.5)

Sketch below showing various layers of a chemical resistant coating on concrete (method)

Sketch below showing the schematic process of electrochemical re-alkalisation of carbonated concrete(method 7.3)

Sketch below showing the schematic process of electrochemical chloride extraction (method 7.5)

Sketch below showing the schematic process of applying electrical potential (method 10.1) using Impressed Current Cathodic Protection

Sketch below showing the schematic process of applying electrical potential (method 10.1) using Galvanic Anodes Cathodic Protection

Photo below showing the painting of reinforcement with coating containing active pigments (method 11.1)

Photo below showing the structural strengthening of vehicle viaducts using post tensioning (method 4.)

New Concrete Repair Specifications.

ACT announces the availability of ACI 563-18: Specifications for Repair of Concrete in Buildings. ACI 563-18. Specifications for Repair of Concrete in Buildings is a reference specification that the Architect/Engineer can apply to any construction repair and rehabilitation project involving structural concrete by citing it in the Project Specifications. Mandatory requirements and optional requirements checklists are provided to assist tile Architect/Engineer in supplementing the provisions of this Specification, as required or needed, by designating or specifying individual project requirements.

The document covers general construction requirements for all repair work; shoring and bracing of the structure or member to be repaired. concrete removal and prepai ation of the concrete substrate for repair and defines common equipment and methods; materials ancl proportioning of concrete; proprietary cementitious and polymer repair materials; reinforcement; production, placing, finishing, and cui ing of repair materials; formwork performance criteria and construction; treatment of joints; embedded items; repair of surface defects; mockups; and finishing of formed and unformed surfaces. Provisions governing testing, evaluation, and acceptance of repair materials as well as acceptance of the repair worl‹ are included. Sections 9 and 10 incorporate by reference two other specifications—ACI 503.7 and ACT 506.2—into this ACI Standard to cover crack repair by epoxy injection and shotcrete, respectively.

ACI 563-18 complements ACI 562 Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures. The ACI 562 code requirements combine the Institute’s historical knowledge with state-of-the-art resources on the evaluation, repair, and rehabilitation of concrete buildings. ACI 562 provides minimum performance requirements that address the unique nature of existing building construction.

Step 4 — Choosinq Repair Method

From the above table, it can be seen that the recommended methods based on the principle are:

  • Method 1.4 — Filling cracks
  • Methods 3.1 to 3.4 — By applying mortar by hand; recasting with concrete; spraying concrete or mortar and replacing elements
  • Methods 4.4 to 4.6 — Adding mortar or concrete; injecting cracks, voids or institices and filling cracks, voids or interstices.
  • Methods 7.2 — Replacing contaminated or carbonated concrete
  • Method 11.1 & 11.2 — Painting reinforcement with barrier coatings with active pigments

 

Step 5 — Speci*vinq Material Performance

Having chosen the method of repair based on the principle, the next step is to determine the properties of product and systems required to comply with the principles of protection and repair, refer to Annex A, Table A1 in BS EN 1504, Part 9. The table indicates the various properties to be met in the product.

The materials typically used are epoxies and cementitious based grouts or mortars for the repairs of crack lines.

The common performance characteristics and requirement for cementitious repair materials (BS EN 1504 — Part 3 : Structural and non-structural repairs) are:

  • compressive strength
  • chloride ion content
  • adhesive bond
  • restrained shrinkage / expansion
  • carbonation resistance

 

The common performance characteristics and requirement for injectable materials (BS EN 1504 — Part 5 : Concrete Injection) are:

  • adhesion strength
  • injectability into dry and non-dry medium
  • workable time
  • viscosity

 

Step 6 — Carry out Repair Works

Preparation Works

All preparation works on the concrete required for the chosen method of repair is given in BS EN 1504 Part 10, clause 7.2. The preparation works here means cleaning, roughening and concrete removal. The reinforcement may require derusting and primer application under clause 7.3.

Site Application

The method statement for the chosen method of repair is not given in BS EN 1504 Part 10. However, it gi\/es pertinent points regarding the method of site application for the Engineer, Specifier or Contractor to consider. The following are to be considered for the different methods of repair:

Hand applied mortar and concrete (clause 8.2.2)

  • Pre-wetting of the concrete if no bonding primer is used
  • Condition of the substrate to be specified
  • Compaction of the repair mortar
  • Exclusion of entrapped air pockets
  • The layer thickness and time between application of layers

 

Cracks and joints (clause 8.2.6)

  • Cleaning of cracks
  • Cracks to be treated to restore structural integrity are to be filled with a bonding product
  • Cracks to be treated to prevent the passage of agents are to be treated
  • Live cracks to be filled accordingly to allow movement to take

 

Surface coatings and other treatments (clause 8.2.7)

  • The concrete surface is to be smoothed where necessary to fill uneven surfaces and surface pores
  • The dry-film-thickness of the coating should be specified
  • The max and min temperature of the concrete, ambient humidity and moisture content of the concrete should be specified when impregnation materials are used.

 

Treatinq Rusted Reinforcement (clause 3.1)

The treatment of reinforcement to prevent corrosion should comply with EN 1504 Part 7

Step 7 — Set Out On-qoinq Requirements

Before, during and after repairs, there are various observations or tests that can be carried out to ensure integrity of the repairs. BS EN 1504 — Part 10 : Table 4 list the available tests and observations for quality control. In the context of crack lines in concrete, I have highlighted relevant test or observations below:

 

Maintenance Followinq Completion of Protection or Repair

Useful information to be kept after the repairs are completed are:

  • A record of the protection or repair works which have been carried out;
  • instructions on inspection and maintenance to be undertaken during the remaining design life of the impaired part of the concrete

 

Summary

Repairs of Crack Lines with Reference to CSTR No. 22

  • Repairs to dormant and live cracks are treated differently
  • Epoxies and cementitious based materials are commonly used for filling of dormant cracks. Epoxies have better adhesion to concrete and should be used when structural bonding is the objective.
  • Polyurethane based materials are commonly used for filling live cracks. Regardless of the material, it should have flexible properties to be able to bridge the expected movement. Alternatively, installing a movement joint may be considered if the cracks are too wide to be repaired. The Hypalon PVC joint system is one method of treating a movement joint.
  • Common repair methods include applying a cementitious slurry layer (to cover multiple fine cracks), patching crack lines by cutting groove lines and filling with appropriate fillers and injecting into crack lines using appropriate fillers.
  • Where cracks are caused by spalling of the concreter cover, the repair typically involves removal of the concrete and patching using a cementitious based mortar.

 

Repairs of Crack Lines with Reference to BS EN 1504

  • In Part 9, the repair method chosen is based upon the principle (purpose)
  • For crack lines in concrete, the relevant principles are: a Principle 1 [P1] (protection against ingress of adverse agents),
    – Principle 3 [CR] (restoring the original concrete to the shape and function),
    – Principle 4 [SS] (structural strengthening by restoring the capacity of the concrete),
    – Principle 7 [RP] (Preserving or restoring passivity whereby the chemical conditions at the surface of the reinforcement is maintained or returned to a passive condition) and
    – Principle 11 [CA] (Control of anodic areas whereby potentially anodic areas of reinforcement are unable to take part in the corrosion reaction)
  • The common repair method for treating crack lines in concrete are filling cracks (method 1.4), apply mortar by hand (method 3.1), injecting cracks (method 4.5), replacing contaminated or carbonated concrete (method 7.2) and painting reinforcement with coatings (methods 11.1 and 11.2)
  • To ensure suitable (compatible) materials are used for the repairs, the performance characteristic of the material should be considered. The performance characteristic of cementitious materials used for crack line repairs are found in Part 3. They commonly include compressive strength and restrained shrinkage / expansion
  • The performance characteristic of epoxies used for crack line repairs may be found in Part S. They commonly include adhesion strength and injectability.
  • The requirement of site preparation, conditions guiding good application of materials and quality of completed repairs are given in Part 10. Various tests and/or observations are given at various stages of the project. These include substrate condition before and/or after preparation, acceptance of products and systems, condition before and/or during application and the final hardened concrete stage.

 

References:

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The following Figures and Tables were extracted from the Concrete Society Technical Report No. 22, Non-structural cracks in concrete, 1992:

Figures 1-7, 11-14, 16-21, 24-25. Tables 1-3

The following Figures were extracted from Ascent Facilities Engineering Pte Ltd, Job No. AJ195/00: Figures 8-10

 

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