Concrete Repair Method

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Repair Strategy

  • Material selection
  • Method selection
  • Support Design
  • Safety Precaution
  • Costs
  • Logistics

 

Process

  • Re-bars
    – Undercutting
    – Cleaning
    – Repair
    – Protect
  • Concrete
    – Surface preparation
    – Bonding
    – Apply new material

 

Performance Requirements of Concrete Repair

  • Protection of Rebars
  • Aesthetics
  • Integrity and Compatibility (with substrate)
  • Carry loads
  • Waterproofing

 

Repair Systems Selection Criteria

  • Stability of element
  • Cost
  • Disturbance to users
  • Existing concrete condition
  • Exposure condition
  • Compatibility of repair material to substrate

 

Concrete Repair Methods

Buildings Department (1998) lists the following common repair techniques for concrete:

– Patch Repair
– Recasting/Partial Recasting
– Sprayed Concrete
– Micro-concrete
– Pre-packed concrete
– Non-shrinkage grout
– Sealing system
– Protective Coating

 

Traditional Repair Methods

– Patch Repair

  • Cementitious mortars
  • Resin-based mortars

– Recasting/Partial Recasting

– Sprayed Concrete

  • Dry process spray
  • Wet process spray

 

Preparation Before Work

– Preparation of concrete surface
– Cleaning of reinforcement
– Coating reinforcement
– Bond Coat

 

Preparation

  • Marking out, Hack back to 10-20mm behind re-bars, ensure concrete cover
  • Saw cut the edge, Brush/grind the re-bar
  • Prime coat for re-bar, bond coat for substrate
  • Batching of repair mortar and apply.
  • Dowel bars:
  • Holes drilled with rotary-percussive drills to provide keys
  • Air blown by pneumatic device
  • Required length of insertion
  • Give it a 1/2 turn and then turn back to original position
  • Give 2 hrs initial set off time

 

Cleaning of reinforcement

  • For cleaning of reinforcement rust, high pressure water-abrasive blasting should be the best ways of ensuring a chloride-free surface for the reinforcement. Probably, it is the only way of removing chlorides from the pits in rusting reinforcement.
  • If the damage is caused by carbonation, it is less critical to remove rust from the reinforcement. Coating with a layer of firmly adhering cement paste will be sufficient to prevent further deterioration. Other material for coating can be used if the repair is not in a cementitious one.

 

1. Patch Repair

  • It is most suitable for small-volume repairs as to restore concrete’s durability. The damaged concrete should be removed and patched up with appropriate repair mortars.
  • In general, there are two common repair mortars used for repairing works:
    – Cementitious mortars
    – Resin-based mortars
  • i. Cementitious mortars:
  • Cementitious mortars is one type of repair mortars for patch repair on small-volume repairs. It is used to restore durability of the structure. The damaged concrete is removed and these areas will be patched up with appropriate repair mortars.

 

1. Patch Repair (Cont’d)

  • Mailvaganam and Taylor (1994) prefers resin based mortars. It is because the resin based mortars have an elastic modulus of about one-tenth of concrete.
  • The coefficient of thermal expansion is five times more than that of concrete.
  • It has the strongest compressive strength than the other type of cementitious based mortars.
  • Recently, polymer is added into the mortar in order to increase their strength.
  • However, there are some limitations in polymer repair materials e.g. it may cause delamination or blistering when applied on the damped areas; and
  • It is not recommended to be applied in areas where fire resistance is required. It has poor fire resisting property and it has thermal incompatibility with substrate concrete.

 

Types of Mortar

Types of cementitious mortars Suitability Reference
Medium-weight cementitious mortar For area where superior resistance is required to chlorides and carbon dioxide. For general purpose concrete reinstatement. Renderoc GP; Renderoc HB40 (with higher performance)
Fibre-reinforced, nonshrinkage mortar For repair of spalling, honeycombing, stair nosing, ramp etc. Lanko 731
Silica fume-modified
Silica fume-modified repair mortar For repair load-bearing concrete Emaco S66 Emaco S88C; Emaco S88C with MCI

 

Types of Mortar (Cont’d)

Polymer modified
Polymer modified repair mortar suitable for different degree of exposure For locations where required high compressive strength, and reinstatement of concrete. Barra 80; Barrafill; Renderoc S; Ronafix Mix A; Ronafix Mix D
Polymer modified repair mortars, feature shrinkage compensating properties Restore concrete and inhibit further deterioration before significant structural damage occurs. Emaco R740
Polymer modified repair mortar, feature lightweight For high build, vertical and overhead repairs (rather low compressive strength will be developed) Barra Mortar HL; Nitomortar HB; Renderoc HB; HB25
Polymer modified repair mortar for high build with different classes of strength Suitable for concrete repair. It can be applied in thick section and is suitable for structural and non-structural application. Unicell 25; Unicell 40, and unicell ultra rapid.

 

Types of Mortar (Cont’d)

Rapid Setting
Rapid setting underwater mortar Specifically developed for underwater repair and restoration work. Emaco S90UW
Rapid setting waterstopping mortar Rapid patching and plugging of concrete segments. Renderoc Plug 1; Renderoc Plug 20 (20mins setting)

 

Types of Mortar (Cont’d)

Types of resin-based mortars Suitability Reference
Expoxy resin mortars
Two-part epoxy resin mortars (possesses outstanding strength and adhesive powers) Low slump for general repair work
For use in exceptionally wet conditions and for underwater applications
Expocrete GP
Expocrete UA
High strength, abrasionresistant epoxy reinstatement mortar For fast and permanent reinstatement of concrete Nitomortar S
Polyester resin mortars
Polyester resin mortars with high strength jointing and multipurpose repair compounds For fast and emergency reinstatement of concrete, bedding, jointing and reprofiling of concrete (cannot be applied to damp or wet surface) Nitomortar PE

 

8 tests on Repair Mortar:

  • compressive strength (28 days) 30-60 MPa
  • Tensile strength (7 days) 2 MPa
  • Mod of Elasticity (28 days) 15-25 kN/mm2
  • Bond Strength (7days) 2.0 MPa
  • Shrinkage Cracking (7 days) Coutinho ring test
  • Permeability
  • Workability
  • Chemical Analysis
  • Open-up inspection: 1 / 25sm patch repair
  • Pull-off test (bond coat adhesion strength): 1/ 50sm patch repair
    – Pass if 1/2 fracture in substrate or pull-off stress > 1/4 of the 7-day bond str
    – Core a 75mm dia. Full thick of the repair with 7days age.

 

Bond Strength of Patch Repair – tested by pull-off test

  • Pull-off test on the adhesion strength of the bond between the repair mortar and the substrate concrete, aided by any bond coat
  • identify re-bar position
    – Repair area shall have a minimum age of 7 days
    – Re-bar sensor to identify the location of reinforcement bars

 

Core

  • A 75mm dia. Core containing the full thickness of the repair was produced

 

Dummy

  • A suitable metal plate was glued to the cored hole and let it dry for one hour;
  • this attachment was pulled with increasing tensile force using a calibrated device until failure occurs.

 

Result 1

Criteria:

– fracture shall be at least half in the substrate concrete, unless

– the pull-off stress is not less than 1/4 (i.e. 0.5 MPa) of the minimum 7 days bond strength of the repair mortar (2.0 MPa).

Result 1

– Fracture at bond surface, bond strength = 27psi (0.9 MPa) – PASS

 

Result 2

  • Half of the fracture at substrate, bond strength = 18psi (0.6 MPa) – PASS

 

Results 3 & 4

  • Result 3: less than half of the fracture at substrate, bond strength = 12psi (0.4MPa) – FAIL
  • Result 4: fracture at surface material, bond strength = 9psi (0.3 MPa) – FAIL

 

2. Recasting

  • In this method, the damaged part of the structural element will be demolished, replace corroded reinforcement and recast it with same or even better grade concrete in order to restore its structural strength.
  • Considerations for practical uses
  • Pay attention to the discontinuity which causes any instability to the linked structural elements due to demolition.
  • Dowel bars and/or replacement steel bars should be placed on the cleaned reinforcement bars.
  • For recasting of slabs, evacuation may be required for flats above or below the element.
  • The joint between old and new concrete should be paid with special attention. Poor joints will result easy seepage of water and causes of corrosion.
  • Shear key is required for large areas of recasting.
  • Pre-soaking of substrate is necessary
  • Avoid air trapping by thorough compaction of repair mortar.

 

Recasting by form and pump

  • Application ranges
  • When defective depth is ranges from 75mm to 100mm.
  • Also for vertical members e.g. wall and column.
  • When structural restoration is required.

 

3. Sprayed Concrete

  • Sprayed concrete is a technique which is widely used in high-volume concrete repair work.
  • The concrete mix will be sprayed with or without admixture or polymers to the defected areas under high air pressure. This process can be done by means of nozzle gun.

 

Dry Spray Concrete

  • There are two different methods of spraying:
    – Dry process spraying
    – Wet process spraying
  • Dry process spraying is the process which the mixture of damp sand and cement is passed through the delivery hose to the nozzle and the water is mixed at that time.
  • This process is often used for repair work.
  • It is because of its fast application process and restoration of structural strength at an early stage.
  • The performance characteristics of dry sprayed concrete are they have good density and high strength.
  • It has very good bond to a suitable substrate.
  • These advantages make it more variable than conventional concrete and wet process sprayed concrete. (SCA, 1999)

 

Wet Spray Concrete

  • Wet process spraying means all the ingredients including water are mixed together before transporting to the site for repairing works.
  • It is not normally applied for building repairs since the size of equipment is much larger than that of dry process spraying.

 

Cracks prevention

  • Concrete surface must be reinforced with small-mesh small diameter reinforcement before carrying out sprayed concrete. This can prevent developments of cracks when the concrete shrinks.

 

Spray Concrete

  • Considerations for practical uses
  • The process should be in a continuous process.
  • Concrete mixed should be complied with acceptable standard and quality.
  • The operation should be carried by experienced nozzleman.
  • Acceptance test should be carried out to test the skill of the nozzleman.
  • The process should be held at right angle to and at one meter from the surface of the repair areas.
  • Min. spray concrete thickness should be considered before spraying process.
  • Reference should be made to Code of Practice for Structural Use of Concrete.
  • Good surface preparation should be done.
  • Sufficient safety precautions must be taken.
  • Application ranges
  • Large areas of repairs with thickness > 50mm
  • In vacant flats and/or public areas, this can minimize the nuisance to peoples.
  • Partial strength is to be restored.

 

Types of sprayed concrete Suitability Reference
Dry Spray
Polymer modified cementitious repair mortar For large area repairs (rather low compressive strength developed) Should not be used when the temperature is <5C and falling. Should not be exposed to moving water. Renderoc DS (aggregates used are classified as ‘non-reactive’.); Renderoc SC; Optimix RM770
Gunite mortar (cementitious based) Specially prepared for use in dry shotcrete applications. (compressive strength: 30-40 N/mm2) Sika Rock 30
Wet Spary
General concrete repair work Sika Mono Top-612; Optimix RM770

 

Other Repair Methods

– Micro-concrete
– Pre-packed concrete
– Non-Shrinkage Grout
– Joint Sealing System

 

1. Micro-concrete

  • By applying this method, the contaminated or damaged concrete are removed and replaced by fluid micro-concrete.
  • It is used to re-form desired concrete profiles.
  • Usually, prescribed mixes are available in the market with shrinkage compensators to control shrinkage in plastic and hardened states.
  • Considerations for practical uses
  • It needs special design by the supplier and specialist advice is required.
  • Formwork with leakproof is required.
  • Pre-soaking of the formwork and substrate is required.
  • Aggregates used should not be greater than 10mm.
  • In order to improve workability, aggregates should be rounded in size.
  • Good curing of the repair is essential.
  • Application ranges
  • For use in area with limited working space or congested reinforcement
  • For use in thin sections where the use of normal concrete is impractical

 

Types of micro-concrete Suitability Reference
Shrinkage compensated For area where access to the repair zone is restricted or where reinforcement is congested which unable to carry out compaction and vibration. Difficult access which make hand/trowel-applied mortars impractical. Renderoc LA
High performance shrinkage compensated Renderoc LA55
Low alkali, non-reactive micro-concrete Renderoc LA27

 

2. Pre-packed Concrete

• Graded aggregates similar to the substrate concrete will be placed firstly in the formwork.
• After the placement of graded aggregates, cement grout will be injected into the formwork from THE BOTTOM.
• The resulting concrete will be in excellent quality in proper implication.• Considerations for practical use • All loose and spalled concrete should be repaired prior to the application.
• Target compressive strength of the pre-packed concrete should be compatible with that of the substrate.
• Trials on compressive strength before operation are recommended.
• Careful design on aggregate size and grading is recommended.
• Aggregate size shouldn’t be too fine in order to avoid blockage of grout patch.
• Similar to micro-concrete, formwork with leakproof is required.
• Pre-soaking of the substrate with water is necessary.
• Aggregate should be clean and free from silt before application.
• Application ranges
• For use at areas where concrete placing is difficult, e.g. at beam soffits.
• For areas with congested reinforcement or depth of repair is greater than 75mm where patch repair is not suitable.
• For where the structural strength have to be restored.
• Advantages
• Compare to sprayed concrete, it has greater density, greater uniformity, lesser permeability, lower shrinkage, less dependence on personal skills of equipment operators, less dust, less clean-up work, and more economical.
• Limitations
• All works in vertical surfaces require formwork. At least 3 to 4 inches of space is required for replacement.

 

3. Non-shrink grout

• cementitious, epoxy resin or polyester resin based
• In repair works, non-shrink grout is used in crack injection repair.
• Also, it is used to repair porous concrete and in those areas with very congested reinforcement.
• Usually, it has a very high workability, it can be applied either flowable or in fluid.
• If mixtures are according to specified instruction, the compressive strength of non-shrink grout can be higher than 50 N/mm2 although it has high workability.

 

Non-shrink grout

• Considerations for practical use
• Please refer to micro-concrete
• Application ranges
• used in crack injection repair,
• To repair porous concrete,
• Very congested reinforcement or in limited working spaces,
• Thickness of repair is thin.

 

4. Joint Sealing System

• This system is generally applied for repairing active cracks.
• The crack must be widened and sealed as joints if any measurements or circumstances suggested that it is live crack.
• When those live cracks cannot successfully be glued together with resin injection, the joints must be widened at the surface and sealed as a joint.

 

Types of joint sealing system Suitability Reference
Versatile sealing system For joints and cracks subject to normal or large movements Sikadur Combiflex
Joint sealing compound (Polyurethane base) Permanent elastic one-compound joint sealing compound Sikaflex PRO 2HP
One component polyurethane sealant/adhesive As an elastic adhesive and elastic joint sealant. Sikaflex-11FC
Instant-settinghydraulicmortar/grout Sealing of waterways Fast setting, Used underwater High resistance to pressure. Some of the products are with different properties, such as, high strength, flexible and elastic for use in different situation. ERTOBLOC 224; Ronabond PU 10; Ronabond PU 20 (High strength); Ronabond PU

 

Protection Methods

• Protective Coating
• Apply Cathodic Protection
• Provide Additional Concrete Cover
• Realkalization or Desalination
• Overcladding and Insulation
• Protection with Corrosion Inhibitors

 

5. Protective Coating

• The protective coating is a thin outer layer which used to protect the substrate concrete by acting as an impermeable barrier or slowing the rate of penetration of active components from the surrounding environment.
• Three main functions of protective coating are:
– Change appearance;
– Improve surface properties;
– Barrier against the penetration of aggressive gases, liquids and vapours.
• Good protective coating should allow water vapour to pass out through it in order to stop the building up of vapour pressure in the substrate concrete which causes blistering. (BSIS)
• Considerations for practical use
• Good surface preparation before application of the coating, such as completed repairing of concrete and removal of surface contaminations and voids.
• Remove all loose part of existing painting and check the compatibility of the existing paints before application.
• If a solvent based protective coating is selected, it should be in an open space with good ventilation for coating to evaporate.
• Application ranges
• It should be applied on repaired concrete substrate to prevent further deterioration.
• In those areas where frequent wetting of concrete surface is unavoidable.
• On surfaces where exposed to aggressive surroundings.

 

Types of protective coating Suitability Reference
Single component epoxy zinc primer (corrosion protection) As an anti-corrosion primer for exposed steel reinforcement for use with concrete repair mortars Nitoprime Zincrich; Frianzinc EPE
Acrylic resin protective coating for concrete Protection and enhancement of concrete and asbestos cement building elements, especially facades. Sikagard 680 S-Concrete Cosmetic
Protective coating with moderate chemical resistance As a barrier to resist chemical resistance, such as carbon ion. Sikagard-62; Ronabond Cover Plus 150
Waterproof protective coating Provides an effective barrier against the transmission of liquid water. It should be applied on concrete, brickwork and blockwork. SikaTop Seal 107; Ronabond Joltec Finish; Ronabond Jotec Transparent; Ronabond Nigatex

 

6. Cathodic Protection

The purpose of applying cathodic protection is to stop the setting up of anodes on the reinforcement by applying a low voltage electric current or by a sacrificial anode.

 

Sacrificial Anode System

There are two main systems of cathodic protection:
– Sacrificial Anode System
– Impressed Current System

In sacrificial anode system, the steel is connected to a metal which is less stable in electro-chemical series, e.g. zinc. Since zinc is more anodic than steel, the current will flow from zinc to steel and zinc will be corroded instead of steel.

 

Impressed Current System

In impressed current system, an external current with sufficient intensity is applied to over-ride the corrosion current.

 

Pros and Cons

Advantages:

– The only way to completely stop steel corrosion
– Permanent solution

Disadvantages:

– Ongoing cost to maintain
– Many structures not suitable (access, noncontinuous reinforcement, pre-stressing steel, etc.)

 

Cathodic Protection

• Considerations for practical use
• Installation and maintenance costs for the systems are much higher than other repair methods.
• It needs special design by the supplier and specialist advice is required.
• Defects must be repaired before implication of cathodic protection.
• Electrical continuity has to be assessed before implication.
• Application ranges
• In area where corrosion is active or chloride content is high.
• Recasting or large scale patch repair is not feasible.
• Corrosion of steel reinforcement is not that serious while application.

 

Types of cathodic protection Suitability Reference
Sacrificial Anode System
Drilled in sacrificial anode for reinforced concrete structures For pre-stressed/post tensioned structures (Not for use with epoxy and polyester repair mortars or primers. Galvashield CC45, Galvashield CC65
Embedded zinc anode which acts to prevent corrosion of reinforcing steel in concrete: To be embedded within repairs performed in chloride contaminated concrete to protect adjacent steel from the onset of corrosion. Galvashield XP
Impressed Current System
Discrete cathodic protection anodes For protection of steel corrosion in reinforced concrete
structures and steel framed buildings
Ebonex

 

Realkalization

Advantages:
– Based on reversing the principles of cathodic protection
– Limited concrete removal
– No ongoing maintenance

Disadvantages:
– Very high installation cost
– Not all structures are suitable
– Environmentally unsound

 

Overcladding

Advantages:
– Greatly improves appearance
– Provides the additional benefit of insulation
– Provides a long-term solution

Disadvantages:
– Very expensive
– Can hide latent defects
– Extended contract period

The Jardine House case

 

Corrosion Inhibitors

See Sika FerroGard-903 Corrosion Inhibitor or Tyfo CIS Inhibitor

Tyfo® CIS Inhibitor Layer 2

DESCRIPTION

Tyfo® CIS Layer 2 is a single-component, clear, waterbased liquid with a mild odor. It may be
applied by roller or spray following the application of Tyfo® CIS Layer 1. Layer 2 controls the pH,
purges and encapsulates water-soluble chlorides, fills voids in the concrete, and inhibits further
penetration of moisture. Layer 2 also acts as a contact corrosion inhibitor on the reinforcing steel.
After receiving both Layers 1 & 2, the exposed surfaces are cleaned by washing with fresh water or
by abrasive blasting. All exposed steel is then treated with the Tyfo® CB material and the concrete
section is restored using Tyfo® P or Tyfo® PF as required.

USE

Tyfo® CIS Layer 2 is used with the Tyfo® Concrete Repair System on new or old concrete surfaces
to seal porosity, increase compression strength, maintain pH, protect from acid and alkali attack,
pacify and extract chlorides, and inhibit embedded steel corrosion.

ADVANTAGES

• Ease of application
• Can be applied to horizontal, vertical or overhead surfaces
• Restores properties of deteriorating Portland concrete cement
• Can penetrate concrete 2 to 3 inches, depending upon existing porosity
• Dramatically improves condition of substrate prior to repairs
• Protects new concrete
• For new and old structures
• Non-toxic

 

European Standards for Repair and Protection of Concrete

The standards related to the repair and protection of concrete of concrete currently available as standards or under development by CEN TC104 SC8- Protection an Repair an Repair of Concrete. It was updated on 6 November 2001.

 

State-of-the-art Repair and Protection Methods

Concrete
– Patch repair with phosphate-based
– Cathodic protection
– Chloride extraction
– Concrete re-alkalization
– Concrete strengthening plates/sheets
– Surface-applied penetrating sealer and coating

Re-bar
– Fusion-boned epoxy bar coatings
– Inhibitive primer

 

1) Patch Repair: Magnesium phosphate-based

It is a unique, two-component with rapid setting concrete repair and grouting system. It cures faster than concrete and it has a very good bonding with new and old concrete. The repair system can be applied at any temperature without shrinkage and is freeze/thaw and deicing salt resistant since it does not require water as an additive.

 

2) Concrete strengthening

An advanced technology of concrete strengthening techniques which do not need to recast the concrete structure for strengthening.

This technique apply the use of placing an bonding CIFIRP (Fibre Reinforced Polymer) plates or sheets to surfaces of structural concrete members

 

Examples of Concrete Strengthening Materials

Types of concrete strengthening Suitability Reference
Composite fabrics for structural and seismic strengthening Shear strengthening in a building beam and slab Flexural strengthening of slab SikaWrap
Fibre reinforced polymer (FRP) system For structural strengthening for post-reinforcement of structural members, such as beams, columns, slabs) Ronacrete FRP system

 

3) Chloride Extraction

If there is a presence of chloride ion, the concrete will deteriorate at a faster rate. It is mainly due to the depassivating effect of chloride ions. Usually, a value of 0.8% (a critical acceptance value) is adopted for practical use.

In this method, chloride ions are removed from concrete by dissolving in electrolyte and drawing it to an external electrode by an electrical potential. The external electrode being used in this method is a titanium wire mesh which is embedded in electrolytic paste of cellulose fibre. The paste is adhered onto the surface of concrete.

 

4) Concrete Re-alkalization

Alkaline is a substance which can protect reinforcement from corrosion. When concrete is carbonated, its alkalinity decreases and hence the protection of reinforcement is decreased. This method is used by diffusing alkaline chemicals like metallic hydroxides into concrete for increasing pH value and cause repassivation.

Recently, this technique is still under testing and not yet been developed for field test. Therefore, it is difficult to find this service in the market.

 

5) Surface-applied Penetration Seal

Aquron® CPT-2000 is a highly reactive catalytic agent which reacts with free alkali and/or alkaline hydrates by internally producing a silica hydro-gel which fills the pore spaces and the voids around aggregate. Furthermore, alkali is converted to a neutral compound structure, reducing the potential for internal chemical reactions. More density is added, additional bonding strength is provided and concrete is permanently hydrostatically sealed from within.

– prevents ingress and migration of moisture and damaging corrosive
– Deplete soluble chlorides in concrete and inhibit corrosion of imbedded reinforcing steel

 

6) Corrosion Inhibitor

• With the addition of calcium nitrite, the standard threshold of corrosion is elevated
• i.e. the protective passivity layer can tolerate a higher level of chloride ions.
• http://www.na.graceconstruction.com/concrete/

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