Causes, Evaluation and Repair of Cracks in Concrete Structures

Suresh Chandra Pattanaik

Team Head, Dr. Fixit Institute of Structural Protection & Rehabilitation, C/O-Pidilite Industries Ltd.,Mumbai- 400059 and Research Scholar, School of Civil Engineering, KIIT University, Bhubaneswar-751024,Odisha E-mail: suresh.pattanaik@pidilite.co.in

[Paper published in the Proceedings of National Seminar held at Institution of Engineers,Bhubaneswar,Odisha from 16-17 July, 2011 on “Recent Advances in Civil Engineering”]

Abstract

Cracks in the concrete structures are early signs of distress which have to be diagnosed properly otherwise the repair of same crack takes place again and again causing loss of time and money. The structural cracks need more attention than non structural cracks. The repair materials and methodology are different depending upon types of cracks, their locations such as joints, structural members etc. and conditions such as dry or moist. The present paper focuses the various types of cementious and polymeric materials that are being used for repair of cracks.

Keywords

Cracks, Crack repairs, Cementious grouts, Polymer modified cementious grouts, Epoxy injection

1.0 Introduction

Concrete inherits certain type of cracks in pre-hardening stage and also develops some other types of cracks in post hardening stage in due course of time due to various reasons, despite our utmost care in prevention of cracks. While concrete becomes older, these cracks become sources, of leakages and seepages and give easy access to the moisture, oxygen, chloride, carbon dioxide, and other aggressive chemicals and gases into the concrete leading to serious degradation of the structure and causing corrosion of steel and damage in the concrete in the form of spalling etc. and subsequently causing structural failure of the member. Cracking is the initial sign of distress of the structure baring other forms of distress and deterioration like deformation, surface deposits and construction defects etc. causing damage to structural strength, durability and serviceability.

2.0 Causes

The deterioration levels of concrete on the basis of percentage of occurrence in descending order can be categorized as cracking, spalling, staining, honeycombing, reinforcement corrosion, efflorescence, pop outs (alkali silica), scaling, and delaminations.

During pre-hardening stage the majority of cracks are due to plastic shrinkage and drying shrinkage which occurs in the process of setting of concrete [1]. The factors which led to formation of plastic shrinkage cracks are higher water/cement ratio, absence of moist curing after post setting of concrete in green stage and excessive water absorption by aggregates from the mix. The factors which led to formation of drying shrinkage cracks are higher temperature and high wind during casting of concrete which helps for rapid drying of concrete, relative humidity and difference in ambient temperature. The other types of pre-hardening cracks are settlement cracks which occur due to improper compaction which may lead to honey-combing and voids formation in the concrete and specially the congested reinforcement at junctions of column and beam, movement of formworks, settlement of sub grades [1]. The construction defect related cracks during this stage are due to absence of required cover, gaps in formworks, excessive vibration, segregation of mix, lack of joints, ineffective joint treatment for expansion and contraction joints, cold joints, construction joints and absence of curing for specified period and prolonged curing up to 28days. Post-hardening cracks are due to bad quality of materials, long term drying shrinkage cracks, thermal shrinkage cracks, chemical and electro chemical corrosion related cracks [1].

The cracks can be classified as structural cracks and non-structural cracks. Non-structural cracks appear due to internally induced stresses in building materials, environmental effects and restraints to these effects and do not endanger the safety of the structure. These inevitable nonstructural or intrinsic cracks are quite harmless to the extent of acceptable limits of cracks as given in the code of practices of IS456:2000 [2] (Table: 1).Again the cracks are classified as active and dormant cracks depending on the movement of cracks in terms of length, depth and width. Dormant cracks can be repaired easily but active cracks are difficult to be repaired and have to be observed for longer period and at least for 6 months before doing any repair to such cracks.

Table: 1.

Exposure conditions Width of Cracking
Members where cracking is not harmful and does not have any serious adverse effects on reinforcement & durability 0.3 mm
Members where cracking in tensile zone is harmful, exposed to moisture/contact with soil or ground water 0.2 mm
Severe exposure conditions 0.1 mm

3.0 Evaluation

Before repair of any type of crack the causes and nature of the crack should be diagnosed properly. The visual observation can be made for surface appearance of the crack which indicates the basic cause of the cracking. Location and pattern of cracking like diagonal, longitudinal, transverse, vertical and horizontal are also to be noted. Some non-destructive tests should also to be carried out to find out the root cause of cracks in the concrete. Ultra-sonic pulse velocity is being used to find out the voids, identifying the cracks and measuring the crack depth. Crack microscope can be used to locate and find out the width of the crack and a digital crack measuring gauge can also be used for the same purpose. For active crack a crack monitor should be used which is used to monitor the changes in the crack by taking observation for a longer period. Concrete endoscope and fiberscope are also being used to find out the cracks inside the concrete. Cracks due to fire damage can be evaluated by petrography. To detect the leakages, voids inside the concrete thermal imaging camera can be used. The Table 2 shows how to identify the pattern of cracks, their possible causes and further tests required.

Table: 2.

Crack Pattern (Symptom)  Possible Cause (Diagnosis)  Further work-up (Tests) 
Rust stain on surface.  Ferrous compounds present in concrete/ mortar, binding wire, nails etc. left, corrosion of rebars.  Chemical analysis of sample, cover check and carbonation test. 
Crack in cover concrete, Rebars exposed, concrete spalls.  Corrosion of rebars (main and secondary) caused by chlorides.  Cover check, loss of rebars, carbonation and chlorides test. 
Vertical and horizontal cracks at interval.  Corrosion of secondary Rebars.  Cover to secondary rebars check. 
Cracks at definite interval.  of rebars. Rebars too near the surface, corrosion  Cover check. 
Map pattern  Alkali-Silicate Reaction, early drying out condition, high cement content, excess compaction, poor curing.  Chemical analysis of constituents. 
Diagonal cracks in beams near the support.  Overload inadequate section of beam, inadequate stirrups.  Reverse calculation of shear strength of beam section 
Vertical cracks in beams near mid span.  Overload, inadequate section of beam, inadequate longitudinal Rebars.  -
Vertical tapering cracks in masonry near vertical joint.  Foundation / substrate / supporting member movement.  -
Horizontal cracks  Corrosion of embedded Rebars  Check the rebar near the crack for corrosion. 
Cracks between RCC and masonry.  Thermal expansion and contraction.  Check the bond between RCC and masonry. 
Water routed through cracks in concrete.  Movement of joint, through crack in RCC member.  Check the Rebar detail. 

4.0 Repair of Cracks

The aim of crack repair has to be established a prior and achieved by proper selection of repair material and methodology. As described in ACI 224.1R [3] the goal of all crack repairs is to achieve one or more objectives such as: restore and increase the strength of cracked components; restore and increase the stiffness of cracked components; improve functional performance of the structural members; prevent liquid penetration; improve the appearance of the concrete surface; improve durability; and prevent development of a corrosive environment at the reinforcement.
Materials for nonstructural crack repair of dormant nature should be a rigid material. The crack should be three to four times wider than the largest aggregate particle. Cementitious, polymer modified cementitious grouts of acrylic, styrene-acrylic and styrene-butadiene should be used for wider cracks. However polyester and epoxy resins should be used for injection of dormant cracks. For live cracks flexible material of polysulphide or polyurethane should be used [4]. Before repair of any non structural cracks the factors have to be considered are: whether the crack is dormant or live; the width and depth of the crack; whether or not sealing against pressure is required, and, if so, from which side of the crack will the pressure be exerted and whether or not appearance is a factor.

4.1 Repair of Dormant cracks

Dormant cracks may range in width from 0.05 mm or less (crazing) to 6 mm or more. The width of the crack has a considerable influence on the materials and methods to be chosen for its repair.The fine cracks are repaired by low viscous epoxy resin and other synthetic resin by injecting. Wide cracks on a vertical surface are also repaired by injection methods. Cracks on horizontal surface can be repaired by injection or by crack filling by gravity.

Dormant cracks, where the repair does not have to perform a structural role, can be repaired by enlarging the crack along the external face and filling and sealing it with a suitable joint sealer. This method is commonly used to prevent water penetration to cracked areas. The method is suitable for sealing both fine pattern cracks and larger isolated defects. Various materials are used, including epoxies, urethanes, silicones, polysulphides, asphaltic materials and polymer mortars [5]. Polymer mortars are used for wider cracks. The crack is routed out, cleaned and flushed out before the sealant is placed. It should be ensured that the crack is filled completely. Where ever a cementitious material is being used, dry or moist crack edges must be wetted thoroughly.

4.1.1 Cementitious Grouts

It is used for repair of cracks that are 6 mm and greater in width. It is a mixture of cementitious material and water, with or without aggregate that is proportioned to produce a pourable consistency without segregation of constituents.

Cement-based grouts are available in a wide range of consistencies; therefore, the methods of application are diverse. These materials are the most economical of the choices available for repair. They do not require unusual skill or special equipment to apply, and are reasonably safe to handle. These materials tend to have similar properties to the parent concrete, and have the ability to undergo autogeneous healing due to subsequent hydration of cementitious materials at fracture surfaces. Shrinkage is a concern in such type of grouts. These are not suitable for structural repairs of active cracks.

For application of cementious grouts generally, some form of routing and surface preparation, such as removal of loose debris are needed. Pre-wetting should be done to achieve a Saturated- Surface-Dry (SSD) condition. Grouts are generally to be mixed to a pourable consistency by using a drill and paddle mixer, and the consistency may be adjusted thereafter. Application should be done by hand troweling or dry packing into vertical and overhead cracks to fill all pores and voids .Finally, a suitable coating to be applied on the repaired surfaces.

One of the potentially effective repair procedures is to inject epoxy under pressure into the cracks. The injection procedure will vary, subject to the application and location of the crack(s), with horizontal, vertical, and overhead cracks requiring somewhat different approaches. The approach used must also consider accessibility to the cracked surface and the size of the crack. Cracks can be injected from one or both sides of a concrete member. If access is limited to only one side, installation procedures may include variations in epoxy viscosities, injection equipment, injection pressure, and port spacing to ensure full penetration of epoxy into the crack. Depending on the specific requirements of the job, crack repair by epoxy injection can restore structural integrity and reduce moisture penetration through concrete cracks 0.002 in. (0.05 mm) in width and greater. However, before any concrete repair is carried out, the cause of the damage must be assessed and corrected and the objective of the repair understood. If the crack is subject to subsequent movement, an epoxy repair may not be applicable.

4.1.2 Polymer Modified Cementitious Grout

It is generally used to repair cracks that are 6 mm and greater in width. It is a mixture consisting primarily of cement, fine aggregate, water, and a polymer such as acrylic, styrene-acrylic, styrene-butadiene, or a water-borne epoxy. The consistency of this material may vary from a stiff material suitable for hand-packing large cracks on overhead and vertical surfaces to a pourable consistency suitable for gravity feeding cracks in horizontal slabs. These materials are generally more economical than polymer grouts, and the performance, with respect to bond strength, tensile strength, and flexural strength, are improved compared with cement-based materials that do not contain any polymers. These materials are filled in the cracks by some form of routing and filling the crack (Figure 1).

Figure 1. Crack to be filled with ready to use polymeric repair material

4.1.3 Epoxy Resin Grouts

This is most common polymer material used for gravity feed crack repairs. It should be formulated to have a very thin consistency (low viscosity) and low surface tension to enable the resin to easily penetrate fine cracks by gravity alone. Viscosities below 200 centipoise (cps) should be a minimum requirement. Many epoxies are available with viscosities below 100 cps. The horizontal concrete elements such as bridge and parking decks, floor slabs, plaza decks, and similar surfaces can be repaired with gravity feed resin. The cracks should be cleaned and free from dust. If required some routing may be required to facilitate pouring of resin. The surface should be cleaned with a compressed air. If water is used during cleaning then it should be dried for 24hr because the moisture present inside the crack may obstruct the flow of resin. The resin has to be mixed in a bucket with a paddle mixer. Small cans or squeeze bottles can be used for pouring into individual cracks. Before pouring of resin the underside of cracks should be sealed temporarily to avoid any leakage. The pouring should continue till the cracks go on absorbing after which the excess resin should be removed with a flat rubber squeegee.

4.2 Structural Crack Repair with Epoxy Injection

Any structural crack repair should be injected with an epoxy resin. The compressive and bond strength of epoxy resin is higher than concrete itself. While injecting with epoxy two parameters are more important; viscosity and pressure required for pumping. Depending upon the width of the crack the viscosity and pressure has to be selected. In general the manufactures supply as low viscous and very low viscous resins as injection material. Epoxy is also available as moisture sensitive and moisture insensitive crack repair material. Before repair one has to check if moisture is present or not. Table 3 provides the guidance for selecting the type of material and pressure.

Table: 3 Materials and methods for treating cracks

Type of Crack  Width mm  Movement  Water present  Treatment 
Hair cracks due to  Applying very low viscosity epoxy 
shrinkage and  < = 0.2  No  No  resin by brush on cracks. Penetration 
creep of concrete  due to capillary action. 
Hair cracks due to  Protective coating that are capable of 
shrinkage and  < = 0.2  No  Not always  bridging cracks by virtue of mineral 
creep of concrete  fibre fillers. 
Structural cracks in concrete  0.2 -1  No  No  Injection grouting with very low viscosity resin, high pressure. 
Structural cracks in concrete  2-Jan No  No  Injection grouting with low viscosity resin, high pressure. 
Structural cracks in concrete  5-Feb No  No  Injection grouting with low viscosity resin, high pressure. 
Structural cracks in concrete  > 5  No  Dry / Moist  Grout or pour expanding high strength cementitious grouts. 
Structural cracks in concrete  > 15  No  Dry / Moist  Gravity pour expanding high strength cementitious grouts. 
Movement cracks in concrete  0.2 -1  Due to temperature  Dry / Moist  High pressure injection with moisture compatible resin. 
Movement cracks in concrete  0.2 -2  Vibration loading, unloading  Dry / Moist  Medium pressure injection with moisture compatible resin 

4.2.1Material Properties and Equipment

Epoxy resins are injected for repair of hair line cracks and fissures as narrow as 0.05 mm due to their unique property of super low viscosity. The appropriate viscosity of the epoxy will depend on the crack size, thickness of the concrete section, and injection access. For crack width of 0.3mm or smaller a low viscosity epoxy injection can be used. For wider cracks, or where injection access is limited to one side, a medium to gel viscosity material may be suitable. Injection can be made of low pressure (Figure 2) or high pressure system (Figure 3) depending on the nature of cracks. It is better to use two-component pumps with a static mix head to prevent premature reaction. The requirement of epoxy resin to bond hardened concrete to hardened concrete as per ASTM C881 “Standard Specification for Epoxy-Resin-Base Bonding Systems for Concrete,” [6] identifies the basic criteria for selecting the grade and class of epoxies is given in Table 4.

Table: 4 ASTM C 881 requirements for epoxy resin to be used for bonding hardened concrete

Type 1*  Type IV# 
Viscosity, centipoise   
Grade 1(Low-viscosity),maximum  2000 2000
Grade 2(medium viscosity), minimum  2000 2000
maximum  10,000 10,000
Consistency, in   
Grade 3(Non-sagging),maximum  4-Jan 4-Jan
Gel time, min  30 30
Bond strength, Minimum, Mpa   
2 days moist cure  6.9 6.9
14 days, moist cure  10.34 10.34
Absorption,24 h maximum,%  1 1
Heat deflection temperature, oC   
7 days minimum, o C  - 48.88
Linear coefficient of shrinkage   
On cure, maximum  0.005 0.005
Compressive yield strength, Mpa   
7 days minimum  55.2 68.94
Compression modulus, Minimum  1034.2 1378.9
Tensile strength,7days Minimum  34.5 48.2
Elongation at break,%  1 1

*Type I: For use in nonload-bearing applications
#Type IV: For use in load-bearing applications

4.2.2 Application of Epoxy Injection

4.2.2.1 Surface preparation

The cracks should be cut and cleaned properly. Any contamination should be removed by flushing with water or some especially effective solvent. Then the solvent should be blown out with compressed air, or adequate time should be given for air drying. The surfaces should be sealed. This keeps the epoxy from leaking out before it gelled. A surface can be sealed by brushing an epoxy over the surface of the crack and allowing it to harden. If extremely high injection pressures are needed, then the crack should be cut into a V-shape, filled with an epoxy, and should be stroke off flush with the surface. The entry ports should be installed thereafter.

4.2.2.2 Fixing of injection ports/nozzles

There are three ways to do this. Fitting of nozzles to be inserted in drilled holes should be made by drilling a hole into the crack for 8 mm dia injection packers @ 200 to 300 mm c/c, penetrating below the bottom of the V-grooved section. A fitting such as a pipe nipple should be inserted or tire valve stem should be inserted into the hole and bonded with an epoxy adhesive. A vacuum chuck and bit will help to keep the cracks from being plugged with drilling dust. The second method is by bonded flush fitting. When the cracks are not V-grooved, a common method of providing an entry port is to bond a fitting flush with the concrete face over the crack. Last method is by interruption in seal. Another way to allow entry is to omit the seal from part of the crack. This method uses special gasket devices that cover the unsealed portion of the crack and allow injection of the adhesive directly into the crack.

4.2.2.3 Mixing

Mixing the two components of epoxy injection grout of base and hardener should be done in a suitable container with heavy duty slow speed drilling machine with paddle attachment. Mixing should be made for 2 to 3 minutes to obtain a uniform colour.

4.2.2.4 Injection of Epoxy

For smaller area or isolated crack a hand pump may be used for injection (Figure 4). Hydraulic pumps, paint pressure pots, or air-actuated caulking guns can be used for larger cracked areas (Figure 5). The pressure should be selected carefully, because too much pressure can extend the existing cracks and cause more damage. If cracks are clearly visible, injection ports can be installed at appropriate interval by drilling directly into the crack surface. The surface of the crack between ports is allowed to cure. For vertical cracks, pumping of epoxy into the entry port should start at the lowest elevation until the epoxy level reaches the entry port above. Then the lower injection port is caped and the process is repeated at successively higher ports until the crack has been completely filled in. For horizontal cracks, injection starts from one end of the crack to the other in the same way. When the pressure is maintained, the crack is filled completely. For injection from underside of ceiling of flat roof a lot of pressure is being exerted. Hence care should be taken while injecting from underside

.

4.2.2.5 Removal of the Surface Seal

After the injected epoxy has cured, the surface seal is being removed by grinding or some other appropriate means. Fittings and holes at entry ports should be painted with an epoxy patching compound.

4.3 Cementious Injection Grouts

Ultra fine polymer stable cementitious grouts are also being used for injections for cracks. It is very much compatible with concrete and can be more effective for densifying the concrete. It penetrates deep into micro-cracks, while ordinary cement slurry can not. It fills effectively all honey combs inside the concrete. Its fineness value ranges from 6000 –16,000 cm2 / gm which are 2 to 4 times higher than Ordinary Portland cement. The 28days compressive strength for cracks up to 0.8 mm is usually 40-50Mpa where as for cracks above 0.8 mm width varies between 60-65 MPa. It is more economical than epoxies or PU injections.

4.4 Polyurethane (PU) injection

Polyurethane grouts are usually used to repair cracks that are 0.12 mm and greater in width, both wet and active, and leaking a significant amount of water through joints or cracks. These grouts are semi flexible; thus, they may tolerate some change in crack width. The reaction time to form the foam may be controlled from 30 to 45 seconds up to several minutes using different catalyst additives.

Polyurethane grouts generally are not suitable for structural repairs. Additionally, a highly skilled work crew is required along with special injection equipment. Finally, these materials typically are not stable when exposed to UV light. This is usually not a major concern because the material is injected into a narrow crack where exposure to UV light is minimal.

Polyurethane chemical grouts are usually injected under pressure as a liquid resin into or in the vicinity of the leak (Figure6). Once the resin contacts water, a chemical reaction occurs. Depending on the material formulation, the grout/water combination forms either expansive closed cell foam (Figure7) or a gel.

Because of the immediate crack arresting nature they are well suited for tanks for the storage of liquids, dams, tunnels, sewers, and other water-containment structures.

 

5.0 Conclusion

For any nonstructural dormant crack any cementious, or polymer modified cementious will be more suitable. The epoxy is the best material for injection in to cracks in structural members. But for densifying and treatment of honeycombs, the cementious grouts will not only be suitable but also economical. The active cracks need to be treated with a Polyurethane sealant. But if the cracks are located in water retaining structures or in damp locations then polyurethane injection is the best option.

References

[1] Concrete Technology Theory and Practice by M. S. Shety, Publication of S. Chand & Company Ltd, Delhi, 2005
[2] IS 456:2000, “Indian Standard of Plain and Reinforced Concrete Code of Practice”
[3] ACI 224.1R-07, “Causes, Evaluation, and Repair of Cracks in Concrete Structures”
[4] Pattanaik Suresh Chandra, “Repair of Active Cracks of Concrete Structures with a Flexible Polyurethane Sealant for Controlled Movement” (2011), Proceed of the National Conference on Advances in Materials and Structures, ‘AMAS - 2011’, Pondicherry
[5] Hand book HB 84-2006: Guide to Concrete Repair and Protection, A joint publication of ACRA, CSIRO and Standards Australia
[6] ASTM C881 “Standard Specification for Epoxy-Resin-Base Bonding Systems for Concrete”