Лако-красочные материалы — производство

The injection of resin-based materials into cracked concrete is a technique widely employed to either restore structural integrity or to seal the cracks for subsequent durability. However, a necessary preliminary to the consideration of resin injection is an assessment to establish the cause of cracking in the first place. Here it is important to distinguish between inactive cracks which no longer move and live cracks which may continue to move with changing loads or temperatures.

In order to assist in establishing the cause of cracking in concrete it is often helpful to distinguish between intrinsic cracking(l) and that caused by an externally applied stress. Intrinsic cracking can then be further subdivided by considering the time period over which the cracking is likely to occur:

(1) Plastic settlement and plastic shrinkage cracks — occur within a few hours of casting.

(2) Early thermal contraction cracking — occurs within a few days of casting.

(3) Cracking due to drying shrinkage — occurs within a few months of casting.

(4) Cracking due to reinforcement corrosion or alkali — aggregate reaction — may take many years to develop.

The most common forms of cracks due to structural loading are those transverse to the direction of the main reinforcement, caused by direct or flexural tension, and diagonal shear cracking.

The existence of a crack or cracks in reinforced concrete members does not necessarily imply that they need to be repaired. Indeed, most members are designed such that controlled cracking is expected under normal loading conditions. The need for repair thus depends on the crack width, the exposure conditions and the direction of the crack. Those cracks which run parallel to the direction of reinforcing steel are particularly important as they may expose long lengths of bar to possible corrosion. In many cases such cracks may be an early sign of reinforcement corrosion already in progress and simple sealing will only provide a very short term remedy(2).

Cracks which have been identified as inactive can be sealed by injection with an appropriately formulated low viscosity resin. If a structural bond is required across the crack faces then epoxy resins are most suitable. For simple sealing against the ingress of moisture, alternatives include polyester and acrylic resins although these are likely to undergo greater initial contraction than epoxies. Crack widths down to 0.1 mm can be successfully filled using resin injection provided the crack surfaces are clean and sound. Those factors affecting the choice of resin will include:

(1) the required pot-life at the application temperature;

(2) the crack width and area which has to be filled;

(3) any requirement to displace water in the cracks;

(4) the rate at which strength must be developed at the anticipated ambient temperature and humidity prior to application of load;

(5) whether or not a structural bond to the crack faces is required.

(6) the method of injection.

Although it is possible to use straightforward gravity to fill some cracks, usually some form of injection under pressure is more satisfactory. Resin is injected at the lowest point so that air and any water it displaces flow upwards. The external facps of the crack are temporarily sealed between injection points to prevent the resin draining out before it has hardened. Alternatively, thixotropic formulations can be employed in situations where it is difficult to guarantee effective sealing along all external faces. Injection techniques vary from simple hand held cartridges to sophisticated machines incorporating metering and mixing devices. The selection of an appropriate pressure for injection is usually based on the need to ensure complete filling of the crack without being so excessive as to cause further crack opening. For the impregnation of highly cracked surfaces vacuum techniques can be employed to remove most of the air in the cracks whilst resin is introduced under atmospheric pressure.

For cracks which have been identified as live the options available are structural repair to prevent further movement followed by resin injection, or widening of the crack surface and treatment as a movement joint. Surface widening of cracks is best performed with a crack router or high pressure water jet. The groove so formed is then sealed with a low modulus material whose strain capacity is compatible with that developed across the joint during subsequent crack movement(3). Use of a rigid material in live cracks will result in crack redevelopment in the adjacent concrete. If the resulting jagged nature of a crack repaired in this way is visually unacceptable a more expensive alternative is to repair the crack using injection techniques and to construct a straight movement joint as close as possible to the original crack line.