General When repairs to a crack are being considered, the following factors should be taken into account: Whether the crack is dormant, ie it is unlikely to extend or open further; or whether it is live, ie it is likely to be subject to further movement. 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. Whether or not appearance is a factor.

Above all, it is necessary to determine the cause of the cracking. Whilst this may seem obvious, it is not always so. The repair of cracks caused by corroding reinforcement, for example, without remedying the cause of the cracking, will inevitably prove a short-term solution to the problem.

Dormant cracks, that is those judged not likely to move further, have traditionally been filled by chasing them and then sealing them with a cement grout or mortar. Whilst this is still an effective method in many cases, many materials are now available which are more effective, albeit also more expensive. They include epoxy resins, polyester resins and synthetic latex.

Live cracks, those judged to be still moving, require a sealant to be flexible if it is to be effective. Again, a wide variety of materials is available including poly-urethane resins, acrylic gels and flexible epoxy resins.

Since there is such a wide variety of these materials available, it is not possible to give detailed instructions on their usage. What follows is intended, therefore, to provide general guidance only. More detailed information should be sought from the manufacturers of particular products.

Dormant Cracks

Dormant cracks may range in width from 0.05 mm or less (crazing) to 5 mm or more. Obviously the width of the crack will have considerable influence on the materials and methods chosen for its repair.

Very fine cracks, eg crazing, are very difficult to repair effectively and in many cases the best option may be to do nothing. Autogenous healing of very fine cracks may occur with time.

If the problem is an aesthetic one, rubbing down the surface with a carborundum stone followed by sealing with a water-repellent material, such as sodium silicate, may provide a solution. Dirt, collecting in very fine cracks, tends to accentuate them.

Fine cracks, those up to about 1 mm in width, may often be sealed against water penetration by simply rubbing in a cement grout or slurry. The grout may be modified with a styrene butadiene or styrene acetylate polymer to increase adhesion.

Fine cracks may also be sealed by injecting them with either a cement grout or an epoxy resin. In recent years, epoxy resins have become the favoured material for this purpose and formulations are reported to be available which will penetrate cracks as fine as 0.1 mm in width, or less.

• They adhere strongly to both fresh and hardened concrete;
• formulations are available which will adhere to most surfaces and harden even under wet conditions;
• they have good mechanical strength and low shrinkage; and
• they are resistant to a wide range of chemicals, including alkalies.

Epoxy grouts are normally injected under pressure. Nipples or injection points are fixed along the line of the crack and the surface is then sealed, on both sides of the cracked element, should this be necessary. The epoxy is then injected under pressure, using specialised equipment. In some instances a vacuum may first be applied to the crack to exhaust the air and assist the inflow of resin when the vacuum is released.

Wider cracks, ie those 1 mm or more in width, may also be sealed by injecting epoxy resin, particularly cracks on vertical surfaces. On horizontal surfaces it may be possible to simply pour the grout into the crack. For cracks wider than say 2 mm, a cement grout may be the most satisfactory, and is often preferred because of its total compatibility with the parent material and its ability to maintain an alkaline environment around reinforcement.

Other materials, such as polyester resins and synthetic latexes, have also been used satisfactorily to seal fine cracks. They can have lower viscosities than epoxies and, hence, can penetrate more easily. However, they may not achieve the same bond strengths and may be less reliable in damp or wet conditions. Polyvinyl acetate, for example, is water soluble.

Live Cracks

Live cracks must be sealed with a flexible materials which can accommodate the movement in the crack. This is especially so when cyclic movements are anticipated.

Flexible epoxy resins are available which will accommodate a small amount of movement but the more usual procedure is to choose a mastic, thermoplastic or elastomer.

Mastics are generally viscous liquids such as non-drying oil, butyl rubber or low melting asphalts. They are used in conjunction with a groove or chase cut into the surface of the crack which is then filled with the mastic. They are the cheapest of the available sealants but their use is restricted to vertical surfaces or horizontal surfaces which are not trafficked. Movement in the crack, particularly in hot weather, may cause the sealant to extrude. 

Thermoplastic materials are those which soften and become liquid or semi-liquid at higher temperatures, normally in excess of 100°C. Although less susceptible to temperature than mastics, they suffer from much the same disadvantages. 

Elastomers include a wide range of materials, such as polysulphides, polyurethanes, silicones and various acrylics. Some are one-apart, some are two-part materials. They have the advantage that the are less susceptible to temperature in the normal range experienced in buildings and other structures; adhere strongly to concrete; and are able to accommodate quite significant movements without failure. Reference should always be made, however, to the information supplied by the manufacturer to ensure that application of particular products to particular situations.