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Samir SurlakerDirectorAssess Build Chem Private Limited
Sunny SurlakerHead Technical ServicesAssess Build Chem Private Limited
The fundamental certainty of concrete is that it will crack.
Concrete is known to have the ability to sustain high compressive loads. The ability of concrete to sustain tensile and flexural loads on the other hand is very limited due to its brittle nature. The concrete is therefore prone to cracking when the tensile stresses in the concrete exceed its tensile strength. When compared, the tensile strength of concrete is barely 1/10th the compressive strength of concrete. That is the reason, why concrete is reinforced with steel.
Concrete may crack due to many reasons, viz., improper mix design, insufficient curing, improper joint design, overloading, shrinkage and many other factors. These may occur due to errors in the construction stage (improper design, improper mix design, improper workmanship) or due to loading after construction (mechanical overloading, accident, fire, thermal changes, chemical attack or biological attack). Some of the types of cracks and/or their causes are shown in Figure 1.
In practice, we cannot eliminate cracks completely, but with proper mix design, production, placing, compaction and curing practices, we can limit the extent of cracking to a degree where it will not adversely affect the structure. For durability purposes international codes recommend limiting the crack widths in concrete to:
The problem with cracks is that they are both a symptom and a cause. General perception of cracks is that they are unsightly, and observers feel that the element that has cracked has failed. In the case of walls, if a crack is not structural, is not too wide and is not leaking water, it may be considered acceptable.
Cracks may affect RCC elements in three fundamental ways:
Fig. 1: Types of Cracks
Before we can start to treat cracks, we need to understand them. Solutions to treat cracks are not universal. They are based on the characteristics of the cracks themselves. Cracks (shown in Figure 2) can be characterized based on:
Prior to selection of a material and method to remediate a crack or void, the characteristics of the defect need to be clearly assessed. Assessing these properties will help us in selecting the correct material and in turn determine the success of the crack repair. Injection is the first step of rehabilitation both when structural distress is encountered as well as when leakages are detected, and durability of the RCC Element/Structure is compromised. The injection technique is often the only viable solution in order to repair/waterproof damaged structures and thus avoid any further ensuing consequential damage. Table 1 shows various reasons for Injection.
Table 1: Reasons for Crack Injections
CRACK INJECTIONS IN CONCRETE STRUCTURES
Structural injection for dry cracks
Structural injection for damp cracks
Sealing of cracks and cavities for waterproofing
Sealing against pressurized water
Injections for imparting stability in Masonry structures
Frictional Sealing of loose masonry
External sealing using curtain injection technology
Grid injection for dampness
Some of the remedial measures based on the characteristics seen above can be summarized in the schematics and sections below.
A summary of these methods is shown in Figure 3. Near Surface Cracks (up to 5 mm) can be repaired by simply coating them with a crack bridging coating, by impregnating the crack with a low viscosity resin grout or by cutting a groove along the crack, filling it with a fine polymer modified mortar and overcoating it with a crack bridging coating.
Fig. 3: Summary of Crack Treatment Methods Based on Crack Depth
The injection grout material and methods also need to be selected on the basis of crack width. The width of the crack will dictate the viscosity and particle size of the injection grout. The types of materials that can be used for different crack widths are shown in Figure 4.
Generally, as a rule of thumb, mineral slurries (cementitious injection grouts) are most suitable for crack widths or voids > 1.5 mm. Hence in many of the cases of waterproofing where cement grouting is used, it is unable to fill the finer cracks in the concrete and the leakage continues. Hence, for Crack Widths < 1.5 mm it is recommended to inject / grout them using a resin [Epoxy or Polyurethane].
Wetness of the crack has quite an impact on material chosen. For e.g. the materials used for repairs or strengthening such as cementitious grouts and epoxies can be applied in dry or damp conditions, but not when a crack is leaking water. In those cases, the only system that can work is an elastic methacrylate gel or polyurethanes. The table based a German Standard for Repair, 2017 is shown in Figure 6. This table guides the selection of various crack filling materials based on the wetness of the crack.
Going by these four considerations, the material and method selection becomes very important in treatment of cracks. Most often than not, these guidelines are not followed in practice and the crack treatment remains ineffective, being referred to as “failed waterproofing”.
Assessing cracks for the properties discussed previously will help us in selecting the correct material and in turn determine the success of the crack repair. Internationally and in India at the moment, the following types of filler materials are being used for crack/void filling:
There are two different ways to fill the filling materials into the cracks and voids of a structural component:
Fig. 6: Guidelines on Selection of Materials Based on Crack Wetness
In Brief, Table 1 below gives an idea of the type of Injection materials available and the conditions these materials can be used under.
After completion of diagnosis and selection of materials for injection the work of injection passes through following stages:
The Injection process comprises of the mixing device, the injection device, packers (filler plugs), possibly an injection hose and insulation, if necessary. The manufacturer generally provides the methodology for the work with the approved system components. The equipment required for crack injection can range from a simple bucket with an outlet to most sophisticated pneumatically compressed machines capable of producing about 500 bar pressure. Other variants are available with hand-controlled nozzles with a mixing assembly to mix the two components at the point of injection. Modern sophisticated machineries are designed to provide better working pressures, better nozzle / packer combinations and to take care of pot life considerations.
Modern Injection Machinery for injection of cementitious materials, a one component injection pump and a two- component injection pump are shown in Figure 7.
The injection method should be clearly specified prior to the commencement of the work and should be supervised to conform to the specifications. After the injection resin or grout has hardened and after the removal of the nipples, the surface sealing material, which is normally quick setting cementitious system or resin should be scrapped off completely and the surface should be prepared for further cosmetic or strengthening treatment.
Packers or Nipple Systems are the link between the structure and face of the crack and the injection nozzle. Packers must be of adequate size to guarantee the flow of injection resin to the desired place with or without being displaced or de-bonded due to injection pressure or rebounds. The critical selection depends upon the access to crack, quality of surface, surface condition as well as pressures used in injection process. Figure 8 shows different type of commonly used packers for Injection Grouting.
Fig. 7: Modern Injection Machinery
Fig. 8: Commonly Used Packers for Injection Grouting
There are normally three types of Packers used under general conditions:
In conclusion, modern injection technology coupled with proper equipment can solve almost all types of waterproofing problems thereby providing economical solution in comparison to removal and replacement of older waterproofing systems. The specifications should be very clear and unambiguous. The specifications should at least cover points like material, viscosity, techniques to be adopted, the equipment to be employed, type of nozzles and spacing, pressure to be applied etc. The repair of cracks is a part of waterproofing and repairs of damaged and distressed structures and cannot replace other remedial measures adopted for successful waterproofing.
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