1. Introduction
Deep cracks in massive concrete members and cracks which pass right through a structural member can often be repaired satisfactorily by crack injection using a selected polymer such as an epoxy resin.
When properly carried out, crack injection will significantly improve the structural strength of the member. If corrosion or rebars and spalling of concrete has already occurred, then a satisfactory method of repair may be to remove defective concrete down to the rebars, clean off the rebars, remove all grit and dust etc., inject the crack with a suitable resin and then fill in the cut-out section of concrete with an SBR modified mortar. In appropriate cases, this repair method can be combined with crack injection, e.g. to avoid cutting out on both sides of the member.
2. Essential features of crack injection
The essential crack injection consists of injecting a suitably formulated resin into the cracks. This should bond the concrete together across the crack and should form an effective seal against ingress of water or other liquids, and reduce the ingress of carbon dioxide.
Correct formulation of the resin is of vital importance; present-day polymer resins provide considerable scope for variations in the formulation so as to obtain optimum characteristics for each particular job.
The resins in general use are epoxy, polyurethane and polyester, in that order. Desirable qualities for the formulated resin include:
a. low viscosity, (to facilitate penetration into the crack);
b. formation of good bond to damp concrete;
c. suitability for injection in a wide temperature range;
d. low-curing shrinkage;
e. toughness (low modulus of elasticity combined with high yield point);
f. curing time to suit injection conditions;
g. resistance to aggressive chemicals may be required;
h. durability under service conditions.
3. The injection process
This work is highly specialized and should only be entrusted to firms with a proven record of success. Unsuccessful crack injection by one firm is likely to result in another (more experienced firm) being unable to rectify the situation; I have come across this unfortunate state of affairs on more than one occasion.
The crack injection process is carried out in the following phases:
a. preparation of the cracks;
b. location of injection points and surface sealing;
c. injection of resin;
d. removal of injection nipples (if used) and plugging the holes
e. removal of sealing strip and application of any surface treatment which may be required.
4. Preparation of the cracks prior to injection
This should consist of the removal of any dirt or loose weak material on the surface, followed by cleaning out of the crack if this is considered necessary. It is seldom that cracks less than 0.5 mm wide require cleaning unless they have been fouled by the use to which the structure has been put. Compressed air can be used for this cleaning work; solvents may be required in special cases.
5. Location of injection points and surface sealing
The distance apart of the injection points will depend largely on the depth and width of the crack. The object is to have as few injection points as possible consistent with maximum resin penetration with low operating pressure. They are either holes drilled on the line of the crack, or nipples screwed into the concrete; the use of nipples is usually reserved for high pressure work (See Figure 1.).
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6. Injection of the resin
As stated above, crack injection work should only be entrusted to specialist firms with a good ‘track record’ and preferably to firms which formulate their own resins. The correct formulation of the resin to suit the particular requirements of each job is of primary importance. If the formulator and the applicator are the same firm, any question of divided responsibility is avoided. For larger jobs, some adjustments in formulation may be required during the execution of the work to meet unforeseen conditions and this requires close co-operation between the staff on site and the laboratory.
Firms specializing in crack injection develop their own techniques and equipment. Some firms favor simple means of injection by gravity feed or pressure guns, the resin being premixed in batches. Others use sophisticated equipment for continuous feed of freshly mixed resin and hardener through separate feed pipes, bringing the two materials together at a specially designed nozzle. One firm supplements pressure-feed by the use of a vacuum mat to assist penetration.
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In appropriate cases, the degree of penetration can be checked by coring or by a UPV survey or impulse radar. The aim of all injection processes is to obtain uniform penetration of the resin and complete filling of the cracks. It has been found in practice that a deliberate fluctuation in the injection pressure can be more effective than an increase in continuous pressure. The volume of resin used in filling cracks is very small.
Where cracks are inclined or vertical, it is usual to commence injection at the lowest injection point and work upwards. For horizontal cracks there is no fixed order of work; the injection can start at one end and work along the crack to the other end, or start in the middle and work first left and then right, or alternately left and right.
7. Final work following injection
It is usual to remove the injection nipples (if they are used) and seal the holes when the resin has set. Crack injection, except where the cracks are very wide on the surface, is likely to be much less conspicuous than cutting out and repairing with mortar. Nevertheless, the injected crack will be visible. Where appearance is important, surface grinding and some ‘cosmetic’ treatment will help to mask the crack.
Figure 2 shows a repair using crack injection.
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