Crack Repair
The time to think about crack repair is before the floor cracks. To prevent (well, reduce) arguments, every floor specification should include a section on crack repair. It should state the conditions under which a crack needs repair, and it should spell out the allowable method or methods of repair. Many cracks – perhaps most do not harm. Cracks rarely need repair if hidden under floor coverings or exposed only to foot traffic. Even in an exposed industrial floor, a crack generally need not be repaired unless it meets one or more of these conditions:
- Its width exceeds 1mm (1/32 in) and it is exposed to hard-wheeled traffic.
- It spalls under traffic, regardless of width.
- It shows differential movement under traffic.
Note
that all three condition are related to traffic. Even in a busy
warehouse or factory, cracks outside the main traffic areas can usually
be left alone. Some users have special requirements, however. In a food
or pharmaceutical plant, and in some electronics plants, concern for
cleanliness may force the sealing of every visible crack (usually
excluding craze cracks). Crack sealing may also be called for in floors
exposed to corrosive chemicals.
Some owners and designers
insist on crack repairs solely for the sake of appearance, but that can
backfire. The repair often ends up more obvious than the original crack.
Once you identify a crack as needing repair, you must decide how to do
the job. Here are the four main methods:
- Fill the crack with a sealant.
- Fill the crack with a semi-rigid joint filler.
- Inject glue into the crack.
- Install dowels across the crack.
The choice of method depends not only on the nature of the crack, but also on how the floor is used.
Sealants
The
choices include hot-poured materials and cold-poured elastomers, which
come in a bewildering variety. Sealants make a good repair if the only
requirement is that the crack be sealed. They do not protect a crack’s
edges from spalling under traffic, nor can they reduce differential
movement. Sealants require a minimum crack width, which varies according
to the product. Most sealants need at least 3 mm (1/8 in). If the crack
is not wide enough to admit the sealant, widen it with a concrete saw, a
crack router or an angle grinder fitted with a mason’s tuck-pointing
blade.
Semi-Rigid Joint Fillers
Based on either epoxy or polyurea, these are usually the best materials for cracks that have spalled, or are likely to spall, under hard-wheeled traffic. They even have some ability to stabilize cracks that show differential movement (Figure 1).
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| Figure 1 This crack was patched with semi-rigid epoxy. Not every crack patch looks this good, however |
Because
semi-rigids are less elastic than sealants, they will fail if cracks
widen much after repair. To minimize the risk, wait as long as possible
before filling cracks, to give the concrete time to shrink. And if the
floor is subject to seasonal temperature swings, apply semi-rigid
fillers during the coldest period, when thermal contraction is greatest.
Semi-rigids should penetrate at least 25 mm (1in) into the slab, and
may need to go deeper where the goal is to reduce differential movement.
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| Figure 2 When repairing a crack with semi-rigid joint filler, always cut back to a clean, square edge |
Unless
the crack is unusually neat and clean, chase it with a concrete saw, a
crack router or an angle grinder fitted with a mason’s tuck-pointing
blade. Make a groove at least 25 mm (1 in) deep and wide enough to take
out all spalled edges (see Figure 2). For the neatest repair, overfill
the crack and sand or grind it flush after the filler has hardened. This
works better with epoxy than with polyurea.
Glues
Some builders inject strong glue into cracks. The glue is usually epoxy (not semi-rigid) or methylmethacrylate. Gluing is attractive because it is the only method that restores the floor’s flexural stiffness to its uncracked condition. Another benefit is that is works in very narrow cracks; some products will go into a crack only 0.05 mm (0.002 in) wide. Glue’s strength is also its weakness. It locks up the crack, eliminating its ability to relieve future stress. If the conditions that caused the crack continue to exist, the floor is likely to crack again. Gluing is usually safe for structural cracks caused by temporary overloading, but not for drying shrinkage and thermal-contraction cracks.
Dowelling
Some of the worst crack problems occur when cracks exposed to heavy industrial traffic lose their ability to transfer load. The two sides of the crack then move up and down relative to each other every time a vehicle passes over them. Once this starts, crack edges can break down quickly and severely. If the differential movement is less than 0.5 mm (0.030 in), filling the crack with semi-rigid epoxy or polyurea may restore the lost load transfer.