Fractal plate

damage to concrete. Instead, cracking is a symptom of damage created by some other cause.

All portland cement concrete undergoes some degree of shrinkage during hydration. This shrinkage produces multidirectional drying shrinkage and curing shrinkage cracking having a somewhat circular pattern (figure 32). Such cracks seldom extend very deeply into the concrete and can generally be ignored.

Plastic shrinkage cracking occurs when the fresh concrete is exposed to high rates of evaporative water loss which causes shrinkage while the concrete is still plastic (figure 33).

Plastic shrinkage cracks are usually somewhat deeper than drying or curing shrinkage cracks and may exhibit a parallel orientation that is visually unattractive.

Thermal cracking is caused by the normal expansion and contraction of concrete during changes of ambient temperature. Concrete has a linear coefficient of thermal expansion of about 5.5 millionths inch per inch per degree Fahrenheit (°F). This can cause concrete to undergo length changes of about 0.5 inch per 100 linear feet for an 80 °F temperature change.

If sufficient joints are not provided by the design of the structure to accommodate this length change, the concrete will simply crack and provide the joints where needed. This type of cracking will normally extend entirely through the member and create a source of leakage in water retaining structures. Thermal cracking can also be caused by using portland cements developing high heats of hydration during curing. Such concrete develops exothermic heat and hardens while at elevated temperatures. Subsequent contraction upon cooling develops internal tensile stresses and resulting cracks at or across points of restraint.

Inadequate foundation support is another common cause of cracking in concrete structures. The tensile strength of concrete is usually only about 200 to 300 psi. Foundation settlement can easily create displacement conditions where the tensile strength of concrete

Guide to Concrete Repair

Figure 32.—Typical appearance of drying shrinkage cracking.

Figure 33.—Plastic shrinkage cracking caused by high evaporative water loss while the concrete was still in a plastic state.

is exceeded with resulting cracking.

Cracking is also caused by alkali-aggregate reaction, sulfate exposure, and exposure to cyclic freeze-thaw conditions, as has been discussed in previous sections, and by structural overloads as discussed in the following section.

Successful repair of cracking is often very difficult to attain. It is better to leave most types of concrete cracking unrepaired than to attempt inadequate or improper repairs (figure 34 and 35). The selection of methods for repairing cracked concrete depends on the cause of the cracking. First, it is necessary to determine if the cracks are "live" or "dead." If the cracks are cyclicly opening and closing, or progressively widening, structural repair becomes very complicated and is often futile.

Such cracking will simply reestablish in the repair material or adjacent concrete. For this reason, it is normal procedure to install crack gages across the cracks to monitor their

movement prior to attempting repair (figure 36).

The gages should be monitored for extended periods to determine if the cracks are simply

opening and closing as a result of daily or seasonal temperature changes or if there is a continued or progressive widening of the cracks resulting from foundation or

load conditions. Repairs should be attempted only after the cause and behavior of the cracking is understood.

If it is determined that the cracks are "dead" or static, epoxy resin injection (section 34) can be used to structurally rebond the concrete. If the objective of the repair is to seal water leakage rather than to accomplish structural rebonding, the cracks should be injected with polyurethane resin. Epoxy resin injection can sometimes be used to seal low volume water leakage and structurally rebond cracked concrete members.

Epoxy resins cure to form hard, brittle materials that will not withstand movement of the injected cracks. Poly-urethane resins cure to a flexible, low tensile strength, closed cell foam that is effective in sealing water leakage but cannot normally be used for structural rebonding.

(Some two component polyurethane resin systems cure to form flexible solids that may be

Guide to Concrete Repair

Figure 34.—Inadequate crack repair techniques often result in poor appearance upon completion.

Figure 35.—Improper crack repair techniques often result in short service life.

Figure 36.—Crack gage installed across a crack will allow determination of progressive widening or movement of the crack. It may be necessary to monitor such

gages for periods up to a year to predict future crack behavior.

useful for structural rebonding.) These flexible foams can experience 300- to 400-percent elongation due to crack movement. It is not uncommon to find that damaged concrete contains cracking not related to the cause of the primary damage (see section 23). If the depth of removal of the damaged or deteriorated concrete does not extend below the depth and extent of the existing cracks, it should be expected that the cracking will ultimately reflect through the new repair materials. Such

reflective cracking is common in bonded overlay repairs to bridge decks, spillways, and canal linings (figure 37). If reflective cracking is intolerable, the repairs must be designed as separate structural members not bonded to the old existing concrete.

22. Structural Overloads.—Concrete damage