19 June 2001
When steel corrodes, the rust product increases in volume many times
over its original volume. Expansion of the rust causes cracking of the concrete.
These cracks run parallel to the reinforcement. Eventually, concrete covering the
reinforcing steel spalls off.
3-3.5.3 Cracking of Concrete. As concrete dries it shrinks, which in turn, can
cause cracks. These cracks may increase in size as the internal water is lost
over time and the concrete cannot contract freely. Temperature changes can
also cause cracking. In addition, freezing water in the concrete can lead to
deterioration, cracking, and spalling. Overload conditions can cause cracks.
Waterfront structures are subject to settlement conditions. When settlement is
uneven, cracks usually result.
Shrinkage cracking can be minimized by proper curing and using a
minimum amount of cement and water in the mix. Temperature cracking can be
controlled in concrete by using expansion joints and temperature reinforcement.
Air entrainment is critical to minimize freeze-thaw damage.
Prevention and control of cracking may be improved by proper design
of the concrete structure and measures taken during construction. The measures
that must be taken during repair of concrete structures are discussed in Chapter
Preventive Maintenance for Concrete. Measures to minimize
deterioration of reinforced concrete must be taken during design of the structure
and during construction. Proper design for concrete is contained in ACI
standards and service design manuals.
The main objectives of preventive maintenance are to:
Keep water out of the concrete.
Protect the reinforcing steel.
Prevent and control cracking.
Prevent chemical actions.
The primary PM measure that should be considered for existing
reinforced concrete structures is the application of a surface coating. Coatings
should be reapplied periodically to concrete that was coated when new. This
measure can be applied to old concrete but rebar corrosion, the principal cause
of concrete deterioration and the reason these PM measure are applied, will not
likely be inhibited.
The treatment of cracks, described briefly in this Chapter, is best
considered as a repair technique and is presented in detail in Chapter 7.