PREVENTION OF PLASTIC SETTLEMENT CRACKS

The restraints that cause plastic settlement cracking are inherent in the construction and generally cannot be avoided. Abrupt changes in section depth could be avoided at the design detailing stage but the main reduction of risk is through mix design and suitable cohesion of the concrete to reduce bleeding. In simple terms this can be achieved by increasing the sand content. However, there is a limit to this at which the bleeding will increase. Very clean (marine-dredged) sand tend to assist water movement, so blending with a ‘dirtier’ sand with a higher fines (<150 micron) content can be beneficial.

A tendency to bleed largely depends on the properties of the cement. Fineness is a controlling factor, possibly because the finer particles hydrate more quickly thereby reducing the rate of sedimentation. Rich mixes are less prone than lean mixes. Pozzolanic additions may also help as they reduce water content and add to the fines. High GGBS contents should be avoided as slower setting times allow for the bleeding to continue longer. Air entrainment admixtures can be used, the entrained air stabilizing the matrix and reducing the water movement. A similar effect is claimed for polypropylene fibres within concrete (British Board of Agreement, 1995).

Remedial measures

Plastic settlement cracks rarely pass through the full section, except in cases similar to trough and waffle slabs, mainly because they stop at the reinforcement that causes the restraint. Structural integrity is therefore not compromised, however the cracks need to be sealed especially on slabs, to reduce the risk of reinforcement corrosion.

Where plastic settlement cracking is apparent in the newly placed concrete, the most effective way of eliminating their occurrence is to revibrate the concrete after the cracks have formed but before initial set. Assessing the most appropriate time is the responsibility of the operative. This will vary depending on the mix characteristics (cement type) and ambient conditions. The concrete must be capable of being re-fluidized by a poker vibrator.

In general, timely and proper revibration can serve only to improve the situation, if not completely restore the bond beneath the bar. Tamping the surface may close the surface but is unlikely to remove the voids beneath the bar. If noticed early enough, and the cracks remain clean and not filled with detritus, cement can be brushed into the openings and allowed to set. This however does not resolve any bond reduction caused by voids beneath the reinforcement.

Treating cracks in older hardened concrete will depend on the service conditions, i.e. exposure class, and the severity of the cracking. They can be treated with resin injection although in some instances full-depth breaking out and reinstatement may be necessary.