Construction World May 2017

DAMS AND RESERVOIRS

Reduce permeability – concrete more durable Many concrete durability problems can be prevented if the concrete is made less permeable, says John Roxburgh, lecturer at The Concrete Institute’s School of Concrete Technology.

structure being produced within the paste with a resulting reduction in permeability. Similar outcomes can be produced with slag-based cements, especially with longer curing times and post 28-day ages. Silica fume and to a lesser extent finer fly ash will also densify the usually less dense transition zone between the paste and aggregate so helping the concrete to become less permeable. “The reduction in permeability can be quite dramatic with the use of silica fume,” Roxburgh observes; and • Finally, the use of well graded smooth rounded sands that pack well will reduce water resulting in a less permeable concrete. Fundamental to producing a cheap but durable concrete is to use a good sand. “Concrete mix design to reduce the permeability of the concrete should also always be considered along with the plastic properties of the concrete. On site, the formation of cracks, voids and bleed water lenses and channels within the concrete will cause permeable concrete so the plastic properties of the concrete should be designed to minimise these. Also the transport, placing, compaction and curing of the concrete plays just as an important role in the reduction of permeability as the original mix design,” Roxburgh adds. 

Roxburgh says air and water will carry chemicals into the concrete where, in the presence of water, chemical degradation will start both in the concrete and the reinforcing steel. Concrete should be made less permeable right at the mix design stage. Here Roxburgh provides three guidelines: • The first is to reduce the amount of voids in the form of capillaries and pores in the paste part of the concrete. This can be done by either reducing the water content or increasing the cement content. By doing the above the water to cement ratio (w/c) is lowered which leads to a stronger concrete – and stronger concretes are typically less permeable than weaker concretes. “This is an older, well tested and popular approach to making concrete content is increased. So, with an optimal amount of cement, mix designers should concentrate on reducing the amount of water. It is the excess water in concrete that is responsible for the capillary and pore sizes and overall volumes. Here admixtures and especially the plasticising admixture will play an important role in the reduction of the water content in concrete,” he advises; • A more modern approach is to use extenders as a portion of the binder. Using fly ash will result in a finer pore less permeable. However, it can be an expensive approach if the cement

LEFT: Over 82 years after being erected, these concrete intake towers of America’s Hoover Dam prove how durable properly mixed concrete can be. BELOW: John Roxburgh, lecturer at The Concrete Institute’s School of Concrete Technology.

Many concrete durability problems can be prevented if the concrete is made less permeable, says The Concrete Institute.

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CONSTRUCTION WORLD MAY 2017