Benefits of Using Fly Ash in Concrete

Fly ash benefits concrete during both its "plastic", or wet (not hardened) phase, and over its lifetime as a hardened product. The benefits during both of these phases are described below.

Benefits for Plastic Concrete

Fly ash's spherical shape adds natural lubricity to concrete. This natural lubricity allows the wet concrete to flow more easily. This leads to two advantages. Firstly, less water is needed to make the concrete flowable enough to place. The less water used in making concrete the less "bleed water" and the sooner the concrete can be finished and the job completed. Secondly, more flowable concrete is easier to work with and this leads to less problems in the finished product.

Another advantage of fly ash in concrete is lengthened workability time. Fly ash may slightly delay the initial and final set of concrete when compared to cement-only concrete. Delayed set can be a substantial advantage in getting the concrete finished during hot weather.

Benefits for Hardened Concrete

Fly ash concrete continues to gain strength after cement-only concrete's strength gain has tapered off. This means that over the long term, fly ash concrete will be higher strength than the same "28-day" strength of cement-only concrete. The following graph shows the strength/time relationship for cement-only concrete versus fly ash concrete.

Higher concrete strength positively impacts many engineering properties of concrete including: abrasion resistance, crack resistance, load carrying capabilities, and long term creep.

Fly ash concrete is significantly less permeable to water than cement-only concrete. The following graph illustrates this.

Lower permeability concrete is better in resisting chemical attack, in protecting reinforcing steel from salt attack, and in improved water tightness for foundations and water containing structures.

Fly ash helps protect concrete from the ravages of Alkali Aggregate Reactivity (AAR), which causes internal stresses in the concrete that can over time cause the concrete to crack and deteriorate as the accompanying photograph shows.

The following graph shows how dramatic the expansion reduction can be with the introduction of fly ash to concrete.

This is a partial list of the benefits that fly ash can impart to concrete. For more information contact Lehigh Cement, Fly Ash Division at 604-946-3842.