Shrinking concrete

Concrete Construction, August, 2005 by Timothy R. Minick

Dr. Hover's article regarding shrinkage in concrete (March, p. 42) was very well written. I will be able to use many of his points in my company's ongoing training on the causes and control of shrinkage cracking. It will also be very helpful in supporting the explanation that I give to my clients regarding the necessity of proper crack control.

Besides the strategies he listed, when spacing joints to control the location of cracks, you must consider the effects of stress caused by "aggregate interlock" during the initial shrinkage. I have seen too many projects where the designed cracks continue across a perpendicular control joint into an adjacent panel. Because it was a small panel, the designer did not think that continuing the controlled crack was necessary and thought that the induced crack would turn ninety degrees into the perpendicular controlled crack.

I would like to see a similar article that explains volume change due to thermal expansion and contraction. My company, which specializes in decorative concrete, knows that joint location for both shrinkage-crack control and for thermal expansion and contraction can be critical to the final look. I have long been an advocate of specifying structural requirements for decorative concrete first and subsequently blending the decorative finish requirements with the structural requirements. I often find that designers have little "common sense" or training in designing expansion joint size and locations while considering the effects of initial shrinkage. Such an article would simplify and support my efforts in providing joint locations specific to each job and located to maximize the finished appearance without sacrificing the basic structural requirements of the concrete. An article with some common, simple design parameters similar to Hover's article would be clearer than trying to explain the coefficient of thermal expansion.

It might be nice to add a simple paragraph on the use of bond breakers/slip sheets to help complete the discussion on reasons and methods employed to control cracks in freshly placed concrete.

--Timothy R. Minick, Sr.

President O.L.S.I./Decorative Concrete