I’m following an active and interesting discussion on LinkedIn moderated by the ACI Group. The dialogue begins with this question: “Why is concrete shrinking more now than ever before?” The group member doesn’t ask if it is shrinking more – he assumes it is. Several other members respond by offering excellent advice for reducing shrinkage: less water; larger aggregate; use of plasticizers; use of set retarders; proper wet curing; lower aggregate temperatures; lower temperatures for everything; and much more. Here’s a summary of the reasons that contribute to concrete shrinkage.
ACI publishes a useful document on the subject entitled ACI 209.1R-05 “Report on Factors Affecting Shrinkage and Creep of Hardened Concrete.“
Higher quality, finer cement
Interestingly, few comments address the original question of why concrete is shrinking more today than ever. One post suggests that the potential for shrinkage is greater now because the quality of cement is better. Cement today is finer than it was decades ago. Finer cements tend to require more water and more water tends to result in more shrinkage.
Faster setting requires proper curing
Today’s cement also sets faster. With ever accelerating work schedules, this is a property that construction managers like. But faster setting can contribute to shrinkage and cracking. The speed of current construction practices also conflicts with proper curing practices. ACI recommends seven days of continuous wet curing. My experience is that most contractors make very little effort at curing, if at all. My common question to the site super: “How do you plan to cure this concrete?” is most often met with a puzzled look, a long pause and eventually, “Whaddya mean?”Many members in the discussion noted the need for better education.
Demands for stronger cement
The demands of concrete today are different as well. Today’s concrete is expected to be stronger and more durable than in grandpa’s day and this often translates into more cement. Higher content of cement paste can contribute to greater shrinkage and cracking.
Evolution of concrete makeup
On the other hand, some members highlighted that concrete has benefited from the technological advances, such as a generally better mix design and the availability of modern concrete admixtures. Chief among these is the advent of high range water reducers or super-plasticizers, which can achieve the plastic properties needed of the concrete for placing while allowing for very low water content requirements.
Better structure design
In addition to the evolving makeup of concrete itself, the design of structures has improved in areas of jointing and reinforcement. This can go a long way toward mitigating the effects of shrinkage by controlling, reducing or even eliminating cracks.
So while there may be a greater potential for concrete to shrink and crack today than before, there is also the opportunity to produce concrete that actually shrinks less and is crack-free. It is simply a matter of taking advantage of the technology available to us today.