Steps to Prevent Concrete from Shrinking

We know that all concrete shrinks and that it is caused by a loss of volume that occurs in different stages of the process.  Shrinkage can affect the long-term strength and durability of the concrete as well as lead to cracking.  According to the Portland Cement Association, “The shortening of concrete slabs can be caused by temperature decreases or moisture loss. These two causes are also related to curling and warping of slabs, respectively. Curling is the deformation of the slab due to a difference in temperature between the surface and the bottom of the slab (temperature gradient).  Slab “warping” is the deformation of the slab surface profile due to a difference in moisture between the surface and bottom of the slab (moisture gradient).”  

Even though we understand what causes shrinkage, it is still a challenge to prevent or alleviate it.  It is especially important to control it in concrete floors as the subsequent cracking will affect the performance of the floor.  This is why when Conco broke the world record for the largest continuous mat pour on the Wilshire Grand in L.A., we worked to carefully monitor the thermal characteristics of the pour to prevent cracking. 

One of the first things we addressed was to increase the proportion of fly-ash in the concrete mix design, which normally results in reduced maximum temperatures.  The next step was to mechanically control the temperatures, and for a project of that size, pre-chilling was not an option.  What we did was to post-chill the concrete by means of cooling pipes that mechanically removed the heat.

For this undertaking, we teamed up with several companies that helped to design and implement a thermal control plan specific to the mix design, the geometry of the foundation, and the configuration of the rebar.  In the end, the system we installed to post-chill the concrete was composed of 18 miles of cooling pipes, over 660 tons of chiller capacity, 40,000 gallons of chilled water, multiple pumps, 3 x 500kva of generators, and over 2,200 valves.  By keeping temperatures well below the ACI limits, we were able to avoid thermal cracking.

Sources:

http://www.cement.org/for-concrete-books-learning/concrete-technology/focus-on-floors/concrete-shrinkage

Our mission is to be the best supplier of concrete services in the Western U.S. and to bring experience, expertise and quality to each project. We continue to upgrade and expand facilities to better serve the growing market for a wide range of construction development.  The Conco Companies have regional offices serving Northern California, Southern California, Washington State, Oregon, Colorado and Nevada.