Once upon a time, on a cold night, a caveman built a nice hot fire on a solid piece of limestone and fell asleep next to it. Many hours later, after his fire had burned out and had cooled, he woke to rain drops on his face. He noticed on the edge of his fire there was a suspicious powder that he had never seen before. As the rain drops fell into this powder it began to steam and then quickly got hard as rock. The rain drops continued and eventually all that was once powder was now solid. Several hours later he came back to the fire and all of the hardened powder had cracked. As he picked up a piece of the cracked concrete to investigate, his wife walked up and said, “Shrinkage sucks, doesn’t it?”
Ever since the invention of using super-heated limestone powder (cement) mixed with water and other ingredients to make concrete, shrinkage has been an issue. Regardless of what you do to it, the fact remains that concrete shrinks. Depending on the other ingredients you use along with cement and water, concrete shrinkage can be reduced to a negligible amount resulting in reduced cracking and curling.
During the chemical reaction between cement and water, called hydration, cement particles are drawn to each other creating a very strong bond. As that process continues through initial set and the available water gets reduced, the now hard concrete continues to shrink causing stress and tension within itself. That tension ultimately results in cracking, curling, or other unwanted results.
How do we know how much our concrete is going to shrink? I’m glad you asked. ASTM C157 is the standard test method for measuring concrete shrinkage. In this test concrete mortar bars are cast, measured and placed in a water solution for 28 days. At the end of this period the bars are then measured again for a change in their length. The total maximum length change depends upon the specifications for a specific job but anything around 0.05% length change is good.
The Solutions – In Design
There are two different schools of thought on how to reduce shrinkage. The first and oldest is to increase the cement content in order to increase the strength of the hardened paste in the concrete. This theory is based on the idea that if you make the concrete strong enough quickly enough it won’t shrink because it is too hard to shrink. Many engineers specify high psi, high cement content concrete in order to reduce shrinkage cracking.
The other theory, and the one Duke City subscribes to, is to reduce the amount of cement in the mix to reduce the mechanism that causes shrinkage in the first place. This reduces the strength and concrete, of course, and you open yourself up to greater risk of not meeting other specification requirements, such as compressive strength. This method of shrinkage reduction requires greater control and consistency in aggregate gradation and proportions in the mix. Gap grading, especially with lower cement content, will be detrimental to reducing shrinkage and compressive strength.
The other concrete design element to reducing shrinkage is admixtures. There are many products on the market that reduce shrinkage, and even some claim to eliminate it. SRA (Shrinkage Reducing Admixture) is a product that we have on hand which has proven to be very helpful in meeting some of the tighter specifications. There are different dosages depending on what you are trying to achieve. Additionally, there are products out there that will reduce shrinkage enough to extended control joint spacing by multiples of three or four.
The Solutions – In Practice
Curling is a problem most flatwork concrete contractors deal with on a daily basis. This occurs when a slab shrinks at a different rate on the top than it does on the bottom. The top layer of the concrete typically dries and shrinks faster than the bottom causing it to pull the top edges together creating a bowl effect. If you have ever driven down a concrete highway and it seems like you are hitting a small speed bump every 10 feet, you have experienced concrete curling. The solution for this type of shrinkage is even and consistent concrete curing; ie: keeping the moisture content of the entire placement even and consistent. (Stay tuned for future blog post on curing)
Increasing the water content in the concrete can cause more shrinkage. When that additional water evaporates, the hydrating cement tries to fill that void and as it does it causes shrinkage, not to mention reduced compressive strength. Instead of using water to increase slump and workability, use a water reducing admixture. These don’t add water to the mix and still give you the workability you are looking for.
Fibers can significantly reduce cracking caused by concrete shrinkage in the first 24 to 48 hours, and beyond. Adding fiber to the concrete creates a binder that holds everything together and while shrinkage is at its worst will significantly reduce early age cracks. Duke City offers fiber and it can be used in any concrete mix to help further combat the effects of shrinkage.
All concrete shrinks. Some mixes shrink a lot, while others don’t. Duke City mix designs are all maximized to meet a very broad range of specifications while at the same time keeping shrinkage to a minimum, even when it isn’t part of specific spec. Our raw material processing and inspections are constantly being evaluated and updated to ensure we provide the highest quality product. Much can also be done on the jobsite to combat the effects of concrete shrinkage. If you have further questions regarding our process or what you can do on the jobsite, please give us a call at 505-877-5777.
Miles Shiver IV
Duke City Redi-Mix