Concrete shrinkage is a common nuisance for a variety of projects. Especially for homemakers working on DIY projects, concrete shrinkage is something to be well aware of. Fortunately, there are a variety of ways to prevent it.
Often, the primary culprit of cracks in concrete is shrinkage. Shrinkage can produce tensile stresses, which cause cracks when they exceed the tensile strength of concrete. Concrete is an extremely versatile structural material that’s affordable to use. Shrinkage is one of its biggest drawbacks, though for many decades projects have employed shrinkage-compensating concrete. Still, it’s important to address concrete shrinkage before it potentially occurs.
Why Concrete Shrinkage Occurs
The first step in preventing concrete shrinkage is knowing precisely why it’s occurring. The primary reason is excess water that’s present while the concrete is drying. Water is an essential function of hydrating the cement, with 16 ounces of water needed for every four pounds of cement. This extra water is needed for proper fluidity and mixing, though this causes water to comprise around 8 percent of the total volume of concrete. This is precisely what causes shrinkage.
How to Handle Shrinkage in Concrete Slabs
It’s important to note using slabs on grade is a common type of building foundation but is prone to shrinkage. This is usually counteracted if you cut control joints or slits that encourage cracks to develop when the slab is still young. Drying shrinkage can still occur even with joint design that’s excellent, though reducing shrinkage is still possible.
One method to handle the issue is to reduce the water used when mixing with concrete. The downside to this is the mixture is tougher to the touch, which can cause openings and cracks to create along the steel and the formwork. These voids can diminish the overall structure, so it’s vital to keep vibrating the mix and promote consolidation as a precaution. These precautions can help decrease the downside of reducing the water amount, even if it’s not a completely foolproof method.
The Role of Additives
If you’re looking for a more definitive solution as opposed to reducing the amount of water used, there are various additives available that help fight against concrete shrinkage. There are two different types of admixtures:
- Water-reducing, which enhances mix fluidity, though water doesn’t act as the agent
- Shrinkage-reducing, which decreases the tension on the water’s surface within the concrete
This diminishes the internal effects of shrinkage. A third option is mineral admixtures with shrinkage compensation, which form expansive crystals to compensate for drying shrinkage in the cement. Effective for a long period of time, drying shrinkage helps to minimize potentials for cracks. These mixtures all share the same quality of being a compressive force that remains within the concrete, which, for as long as it’s active, will prevent shrinkage.
Concrete Shrinkage Prevention in Action
While the aforementioned methods may sound great, it’s reasonable to want some examples. Fortunately, as concrete is such a prominent staple in society, there are many involving shrinkage-compensated concrete.
One case is a concrete roof deck constructed in 1963, which used this type of concrete in a reinforced concrete home in Los Angeles. The owner reported no cracks or leaks in its 40 years. This is certainly a best-case scenario, though one that’s very likely with a quality foundation and precautionary care.
Other case studies have provided food for thought in regard to potential external factors that can cause shrinkage as well. For example, this type of concrete used in an airfield pavement project at the Rockford Regional Airport in Rockford, Illinois, led to findings by the FAA that degradation of concrete at the joints can cause cracks, so it made sense to reduce the number of joints in the pavement.
Doing this over a 10-year-span, the fiber-reinforced shrinkage-compensating concrete performed better due to a reduction in joints.
Overall, the ideal solution to drying shrinkage is not one in particular, though shrinkage-compensating mineral admixtures are often the go-to. They reduce shrinkage and also counteract it, placing the concrete into compression and resulting in no alteration to the concrete. Water-reducing, shrinkage-reducing and shrinkage-compensating mineral admixtures are all worth considering for your DIY needs.