Why do concrete floors & foundations crack in Florida?

Many builders and Realtors in Florida when asked by clients about floor cracks often state that cracking of the concrete floors is common. Unfortunately this is true but it is generally caused by poor construction management rather than a natural occurrence. Most properties now built in Central Florida are built upon a solid concrete slab and foundation called a Monolithic slab and the speed and drying process is unfortunately not the most important consideration to many builders. Many people are actually unaware that the floor beneath them has cracks inviting a selection of problems over a period of time. If you have flexible floor coverings such as carpet, wood or vinyl you probably will never know until you get a structural defect, moisture or insect problems. If you have a non flexible floor covering such as tile and you have concrete cracking you will know very soon after the tile has been installed. Concrete is a carefully measured quantity of cement, sand, rock aggregate and water. The strength of concrete is determined by the mixture ratio and is often based on the primary requirement for the structure and the load bearing requirements. As concrete sets, the water in the mixture forms a chemical reaction with the cement; although the concrete dries in a sense that no liquid water remains the water is still there and is a very important element of the structure. The drying or setting of concrete is normally known as the Curing. In fact it is more important to keep the concrete wet during the early stages of curing. This is because of  the chemical reaction produced called Hydration which is the generation of heat produced in the curing process, while bonding the raw materials of the concrete together. The heat that is self generated by this process together with the ambient temperature and direct exposure to the sun can cause the concrete to have what is known as accelerated drying causing expansion cracks to appear on the surface and often run deeper into the concrete slab. Prevention is the best cure, it is important to try and reduce the accelerated drying process as the excessive heat will prevent the natural bonding together of the raw materials and cause the concrete to crack becoming weaker. A heat reduction process can be as simple as covering the concrete reducing the sun’s exposure but the most effective method is spraying the concrete with a fine mist or water when the surface has become hard to touch. Ideally 7 days would be a satisfactory time to leave the concrete before building upon it. Unfortunately we often see contractors in Florida building on it after just 24 hours. The stress produced on the concrete at this stage will defiantly produce cracking and effect the structural integrity of the structure. Expansion joints should be cut into the concrete as soon as it has been set to allow for the continuous movement from expansion and contraction generated by the heat. The joints should be spaced correctly to be effective and they should be no more than 2-3 times the slab thickness (in inches). Therefore a 4 inch slab should have joints 8 -12 feet apart. The joints should be cut 25% of the depth of the slab; therefore a 4 inch slab would have joints cut 1 inch deep. In hot weather the concrete will crack if the joints are not cut within 6 -12 hours after finishing the concrete pour. Once the joint is cut a material with less density than the concrete can be inserted as an expansion joint material. Specialist joints can also be used which is basically a plastic or aluminum type material which is in the shape of a V inserted into the cut joint.