Concrete_Crack_Repair
The Problem
Why does concrete crack?
Concrete Shrinkage
One of the key ingredients in concrete is water. Water is also one of the key reasons concrete cracks. The ideal water content in concrete about to be placed would make the concrete difficult to place and finish. Additional water is added to make the placement of the concrete easier. Once placed, the concrete begins to cure and water will be absorbed into the substrate and evaporate into the air. Shrinkage cracks will occur during this process. These types of cracks usually appear during the first twelve months and can appear as soon as one hour after the concrete has been placed in very warm conditions.
Contraction Joints/Sawed Joints
Contraction joints are created in slabs to accommodate shrinkage. Commonly, a groove is created in the slab (patio, sidewalk, driveway, etc.) ½" to 1" deep. Sidewalks are grooved every 5 or 6 feet in a transverse direction. Slabs are usually grooved every 15 or 20 feet. Slabs 20 feet or wider should be broken up with a groove so that no section is wider than 20 feet. The tooled joint encourages the concrete to crack along the tooled line maintaining a neat appearance. Un-tooled slabs will still contain transverse cracks but they will zig zag across the plane.
Concrete Placement
Construction joints are often seen on large pours where the placing of concrete will take more than one day. They can also occur on walls when there is a long delay between the trucks delivering concrete or pouring new concrete against old concrete, often referred to as a cold joint. Construction joints are typically tied together using rebar inside the initial pour. The end length of rebar is left bent against the forms holding the initial pour. After the forms are stripped, the rebar is bent out into what will be subsequent pours thus tying the two slabs together. Rebar can be mechanically inserted using drills and epoxy between pours. Keyways and smooth dowels are sometimes used to tie slabs together between pours.
Environmental Factors
Expansion joints allow changes in volume of concrete using due to changing temperatures or humidity. Expansion joints should always be at least ¼" wide and are often made of fiber, cork, foam, or cork. Fiber is the most common in the residential industry and has a telltale black color. The characteristics of a good expansion material include flexibility and water resistance. Expansion joints should always be created where old concrete meets new concrete. Patios, sidewalks, driveway slabs, etc. poured up against an existing foundation are excellent examples of places to create expansion joints. Foundations do not "float" during the year due to humidity or temperature changes while slabs do. The slab will disintegrate at the point where it meets the foundation unless separation is created with the expansion joint.
The Solution
The solution for repairing cracks in concrete floors (slabs) is to fill the crack completely with a product as strong or stronger than the concrete itself. The question is "what do you fill it with?". Caulk is the easiest and cheapest but has a number of significant limitations. First, it often cracks again under the constant change in the temperature and humidity. Secondly, it will often fail after extended exposure to UV light. Epoxy is usually the other choice. It is certainly strong and normally very fluid to fill the entire crack. The limitation epoxy presents is it will often continue to flow into the substrate or ground and disappear from the crack. It can lead to a very expensive repair. The solution to epoxies' fill problem is to open the crack up enough to sand the bottom of the crack to stop the flow of material out of the crack. Obviously the problem with this method is the crack is now significantly larger. Hybrid products such as Everfill Fast offer the best of both worlds and further information
can be found at http://www.thebasementoasis.com or by calling (630) 771-0380.