Cracking in concrete slabs occur for a variety of reasons, most typically resulting from concrete shrinkage and expansion, or unbalanced soil movement.
Shrinkage Cracks
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 12 months and can appear as soon as one hour after the concrete has been placed in very warm conditions.
A number of actions can be taken to minimize the amount and degree of shrinkage cracks:
- Keep the water-cement ratio as low as possible
- Use clean, well-graded aggregates
- Use of water reducing agents
- Low slump cement
- Proper consolidation of the concrete
Any and all of these actions will strengthen your concrete slab and minimize shrinkage cracks.
Construction Joints
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.
Expansion Joints
Expansion joints allow changes in volume of concrete using due to changing temperatures or humidity. Expansion joints should always be at least 1/4” wide and are often made of fiber, 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.
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 five or six 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.
