Although water is a vital ingredient in concrete, it's also the biggest enemy of this popular construction material. Water mixes with cement to make the glue that binds sand and gravel into rock-hard concrete. However, when water seeps around or under concrete, you end up with cracking and buckling. This failure results from a variety of factors, including improper mix, wrong curing process, poor installation, improper subgrade and lack of expansion joints/reinforcement.
Not Too Wet, Not Too Dry
Pouring concrete is tricky. It must not be too wet or too dry when you pour it. If it's too wet, concrete will shrink as it dries, which literally pulls it apart. Those cracks allow water to enter the substance, eventually causing the surface to heave or buckle. Although cement does not take much water, too little will also cause it to dry too quickly, which also leads to cracking and buckling.
It can take days or weeks for concrete to fully solidify or "cure." It must be kept damp, but not wet, during this process. Otherwise, shrinkage will cause cracks that lead to buckling. Curing begins with pouring -- the temperature must be right (40 degrees Fahrenheit and rising is one old engineer's motto) and not too hot or wet. Never pour concrete in the rain or on frozen ground. Cure concrete for seven days by dampening it and covering it with a tarp or plastic sheeting. It will continue to cure for at least a month.
A Firm Foundation
A solid base or subgrade of compacted gravel is essential for concrete, because it allows water below the surface to drain or seep away into the ground. Without a proper base, underground water can pool or accumulate and then freeze, forcing the concrete up -- a major cause of heaving and buckling. Once a space is opened for water, the problem will escalate. More water will get in, making the opening bigger. Eventually, this situation will force the concrete up or erode the base, producing buckles.
Joint and Reinforce
Expansion joints and reinforcements are vital to concrete. Concrete expands and contracts with temperature changes. Unless space is provided for this process, expansion/contraction will cause the concrete to break, which makes openings for water, which accelerates the process and produces surface breaks or buckles. Expansion or control joints let concrete crack where you want it to -- like seams in a sidewalk. Place joints at intervals 2 1/2 times the thickness of the slab. Add steel reinforcing bar or steel wire mesh for added strength. Keep in mind that concrete is strong under compression but weak under tension.