There are several tests that can be performed to check the concrete quality. A slump test utilizes a cone within which concrete is placed in three equal layers. It is then “rodded” multiple times to consolidate the concrete inside. The cone is lifted, then the concrete “slumps” down. The distance from the top of the cone to the top of the slumped concrete is known as the “slump number”. The higher the number, the wetter (and usually, weaker) the concrete is.
An important issue to remember to ensure a quality job is curing. After concrete is placed, the standard length of time to cure out the concrete is 28 days. A curing agent/sealer is essentially a nonporous liquid coat that is applied to the concrete to prevent chemicals and moisture from migrating into the concrete and causing defects. Although not required, sealer is recommended, especially in areas exposed to chemicals, water, and temperature fluctuations into freezing / thawing conditions.
Another test is for air content. Air entrainment provides thousands of microscopic air bubbles inside the concrete within which water can freeze and expand, without damaging or stress-cracking the concrete. Air entrainment is vital in areas which experience freezing weather in the winter.
DEFECTS in CONCRETE
After concrete is placed and it begins to cure, one must be careful to follow the right process. Concrete which is incorrectly poured, mixed, or finished can exhibit some of the following defects (most of which are purely aesthetic in nature, some of which are structurally damaging)
Blisters are the same as what you might expect – bumps formed from air or water rising underneath a surface which is already sealed to the point that it will not release this pressure. Causes of blistering can include too much entrained air, to little vibration to settle the concrete, or finishing the surface to early.
Crazing is a minor version of cracking where very small hairline cracks occur on the surface of the concrete only. This is usually due to rapid setting of the surface of the concrete.
Delaminations occur when blisters finally do release the pressure, essentially peeling off a layer of the concrete. Again, this is typically caused by finishing the surface too early.
Discoloration can occur from changes in mix, uncontrolled calcium chloride admixtures, overly finished surfaces, or inadequate curing.
Plastic Shrinkage Cracks occur when water is rapidly lost while the concrete is still 'plastic'. Fiber Mesh is effective in helping control plastic shrinkage cracks.
Popouts are chunks of concrete that break out after curing (usually within the first few months of the pour), and usually happen due to moisture still inside the surface freezing and expanding, popping out the concrete on top.
Spalling is larger separation of sections of concrete usually due to bad joints or corroded rebar