Common Concrete Problems
Concrete is a popular construction material due to its versatility and durability. Under the right conditions, concrete is a sustainable material. However, there are several common concrete problems that can prevent the material from achieving its longevity. Recognizing the problems and understanding the solutions would improve the performance of concrete.
Common concrete problems include blistering, dusting, plastic shrinkage cracking, and scaling.
Blistering
Blistering appears as bumps on fresh or hardened concrete surface. These imperfections occur when entrapped air or bleed water rises to the sealed concrete surface. Bleed water is water that separates from the concrete. Blistering is problematic as it can break under stresses such as traffic. There are many ways this defect can form. Inadequate or excessive use of vibration during compaction of concrete, sealing concrete surface early, and setting concrete over vapor barrier all contribute to blistering. In order to prevent these issues, proper concrete compaction procedures should be followed, trowel blades should be kept flat to avoid sealing the surface early, and concrete should not be placed over vapor barrier if possible.
Dusting
Dusting appears as powdery material on hardened concrete surface. Laitance, a layer consisting of water, cement and fine aggregates is responsible for dusting. The laitance layer is thin, weak, and permeable. As a result of this, dusting is problematic as it indicates that the hardened concrete surface is weak and vulnerable. Dusting can appear for a few reasons. Improper cure of concrete, exposure of carbon dioxide while concrete is in its plastic state, conducting finish operation while bleed water is at concrete surface. In order to prevent these issues, proper curing procedures should be followed, equipment producing carbon dioxide should be vented while concrete is in its plastic state, and finishing operation should be conducted after bleed water at concrete surface is removed.
Plastic Shrinkage Cracking
Plastic shrinkage cracking appears as crack on fresh and hardened concrete surface. As the name suggests, these are cracks that develop while the concrete is still in its plastics state. These cracks occur when there is a rapid loss of water from the concrete surface before the concrete sets. Although plastic shrinkage cracks are generally an aesthetics issue rather than a structural issue, the cracks do allow chemicals to penetrate the surface, which can result in structural issues later on. The main cause of these cracks is rapid loss of water from concrete surfaces and these can occur for several underlying reasons. Low relative humidity, high ambient air or concrete temperature, and high winds. In order to avoid these conditions, fog spraying equipment should be used, concrete should not be placed midday, windbreaks and windscreens should be installed.
Scaling
Scaling appears as flaking or peeling on hardened concrete surface. These defects occur when the concrete is exposed to freeze-thaw cycles. There are several degrees of scaling (light, medium, severe, very severe) that vary depending on the depth and radius of flaking or peeling. Scaling is an issue as it results in loss of surface. The overall cause of these scaling is freeze-thaw cycles but the fundamental causes are inadequate cure, exposure of deicing chemicals, and conducting finishing operation while bleed water is at concrete surface. In order to avoid these issues, proper cure procedures should be followed, deicing chemicals should only be used moderately if needed, and finishing operation should be performed after removal of bleed water from concrete surface.
There are several more common concrete problems that impact the structural integrity of concrete including curling, honeycombing, and popouts. There are also common concrete problems that affect the aesthetics of concrete including discoloration and crazing. Regardless of the type of problem, each of these issues can be avoided by being aware of the common mistakes and oversights in concrete placement.
