Crusting is when the top surface of the concrete begins to set while the concrete underneath is still plastic. It is caused by differential setting of the concrete. In this condition, the finishing operations must be done over a jelly like concrete base. This causes differential movement of the slab surface due to finishing which results in wavy and/or cracked surfaces. Some causes for crusting might be: a significant temperature and/or moisture difference exists between the top and bottom of the slab, concrete is placed on a cold subgrade which acts as a heat sink (especially if a heater is being used to maintain temperature of top slab), there is rapid surface moisture loss (environmental conditions leading to this are high winds, direct sunlight, and low relative humidity) One way to reduce the likelihood of crusting is by utilizing a concrete mix design with low bleeding characteristics and/or utilizing a retarder admixture in the concrete mix. Below are some ways to reduce the likelihood of such problems:
-Heat subgrade during winter months, in summer schedule placement/finishing at times when temp is not rising significantly
-Use proper initial curing strategies such as fog spraying and evaporation retardant.
-Use finishing tools that open the surface of the concrete and don’t result in surface scaling. Delay floating as long as possible as this is the activity that causes the most humps.
-Be sure all floating and troweling equipment is used in a flat position to minimize the chance of sealing the concrete.
-Consider using chemical admixtures to increase the setting rate. Don’t use chemical retarder admixtures, this will only intensify the problem.
-Minimize variations between loads of concrete and avoid unloading delays.
-Delay power troweling as much as possible to minimize the waviness that will be created during the finishing operations.
-Use the pans on power trowels to break open the concrete surface immediately after the bleed water period ends to promote uniform concrete stiffening.