Avoid Surface Defects on Exterior Slabs

While bull floating or darbying, do not seal or close the surface by tilting a float or darby so that only the leading or trailing edges are in contact with the concrete. Keep bull floats and darbys as flat as possible to increase the contact area on the fresh concrete. This reduces the pressure applied to the concrete and reduces the potential for densifying and sealing the surface. To check if the floating operation is sealing the surface, do not float a small test section. Cover the test section and an equal floated area with a sheet of clear plastic. If bleed water appears on the test section (non-floated area) but not on the area floated, the floating operation sealed the surface.

As stated in Figure 1, steel troweling is not recommended for exterior slabs exposed to winter conditions and deicing chemicals. Surface sealing is more likely to occur when using steel trowels or fresnos to finish air-entrained concrete. A fresno is a long-handled trowel with a 5-in.-wide and 24- to 48-in.-long blade that looks like a bull float. However, it is a steel finishing trowel and should be used like a trowel. Steel finishing tools both densify and close the surface. Thus, bleed water and air rising to the surface can be trapped below the surface reducing the scale resistance of the concrete. Many finishers commonly use steel trowels and fresnos to finish exterior, air-entrained concrete with success by waiting as late as possible before using these tools. Be cautious if using steel tools to finish exterior concrete exposed to winter conditions and understand there are higher scaling risks associated with their use.

Overworking the surface

Overworking can create excessive mortar at the top surface but primarily overworking the surface can significantly reduce or even destroy the air-entrainment along the surface of the slab as shown in Image 3. The absence of entrained air voids along the top zone makes the surface very susceptible to scaling when exposed to winter conditions and deicing chemicals. Overworking the surface commonly occurs when the concrete is overly wet during the floating operations.

Inadequate curing

Favorable moisture and temperature conditions must be maintained throughout the entire construction process starting with the concrete placing operation and stopping when the specified cure period ends. Curing or concrete protection can be subdivided into two time periods: 1) before texturing and 2) after texturing.

Plastic shrinkage cracking. If the evaporation rate exceeds the bleed rate before texturing or during the finishing process, the water sheen discussed previously does not form and surfaces are prone to early drying resulting in plastic shrinkage cracking and surface crusting. When surface moisture evaporates faster than it can be replaced by rising bleed water, concrete shrinkage at the surface exceeds the shrinkage of the underlying concrete resulting in surface tears or cracks.

Crusting. If excessive moisture loss occurs, a surface crust may form on the top surface. When this occurs the top surface becomes stiff and hard to finish due to surface drying, but the underlying concrete is still plastic. Surface stiffening should occur because of the chemical hydration process of the cementitious materials — not by premature surface drying. Slabs that crust are difficult to finish and typically result in a wavy, cracked surface that is prone to premature surface wear and scaling. As shown in Image 4, the tined surface is highly irregular due to the loss of plasticity during tining and open void spaces occurred because of trapped bleed water and loss of surface plasticity.

Mortar flaking. Mortar flaking over large aggregate particles is a form of surface scaling related to early surface drying. Aggregates with flat surfaces are prone to mortar flaking because these aggregates block bleed water from replenishing the evaporated moisture from the mortar covering the aggregates. This results in a dry-mortar layer of low strength, poor aggregate bond and low durability. Upon exposure to winter conditions and deicing chemicals, these thin layers of weakened mortar flake off. Occasionally mortar flaking precedes wide spread surface scaling but many times it does not.

Premature surface wear and scaling. In addition to improper finishing, failing to maintain favorable moisture and temperature conditions during the finishing process and delaying or improperly curing after texturing increases the potential for premature surface wear and scaling. In order for concrete to develop its full strength and durability potential, it must be properly protected and cured. Image 5 shows surface defects caused by failing to protect freshly placed concrete from freezing. This concrete exhibited extensive scaling and deterioration during its first winter of service.