Avoid Surface Defects on Exterior Slabs

Premature finishing, overworking the surface and inadequate curing are typical causes of surface defects on exterior slabs. Some of these defects include premature wear, scaling, mortar flaking and plastic-shrinkage cracking. Figure 1 in the sidebar on page 34 shows the construction steps or sequence to properly place, finish and cure exterior slabs. If these steps are not properly followed, surface defects can occur resulting in costly repairs or slab replacements for the concrete contractor.

Premature finishing

As shown in Figure 1, premature finishing occurs when the second floating starts too soon. Premature finishing typically involves finishing or mixing bleed water into the top surface of the concrete or prematurely sealing the surface. Sealing the concrete prematurely traps rising bleed water and air beneath the top surface creating weak or soft zones directly beneath the slab’s surface. Mixing bleed water into the surface or trapping bleed water and air directly beneath the surface typically results in premature surface wear and scaling.

Finishing bleed water into the top surface. Upon concrete placement, mixing water starts to migrate upward as cement and aggregate particles begin to settle. This phenomenon is called bleeding. If the evaporation rate at the surface of the slab is less than the bleed rate of the concrete, bleed water accumulates and creates a sheen or layer of water on the top surface.

As illustrated in Figure 1, wait for the bleed water to evaporate or physically remove the bleed water before starting the second floating process. Also, the second pass of edging and jointing should not start until most of if not all of the bleed water is gone. Mixing or finishing bleed water into the top surface increases the water to cementitious material (w/cm) ratio on the top surface as shown in Image 1. Increasing the w/cm ratio decreases strength and watertightness of the concrete making the surface more prone to premature surface wear and scaling. If the surface has stiffened sufficiently for floating but bleed water has not evaporated, drag a rubber water or compressor hose across the surface to remove the surface water.

Trapping bleed water and air. More often, premature finishing or improper first (bull floating) or second floating results in trapping rising bleed water and air below the top surface of the concrete (Image 2). When this occurs, the resulting weak or soft zone below the surface causes the top surface of the concrete to flake or scale off. Of course, other factors including exposure conditions, overall quality of concrete (strength, mix proportions, aggregate properties, etc.) and quality of the air-entrainment and curing contribute to the scale resistance of concrete.

However, finishing slabs before bleeding has stopped is a common cause of surface scaling, especially in areas of the country where surface evaporation rates commonly exceed the bleed rates of concrete. When evaporation rates exceed bleed rates, finishers many times mistakenly believe the waiting period is over and start the second floating operation. However, the concrete has not stopped bleeding. The surface has stiffened and appears ready to be floated but stiffening occurred due to drying of the surface and not by the hydration process of the cementitious materials. To offset surface drying, use foggers or spray-on evaporation retarders.

In the early spring and late fall, another similar phenomenon may occur when concrete is placed on cold ground or base materials. Due to warm air temperatures and solar radiation, differential concrete setting occurs or the concrete sets from top down. When this occurs, the surface is ready for finishing but the underlying concrete is still plastic and bleeding. This situation presents a finishing challenge. Consider offsetting differential concrete setting by using hot water and chemical accelerators to offset the slower setting concrete below the surface or warming the ground or base materials with electric curing blankets or hydronic (ground thawing) systems.