Building owners expect steel troweled floors to be smooth, hard, attractive and free of surface defects. As you know, easier said than done! With environmental conditions changing daily and perhaps hourly, flatwork finishers must be able to read the concrete and respond appropriately to avoid surface defects.
Avoid premature finishing
Wait to start power floating until the bleed water sheen has disappeared and the concrete has stiffened sufficiently to support a finisher with no more than a ¼ inch footprint indentation. Failure to do so results in bleed water being finished into the top surface or the surface being prematurely sealed.
Finishing bleed water into the surface. Bleed water is extra mix water that rises and collects on the surface of the concrete when the bleed rate of the concrete exceeds the surface evaporation rate. Finishing the bleed water into the top surface increases the water to cementitious material (w/cm) ratio on the top surface, which decreases the surface strength of the concrete and makes the surface more prone to premature wear, dusting and crazing. Other causes that produce the same effect include too much mix water, high w/cm ratios, overworking overly wet mixes, premature floating, improper or inadequate troweling, and inadequate curing.
If surfaces have stiffened or hardened sufficiently for power floating but the bleed water has not evaporated, drag a rubber hose or compressor hose across the surface and remove the water. Also, do not add and finish water into the surface to facilitate finishing because the surface has dried due to rapid evaporation. Finishing added water into the top surface has the same detrimental effects as finishing bleed water into the surface, as seen in Image 1.
Dusting is the development of a fine powdery material consisting of water, cement and fine particles that easily rubs off the top surface of interior floors. Mixing bleed or added water into the surface dilutes the mortar phase of the concrete and creates a thin, weak layer of mortar called laitance along the top surface. Other causes of dusting are overusing vibrating screeds, finishing overly wet mixes, insufficient cement, excessive clay or dirt in the aggregates, using dry cement to soak up bleed water, carbonation of the surface related to unvented heaters, freezing of the surface and inadequate curing.
Crazing is the chicken-wire-like pattern of fine cracks that are barely visible and sometimes only visible when the concrete is drying after the surface has been wet. Since crack depths are very shallow, this form of surface cracking is primarily an aesthetic concern. Crazing seldom creates structural or serviceability issues, even for floors exposed to heavy forklift traffic. Crazing is caused by minor surface shrinkage related to rapid surface drying and wetting-and-drying cycles. Many of the causes listed above for dusting also contribute to crazing, especially overworking overly wet mixes and finishing bleed or added water into the top surface.
Trapping bleed water and air. When mix water migrates upward because the cement and aggregate particles are settling, the surface must be “open” so as not to trap the rising bleed water and air directly beneath the top surface. Trapping bleed water and air creates a thin, weak zone directly beneath the surface and results in surface blisters or delaminations. Blisters and delaminations typically form during the onset of troweling. Blisters range in diameter from 14 to 4 inches and typically about 18-inch deep, whereas surface areas for delaminations are larger and can range from a few square inches up to several square feet or more and depths vary from about 18 to 38 inch.
Premature sealing often occurs when the top surface stiffens due to surface drying or the top surface is setting faster than the underlying concrete. Top down setting often occurs when the ground temperatures are cool but the ambient conditions are warm and sunny. When this happens, flatwork finishers mistakenly believe the slab is ready to be power floated or troweled. However, the bleed water and air are still rising. If the finishers seal the surface prematurely, then rising water and air become trapped beneath the surface.