Question: This time of year always brings numerous complaints about driveway wearing surfaces. Conditions that mar a driveway's beauty-and are the homeowner's most common complaint-are popouts and exposed rocks at the slab's surface. More precisely, crushed limestone aggregate is exposed after the medium broom finish has worn away. Why is it that, with less than a year's service, a broomed concrete surface might wear to the point of exposing major portions of coarse aggregate? In many areas, the coarse aggregate even dislodges.

Answer:Daniel J. Green, technical service manager for National Cement Co. of Alabama in Birmingham, responds: Over the past few summers, I have been analyzing my inspections in my territory, and I'm offering my take on this problem.

At first glance contractors always want to suggest that the No. 1 cause for this situation is low-quality concrete as delivered from the ready-mix plant. They suggest many contributing factors. Perhaps not all of the cement had been batched. Were there fly-ash contaminants in the cement silo? Possibly the air content was excessively high, resulting in low strength. Maybe the plant operator opened the water valve too long, producing a very high slump. Any one, or a combination of these, would explain a weak concrete surface.

All of these scenarios make the ready-mix plant manager an easy target for the already upset home builder or homeowner. In one case, the homeowner even had the foresight (perhaps in preparation for a lawsuit) to contact a testing lab. The lab suggested drilling core samples for compressive-strength tests. Imagine the homeowner's surprise when results showed core compressive strengths approaching 4000 psi.

My explanation is that the saying, "You can't judge a book by its cover," applies to driveway surfaces.

Almost every core break I have observed on similar driveways has shown that the body of the concrete has acceptable strength. I say "body" because lab technicians trim the ends (which include the finished surface) during the core sample preparation. So only a visual inspection shows that the surface of the driveway is eroded and worn, is not durable, and therefore is significantly lower in strength, lower in durability, and lower in wear resistance than the body of the concrete.

What can cause the surface of the concrete-that upper 1/8 inch, 1/4 inch, 1/2 inch or even more-to be so different in physical characteristics, strength, and durability than the body of the slab? As technicians, we know what causes strength and durability reductions in concrete. And further, we know what causes low wear resistance in concrete slabs. We don't even have to speculate. We only have to refer to ACI 302, "Concrete Floor and Slab Construction," which I summarize.

"Low wear resistance is due primarily to low-strength concrete, particularly at the surface. Such low strengths result from:

  • Too much mixing water
  • Use of concrete with too high a water/cementitious materials ratio
  • Excessive slump, which promotes bleeding and carries softer, lighter material (laitance) to the surface
  • Overworking overwet concrete
  • Premature floating and troweling that works bleedwater into the surface (increasing the w/cm ratio at the surface and lowering the strength at the surface)
  • Excessive use of water by finishers
  • Deficient curing
  • Opening the slab to abrasive traffic before sufficient strength has developed
  • Poor finishing techniques and improper timing during and between finishing operations, i.e., floating the concrete while bleedwater is still present."

The repair for this problem is time-consuming and costly: resurfacing, overlays, or removal and replacement. In the future, let's all make a renewed effort to rid our industry of these problems completely.