Aggregate Matters:

About this Project

Location: Pasadena, TX

Summary of Project: Cracks and patches of surface delamination was noted on a concrete slab intended for commercial use with heavy equipment shortly after concrete placement. Cracks were observed in about 14,000ft² section of the 20,000ft² slab. The cracks range from hairline to width sizes up to 0.030in (0.75mm), and lengths ranging from 5-ft to 30-ft. Some of the cracks cross and join. Surface delamination of the slab also occur in several locations, about 6 to 8 of them, covering areas of about 2ft² to 10ft².

Stakeholders were concerned and contracted C3S to evaluate the slab and advise them on the probable cause(s) of the defects and its effect on the structure’s long term durability.

Site inspection and samples collection were performed in May 2020.

Findings from Assessment:

Petrographic analysis of samples collected showed over-sanded concrete with inadequate coarse aggregate. Both the coarse and fine aggregates do not conform to certain standards for optimum engineering performance, namely, they were weathered with inherent cracks in them with a tendency to chip off during mixing of the concrete (see inherently cracked aggregates marked in red circles). The chipping off tend to occur during the latter stages of mixing at which point the cementitious paste has thickened to the point where it cannot flow to fill the voids, thus leaving voids in the hardened concrete. These voids created in the samples are marked in yellow circles.

The test also showed that the paste content in all four samples collected were not uniformly dense, which is indicative of concrete with moderate re-tempering.

Concrete with these noted characteristics is likely to have worse cracking issues when put into service, especially if slab is to be subjected to frequent loading. With applied loads, propagation of cracks will be accelerated aided by sporadically-distributed voids in the hardened concrete,

The client decided to remove and replace the slab.

In Contrast

Below are photomicrographs from the petrographic analysis of samples taken to evaluate the current condition of a 40 year old concrete runway at Love Field International Airport, Dallas, Texas. This assessment was part of efforts to rehabilitate the runway. This analysis showed that the significantly older concrete structure, about to be removed and replaced, is in good condition and will be a good source of durable material to use as recycled aggregate in concrete.

Six samples were evaluated during this assessment, and they revealed that coarse aggregate used is predominantly limestone, some argillaceous (clay-bearing), others ferruginous (iron bearing). The samples showed dense cement paste with varying amounts of unhydrated cement particles. In some areas examined the paste surface is clear, with virtually no unhydrated particles, and in other areas, the particles almost cover the entire paste surface. The cement content used in formulating the concrete appear to be high, estimated at 6-1/2 ± ½ sacks per cubic yards.

The water-to-cement ratio is estimated to be 0.41 ± 0.01. The estimated entrained air content in the six samples is 6 ± 1%. Petrographic analysis also showed that the concrete appears to have been formulated with fly ash.