Investigating Probable Causes of Cracks & Defects in Concrete

Summary

Concrete placed for the slab of the referenced building in Cypress, Texas has sustained cracks in the slab and stakeholders want to know the cause of these cracks.  These cracks became noticeable and worsened following Hurricane Harvey in August of 2016.

A sample taken from a cracked area of the slab was presented for an evaluation by petrographic analysis, as per ASTM C 856.

Test performed on the sample revealed the following as probable causes:

  1. The quality of the aggregate does not conform to standards for optimum engineering performance – The aggregate is predominantly a combination of clay-bearing limestone and limestone with inherent cracks in them; others had pores in them.
  2. Fly ash used in formulating the concrete is estimated to be high; over 40% by weight of cement used.
  3. An uneven setting in the sample, namely, the differential hardening of the concrete noted in the sample, with applied loads, may have contributed to the initiation and propagation of cracks formed.

. Visual Observation & Petrographic Analysis

3

a. Full-depth crack in core

4

b. Chipped-off sections of aggregate creating voids.