The dozen biggest recurring slab-on-grade problems—both real and imagined—involve: cosmetics, cracking, curling, delamination, dowel alignment, profile tolerances, joint filling, joint stability, joint spalling, sweating, thickness, vapor transmission, and WWF location. Both owners and designers, of course, presume the science embodied in the established design and specification procedures is capable of avoiding all these troubles. Indeed, so the thinking goes, if the contractors would just follow the plans and specs then everything would be fine.
Because contractors—inveterate miscreants that they are—can't be trusted, prudence requires the imposition of strict formal quality-control procedures. To that end, the ASTM provides an impressive array of standardized tests. See the ASTM Standardized Concrete Tests table.
As evidenced by the fire drill that attends any adverse result, great faith is placed in the power of these tests—especially those for compressive strength and slump—to push the slab toward success. Sadly, this faith is badly misplaced:
Rule No. 8a: Neither the Compressive Strength Test nor the Slump Test provide meaningful information regarding future performance.
No slab has ever failed in compression and no correlation has been established between slump and cracking, curling, and cosmetic tendencies.
Here again the bizarre world of the slab on grade is exposed, where everything is frustratingly uncertain and counterintuitive, even to the point where the two tests comprising the backbone of its quality control are pointless. Slabs on grade have always been burdened by their passing resemblance to structural concrete. As long as the two remain confused, serious problems will persist.
Rule No. 8b: The ASTM flexural strength, shrinkage, air content, abrasion resistance, relative cracking tendency, and flatness/levelness tests are, in varying degrees, at least plausibly pertinent to future performance.
The slab's ultimate shear and tensile properties will affect its joint spalling and live-load cracking tendencies. Its ultimate shrinkage will affect its curling. Its air content will affect its tendency to delaminate. Its abrasion resistance, relative cracking tendency, and flatness/levelness are all important to the end user. Nonetheless, because the Big 3 problems—curling, “random” cracking, and delamination—all initiate soon after placement:
Rule No. 8c: Most serious slab-on-grade problems occur in a material whose properties are both unknown and untestable.
With two exceptions, the C-1581 ring test and the E-1155 surface profile test, all of the ASTM tests listed investigate the properties of a material that in no way resembles the just-set concrete in transition that is starting to crack, curl, and delaminate. Currently there is no way to generate the very early-age time plots (versus depth) of any of the mechanical properties essential to curling, cracking, and delamination prediction: the elastic modulus, shear modulus, Poison's ratio, tensile strength, shear strength, and shrinkage, among others. When it comes to material science, the industry is mired in endemic self-delusion. The testing of irrelevant properties will continue, only because they're important in structural work, not because they contribute to the production of good slabs. In any event, when bad things continue to happen, some way can always be found to blame the contractor. All things considered, therefore:
Rule No. 8d: Placing a knowledgeable observer at the batching location is the most effective means currently available for controlling the quality of slab-on-grade concrete.