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Concrete Floor Technology

A House of Cards

Originally Published: Oct 2, 2009


ACI 318 excludes it from consideration as part of the structure. ACI 302 and 360 both focus on its utilitarian character. So obvious and pedestrian is its nature that neither American Concrete Institute, Portland Cement Association, Post-Tensioning Institute, Wire Reinforcement Institute, or U.S. Army Corps of Engineers state what constitutes a failure in its design. For all its unpretentiousness, the lowly slab on grade is consistently the center of more disputes over alleged quality deficiencies than any other building element. It’s been 40 years since Neil Armstrong confirmed man’s engineering prowess. So why is it that the concrete flooring industry still finds it difficult to please its customers?

The same few parties have been engaged in the construction business since antiquity: owners, design professionals, general contractors, subcontractors, and suppliers. Everyone’s responsibilities and statuses are well understood, and when everyone performs competently, then success is assured. The logical converse also is accepted: if a project is unsuccessful, then someone must have done something wrong—but who exactly?

Before the lawyers get involved in a slab-on-grade dispute, the following biases have usually hardened everyone’s position beyond adjustment.

The owner is strongly predisposed to suspect faulty construction rather than faulty design. He is mistrustful of contractors and suppliers because of their opposing financial interests; impressed by the designer’s credentials, presumed objectivity, and comparable status; and financially motivated to find fault to justify the withholding of payment.

The design professional also is strongly predisposed to suspect faulty construction rather than faulty design. He is motivated by intimate reputation and liability concerns and his superior education, status, and adherence to standard practice; and holds similar suspicions regarding the probity of those in the construction business.

The general contractor tends to side with whichever position makes his life easiest in the short run, regardless of what might be the actual cause of the problem. He is concerned much more with scheduling and cash flows than with forensics, often able to leverage the subcontractor by withholding payment and/or threatening replacement for cause, and well-aware of the very high cost-of-litigation barrier.

Given his typically inferior education and status, suspected character deficiencies, and exposed financial position, the flatwork subcontractor usually becomes the guilty-until-proven-innocent fall guy who is pressured to absorb the repair costs—again, regardless of what actually might have caused the problem. By providing a plausible and convenient cover for everyone else’s mistakes, the subcontractor ensures that similar problems will occur on future projects, because no feedback or learning occurs.

This system is founded upon the presumed competence of the standard practice employed by the design professional. But how has the typical slab-on-grade designer come to know what he knows? There are no courses in floor slab-on-grade design. There are no documents published by expert sources, that even define successful behavior, no less guarantee its achievement. As explained, practical experience usually is flawed fatally by the absence of any definitive technical feedback. As a starting point to improve matters, the industry is compelled to adopt this principle:

Rule No. 1: The typical structural engineer knows very little about slab-on-grade physics.

Even worse, most of what they think they know is wrong. Except for flatness, levelness, and cosmetic issues, the cause of most slab-on-grade problems is faulty design, not faulty construction.

This column will present a step-by-step approach to slab-on-grade design, construction, and quality control. If followed, it should easily ensure successful concrete floor installation. The reader will have to abandon many ideas that have become sacrosanct and embrace certain concepts (e.g., Rule No. 1) that now border on the heretical. It promises to be a long and challenging exercise, but the resulting enormous reduction in risk will be more than adequate compensation.

Originally Published: Oct 2, 2009