Inspecting poured concrete roof structures

98% of homes in Mexico have roofs constructed of poured concrete. In cities, they have concrete trucks and pumps, just like in the US. Other places, they have trucks but no pumps. Other places they mix the concrete on the ground and its carried up in buckets and poured into place.

The question is, is there any non-invasive way to evaluate concrete roofs?

Bear in mind that many of these roofs are covered with clay or concrete tile, although in many cases, the tile is not fastened down.

A Schmidt hammer might tell you about compressive strength, but the roof is really in tension. Does compressive strength contribute to the ability of the roof to resist tension? If not, what does?

Open to suggestions here.

Kenton, my outlook on this is you can only observe the condition of the concrete slab whether it is on grade or elevated.

The condition that it is in would be commented upon when flaking, curling, cracking, delamination, bowing, exposed aggregate, etc. is observed.

Elevated slabs is typically engineered and cast in place using form deck, or plywood forms.
Form decks such as composite decking holds the concrete in the liquid state and becomes part of the engineered system as the concrete is hard.

I no that was not the case in Mexico, but in order for the roof slab to span whatever held it up, it was exposed to tension as you mentioned.

With that said and knowing that concrete has very little tensile strength, it must have been reinforced concrete with rebar mats at the bottom of the roof slabs to counteract the tension.

The rebar would take care of the tension exerted while the slab itself would be in compression at the top portion.
Essentially working together.

Ground Penetrating Radar (GPR) is the only Non-invasive tool that I know of that could be used to inspect for reinforcement and voids in a concrete slab.
Metal Detectors for Re-Bar are also available.


Marcel covered all the basics. A suspended slab needs to be designed under modern building codes, however in central america and other parts of the 3rd world concrete roofs have been around for a long time. I am sure many of them were built by trial and error with general knowledge handed down by older contractors. The load carrying capacity is very sensitive to the quality of materials and reinforcement placement. Modern reinforced concrete design ensures a suspended concrete slab or beam fails in tension not in compression. Tension failure is ensured by limiting the amount of reinforcement, which results in deflections and cracking on the tension side so failure is gradual and noticeable. Over reinforced concrete will fail in compression which is a sudden failure, NOT good.

Ken, in grad school I worked closely with Dr. Antonio (Tony) Nanni. He is internationally known in his work doing nondestructive load testing and strengthening of concrete slabs and beams with carbon fiber. If he can’t help you, he will know the person who can. I have listed his contact information below. Tell him I sent you:

Dr. Antonio Nanni
(305) 284-3461

Professor and Chair, Civil, Architectural, and Environmental Engineering, Department

Room 324 McArthur Engineerng, Coral Gables Campus
P.O. Box 248294
Coral Gables, FL 33124-0630