Determining Concrete Slab Thickness that ANYONE can DO!

This is an inspection technique I have used for many years and this has been nearly spot on for all that time.

PRECAUTION - In projects where there may be a significant difference between what is potentially discovered during this inspection, coring MUST be done as a determining factor. This technique has been very reliable for me even if it isn't 100% accurate and is ONLY suitable for a standard concrete mix design.

Concrete Thickness and Joint Cut Scheduling

Concrete tends to square itself off as it cures, dries and shrinks. The rate of shrinkage is consistent enough to have established a one by two rule; for every inch of concrete thickness, the cut joint scheduling should be at a two foot by two foot increment.

As a general habit, I will step off joints AND cracks to determine a "squaring off pattern".

With very few exceptions, a 5 inch concrete will have joints and/or cracks every 10 feet by 10 feet, 6 inch concrete 12 feet by 12 feet and so on.

Defining Moment

I was asked to come out and inspect some large commercial buildings where they were still making final determinations of the flooring choice and I met the moisture testing crew out there to make placement determinations.

During the walk-through, I had noticed there were joint cuts every 15 feet by 15 feet, but the cracking was persistent and had a somewhat consistent pattern.

This was one of the reasons I was called out to determine if the joint cutting was delayed to the point of the cracking to be independent from the joint cutting.

I stepped off the average spacing of the cracks, and the cracking pattern was roughly squared (it will almost always vary a bit), but what was concerning, the slab was supposed to be 7 inches thick for the anticipated heavy loads; but the cracking was averaging a 9-10 foot span, consistent throughout the facility(s).

There was a receiving area where the concrete design was scheduled to be 5 inches thick. The joint spacing was 10 ft by 10 ft, with little to no extraneous cracking.

I then spoke with the testing crew and asked them to wait until I contacted the owner (who contracted with me).

I said I didn't want to cause undue alarm, but I suspect the concrete as placed was 5 inches thick rather than the specified 7 inches due to the crack pattern.

The owner came out and stepped off the cracks with me and agreed to discreetly contact a concrete coring company to see if I had made a mistake, or that the concrete thickness was out of spec.

First core, concrete was exactly 5 inches thick. Based on that, the owner had the coring company core two more areas where the concrete SHOULD have been 7 inches thick.

I gave my opinion that the receiving area was likely 5 inches thick as well due to the relative absence of cracking, which also confirmed the joint cutting was done within a correct timeline.

Sure enough, the receiving area was 5 inches thick.

DANGER Will Robinson, DANGER!

Well, this triggered a bit of excitement in the owner and prospective tenant since the concrete may not be adequate for the intended usage due to the tolerance restrictions.

At the end of the day, the concrete needed to be replaced, which created some significant delays, but the tenant, because they WERE involved early and had a great relationship with the owner, showed incredible patience.

Conclusion

Let's just say the concrete company had some pretty dramatic issues to take care of.

Thankfully, I had the foresight to advise the owner to check what was ordered from the ready-mix producer to see who was responsible, or who dropped the ball.

The ready mix producer supplied what was requested by the contractor, who had discovered the forms as placed were for a 5 inch slab, NOT a 7 inch slab and rather than stop the project and make the necessary corrections, the ill-advised determination was to simply place the concrete, obviously thinking this wouldn't be noticed, or an issue at a later time.

So the ready-mix producer wasn't at fault, but the contractor got clobbered!

It is likely that the contractor was trying to give a break to the person responsible for setting the forms, but shouldn't have. If the contractor had done the right thing, there may have been a lot of temporary stress, but in the end, all ANYONE remembers is the quality of the job.

In ALL cases, the emphasis should be for the owner's best interest.

Post Script

Considering the unpredictability of California Laws and responsibility, if a problem had arisen later, there is little doubt the owner, even having completely clean hands in this regard, would have little choice than to contribute monetarily to needed remediation and repairs, which could have been substantial.

Even though stepping off crack and joint patterns may never amount to much other than as a curiosity, in these rare situations, the little things can become the BIG things!