concrete slab compaction, settlement, and cracking

by laura
(San Benito Tx)

Question:

My builder is going to use sand as the top layer for a concrete slab for my new home. And he said that the tractor will compact the soil, so he doesn't need a compactor. Other people are telling me horror stories of slabs settling, cracking and etc. here in south Texas. Do I need to get my builder to do more to get a good foundation?

answer:
Hi Laura, thanks for visiting

I would ask the concrete contractor to use proper way to compact the sand but be prepared for an added cost. This should had been a conversation before starting any work.

But this is not an easy question because the issues of slab settlement and concrete slab cracking are very complicated subjects. If you look around at any structure that has exposed concrete, be it slab, driveways, walls, etc you will find concrete cracks happens everywhere.

This by the way is not a Texas problem it’s just the nature of concrete no matter how much we try to prevent it from happening.

Sand is a very poor and difficult material to compact which explains your contractor’s choice of action. Some contractors consider sand as self compaction material which is not true.

Sand compaction need to be done in small lifts about 4 to 6 inches thick meaning you spread about 4 to 6 inches thick of sand than run the compactor, and add another lift and repeat until you get to the desired height.

Compacting 12 inch thick sand in one lift can be very difficult because the compactor will instantly sink to the bottom once activated making it difficult to move around for proper compaction.

Since sand is more consolidated than clay or gravel, most cement contractors prefer using sand because it poses lesser risk for slab settlement compared to other materials such as poorly compacted clay or gravel.

The cost factor can’t be ignored in this discussion. The proper way to achieve compaction requirements will take more equipment and more time which usually lead to higher cost. If the cost of proper compaction was not part of the bidding process, it can create tension between parties involve.

As mentioned above slab cracking and floor settlement is very complicated because it is difficult to pin point the cause.

In most cases it is natural to point finger at concrete contractor when slab settlement or cracks occur.

The ground that supports our homes has natural behavior that we don't fully understand. Some soils expand when wet, some collapse if absorb water. There are others that shrink when loose water during summer months, etc

We can spend thousands focusing on the top 12 inches but if the rise in water table can cause the slab to settle and crack, the effort and money spent does nothing to alleviate the problem.

Best of luck with your project.
Tonga