My cousin and her husband decided to sell their house and move into a new condo. Everything was fine except for the basement floor, which was cast on a very cold day over a year ago in the middle of January. The concrete contractor covered the fresh concrete with straw the following day. Over the next several months the floor cracked in many places in its 30x48-foot area. At each Lally column, 3/8-inch cracks intersected in both directions with slab curling at the cracks as well. The total width of the cracks perpendicular to the 48 foot length added up to 1 1/8 inches not counting the shrinkage between the edge of the slab and the foundation walls on either side. If the slab was cast with a good concrete mix having a 0.40 water/cement (w/c) ratio you would expect about 3/8 of an inch of shrinkage in the 48 foot length. So what happened? Without expensive testing, only educated guesses can be made. But here are some of the possibilities:
- The concrete contractor used an excessive amount of calcium chloride to accelerate the initial set of the concrete.
- Additional portland cement was added to the mix to reduce the initial setting time. This would increase shrinkage.
- The concrete slump was too high. It's also probable that the contractor added water as the placement proceeded to counter the affect of the calcium chloride stiffening the concrete still in the ready-mix truck.
- There were no control joints in the slab. They should have been placed at intervals no greater than 15 feet in both directions—10 feet would be even better.
- The thickness of the slab wasn't checked but it's probable that the subgrade was neither flat nor compacted causing the thickness of the concrete to vary between 2½ and 4 inches across the floor.
- All of the above.
After signing the contract, my cousin and her husband were given a one-year warranty against construction defects. They immediately brought up the issue of the floor. The builder responded by telling them there were no defects in the floor (although the concrete contractor returned after casting the slab to grind down the more significant curls at some crack locations). He told them that “concrete naturally cracks and the concrete contractor did nothing wrong.”
So who is responsible for the floor? Is this really what you should expect from concrete? It's easy to point a finger at the concrete contractor in this situation; they probably bear the greatest responsibility. But others are responsible too. The builder probably told the concrete contractor the schedule required the slab installation and there wasn't money in the budget to ensure the proper conditions for cold-weather concreting. It's also unlikely that the builder supervised any part of the installation. There probably were no concrete specifications included with the plans for the condos in the project—architects and engineers always include specifications and testing services for commercial work but rarely for residential projects. And although this happened in a community that could well afford good inspection services, the building inspector probably didn't know much about concrete, visited the site briefly the day before concrete placement, didn't return during placement, and didn't evaluate the work afterward. And my cousin, being the consumer, should have raised the issue before signing the contract, though it's doubtful the builder would have made any adjustment. My cousin will probably be the one to pay for the mistakes in the form of concessions to the next owner when the house is sold.
The sad thing about this common situation is that in the minds of consumers, concrete is to blame—not the concrete contractor, builder, architect, or building inspector. The most used building product on earth is seen as an inferior product. But it's really ignorance and the unwillingness to pay attention to the details that causes the problem. Residential concrete work is like the old west; anything goes because there is little accountability.
We publish this magazine to provide information about concrete to all professions connected to it. Hopefully your combined efforts will make residential work turn out more like commercial work.