Question:

We poured a concrete slab and did it ourselves. We followed a handbook from the local lumber store and it was helpful in getting the area ready for the concrete. Some of the people did not show up as they had promised and we had to pour the concrete with three people, only. The concrete was real "soupy" and flooded into the forms from the truck. We leveled it and had to wait to smooth it. The breeze dried the water off the top and we finished the top with hand tools. Now we have a hump in the middle and the ends of the slab are bent up and there are cracks running through the slab. What did we do wrong? We did not want wavy, cracked concrete.

Answer:

Thank you for your question. The things that caused the wave or curl and cracking in the concrete slab is most likely the amount of water in the concrete. Water in concrete is needed to hydrate the cement paste in the concrete. Too much water makes the concrete have problems if the concrete sets with the high water content still present in the fresh mix. I suspect the small crew you worked with needed wet concrete. The better thing to do is place smaller amounts of concrete in smaller areas if the crew is too small to handle the larger area.

This can be accomplished by building some forms inside the original formed area to make an area and more in line with the ability of a smaller crew. Let us say the original area was 20 feet by 40 feet. You could take lumber or forms and make a series of 10-foot by 10-foot squares in side the larger rectangle and a small crew could handle that smaller size area.

Back to the original questions. The "hump" in the middle of the slab is a type of curling or symptom of the slab shrinking and pulling in from the form faces, the upturned edges are curling of the edges of the slab, because the surface dried faster than the bottom of the slab and the edges lifted up.

The cracking could have been controlled by jointing the slab with a jointer tool or a concrete saw cutting joints into the hardened but still young concrete. Concrete shrinks from its original volume. You can prove that to your self. You can fill a tall five-gallon bucket with the same wet concrete you used in your slab, fill it to the brim of the bucket. Let the water rise to the top and evaporate. Once the concrete has hardened for about a week, check the surface of the concrete, it will have shrunk or receded down from the brim of the bucket.

A slab of concrete shrinks, too. The problem is there is bucket to contain it, it can lose volume across a wide flat area and the surface of the slab will crack from the change in volume.

You may able to wet the slab with water and get the curl to minimize along the edges, the breeze sucked too much water out of your concrete and caused rapid drying and shrinking. Next time consider a smaller area and use our E-CON to prevent early drying, and be sure to joint the slab as soon as the surface will hold a joint or saw.