Q.: Last December we cast the first and second floors of a building by placing concrete 2 1/2 inches thick over precast plank. Late in April the owner reported that extensive random cracking has appeared on the first floor and a small amount on the second floor. We don't know how long ago these cracks were first noticed.
During construction we placed and finished 1800 square feet per day for a total of 30,000 square feet. On the first floor we used a 6-bag mix but because we pumped the concrete for the second floor we used a pea gravel mix with 6 1/2 bags of cement. Both mixes contained 1 percent calcium chloride, though they were used in an enclosed building where temperatures were 70 to 80 degrees F at the time of concreting. Cylinders from the first floor produced 5200 psi and those from the second floor produced 4400 psi strengths at 28 days. I do not have a record of when the curing and sealing agent was applied, but Schmidt hammer tests made in May of this year indicate concrete strengths of 4400 to 4700 psi for various locations on both floors. As far as I know there is no cracking problem in the basement slab on ground, which was constructed with the same materials and methods except for the use of precast planks.
Do you know what could have caused the cracks? They do not look like plastic shrinkage cracks. Could the cracking be related to the fact that the precast plank were not wetted before casting the floors? Is there anything that we can use to fill the cracks? The building is to be used as a food manufacturing plant and the floors will have to carry some lift truck traffic.
A.: Without seeing the floor and examining records in detail it is hard to be sure what might have caused the large amount of cracking. The cause might be the fact that the precast plank, which are often quite dry when installed, were not wetted before casting the slabs. The plank could have absorbed a large amount of water from the fresh concrete, creating incipient cracks on the bottom of the slab. These cracks could later come through to the top of the slab.
One of the least expensive ways to repair the cracks would be to fill them with one of the so-called self-leveling cements now on the market. These products can be flowed over the surface to produce a thin, level topping. Manufacturers of such materials can also supply other cementitious products to be squeegeed over the cracks to fill them before the topping is applied. You may want to discuss the procedures and materials with a manufacturer and to ask whether anything can be done to enhance the resistance of the new topping to the wear that the lift trucks will impose. Manufacturers of such products are listed in the yellow pages of the December issue of CONCRETE CONSTRUCTION and in the Concrete SourceBook. Look under the category "Underlayments." (Only a small number of the manufacturers of underlayments make the type of flowing, portland cement-based compound you are looking for.)
There is, however, a question whether a concrete plank floor with a 2 1/2-inch topping will carry lift truck traffic without undergoing so much deflection that it will crack from repeated loading. It might be advisable to have the floor examined by a professional engineer to determine whether it needs to be strengthened structurally.