Please tell me how to correctly prepare and apply a concrete topping. We recently placed a slab, and then topped it with a mixture of 1 part cement and 3 parts sand. The slab was given a broom finish before the topping was applied. We did not use a bonding agent between the slab and the topping. After the topping had been placed, we were caught in a rain storm and the topping now is sandy and loose. What do you suggest?
If the topping you applied is loose it should be removed. The old base then must be roughened before applying another topping which should not be less than 1 inch thick. Instead of using a 1 to 3 sand mix, use a mix consisting of 1 part cement, 1 1/2 parts sand and 2 parts 3/8-inch gravel or stone. After the old base has been roughened and cleaned, prepare a grout of 1 part cement and 1 part water. Mix this to a creamy consistency and brush it into the base just ahead of placing the new topping. Be sure that the base concrete is damp so that it will not pull too much water from the topping mix. Avoid an overly wet mix and excessive troweling. Troweling a wet mix will bring fine material to the top and this will result in a dusting surface after the concrete has hardened. Finally, it is most important to prevent the concrete from early drying, which may cause cracking. Water for curing should be applied to the new concrete as soon as this can be done without marring the surface. A fog spray is very effective.