"That salt you put on your drive last winter is causing the pitting on your concrete". This is an often repeated claim but deicers are not the problem. You might ask in response: " Then why doesn't the street next to my drive exhibit the same pitting?" The answer: The city made sure that the concrete on the street was laid properly. Yes salt can have a minor effect on concrete but good quality concrete that is laid properly will not be harmed by deicers with one exception; never use fertilizers on concrete.
Many garden and lawn fertilizers will chemically attack concrete. Spalling in concrete is usually caused by the expansion of water in concrete during freeze and thaw cycles. Water expanding during freeze cycles can produce excessive pressure and fracture weak concrete causing the concrete to pull away from the aggregate and create voids or pitting in the surface of the slab.
Four things that you must watch out for when having new concrete poured:
1. Water to cement ratio: Concrete consists of cement, sand, aggregate and water. Cement should make up 10% to 15% of the total weight. The weight of the water in a concrete mix should not exceed 45% of the weight of the cement. Water has to evaporate through the surface of a concrete pour and too much water will leave voids in the top of the pour and weaken the surface of the concrete.
2. Adding water to concrete during placement: will allow the aggregate to settle to the bottom leaving the surface weak.
3. Air entrainment: Concrete plants add a surfactant to bring the air content to an ideal 6% which causes small air bubbles to form leaving tiny air spaces in the concrete. When water in the concrete freezes and expands these air pockets absorb the pressure from the expanding water thus preventing excessive stress on the concrete and protecting against cracks and spalling.
4. Improper finishing: Overworking the concrete as it begins to stiffen will collapse the air entrainment pockets and will also cause the aggregate to fall to the bottom of a pour leaving the surface weak and susceptible to fracture. Also finishing the slab before all the excess water evaporates forces too much water back into the top surface leaving it weak.
So use a reputable contractor who will order the correct mix of concrete and finish it properly. If you are still worried about spalling then wait at least 30 days after a pour and seal the surface with a penetrating silane/siloxane water repellent.
If you already have spalling concrete you can use a polymer modified cement resurfacing system that transforms your tired concrete into a beautiful stone look while eliminating your spalling problems.
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