Concrete, Snow and Ice
DEAR TIM: Winter is fast approaching. Within the past month, I had a new concrete driveway and sidewalk installed. I am concerned about surface scaling caused by deicing salts and freezing temperatures. Is my new concrete at risk and what, if anything, can be done to minimize the damage caused by salt and ice? A. L.
DEAR A. L.: Many homeowners ask this same question. Because of the relatively high initial expense of installing concrete, homeowners want their driveways and sidewalks to last as long as possible. Virtually everyone knows that deicing salts and freezing temperatures can damage concrete - concrete that has not been ordered, placed, finished and cured properly that is.
Your new driveway is extremely susceptible to damage in its first year. Only use sand for traction. Concrete, contrary to popular belief, is not a totally 'solid' object. It contains microscopic passageways. These passageways are created during the initial crystallization process as concrete transforms from the liquid to the solid state. These tunnels are created as the needles of the interlocking cement paste crystals grow.
Water cannot be compressed. When water freezes and turns into ice, its volume expands by about nine percent. If there is not sufficient room within the passageways to accommodate this expansion, the ice can begin to break the needles of the interlocking cement paste crystals. Your freshly poured concrete already has a high water content. Although it appears dry, there is quite possibly a large quantity of free water still within the crystal structure.
Deicing salts can also cause similar problems. The presence of salts in concrete can create high internal pressures. Deicing salts melt snow and ice and create a salty brine which is absorbed into concrete. As the temperature drops and ice crystals begin to form, the concentration of the unfrozen salt brine begins to increase. This brine actually attracts water creating a pressure difference within the network of cement paste crystals. This pressure can and does fracture some of the crystals.
Fortunately, there are things that can be done to minimize the damage from these forces. When concrete is mixed, additives can be included which create very small air bubbles within the concrete as it dries. These air bubbles provide a place for the expanding ice crystals to relieve their pressure. Residential concrete should have an air content of five to seven percent.
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Concrete strength is a function of the amount of cement powder that is added to the mix. This strength is often measured in pounds per square inch or PSI. Residential concrete subjected to freezing or deicing salts should attain a minimum strength of 4,000 PSI. You can order concrete with higher PSI strengths. The concrete can only attain this strength if it is properly cured. A liquid curing compound should be applied to your concrete as soon as the final finish has been applied.
When finishing or placing concrete, never add extra water to the mix. Also, do not sprinkle water on top of the concrete as it is being finished. Never finish concrete that has standing water on the surface.
These practices dilute the amount of cement paste at the surface of the concrete. With less cement, there are fewer crystals to hold things together.
You can also apply special sealants to the concrete which minimize the absorption of water or salt brine. Many of these sealants are highly effective and can often help to protect concrete which has been improperly installed. These sealants should have the capability to breathe. Water vapor from the soil beneath the slabs must be able to evaporate into the air.