Winter Maintenance of Exterior Concrete Flatwork
Winter weather is here! For much of the country that means freezing rain, sleet, snow and ice on sidewalks, driveways, parking lots, and roads. The easy solution – deicing salt. While salt itself does not react with concrete, the use of deicing products to lower the freezing temperature to promote thawing of frozen surfaces can affect concrete durability. The roller-coaster ride of temperature fluctuations can wreak havoc on exterior flat concrete surfaces in the presence of wetness.
When the snow or ice melts, water can be absorbed into the concrete’s surface and settle into microscopic pores. When the temperature drops again below freezing, the trapped water freezes and expands. The expanding ice can exert pressures greater than the strength of the concrete. When the external pressure of the expanding ice exceeds the internal strength of the concrete, scaling and/or spalling occurs on the concrete surface. A damaged surface can eventually result in progressive failure of the concrete. There are a number of factors affecting scaling resistance, but here are a few of them.
First, starting with the concrete mix design, air-entrainment additives are commonly used in concrete mix designs since it results in discontinuous pores, as opposed to more interconnected pores associated with non-air-entrained concrete, or concrete with too little air entrainment which allow water to get deeper in the concrete surface. Air-entrained concrete is certainly an advantage to reduce the pop outs from ice damage, although other issues need to be addressed to fully resist scaling and spalling.
Concrete strength has an effect on the durability of the surface, because a higher compressive strength also means a higher tensile strength that resists the forces being exerted by the ice crystals. Specifying a 4,000 psi mix with a sufficient amount of air-entrainment for the anticipated exposure would provide greater protection than would a 3,000 psi mix with air-entrainment.
Secondly, proper construction techniques such as not overworking the surface of the concrete, along with proper curing can improve resistance to scaling. The application of a “breathable” sealer on the concrete surface after finishing can reduce the potential for water to enter into the pores of the concrete during thawing.
Finally, comes maintenance. Concrete flatwork exposed to freezing and thawing cycles is more vulnerable to damage during the first year of exposure. The National Ready Mix Concrete Association (NRMCA) recommends that deicing products not be used in the first year, and many deicers come with a warning not to use on concrete less than one year old. In lieu of deicing products, sand can be used to increase traction to reduce slips and falls.
When selecting deicing products, sodium chloride based products (deicing salt) are generally less damaging to concrete when used in smaller amounts than calcium or magnesium chloride products, or calcium magnesium acetate. Ammonium sulfate or ammonium nitrate as deicers should be avoided since they are chemically aggressive and destroy concrete surfaces.
With proper planning and execution, scale resistant concrete can be achieved for exterior flatwork. A good two-page resource is CIP 2 – Scaling Concrete Surfaces published by the NRMCA (https://www.nrmca.org).
We hope this “Lessons Learned” has been informative. For more information, contact one of our offices listed here.
ECS Corporate Services, LLC