Byron D. Hanson
ASTM, CDT, Lead
Frequent cycles of extreme variations in temperature and weather contribute to scaling, spalling, and eventual disintegration of concrete surfaces. Parking decks, garages, driveways, sidewalks, steps, ramps and curbs are often subjected to freezing and thawing, excessive use of de-icing agents, and wetting and drying cycles.
Air-entrainment is the first thing that comes to mind when considering durability of concrete and its resistance to freeze/thaw; however, there are other considerations. The concrete mix design, quality of aggregate and materials, its placement, and long-term protection of the concrete thereafter must all be examined.
DO: Take an active role in the mix design... You're ultimately responsible!
Specify air entrainment for exterior concrete. Air entraining admixtures must conform to ASTM-C-250 and follow the guidelines of ACI 302.1R, Guide for Concrete Floor and Floor Construction.
Admixtures must conform to C-494 standard types. Water-reducing admixtures reduce the amount of water required for a given consistency and contribute to the degree of cement hydration and, in turn, produce a more durable cement paste. Increased hydration results in more durable concrete.
Other important components of the mix design are the aggregates and portland cement. Aggregates must conform to ASTM-C-33, cement to ASTM-C-150 standards. Contaminated aggregates can cause pop-outs or alkali silica reactivity (ASR), and off-spec cement can cause finishing problems with inconsistent setting times.
The amount of water specified may be the single most important component of the mix design. A low water-cement ratio between 0.40 and 0.50 is normally recommended for concrete frequently subjected to severe exposure. Do not add extra water on site. Additional water will lower strength and increase permeability and the likelihood of scaling.
DO: Make sure to prevent premature moisture loss during the placement of concrete
Proper placing and finishing, as outlined in ACI302.1R, are essential for durable concrete. Excessive or premature finishing contributes to blistering and reduces surface air content and durability. The popular practice of "blessing" concrete with water during troweling, increases the water-cement ratio at the surface, and, may weaken the concrete at the near-surface wear zone. This common practice increases the probability of surface scaling and spalling.
The use of an evaporation retardant, such as L&M's E-CON, in rapid moisture loss conditions eliminates the need for "blessing." E-CON will prevent surface crusting and plastic shrinkage cracking and will reduce overall placement costs while increasing placement crew productivity and overall concrete quality. Keeping the concrete moist and at an adequate temperature for as long as possible helps to provide proper hydration of the cement for maximum strength. More complete hydration and higher strength mean a concrete with increased density, lower permeability, and greater resistance to scaling.
DO: After curing, protect concrete from de-icing salts
Frequent wetting and drying during this early age of concrete will cause crazing, lower concrete strength, and increase the possibility of scaling. L&M CURE R or CURE W are curing compounds that exceed ASTM C309, and when properly applied, will aid in both hydration and strength. It is essential that concrete be protected from freezing temperatures during the first days of hydration; when its low strength and high moisture content make it most susceptible to freeze/thaw damage.
The most cost effective treatment to increase the concrete's resistance to scaling caused by freeze/thaw cycles is L&M's AQUAPEL. AQUAPEL prevents the migration of nearly 90 percent of deicer salts and effectively repels a high percentage of the moisture that will cause deterioration. AQUAPEL treated surfaces are breathable and prevent the entrapment of moisture beneath the sealer, which aggravates scaling and spalling.
Concrete that is less than a year old is very susceptible to scaling, especially during the first winter. Where the presence of deicer agents cannot be eliminated, the application of AQUAPEL before the first winter will help reduce scaling and promote long term protection.
© 2001 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2001.