By: Steve Morrical
Each year, millions of dollars are spent in North America removing
and replacing, rehabilitating or financially compensating owners for concrete
that is unacceptable. In many cases, the cause of dissatisfaction was surface
scaling or spalling. Concrete scaling is defined by the American Concrete
Institute (ACI) as "local flaking or peeling away of the near-surface
portion of hardened concrete or mortar," which defines various degrees.
These are light, medium, severe and very severe depending on the depth of the
scaling. Concrete contractors, suppliers, homeowners and transportation
officials are very familiar with this kind of surface deterioration.
Trying to stop infections
In many cases, the use of deicing chemicals is cited as the
primary cause of concrete scaling. In the U.S. there are approximately 30
states that expend a large percentage of their transportation budget on winter
maintenance. Providing safe roadways for the traveling public and helping
ensure steady commerce through the winter season are the primary justifications
for this effort. Traditionally, this service has been accomplished by
mechanically removing accumulated snow and ice from roadways and
re-establishing traction through the use of abrasives and solid chemicals.
Winter road maintenance has undergone major changes in the
course of the last decade. There has been a shift away from the use of solid
chemicals and sand-salt combinations that are costly to purchase, store, use
and clean up, and toward the use of liquid deicing chemicals.
The use of liquid chloride deicers has enabled many transportation
departments to take a proactive approach to winter road maintenance through
anti-icing (treating roadways before a winter storm) and improved, more
efficient deicing procedures. This new strategy has resulted in questions by
the concrete industry as to the effects of these materials on concrete
integrity.
Water-based solutions containing combinations of magnesium
chloride, calcium or sodium chlorides also are being used, the latter two as
concentrated deicing brines. The volume of these materials being applied is
expanding rapidly.
It has been demonstrated that deicing chemicals can cause
and aggravate surface scaling, particularly in concrete with poor air void
systems and low air contents. The presence of chlorides in the surface matrix
of exposed concrete can cause a buildup of osmotic and hydraulic pressures
produced when water in the concrete freezes. If this pressure exceeds the
tensile strength of the concrete surface, scaling may result. Additionally, the
hygroscopic (moisture absorbing) nature of salts keeps the concrete saturated,
increasing the potential for freeze-thaw damage to the concrete.
Not all field concrete exposed to chloridebased deicers
(including magnesium chloride) has shown premature scaling or spalling. What,
then, is the mechanism behind failure? This question is being addressed by many
entities including a pooled fund study led by the South Dakota Department of
Transportation with cooperation from the Federal Highway Administration.
Experience has shown that maximum resistance to scaling can
only be expected and achieved with a combination of multiple procedures
executed correctly. These include (a) proper selection and proportioning of the concrete mixture;
(b) use of high-quality materials; (c)
achievement of an adequate, in-place air void system; (d) proper
finishing procedures that do not impair the near-surface air void system; (e)
application of proper curing practices designed for existing field conditions;
and (f) drying and sealing of the concrete before the first freeze-thaw cycle
and deicer application.
About The Author: Morrical is a technical service engineer for Holcim Inc., Ann Arbor, Mich.
