The following are some of the top technical inquiries received by the American Concrete Pavement Association (ACPA). This article focuses on some of the challenges of paving and maintaining concrete pavements in winter months. Some of the following technical tips cover winter topics including frost heave, studded tires and insulation during cold weather paving.
Frost heave
Frost action is best described as the expansion and eventual consolidation of fine-grained soils due to freezing. A number of factors must be present for frost action to occur including:
* A frost-susceptible soil (generally a silt or silty clay);
* An adequate supply of moisture (due to infiltration, ground water movement, capillary rise and others); and
* Sustained temperatures below freezing (the soil must freeze--ambient air temperature can be used as a predictor, as can historic climatic data).
Frost heave occurs when adequate moisture is present in a frost-susceptible soil that is then frozen. These conditions lead to the formation of "ice lenses" in the soil. Because ice occupies a greater volume than water, a wedging action or expansion of the soil results. As the ice lenses form, additional water is drawn in, leading to further expansion. When the soil thaws, the ice lenses melt and consolidation of the soil occurs.
Frost action affects all pavement types, although concrete pavements are less susceptible to it than asphalt pavements. It is most detrimental during the formation of the ice lenses, which result in expansion of the soil. Pavement distress typically involves longitudinal cracking and differential vertical movement of the slabs. The most problematic areas are transition zones between materials of different frost susceptibility.
Methods to minimize or eliminate frost action include:
* Removal of the frost-susceptible soil and replacement with a more suitable material;
* Cross hauling to eliminate differential frost susceptibility;
* Addition of soil modifiers to reduce frost susceptibility; and
* Minimizing the level of moisture present through proper drainage, pavement maintenance and design features.
For more information on frost action, see ACPA publication TB011P which covers subgrades and sub-bases for concrete pavements.
A pinch of salt
With winter weather comes seasonal pavement maintenance and lots of salt. Although most concrete pavements placed in the last year should have resistance to any detrimental effects of deicing salts, a certain amount of caution should still be exercised before salting.
The concern with putting deicing agents on concrete is based on the ability of the saltwater solution to penetrate into the surface of the concrete and then refreeze. The basic reaction is mechanical--freeze-thaw--and not chemical. Regardless of the deicing chemical, it still melts the ice/snow, which can make the concrete susceptible to scaling (the pavement distress that results from freeze-thaw damage at the surface). However, concrete pavements are designed to be resistant to this effect.
Deicing salts are detrimental to concrete pavements in the following situations:
1. The new pavement has not undergone an air-drying period of 30 days after placement. The 30-day period of air drying allows the concrete to seal and prevent a saltwater solution from penetrating into the concrete;
2. There is inadequate air entrainment. It is best to have around 6% (no less than 3.5%) for harsh freeze-thaw environments; and
3. There was insufficient curing. Acceptable curing methods include: curing compound, plastic sheeting or misting, among others.
Therefore, if the concrete pavements are at least 30 days old, had adequate air and underwent an adequate curing regimen they should survive a harsh, salty winter without problems. For more information on scaling and deicers, refer to ACPA publication "Scale-Resistant Concrete Pavements" (IS117P). It is available as a free download on ACPA's website (www.pavement.com).
Chill factor
A common question this time of year has to do with the difference between melting rates on light-colored and dark-colored pavements.
There is no clear relationship that darker pavements (new asphalt or dark-colored concrete) will affect ice and snow control to an advantage over lighter surfaces. Older reports have shown a possible 6- 8°F temperature differential between concrete and asphalt surfaces exposed to sunlight in cold climates. However, this is primarily applicable to high-altitude locations where sunshine is predominant during cold temperatures. Many northern states and Canadian provinces receive very little sunshine during the winter months, especially during snowfall events, reducing the effect of solar radiation.
In general, other factors such as temperature, wind velocity and direction, sunshine, terrain, roadway grade and deicing chemicals have a larger influence on snow control than pavement surface color. In fact, the few studies (Montana 1967 and SHRP H-643, 1993) have shown that salt demand is slightly higher on asphalt surfaces than on concrete.
Concrete coming of age
The maturity method is a non-destructive approach for estimating the strength of concrete.
It accounts for the combined effects of time and temperature on concrete strength development. The strength of a given concrete mixture that has been placed properly, consolidated and cured is a function of its age and temperature history.
If the temperature of a freshly placed concrete pavement is measured over time, and those data points are plotted on a graph, the area under the curve can be called the time-temperature factor (TTF), which is a measurement of the concrete's maturity. The logarithm of the TTF directly relates to the strength of the concrete.
In developing the maturity curve for a particular concrete mixture, multiple specimens must be cast, their temperatures measured over time and their strength determined by conventional destructive testing.
The benefits of maturity include:
* Identifies earliest possible opening to both construction and public traffic;
* Allows determination of optimum time to sawcut joints;
* Facilitates both fast-track and cold-weather construction operations;
* Requires fewer specimens to fabricate and test thereby reducing QC/QA costs; and
* Facilitates earlier agency acceptance and contractor payment.
For more information on maturity testing, see ACPA publications PL518P and IS257P at www.pavement.com.
Insulation blankets
Cement hydration in a freshly placed concrete mixture is an exothermic reaction, which means that it gives off heat. Most of the heat of hydration is generated during the first three days after placement and finishing. However, the concrete must be protected from freezing so that the free water can combine with the cement in the mixture and form the hardened paste.
Insulating blankets, mats or foam sheets are commonly used in cold weather concreting to protect fresh concrete from freezing, allowing hydration to occur at a more rapid rate, because higher temperatures promote faster strength gain. However, insulation blankets do not negate the need for curing compound, which should be applied prior to the blankets.
Concrete should be protected from freezing if air temperature is expected to fall below 40°F in the three days following paving. It is advisable to maintain the concrete temperature above 40°F until the pavement reaches 2000 psi compressive to continue the cement hydration reaction. The use of maturity meters is encouraged, because they monitor the temperature and are a very good predictor of strength, as long as the laboratory work and strength correlation curve are determined using project materials prior to paving.
Insulation blankets are capable of holding concrete slab temperatures around 120-140°F, even in cold weather. A repair project constructed in the late fall of 1999 on Highway 401 in Ontario, Canada, used insulation blankets for curing and maintaining the concrete temperature. The full-depth repairs were constructed at night starting at 10 p.m. and opened each morning at 6 a.m. When the blankets were removed to open the lanes for traffic, a thermal shock occurred because the average low temperatures were between 20 and 55°F. Combined with traffic loading, the thermal shock caused most of the patches to crack.
ACI 306R-88 recommends that concrete be cooled gradually--pavements less than 12 in. thick should not experience more than a 50°F drop in temperature. It also states that concrete that is placed and cured at a low temperature (40-55°F) is more durable concrete as long as it is protected from freezing and frost.
For more information, consult ACPA publications TB004P, IS257P, SP204P, or ACI publication 306R-88.
Studded tire wear
Studded tires only give traction on ice or hard-packed snow. There is no added benefit to the road user when the driving surface is wet or dry pavement. Studded tires are a problem for both concrete and asphalt pavement types, although the damage appears more quickly in asphalt pavements. The steel studs embedded in the tires cause rapid abrasion and wearing of the pavement surface in the wheelpath. This can lead to standing water during rainfall and increased possibility of hydroplaning, as well as black iceduring winter months.
The first step in trying to eliminate the problem is to ban the use of studded tires altogether. Numerous studies have shown that the added benefit studded tires offer is far outweighed by the additional cost to repair the pavement damage.
To address problems on existing pavements, diamond grinding will remove the pavement surrounding the worn wheelpaths, in addition to other benefits, including smoother ride, excellent friction and longitudinal texture for low tire-pavement noise.
If studded tires are allowed in the state or region, concrete pavements can be designed to withstand the abrasion from the studded tires. An abrasion-resistant concrete mixture would contain high-quality aggregate, between 540 and 760 lb/cu yd of cement, plus silica fume, fly ash or slag, with water-cement ratios of 0.22 to 0.36, and would achieve compressive strengths in the range of 12,000 to 19,000 psi.
These abrasion-resistant mixes come with a big price tag. It may be best to focus on eliminating studded tires by statute.
