The Washington State Standard Specifications for Road Bridge and Municipal Construction have untold references to weather conditions that are current to the work site, or predicted to occur during the work shift or cure period for the item of work being completed. The contractor needs to know what the weather will be for scheduling purposes. The Department of Transportation, in their oversight role, needs to monitor current weather to insure that specification requirements are being fulfilled.
In projects that involve earthwork, it is typical to include an erosion control plan. The state develops a plan to treat surface runoff before it leaves the work site. The contractor installs the various erosion control devices and is required by specification to inspect the devices every day during a “runoff producing rain event.” Damaged or inadequate erosion control measures are repaired or replaced as necessary.
Paving projects using hot mix asphalt are very sensitive to weather conditions from the time the hot mix is loaded into trucks until it is placed and compacted. Monitoring air and surface temperatures, and wind conditions are keys to long lasting pavements. The hot mix is to be placed on a dry surface, so forecasting of rain is important in the decision to go or not go at the start of the paving shift. A near disaster occurred on one project when a rain shower came through shortly after the emulsified asphalt tack coat had been placed on the surface of the existing pavement. The runoff that resulted from the rain shower, mobilized the asphalt emulsion that had not yet cured prior to placing the hot mix pavement. The tack coat was ultimately contained with no environmental damage, but not before it entered a river flowing under the highway bridge, causing delay and added expense for clean up activities. An accurate forecast of the rain shower and the timing of its arrival would have helped in the decision to proceed with the paving operation on that night.
The ability to adequately compact the hot mix is dependent on the mix temperature between the time the paving machine places the mix until the rollers have a chance to compact it to the proper density on the road surface. Thinner pavement sections cool more rapidly than thicker sections. For placement of thin sections of pavement (1.25” or less), surface temperatures must be 55°F or greater. Thicker sections (2.5” or greater) may be placed at surface temperatures of 35°F or greater. Surface temperature forecasts would help the contractor with scheduling of work crews and equipment, and minimize standby time.
Concrete paving is also influenced by weather conditions. The rate of cure of concrete decreases with decreasing temperatures. One project on Interstate 5 in Tacoma used a fast setting concrete mix to patch small areas of concrete pavement. The project required working at night and having the road open to traffic by 5:00AM. The product used for patching promised full strength gain in two hours or less. The first patches that were completed began to wear very quickly under traffic. Upon analysis of the situation, the cure times given for the patching material were based on cure temperatures of 70°F. Nighttime temperatures in the spring, when the project began, were in the low 40’s. The two hour cure time the contractor and the state inspectors were expecting was taking much longer and traffic was being released before the concrete was able to come to full strength.
By contrast, warm temperatures, low humidity and wind cause the concrete to cure too rapidly. Random cracking through the concrete panels or surface cracking will result if curing is too rapid. Unexpected rain or hail on the surface of freshly placed concrete pavement will damage the surface of the concrete and decrease its durability under the wear and tear of traffic.
For structural concrete used in bridge construction, air temperatures are important in the curing of the concrete. As stated for concrete pavement, the rate at which the concrete gains strength decreases with decreasing temperature. Concrete is best kept at temperatures above 50°F to assure that it cures quickly enough to allow removal of forms. Cooler temperatures require longer times before forms can be removed. Continued cooler weather may require the use of heated enclosures for curing. Curing concrete requires a moist environment, so a determination of humidity is important whether the temperatures are cool or warm. Warm, dry, windy conditions cause concrete to cure too rapidly and results in less durable concrete over the long haul.
In summary, the ability to know and collect information on current weather conditions for a particular jobsite location, and having access to a credible forecast of the weather for that location is important to successful highway construction. Through long and short range localized forecasts, the contractor is better able to plan and schedule work activities to produce a quality product in a timely and economical fashion. The Department of Transportation needs the ability to monitor and document current weather conditions to assure that the contractor’s work is being performed under optimum conditions, as required by the contract and to achieve the best quality product for the motoring public. Much of this information is becoming easily available on the internet, or through weather forecasting services. The Washington State Department of Transportation has expanded the numbers of road weather information systems throughout the state highway system. These systems are primarily used by maintenance forces for snow and ice control. The construction engineering side of the agency could benefit by becoming more familiar with what these systems have to offer and what benefit they could be during construction.
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