Going from a 10-inch concrete pavement design to an 11-inch concrete pavement design can nearly double the life of the pavement. That's what Tom Van Dam, Ph.D., P.E., associate professor in the Department of Civil and Environmental Engineering at Michigan Tech University, told attendees of the recent Concrete Workshop in East Lansing sponsored by the Michigan Concrete Paving Association (MCPA), the Center for Transportation Research and Education at Iowa State University, the Federal Highway Administration (FHWA), the Construction Quality Partnership of Michigan, Michigan's Local Technical Assistance Program, and the National Concrete Pavement Technology Center.
"The new design method that I believe Michigan is looking at is the mechanistic empirical pavement design method," Van Dam said. The mechanistic empirical pavement design method represents a new and different approach to pavement design.
This method combines a scientific approach (mechanistic) with observed pavement performance (empirical). The mechanistic empirical pavement design requires many new materials-related variables, some of which are controlled during construction. The following list represents a small portion of the new inputs influenced during construction:
- The coefficient of thermal expansion
- Concrete mix design parameters
- Curing type
- Joint sealant type
"This is a big, big advantage over what was done in the past," Van Dam said. Early age concrete cracking was also discussed.
Concrete expands as the temperature rises and it contracts as the temperature falls. Concrete also expands as moisture increases and contracts as moisture decreases. These are primary factors of early age cracking.
"So, when it's hot outside the concrete is expanding. But it's cooling inside of the concrete. So it's contracting. What happens? Stress builds up," Dale Harrington, P.E., senior project engineer with Snyder & Associates, of Ankeny, Iowa, said. The objective is to manage the change.
"The way we manage change in the first 72 hours of the concrete's life is that we minimize that temperature-moisture change in the concrete if we can. We minimize the build-up of stress if we can," Harrington said.
The following are key points for reducing early age concrete cracking:
- Optimize the size and amount of coarse aggregate.
- Use low-shrinkage aggregate in order to minimize shrinkage that may cause cracking.
- Consider using a water reducing admixture to reduce paste content.
- Use supplementary cementitious materials (SCM) to help reduce the set temperature and the temperature peak.
- Avoid calcium chloride admixtures, which can significantly increase drying shrinkage.
- Time concrete placement so that the temperature peak does not coincide with the hottest time of the day.
- Prevent rapid loss of surface moisture while the concrete is still plastic.
- If the ambient temperature is likely to drop significantly, cover the pavement surface with blankets to slow heat loss and prevent extreme differentials in temperature through the slab.
