Edward Binseel, Prince George’s County, Maryland
Prince George’s County, Maryland, plans to build 12 new bridges in the next three years. Some of the bridges will be designed and built using high performance concrete (HPC). All bridges will have simple spans ranging from 24 to 80 ft (7.3 to 24.4 m) in length.
The move toward HPC began several years ago, when the County’s bridge inventory grew to more than 170 structures. Financial demands related to the maintenance and repair of the bridges grew to a level that was in excess of the available resources. Several of the fundamental decisions that had been guiding the design of new bridges in the County were changed. At the expense of higher initial costs, the County would now design its bridges to be more durable with extended longevity, while also decreasing each structure’s long-term maintenance and repair costs. Decreasing the life-cycle costs associated with each bridge became a priority. We believe HPC will give us durability and longevity at a lower overall cost.
The lack of familiarity with HPC within the design community and the construction industry poses a potential barrier to our use of HPC. It’s one thing to design specifying the use of HPC and it’s another to build with it. These barriers can be overcome by recognizing the education and resources of the consulting engineers that we use for design, construction management, and inspection, and then by facilitating their training. Since we intend to require designs utilizing HPC, the consultants will have to learn what’s necessary to prepare the design and specifications accordingly.
Five years ago, the County began restricting the water-cementitious materials ratio for all bridgerelated concrete. By specifying a maximum ratio of 0.40, we are achieving denser, less porous concrete that is also stronger. The low ratio, which implies an additional amount of cement, appears to be giving us the longevity and durability that we need. If one consequence of the design is added strength, we’ll accept that—although we’re not seeking that result directly or depending on it in the way a traditional design would.
Curing of the decks has been an important factor in reducing concrete shrinkage cracking. In winter, decks have been wet cured under burlap and plastic sheeting with flooding of the deck several times a day. In summer, burlap without plastic sheeting is used since we do not wish to retain the heat. Sprinklers are used on the deck to keep the deck flooded for seven days.
We are prepared to pay higher initial costs for HPC, but this has not been necessary. In recent projects, where we have restricted the watercementitious materials ratio and increased the minimum concrete cover over all steel reinforcement to 2 or 2-1/2 in. (50 or 65 mm), we have not experienced higher bid prices or driven up the overall construction cost. We have also seen fewer construction-related problems than were expected using the stiffer mix, and there have not been any change orders as a result of surprises or any difficulties encountered. Each of the design changes that we have incorporated has also been directed toward decreasing the porosity of the concrete to prevent chloride ions from reaching the reinforcement.
HPC may also offer economic advantages because it results in greater strength and requires fewer structural members. We will be exploring this benefit as we become more familiar with the material. With dwindling resources, we are determined to achieve the best value for our construction dollar.
The State and the other counties have been slower to move and adopt these design changes. We have decided to lead, having determined that we cannot afford to wait for the local design community or for the local construction industry to mature and become proficient in the use of HPC. We believe in the benefits of HPC, and will help lead the local industry in its use by putting contracts on the street for construction.