Concrete: A hard material engineers hope to make harder

For thousands PITTSBURGH of years, people have built civilizations with concrete made from local materials. Just mix and heat. Add sand, stone and water. Put it where you want it. Give it time to harden.

So it’s no surprise that concrete is the world’s most widely used building material, says concrete expert Julie Marie Vandenbossche, a University of Pittsburgh civil engineer.

But, as it turns out, there’s far more to concrete than meets the tire tread. How these materials glue themselves together, harden and maintain their shape over long periods involves common materials and complex chemistry. Various factors influence strength and durability, all theway down to the concrete’s concentration of air bubbles.

It explains the continuing academic interest in concrete, with Pitt serving as a research mecca, given its team of four notable research scientists devoted to the hard stuff.

That team includes Vandenbossche; her husband, Mark B. Snyder, a concrete consultant; Lev Khazanovich, who holds an endowed chair in engineering; and Steve Sachs, an assistant professor in civil engineering. Vandenbossche said no other university in the nation has as many researchers focused on concrete pavements.

Snyder was key in convincing the Pennsylvania Department of Transportation to develop specifica- tions for long- life concrete, designed to last 40 to 50 years and as long as 60, rather than the current life expectancy of 20 or 25 years. The ultimate goal is formulating concrete to last a lifetime rather than a generation.

Neal Fannin, PennDOT’s pavement materials engineer, wrote the state specifications in recent years for long- life concrete that’s already being used in building interstate highways.

Time will tell, he said, if the long- life concrete mixture will meet expectations. Gradations of stone is key in allowing the concrete to compact better than stone of one size. Better compaction helps prevent slumping of pavement especially at the edges. Also the new concrete involves less cement, which contributes to concrete shrinkage after it hardens. Less shrinkage means more stability, Fannin said.

The U. S. Department of Transportation reports that 2.74 million miles of paved roads crisscross America, with 83 percent paved with asphalt. In the United States, there’s an estimated 465,800 miles of concrete highways, most of them interstate highways that account for about one- quarter of the total miles driven each year.

When talking apple pie, you need a good recipe. The same holds true for concrete. Portland cement, the most common concrete, is made from “a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients,” the Portland Cement Association says.

Materials used can include “limestone, shells, and chalk or marl” combined with another large assortment of materials _ shale, clay, slate, blast furnace slag, silica sand, and iron ore.

Combinations of ingredients are heated at high temperatures — about 2,700 degrees Fahrenheit — “to form a rock- like substance that is ground into the fine powder that we commonly think of as cement,” the association states. When mixed with sand, stone and water, and used as intended, the results include strong walls, roads and sidewalks.