Aggregate For Concrete Is High Tech
By Harold Hough
Think there is nothing new about aggregate for concrete? Well, you are wrong. There is a lot more to concrete and aggregate than meets the eye. That’s why the Pantheon, which was built 1,900 years ago with unreinforced concrete is still standing and taking in visitors, while some modern concrete is crumbling after a decade. And, that’s why aggregate production is more than pulling rock out of the ground and modern aggregate companies are more sophisticated than they were a decade ago.
Ironically, what the Romans inadvertently did nearly two millennia ago is only now being understood. And, it is being used now to produce more durable concrete structures.
The Pantheon in Rome is the world’s largest unreinforced concrete dome. The dome is 142 feet across and the building has been in continuous use throughout its history. But, when the Romans were designing it, they realized the weight of the concrete was too great (4.535 tons) to support itself. The solution was to use successively less dense aggregate in higher layers of the dome. The result is that the stress on the outside walls is nearly half of what it would have been with regular Roman Concrete.
The secret of the Pantheon’s longevity is pumice concrete. The pumice aggregate was lighter and reduced stresses. However, in the last ten years, aggregate and concrete specialists have come to understand another advantage of pumice. This lightweight pumice concrete used to build the dome had set and hardened from the inside out. This internal curing process enhanced the material's strength, durability, and resistance to cracking.
Not only is pumice light weight, it tends to absorb water like a stone sponge. When this material is substituted for some of the sand and/or coarse aggregates, the water-filled lightweight pumice serves as a reservoir that releases water to nearby hydrating cement particles. Studies show that bridges made from this type of concrete can last 60 years versus 20 years for conventional concrete. No wonder that within the last decade or so, internally cured concrete has begun to have an impact on modern construction. Increasingly, internally cured concrete is being used in the construction of bridge decks, pavements, parking structures, water tanks, and railway yards.
But this isn’t the only new technology being used in aggregates. Technical specifications of material have become increasingly important in the business. This is especially true since government entities represent a major part of the construction business and their specifications have tightened up.
One problem is that concrete is being relied upon by structural engineers for more strength. In the past, the composition of concrete varied considerably by location. For instance, if the aggregate becomes just a little more irregular and rough, during the processing and washing phase, more paste will be required or the final product won’t be as stiff.
But quality isn’t enough, unless it remains consistent. Aggregate companies also make sure that the product provided tomorrow is the same as that provided yesterday. Otherwise, the variations in quality can create stresses in the finished product that may lead to unexpected failures.
But, aggregate quality doesn’t end with the producer. It extends to the user too. Stockpiles at the worksite must also be properly managed. They should be built in horizontal or gently sloping layers to reduce segregation and be adequately spaced to allow working the piles properly. In areas where space is limited and stockpiles are difficult to keep separate, barriers should be used to avoid intermingling and cross-contamination of different-sized materials.
But the quality of the original aggregate remains the most important criteria. And some companies are producing synthetic aggregate that has the exact specifications they need. Ring Industrial Group has an EZflow product line produced with geosynthetic aggregate pieces that are more than 99.9% recycled polystyrene. This polystyrene, which would have gone to a landfill in the past, is gathered, melted, mixed, reformulated and expanded to create low density aggregates that maintain high strength properties while under compressive loads much as the Roman’s lightweight concrete did in the Pantheon. These synthetic aggregates replace conventional gravel and provide the porosity for internal curing that the Romans provided for in the Pantheon.
It’s ironic that one of the most modern aggregate solutions merely replicates what Roman engineers used nearly 2,000 years ago.