Listen to Mats Drugge, Minelco's North American president, describe how his product is used in concrete construction.
Heavyweight concrete is not new. In the U.S., producers have supplied this specialty performance product to hospitals for x-ray protection, to bridge designers to provide counterweights on lift bridges, and to designers of nuclear power plants.
Recently, engineers have used heavyweight concrete to contain hazardous waste. And for many years, engineers and producers have used “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing” (ACI 304.3) to help alert specifiers to the special techniques they need to properly handle this unique product.
When considering producing heavyweight concrete, producers will find information in Chapter 7 of ACI's “Guide for Use of Normal Weight and Heavyweight Aggregates in Concrete” (ACI-221). It begins with properly selecting materials. For projects on which the heavyweight concrete is used as shielding, the material must meet a special specification.
ASTM C 63, Aggregates for Radiation-Shielding, details the requirements for special aggregates where composition, high specific gravity, or both, are of prime consideration. There also is ASTM C 638, Descriptive Nomenclature of Constituents of Aggregates for Radiation-Shielding Concrete.
It's also important to establish a very thorough material testing program for both aggregates and fresh concrete before and during construction and specifications. ASTM C 637 indicates that fine and coarse aggregates should meet the conventional concrete aggregate gradings in ASTM C 33. Often the fresh concrete tests also follow traditional guidelines.
Test core issues
The problem occurs when something seems to be awry. Heavyweight concrete is normally placed on very complex structures, many of which can only be examined using non-destructive testing procedures. This can preclude the possibility of taking test cores. And should test cores be necessary, their extraction is very difficult and expensive.
Concrete containing coarse metallic aggregate particles is subject to non-representative test results. Unless using great care, the metallic particles will be torn loose from the matrix, which may destroy the bit and core sample.
Fortunately for producers, material availability is not a major problem. Over the last few years, many of the sources listed in the ACI document have been shut down. But about two years ago, Minelco, a Swedish material supplier, began shipping magnetite products to North America by bulk freighter. From their unloading port in the Southeast, producers can order smaller shipments by rail, barge, or truck.
Currently available in two gradations, Magnadense 1 corresponds to a fine aggregate, a 2-mm down product; Magnadense 2 is an 8-mm down product. Minelco says its magnetite products have a higher level of iron, so they are more dense than alternatives.
Minelco plans to bring a new perspective to using heavyweight concrete in the U.S. Engineers have found interesting uses for heavyweight concrete in Europe for many infrastructure projects, including bridge piers. Minelco has several case studies at www.minelco.com.