Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing
|Publication Date:||1 November 2020|
Heavyweight concrete is used in counterweights of bascule, lift, and cable-stay bridges. It is used for counterweights for both cranes and concrete-conveying equipment. It has also been used for sound and vibration attenuation in subway systems where extra mass helps in protecting sensitive instruments. Large quantities are used as coarse dry-pack cementitious coating on offshore oil and gas transmission lines. However, the most common application of heavyweight concrete is for providing biological shielding against radiation in nuclear-power-genera
When heavyweight concrete is used to absorb gamma rays, the density and materials costs are of prime importance (Pihlajavaara 1972). For some radiation shielding applications, heavyweight shielding concrete is also used to attenuate neutrons. The neutron energy accompanying gamma radiation requires lightweight or low-atomic-weight elements such as hydrogen and boron to slow these neutrons by inelastic collision. The resulting gamma radiation from these collisions is stopped by heavyweight elements, such as iron, that are incorporated in the heavyweight concrete. Hydrogen is always present in concrete because of the hydration of the cement and mixing water. In addition to the portland cement, supplemental cementitious materials can provide hydrogen that is "locked in" during the process of hydration (Davis 1972a). Once the cement is hydrated, the water of hydration cannot escape unless the concrete is heated at high temperatures for extended time periods.
Some heavyweight mineral aggregates such as goethite (α-Fe3+O(OH)) and limonite (FeO(OH)·nH2O) contain water in their chemical structure and are known as hydrous minerals. They can be used as sources of hydrogen because they can retain water of their crystallization at elevated temperatures. Use of these minerals ensures a presence of hydrogen not necessarily present in conventional aggregates. Use of minerals containing hydrogen in their chemical structure will assure a content of hydrogen, even when the hydrated cement loses its chemically combined water at high temperatures.
In addition to hydrogen, another desirable element is boron, which has the ability to capture thermal (slow) neutrons produced by collision of high-energy neutrons with hydrogen. The gamma radiation resulting in this energy transfer is captured by heavyweight elements, such as iron in heavyweight aggregates in heavyweight concrete. Care should be taken to minimize possible retardation of the concrete mixture due to influence of boron and its compounds, boric acid and borax, on the hydration of portland cement.
ACI Committee 349, Concrete Nuclear Structures, has several standards and code requirements concerning this type of construction, including ACI 349-13, "Code Requirements for Nuclear Safety-Related Concrete Structures and Commentary." This code covers the proper design and construction of concrete structures that form part of a nuclear power plant and that have nuclear safety-related functions, but does not cover concrete reactor vessels and concrete containment structures (as defined by Joint ACIASME Committee 359). The structures covered by the code include concrete structures inside and outside the containment system.