When designing a concrete mix, there is an entire smorgasbord from which to choose, depending on the application. Engineers usually need just a normal-weight product that will carry a heavy load or to stay put when it’s buried, or they may prefer something quite a bit lighter for hoisting large panels into place to create an attractive building facade. There are plenty of choices in between, and some push the envelope beyond either extreme.
But what if the engineer wants something really dense and really heavy? Why would anyone want a denser, heavier concrete? As this article will show, a market does exist for high-density (HD) concrete, and there seems to be a resurgence in its popularity.
Why use high-density concrete?
Most times, dense concrete is used for radiation shielding purposes, where the required thickness of walls made with normal concrete can be decreased and thereby provide more interior workspace. In the ’60s and ’70s, HD concrete was used quite a bit due to the boom in nuclear power plant construction. It is also used in nuclear weapon development facilities. Higher-density concrete costs more than normal concrete, but provides excellent shielding from harmful radiation due to its mass. Add to that concrete’s structural superiority, durability and lower cost than the alternatives (composite lead or steel shields), and the decision to use HD concrete becomes simple. The nuclear power plant construction industry may not be as robust as it once was, but a need for storage of radioactive waste or materials still exists, and precast concrete may be able to fill that need.
There are also many other uses for HD concrete, as in situations where a lot of weight is needed in smaller volumes such as ballasts for offshore pipelines, breakwater structures or in counterweights. Other uses include sound or vibration attenuation, linear accelerators, and increased resistance to buoyancy.
Typical concrete density is around 150 lbs/cu ft. Lightweight concrete can weigh as little as 95 lbs/cu ft. High-density concretes range from 190 to 260 lbs/cu ft. Even higher densities (≥ 350 lbs/cu ft) can be attained but are rare.
Aggregates: the most critical component
The key to high-density concrete is the aggregates. Therefore, the quality and types of aggregates are the most important factors in the selection process. No matter what types of heavy aggregates are chosen, they should first meet the same standard of quality as normal aggregates such as ASTM C33, “Standard Specification for Concrete Aggregates.” However, they must be clean, inert, free from deleterious substances, and not be contaminated with normal-density aggregates.
Particle shape and grading should also be similar to that of normal-density aggregates. But the relative density of the aggregates should be suitable to produce the desired concrete density. Normal weight aggregate densities are around 160 lbs/cu ft. High-density aggregates are in the 270 lbs/cu ft range. Aggregates such as magnetite, ilmenite, hematite and barite have been used for low- to medium-weight ranges in the past.1 For densities ≥ 250 lbs/cu ft, iron punchings or ferrophosphorous can be added.
Special production concerns
The procedures for measuring, mixing, transporting and placing high-density or heavyweight concrete are somewhat similar to those used in conventional concrete construction, yet special expertise and thorough planning are essential to avoid any problems. Batching and mix proportion recommendations for HD concrete can be found in ACI 211.1-91.2 All cements conforming to ASTM C150 or ASTM C595,3 which are suitable for conventional concrete, should also be suitable for use in HD concrete. Low-alkali cement should be used when alkali-reactive constituents are present in the aggregates.
As a heavyweight concrete, HD concrete design is similar to that of normal-weight concretes, but the additional self-weight must be taken into account. Conventionally placed high-density concrete may contain a water-reducing admixture meeting ASTM C4944 requirements. Air-entraining admixtures are not generally used in HD concrete, which is not exposed to freezing and thawing, because their use would tend to decrease the density of the concrete. However, if the concrete mixtures have sufficient density to allow some amount of entrained air, there are definite advantages to be realized, including reduced bleeding, greater workability and a more homogeneous concrete. Keep in mind that HD concrete with a high cement content and a low water-cementitious ratio may exhibit increased creep and shrinkage.
There are two basic methods of HD concrete placement. The first is commonly referred to as the pre-placed aggregate method. In this method, the high-density aggregates are placed in the form, and a grout is poured into the aggregates, filling the voids. For the purposes of this article, we will deal with conventionally placed HD concrete.
Standard mixing equipment can be used to mix HD concrete, although the mixer manufacturer should be consulted first. Take special care to not overload the mixing and placing equipment, including conveyor belts. In general, the allowable volume of HD concrete mixed should be equivalent to the mix weight of normal density concrete rather than the volume capacity of the mixing equipment.
The formwork for conventionally placed high-density concrete must be carefully selected and inspected, as it will be subjected to considerably higher stresses than comparable forms for ordinary concrete.
Transportation and placement of HD precast concrete is also similar to that of normal-weight concrete, but its higher weight must be considered with respect to the load-rated capacities of transport vehicles, roadways and installation cranes. Transporting high-density concrete for extended periods of time can result in excessive consolidation or packing.
Prone to segregation
Placement of conventionally mixed HD concrete is subject to the same considerations of quality control as normal-density concrete, except that it is far more susceptible to variations in quality due to improper handling. Heavyweight aggregates are particularly subject to segregation during placement. Segregation of HD concrete results not only in variation of strength but, more importantly, segregation affects concrete density. Inconsistent density is not acceptable for radiation-shield enclosures, because density is directly related to the effectiveness of concrete’s shielding properties.
Uniform consolidation is key to achieving uniform and optimum density. In HD concrete, vibrators have a smaller effective area, or radius of action; therefore, greater care must be exercised to ensure that the concrete is properly consolidated. If stinger-type vibrators are used, they should be inserted at closely spaced intervals and only to a depth sufficient to cause complete intermixing of adjacent layers.
Quality control tests of freshly mixed concrete are very similar to conventional concrete and should include tests of unit weight, temperature, slump and air content made in accordance with appropriate ASTM test methods. Testing for compressive strength may be carried out according to ASTM C39.5 The tolerances for rejection of HD concrete should be established in the construction specifications to conform to the design parameters of the structures involved.
The use of long, rigid chutes or drop pipes should be avoided, because high drop lengths can cause mix segregation. Where concrete must be placed in narrow forms or through restricted areas, a short, flexible-type drop chute that tends to collapse and restrain the fall of high-density concrete should be used.
In the event you find yourself with an opportunity to bid on a project involving high-density precast concrete, make sure you are properly equipped to handle this material prior to bidding and that you understand related production issues. HD concrete could become a niche market for precast producers who are ready and able to adapt.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.
1 See also ASTM C637-09, “Standard Specification for Aggregates for Radiation-Shielding Concrete”
2 See Appendix 4 of ACI 211.1-91, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete (Reapproved 2009).”
3 ASTM C150-12, “Standard Specification for Portland Cement,” and ASTM C595-12e1, “Standard Specification for Blended Hydraulic Cements”
4 ASTM C494-12, “Standard Specification for Chemical Admixtures for Concrete”
5 ASTM C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens”
References:
http://www.concrete.org/BookstoreNet/ProductDetail.aspx?itemid=211191
http://www.concrete.org/PUBS/JOURNALS/OLJDetails.asp?Home=JP&ID=11143
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