High-Performance Cellular Concrete [HPCC] has all the properties of cellular concrete and can achieve 55.37 MPa [8,000 psi]. Higher strengths can be produced with the addition of supplementary cementitious materials. Density and strengths can be controlled to meet specific structural and nonstructural design requirements. Where as in conventional cellular concrete these can not be achieved.
High-performance concrete is defined as “concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventionalmaterials and normal mixing, placing and curing practices.” The requirements may involve enhancements of characteristics such as ease of placement and compaction without segregation, long term mechanical properties, density, volume, endurance, stability, or service life in severe or hostile environments.
Density is the best characteristic feature of cellular concrete. The lowest densities being used for fills and insulation; and the higher densities being used for structural applications, leading to a substantial reduction in the dead weight of a structure. 0.028 cubic meter [one cubic foot] of foam in a matrix replaces 28.30 kilograms [62.4 lbs.] of water, or 0.028 cubic meter [one solid cubic foot] of aggregate weighing 74.84 kilograms [165 lbs. per cubic foot]. HPCC has excellent insulation properties that significantly reduces the transfer of heat through the concrete member, This bubble is accountable for a superior freezing and thawing resistance and thermal reduce conductivity, low water absorption, high tensile strength, high fire resistance and sound retention, and corrects deficiencies in the sand that causes bleeding.
Forming, conveyance, placing and finishing systems for cellular concrete are no different than current methods in the construction industry. HPCC also has the advantage of being conducive to mobile and remote projects where building materials are difficult to obtain or reach.
How does it work? It works in two ways.
One it mechanically reduces the water/cement ratio [w/cm] similar to a high-range water-reducing admixture. This technique can generally reduce water requirements of a concrete mix by 20% to 54% providing a similar increase in concrete strength. 0. 69 MPa [100 psi] increase in strength for each 0.01 decrease in water/cement ratio w/cm]. The second technical feature is that it also performs as an aggregate. Air
Conventional cellular concrete produced with a pre-formed foam mixture is produced by discharging a stream of preformed foam into a mixing unit on site or a transit mix load of sand-cement grout or cement-water slurry. This foam [surfactant] resembles shaving cream or the foam used for firefighting. Most of the foam concentrates are hydrolyzed proteins or synthetics and are available through proprietary sources. Amine and amine oxides, naphthalene sulphonate formaldehyde condensates are examples of these. Some of these products can contain a substance or substances classified as dangerous or hazardous to the environment, cautious attentiveness should be considered when using these products, especially towards the formaldehyde condensates, butyl carbitol, and glycol ethers. Depending on an application using foam produced from a surfactant usually is not an environmental issue. However in some countries this can be a religious concern/significance. This would be the case when using hydrolyzed protein based surfactants that contain keratin or casein derivatives.
Surfactants are surface-active substance or agent [detergents, wetting agents, emulsifiers] that when added to water lowers surface tension and increases the “wetting” capabilities of the water, thus improving the process of wetting and penetrating that surface or material. When agitated forms a large mass of micro/macroscopic bubbles.
With this device or process [HPCC] a surfactant [wetting agent] or foam concentrate is diluted with water to form a foam solution. This solution is then injected with compressed air through a blending device or foam generator. The quantity of the foam injected into the mixture proportions is in the range of 0.07 to 0.40 per cubic meter [2 to 11 cubic foot per cubic yard] of concrete. The water/cement ratio [w/cm] is in the range from 0.23 to 0.32 and a foam of microscopic bubbles with at least a majority being in the range from 25 µm to 100 µm [0.025mm to 0.1 mm / 0.001 inches to 0.004 inches] in diameter. * Normal cellular concrete bubble range is 0.3 mm - 0.8 mm [0.012 in. - 0.032 in.] in diameter.
Concrete is formed by mixing the liquid cement paste with predetermined qualities of aggregate material. The aggregate is typically made up of medium and coarse aggregate or rock and fine aggregate or sand. Or the next generation of fillers that are artificial or recycled. These to include natural/artificial pozzolans, recycled glass, ceramic, expanded polystyrene beads, plastic, organic or inorganic materials.
In conventional concrete, the percentage of sand in the aggregate is 30% to 40%. However, the foamed cement of this process/invention is preferably mixed with an aggregate having a higher ratio of sand. Preferably in the range of 40% to 50%. This reduces or eliminates voids in the concrete mixture, since gaps between larger rock particles may be filled with a combination of smaller rock, sand, and air bubbles. The smaller the spacing factor, the more durable the concrete will be. These microscopic bubbles are smaller than the size of the sand particles increasing the plasticity or flowability of the mix.
As the concrete hardens, the bubbles disintegrate or transform, releasing the water which is absorbed into the cement matrix, thus aiding in the hydration process and leaving air voids of similar sizes. Thus, there is less need to wet the concrete during curing, as is normally necessary with conventional, unfoamed concrete. An air-entraining admixture must produce stable air bubbles that won’t coalesce to form larger bubbles during mixing. For a given air content or volume of air, if bubbles are too large, there won’t be enough of them present to properly protect the paste. Large bubbles are also more likely to break while the concrete is being mixed, transported, placed and vibrated. If too much air is lost during these operations, the remaining air voids may not adequately protect the hardened concrete during cold weather or thermal conductivity. To prevent air loss, the bubble skin must be stable and tough enough to resist breaking and coalescing, and the size must be extremely small, minute or microscopic.
Grading and Segregation
Aggregate gradation significantly affects concrete mixture proportioning and workability. The particle size distribution, particle shape and surface texture are all important elements in the assurance of concrete quality and durability. Variations in grading materials, either by blending selected size aggregates or an adjustment of concrete mix proportions involves constant attention to meet a performance specification. When particles are poorly distributed within the mixture or if there is a deficiency of intermediate [medium] aggregates, mechanical properties of the mix, as well as placing and finishing will result in an inferior product. Eventually, the mechanical and physical properties of the concrete will continue to deteriorate creating additional problems.
AC I 116R-00 "Cement and Concrete Terminology" defines grading as "the distribution of particles of granular material among various sizes; usually expressed in terms of cumulative percentages larger or smaller than each of a series of sizes (sieve openings) or the percentages between certain ranges of sizes (sieve openings) ". Proportioning should be made in accordance with ASTM C33-99ae1 [Standard Specification for Concrete Aggregate] and ASTM C136-96a [Standard Test Method for Sieve Analysis of Fine and Coarse Aggregate].
The quantity of the fine aggregate and coarse aggregates in a mixture must be in balance with one another so as to create a particle size distribution to produce a specified accumulated density. However selection of the aggregates is or sometimes not always consistent. Accessibility, environmental mandates and the cost to import supplementary natural or artificial intermediate aggregates are issues that must be addressed so that a maximum optimized concrete can be produced economically for performance, durabilty, and structural construction methodology.
High-Performance Cellular Concrete is an excellent choice to use as an intermediate aggregate when these material sizes are substantially absent, creating an improved concrete uniformity or an optimal particle size distribution. Segregation is greatly decreased, especially in concrete where sand gradation is poor. [Segregation is when coarse aggregate separates from the water, settles to the bottom and the water rises to the top producing poor workability and excessive bleeding.] How is segregation controlled? These microscopic strong super bubbles puts the matrix in suspension.
This air-entraining admixture is also advantageous for special applications where extended set or where delivery over long distances is necessary. One way to extend or prolong this is the use of hydration stabilizers. With this process [HPCC] a mix can be transported/placed exceeding ASTM C94-96el, without the addition of a stabilizer, water or admixtures. In paragraph 11.7 of ASTM C94-96el, "Standard Specification for Ready-Mixed Concrete" states "discharge of the concrete shall be completed within 1½ hours, or before the drum has revolved 300 revolutions, whichever comes first, after the introduction of the mixing water to the cement and aggregates or the introduction of the cement to the aggregates."
This process can be vibrated and will not cause segregation of the mortar and coarse aggregate. In most applications no vibration is necessary.