High-strength Structural Lightweight Concrete

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High-strength Structural Lightweight Concrete

A new direction towards advanced construction techniques using High-Strength Lightweight Cellular Concrete in the development of concrete in building and civil engineering construction 

Specified Density Concrete 

Introduction


Rock, Stone, the Earth's perfect building material

Cement, with different characterisitics, is also a rock,. We crush, grind, then blend additional elements under intense heat to create this process. When properly proportioned with water, fine, medium and coarse aggregate and cured we then shape it back into a rock, but this time a shape that we want it to look like. We build highways, bridges, and structures.

However this new rock, or concrete is heavy and is limited because of it is composition to protect from heat and cold. It protects us somewhat from the elements: rain, wind, and fire. Rock, stone and concrete are to a degree good insulator but only as a thick or expanded volume.

What if we could find a lightweight building material that has the quality or durability and strength of rock contain freezing - thawing properties, and thermal conductivity as well.

What if we could find a lightweight building material that has the quality or durability and strength of rock contain freezing - thawing properties, and thermal conductivity as well.

How could we achieve this? We would have to change the composition: the weight or density.

One way is to incorporate a lightweight  aggregate. Natural lightweight aggregates such as pumice (most widely used), scoria, volcanic cinders, tuff and diatomite. Or artificial aggregates or rotary kiln produced lightweight aggregates such as expanded clays, slates, slag, perlite or shale's. Cenospheres (hollow sphere comprised largely of silica and alumina with cavities filled of inert gases such as nitrogen and carbon dioxide) and recycled glass beads are additional multi-functional fillers. These low-density aggregates may function well; however they are economically available only in the vicinity of blast burnaces and are becoming increasingly expensive with increased material, fuel and labor costs.

What if we could find a way to lower the density with air? And add a relatively large amount that can be entrained in the concrete without substantially reducing the strength of the ultimate structure. Control the density, control the strengths.

This can be done by using a new, improved mechanical air-entraining admixture or concrete containing air cells or voids throughout its volume. A Specified Density Concrete (SDC) called High Strength Lightweight Structural Cellular Concrete or High-Performance Cellular Concrete (HPCC) / Air-Entrained Aggregate Concrete.


Cellular Concrete


Cellular Concrete is a cementitious paste of neat cement or cement and fine sand with a multitude of micro/macroscopic discrete air cells uniformly distributed throughout the mixture to create a lighweight concrete.

It is commonly manufactured by two different methods. Method A, consists of mixing a pre-formed foam (surfactant) or mix-foaming agents mixture into the cement and water slurry. As the concrete hardens, the bubbles disintegrate leaving air voids of similar sizes.

Method B, known as Autoclaved Aerated Concrete (AAC) consists of a mix of lime, sand, cement, water and an expansion agent. The bubble is made by adding expansion agents (aluminum powder or hydrogen peroxide) to the mix during the mixing process. This creates a chemical reaction that generates gas, either as hydrogen or as oxygen to form a gas-bubble structure within the concrete. The material is then formed into molds. Each mold is filled to one-half of its depth with the slurry. The gasification process begins and the mixture expands to fill the mold above the top. Similar to baking a cake. After the initial setting, it is then cured under high-pressured-steam "autoclaved" for a specific amount of time to produce the final micro/macro-structure.

Recently, a direction to concrete compositions prepared by using aqueous gels is being considered as all or part of the aggregate in a concrete mix. Aquagel spheres, particles, or pieces are formed forom gelatinized starch and added to a matrix. Starch modified or unmodified such as wheat, corn, rice, potato or a combination of modified or unmodified starches are examples of aqueous gels. A modified starch is a starch that has been modified by hydrolysis or dextrinizaton. Agar is another material that can create a pore or cell in concrete. During the curing process as an aquagel loses moisture, it shrinks and eventually dries up to form a dried bead or particle that is a fraction of the size of the original aquagel in the cell or pore in the concrete. This results in a cellular, lightweight concrete.

High carbon ash, recycled aluminum waste and zeolite powders are additional mechanical structures suitable in the production of cellular lightweight concrete.

These cells may account for up to 80% of the total volume. Weight of the concrete mixtures range from 220 kilograms per cubic meter to 1922 kilograms per cubic meter and compressive strengths vary from 0.34 megapascals to 20.7 megapascals.


High-performance Cellular Concrete


High-Performance Cellular Concrete (HPCC) has all the properties of cellular concrete and can achieve 55.37 MPa. 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 perfomance and uniformity requirements that cannot always be achieved routinely by using only conventional materials 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 of foam in a matrix replaces 28.3 kilograms of water, or 0.028 cubic meter of aggregate weighing 74.84 kilograms.

HPCC has excellent insulation properties that significantly reduces the transfer of heat through the concrete member. This bubble is accountable for the 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 similar to a high-range water-reducing admixture. This technique can generally reduce water requirements of a concrete mix by 20% to 545 providing a similar increase in concrete strength. 0.69 MPa 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.

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 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, napththalene 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 toward the formaldehyde condensates, butyl carbiitol, 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 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 a surfactant 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 0.4 per cubic meter of concrete. The water/cement ratio 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 um to 100 um in diameter. Normal cellular concrete bubble range is 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 iparticles 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 smal, minute or microscopic.



Grading and Segregation


Aggregate gradation significantly affects concrete mixture proportioning and workavility. 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 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 1 16R-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 ". Proportioning should be made in accordance with ASTM C33-99ae 1 and ASTM C136-96a.

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, wettles 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.



Extended-set:


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 1.5 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 cemnt to the aggregates."


Vibration:


This process can be vibrated and will not cause segregation of the mortar and coarse aggregate. In most applications no vibration is necessary.


APPLICATION

Brief History


Cellular Concrete was first developed in Stockholm, Sweden in the early 1900's. The original material was known as "gas concrete" to be used in producing heat-insulated building materials. This led to the development of related lightweight concrete which are now known as cellular concrete, foamed concrete, aerated concrete and autoclaved cellular concrete.

After the Second World War, this technology quickly spread to different parts of the world, mostly Europe and the Soviet Union. The applications were for economical large-size structural panel units. These were used in site reconstruction and low-rise structures.

It wasn't till the late 1950's when this was introduced to the US as foamed or cellular concrete. The application were for floor, roof and wall units. Having low compression strengths, it limited this product to fills and insulation only.


Currently


The major use in recent years in the United States has been over plywood on wood floor systems or over hollow-core precast slabs. The material is also used in light density for roof fills 481 kilograms per cubic meter (30 pounds per cubic foot) providing good insulatig properties.

Even today this material still generate low compressive strengths limiting it to these two applications. Range options are 3.45 MPa to 6.89 MPa for midrange nonstructural densities and 10.3 MPa to 24.1 MPa for higher densities 1762 kg/m3.


New Direction


Concrete design has evolved rapidly in the last 30 years. Construction technology has seen the introduction of a variety of concrete products to the market as well as an increased use of supplementary cementitious materials and recently blended cements. Emphasis has been placed on creating more durable concrete through changes to the mix constituents and proportions, including the aggregates, admixtures and the water-cement ratio. These changes have been reflected in national and hopefully will lead to global/International design, standards, performance specifications and codes which address such factors as performance, durability, permeability, cement constituent ratios, and limitations on impurities. This evolution, along with improved reinforcing steel strength, has lead to modifications in design philosophy - most notably the use of thinner structural members.

As for a lower weight of these structural members, there are many applications for which a 1602 kg/m3 or lower structural concrete would be beneficial. With normal lightweight concrete in the 1442 - 1681 kilograms per cubic meter (90 - 105 pounds per cubic foot) range requires lightweight fine aggregate as well as coarse. When natural sand is used with lightweight coarse aggregates, strengths of 34.5 - 48.3 megapascals (5000 - 7000 pounds per square inch) can be obtained but the weight runs from 1842 - 2002 kilograms per cubic meter (115 to 125 pounds per cubic foot) adding to the weight. With High Perfomance Cellular Concrete (HPCC) the weight is significantly reduced to a 1041 - 1522 kilograms per cubic meter (65 to 95 pounds per cubic foot) with a 34.5 to 48.3 megapascals (5000 - 7000 pounds per square inch) resulting in improved structural efficiency in terms of strength/weight ratios, with fewer structural components, and a consequent reduction in the number and size of reinforcements. Panel width can be manufactured as thin as 63.5 millimeters. 


What are the applications and benefits


Architectural


Improved structural efficiency in terms of strength/weight ratios resulting load reduction on the structure and substructure, fewer structural components resulting in more usable space in the structure, a reduction in the number and size of reinforcements, increased flexibility in absorbing strains and improved thermal properties minimizing the effects of differential temperatures resulting in building energy conservation as well as improved fire/spalling mitigation.

It is ideally suited for precast concrete products as larger units can be handled with the same handling equipment or manually for same size units, resulting in speed and economy in construction. These units in addition to smaller ones can be lifted or managed by down-sizing machinery resulting in reducing site cranage requirements and miximizing the number of concrete elements on trucks without exceeding highway load limits reducing transportation delivery cost.



Advanced Construction Techniques for Precast Panel System
Implementation of advanced construction techniques using cellular concrete technology.

One example of areas to which High Performance Cellular Concrete can be applied is in the development of a lightweight binary density insulating concrete panel system. This product could be a consideration for future applications in a new generation of buildings. These elements can be used for the construction in all types of building or structures worldwide. For instance affordable housing, schools, senior citizen's centers, industrial, military and municipal facilities, and structures requiring service life in severe or hostile environments. Another precast application would be for new or replacing metal sheeting on the exterior skins on metal buildings.


High-Performance Lightweight Plaster Application on Cold Steel Framing Precast Composite Binary Insulated pael + cold metal system

Geotechnical 

Primary application of CLSM 9Controlled Low Strength Material) is as a structural fill or backfill in lieu of compacted soil. Compaction is not requred and is ideal for use in tight or restricted-access areas where placing and compacting fill is difficult.

Low density CLSM is especially advantageous where weak soil conditions are encountered and the weight of the fill must be minimized. Provides superior thermal insulation and shock mitigation properties.




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