Concrete Construction

Information on the materials, methods, and technologies used in concrete home construction, including concrete walls, foundations, insulated concrete forms, and concrete admixtures.

Innovative Products & Processes - PATH Technology Inventory

• Autoclaved Aerated Concrete (AAC)

Manufactured building block made of all-natural raw materials

Summary

Builders in the U.S. can use an innovative concrete material that Scandinavians have built their homes with for decades. Autoclaved Aerated Concrete (AAC) is a precast structural product made with all-natural raw materials. In 1914, the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminum powder. The material was further developed to what we know today as autoclaved aerated concrete (also called autoclaved cellular concrete).

It is an economical, sustainable, solid block that provides thermal and acoustic insulation as well as fire and termite resistance. AAC is available in a variety of forms, ranging from wall and roof panels to blocks and lintels. Although it has been a popular building material in Europe for over 50 years, AAC has only been introduced to the U.S. in the past two decades.

To manufacture AAC, Portland cement is mixed with lime, silica sand, or recycled fly ash (a byproduct from coal-burning power plants), water, and aluminum powder or paste and poured into a mold. The reaction between aluminum and concrete causes microscopic hydrogen bubbles to form, expanding the concrete to about five times its original volume. After evaporation of the hydrogen, the now highly closed-cell, aerated concrete is cut to size and formed by steam-curing in a pressurized chamber (an autoclave). The result is a non-organic, non-toxic, airtight material that can be used for wall, floor, and roof panels, blocks, and lintels which according to the manufacturers, generate no pollutants or hazardous waste during the manufacturing process

AAC units are available in numerous shapes and sizes. Panels are available in thicknesses of between 8 inches to 12 inches, 24-inches in width, and lengths up to 20 feet. Blocks come 24”, 32”, and 48” inches long, between four to 16 inches thick, and eight inches high.

AAC features include structural capacity, thermal, fire, and acoustical resistance properties. With an R-value of approximately 1.25 per inch, dependent on density, AAC significantly outperforms conventional concrete block or poured concrete. Consistency in quality and color may be difficult to obtain in AAC made with fly ash. Unfinished exterior walls should be covered with an exterior cladding or parged with mortar when exposed to physical damage, dirt, and water, because atmospheric debris can collect in the open cells. If installed in high humidity environments, interior finishes with low vapor permeability, and exterior finishes with a high permeability are recommended.

Because of the thermal mass of AAC and its ability to store and release energy over time, AAC may be beneficial in climates where outdoor temperature fluctuates over a 24-hour period from above to below the indoor temperature conditioned air set point.

PATH Attributes

		The insulating properties of AAC, when compared with conventional concrete, make it an energy-efficient choice.
		AAC is recyclable.
		AAC is naturally resistant to fire, termites, and fungal decay. Concrete Admixtures Improving the properties of concrete Summary Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as workability, curing temperature range, set time or color. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance. Not all admixtures are economical to employ on a particular project. Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high quality concreting practices. The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. A general understanding of the options available for concrete admixtures is necessary for acquiring the right product for the job, based on climatic conditions and job requirements. Based on their functions, admixtures can be classified into the following five major categories: Retarding admixtures Accelerating admixtures Super plasticizers Water reducing admixtures Air-entraining admixtures Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring. The following paragraphs provides details on the above-mentioned categories of concrete admixtures. Retarding admixtures Retarding admixtures slow down the hydration of cement, lengthening set time. Retarders are beneficially used in hot weather conditions in order to overcome accelerating effects of higher temperatures and large masses of concrete on concrete setting time. Because most retarders also act as water reducers, they are frequently called water-reducing retarders. As per chemical admixture classification by ASTM-ASTM C 494, type B is simply a retarding admixture, while type D is both retarding and water reducing, resulting in concrete with greater compressive strength because of the lower water-cement ratio. Retarding admixtures consists of both organic and inorganic agents. Organic retardants include unrefined calcium, sodium, NH4, salts of lignosulfonic acids, hydrocarboxylic acids, and carbohydrates. Inorganic retardants include oxides of lead and zinc, phosphates, magnesium salts, fluorates and borates. As an example of a retardant's effects on concrete properties, lignosulfate acids and hydroxylated carboxylic acids slow the initial setting time by at least an hour and no more than three hours when used at 65 to 100 degrees Fahrenheit. The concrete contractor, however, need not memorize these chemical-specific results. Given the specific job requirements and goals, the concrete supplier should offer appropriate admixtures and concrete mixes from which to choose. Accelerating admixtures Accelerators shorten the set time of concrete, allowing a cold-weather pour, early removal of forms, early surface finishing, and in some cases, early load application. Proper care must be taken while choosing the type and proportion of accelerators, as under most conditions, commonly used accelerators cause an increase in the drying shrinkage of concrete. Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain. It should meet the requirements of ASTM D 98. Excessive amounts of calcium chloride in concrete mix may result in rapid stiffening, increase in drying shrinkage and corrosion of reinforcement. In colder climates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete. Super plasticizers Super plasticizers, also known as plasticizers, include water-reducing admixtures. Compared to what is commonly referred to as a "water reducer" or "mid-range water reducer", super plasticizers are "high-range water reducers". High range water reducers are admixtures that allow large water reduction or greater flowability (as defined by the manufacturers, concrete suppliers and industry standards) without substantially slowing set time or increasing air entrainment. Each type of super plasticizer has defined ranges for the required quantities of concrete mix ingredients, along with the corresponding effects. They can maintain a specific consistency and workability at a greatly reduced amount of water. Dosages needed vary by the particular concrete mix and type of super plasticizer used. They can also produce a high strength concrete. As with most types of admixtures, super plasticizers can affect other concrete properties as well. The specific effects, however, should be found from the manufacturer or concrete supplier. Water reducing admixtures Water reducing admixtures require less water to make a concrete of equal slump, or increase the slump of concrete at the same water content. They can have the side effect of changing initial set time. Water reducers are mostly used for hot weather concrete placing and to aid pumping. A water-reducer plasticizer, however, is a hygroscopic powder, which can entrain air into the concrete mix via its effect on water's surface tension, thereby also, obtaining some of the benefits of air-entrainment (see below). Air-entraining admixtures Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water-cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. As always, admixtures should only be combined in a concrete mix by a competent professional because some of them can interact in undesirable ways. Bonding admixtures, including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete. Coloring agents have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available. Waterproofing and damp proofing admixtures, including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete. "Antifreeze" admixtures typically are accelerators used in very high doses, with a corresponding high price, to achieve a very fast set-time, though they do not have properties to protect against freezing on their own. However, in general, these are not used for residential work. Cement substitutes also change concrete properties, but typically are not classified as admixtures. See the Technology Inventory article, "Cement Substitutes." Most organic chemical-type admixtures are affected by cement type and brand, water-cement ratio, aggregate grading, and temperature. Damp proofing and waterproofing admixtures still have uncertain value and hazards. These are just two cases that point to the learning curve required of anyone working with admixtures. In some cases, if exacting directions are not followed, including addition of supplemental materials to balance the negative or undesirable side effects of an admixture, the resulting concrete mix may be compromised. For example, retarding admixtures generally have a possibility of rapid concrete stiffening, resulting in difficulty in concrete placement and finishing. Therefore, an in-depth knowledge of the potentially complex interrelated effects, besides specifications, is required to successfully use a number of admixtures. This is even more critical when a number of parties are involved in the manufacture of the concrete, for example the producer, the placing contractor and the builder, where the finished concrete is a combined result of a number of individual decisions. Choosing an appropriate admixture for a specific job should be the responsibility of an experienced expert. Alternatives to the use of admixtures should always be considered. The environmental impact of certain admixtures is questionable. Some super plasticizers may impact the environment through pollution of ground and surface waters. More research remains to be carried out in this area. Finally, admixtures cannot compensate for bad practice and low quality materials. PATH Attributes On the low-end, water reducers should only cost $3 to $5 per cubic yard of concrete. The primary benefit of a particular admixture is generally self-evident from the type of admixture, such as a retardant or a water reducer admixture. Concrete Aggregate Substitutes Alternative aggregate materials Summary Conventional concrete aggregate consists of sand (fine aggregate) and various sizes and shapes of gravel or stones. However, there is a growing interest in substituting alternative aggregate materials, largely as a potential use for recycled materials. While there is significant research on many different materials for aggregate substitutes (such as granulated coal ash, blast furnace slag or various solid wastes including fiberglass waste materials, granulated plastics, paper and wood products / wastes, sintered sludge pellets and others), the only two that have been significantly applied are glass cullet and crushed recycled concrete itself. Even though aggregate typically accounts for 70% to 80% of the concrete volume, it is commonly thought of as inert filler having little effect on the finished concrete properties. However, research has shown that aggregate in fact plays a substantial role in determining workability, strength, dimensional stability, and durability of the concrete. Also, aggregates can have a significant effect on the cost of the concrete mixture. Certain aggregate parameters are known to be important for engineered-use concrete: hardness, strength, and durability. The aggregate must be "clean," without absorbed chemicals, clay coatings, and other fine materials in concentrations that could alter the hydration and bond of the cement paste. It is important to note the difference between aggregate and cement, because some materials have found use both as a cementitious material and as aggregate (such as certain blast furnace slags). Materials that have been researched or applied only as cement substitutes are addressed in another Technology Inventory article - Cement Substitutes. Aggregate composed of recycled concrete generally has a lower specific gravity and a higher absorption than conventional gravel aggregate. New concrete made with recycled concrete aggregate typically has good workability, durability and resistance to saturated freeze-thaw action. The compressive strength varies with the compressive strength of the original concrete and the water-cement ratio of the new concrete. It has been found that concrete made with recycled concrete aggregate has at least two-thirds the compressive strength and modulus of elasticity of natural aggregate concrete. Field-testing has shown that crushed and screened waste glass may be used as a sand substitute in concrete. Nearly all waste glass can be used in concrete applications, including glass that is unsuitable for uses such as glass bottle recycling. Some of the specific glass waste materials that have found use as fine aggregate are "non-recyclable" clear window glass and fluorescent bulbs with very small amounts of contaminants. Possible applications for such waste-glass concrete are bike paths, footpaths, gutters and similar non-structural work. Lack of widespread reliable data on aggregate substitutes can hinder its use. To design consistent, durable recycled aggregate concrete, more testing is required to account for variations in the aggregate properties. Also, recycled aggregate generally has a higher absorption and a lower specific gravity than conventional aggregate. Research has revealed that the 7-day and 28-day compressive strengths of recycled aggregate concrete are generally lower than values for conventional concrete. Moreover, recycled aggregates may be contaminated with residual quantities of sulfate from contact with sulfate rich soil and chloride ions from marine exposure. Glass aggregate in concrete can be problematic due to the alkali silica reaction between the cement paste and the glass aggregate, which over time can lead to weakened concrete and decreased long-term durability. Research has been done on types of glass and other additives to stop or decrease the alkali silica reaction and thereby maintain finished concrete strength. However, further research is still needed before glass cullet can be used in structural concrete applications. PATH Attributes Certain substitute aggregates, such as glass, can help reduce the cost of concrete, especially if aggregate is in short supply. Use of any recycled material helps to keep that material out of landfills. Recycling practices also can decrease the environmental impact of obtaining / manufacturing the material from virgin resources. Glass aggregate can improve the durability of concrete and can also make decorative finishing easier. Source : http://www.toolbase.org