Standards for Autoclaved Aerated Concrete Promote Sustainability
Autoclaved aerated concrete was originally introduced in Sweden in the late 1920s. It is a lightweight material with no coarse aggregate, produced by mixing cement, lime, sand or fly ash, aluminum powder and water. The aluminum powder produces hydrogen gas upon mixing with cement paste, which in turn forms air bubbles in the concrete matrix that leads to the porous structure of the AAC.
In terms of sustainability, AAC is an exemplary construction material since it is produced from natural materials that are readily available. Because of its light weight, AAC has a clear advantage in transportation energy in comparison to denser materials such as concrete or masonry. AAC waste during construction is minimal since pieces can be cut to exact dimensions using standard wood cutting tools. AAC is a highly durable material and can be easily recycled after disposal. Because it is made with inert materials, AAC does not adversely affect indoor air quality. Most important are the energy savings due to the remarkable thermal properties of the material. Figure 1 shows construction in the U.S. with AAC.
AAC has been used extensively through Europe, but only recently have major production plants been constructed in the United States. The slow arrival of AAC to the U.S. market may be primarily attributed to the marketing conditions, lack of technical information, and limited understanding of the advantages of AAC by the design and construction industry. The first major production plant was not constructed in the U.S. until 1996. Two large European conglomerates have led the introduction of AAC in the U.S. The German Hebel Group, through Hebel USA, and Ytong Inc., subsidiary of a German company Ytong Holding, began production of AAC in the United States in 1996 and 1997 respectively. Initially houses were constructed from imported AAC blocks and reinforced panels from Europe. A building system composed of floor and roof panels with external block walls was marketed in the U.S. and successfully used for residential construction in resorts and housing developments in Florida.
Today, AAC is gaining rapid acceptance as a new building product in the U.S. as a result of the increasing importance placed on the conservation of energy (both the energy savings produced by thermal insulation and the amount of energy required for the mass production of the product). The rising cost of lumber and increasing environmental concerns have also played a role in the increasing interest in AAC. The availability of technical information and ASTM standards for AAC will serve as a tool to further promote and expand the use of the material.
DEVELOPMENT OF ASTM STANDARDS FOR AAC
Research activities on AAC in the U.S. began in the late 1980s on imported AAC blocks and panels. However, once manufacturing plants were established in the late 1990s, it was evident that additional research on U.S.-manufactured AAC was needed. Research was sponsored by the AAC manufacturers and focused on the material properties of AAC and the structural behavior of steel reinforced AAC elements. The research studies, which were generally led by the University of Alabama at Birmingham and the University of Texas at Austin, formed the basis for the ASTM standards on AAC. Figure 2 shows testing of AAC at the UAB laboratory.
The process of developing ASTM standards for AAC started in 1992 through the efforts of ASTM Subcommittee C27.20 on Architectural and Structural Products, part of ASTM Committee C27 on Precast Concrete Products. As the work on AAC standards increased to a level beyond the subcommittee scope, it was decided to organize a separate ASTM subcommittee that would specifically focus on AAC. In 1999 Subcommittee C27.60 on Precast Autoclaved Aerated Concrete was formed, and around the same time Subcommittee C15.10 on Autoclaved Aerated Concrete Masonry was also organized as part of Committee C15 on Manufactured Masonry Units to address issues that are specifically related to the construction of AAC masonry. Since that time, the members of both subcommittees have been working collaboratively in an effort to produce ASTM standards that would most benefit the AAC industry.
Through the efforts of ASTM Committees C27 and C15, four AAC standards have been developed and published by ASTM over the past 10 years. These standards are:
• ASTM C 1386, Specifications for Precast Autoclaved Aerated Concrete Wall Construction Units
• ASTM C 1452, Specification for Reinforced Autoclaved Aerated Concrete Elements
• ASTM C 1555, Practice for Autoclaved Aerated Concrete Masonry
• ASTM C 1591, Test Method for Determination of the Modulus of Elasticity of AAC
HIGHLIGHTS OF CURRENT AAC STANDARDs
ASTM C 1386
Standard C 1386 covers AAC block units used in the construction of solid nonload-bearing and load-bearing walls. The standard provides test procedures for determining the compressive strength, the moisture content, and the bulk density of AAC. Details of a drying shrinkage test are also provided. The physical requirements of AAC, including the compressive strength, nominal dry bulk density, and average drying shrinkage for each of three strength classes of AAC are specified in the standard (Table 1). Dimensional tolerances for AAC standard units are also prescribed.
The compression test prescribes cube specimens of 4 in. (10 mm) edge length tested in an air-dried condition at about 5 to 15 percent by mass moisture content. A minimum of three specimens are to be tested and whenever possible one specimen is obtained from the upper third of the block unit, one from the middle, and one from the lower third, determined in the direction of the rising of the mass during manufacture. The direction of rise is noted on all specimens.
The moisture content of AAC is determined by drying the samples in a ventilated oven at 100 to 110 °C until two successive determinations of mass at 2-hour intervals show an increment of loss not greater than 0.2 percent of the last previously determined mass of the specimen. The exact dimensions are measured with a caliper gage and the volume of the specimens is determined. The dry bulk density of each specimen is determined by dividing the dry mass by its volume.
To determine the drying shrinkage, the specimens are packaged in plastic and stored for a minimum of 24 hrs at 18 to 22 °C for uniform moisture distribution. They are then stored on a grid to allow sufficient movement of air at a temperature of 20 ±2 °C and a relative humidity of 45 percent. The specimen’s mass and length are determined regularly until the moisture content has decreased to below 4 percent. The moisture content of these samples is then determined and the average values for the relative change in length and the moisture content for each reading is shown graphically and connected by a curve.
ASTM C 1452
ASTM C 1452 covers load-bearing and nonload-bearing steel reinforced autoclaved aerated concrete floor, roof, wall, and stair elements used as components for building construction. For the wire reinforcement, the standard specifies minimum yield strength of 70 ksi (485 MPA) and minimum tensile strength of 80 ksi (550 MPA). A minimum concrete cover over the steel reinforcement of 0.375 in. (10 mm) is specified.
The test for corrosion protection of the steel reinforcement in AAC is performed by immersing the reinforced AAC specimens in an aqueous sodium chloride solution, 3 percent NaCl by mass, for two-hour periods at intervals of three days. This is repeated for a total of 10 test cycles. After completion, the specimens are allowed to air dry for four hours and the AAC around the steel reinforcing is removed from both the reference and the test specimens. The area of rust covering the steel is determined by visual inspection and is expressed as a percentage of the total area of specimen.
The standard also provides a procedure to determine the weld-point shear strength of the prefabricated steel wire cages. The specimen is taken at random from the welded reinforcement cage before coating with the corrosion protection compound. The wire with the largest diameter shall be selected as the test specimen. The shear specimen is gripped in the test fixture such that the tension bar is centrally located and rotation of the anchoring bar is prevented. The loading rate does not exceed 112 lbf/s (0.5 kN/s). A typical device for gripping the test specimen is illustrated in the standard.
As part of this standard, a full-scale test procedure to determine the lateral deflection and load bearing capacity of reinforced AAC floor and roof panels in bending is outlined. A symmetrical four-point loading scheme is used (Figure 3). The bearing length (Sb) is adjustable such that the minimum bearing length is equivalent to the least bearing length supplied by the manufacturer or a minimum of 1.6 in. (40 mm). The two point loads are transmitted to the surface of the AAC element by steel profiles having sufficient bearing area to produce an even bearing pressure not exceeding 50 percent of the compressive strength of the AAC material. The ultimate load is recorded and observations made as to the mode of failure of the specimen.
ASTM C 1591
Standard C 1591 is the most recent ASTM test standard on AAC, which appeared in print in 2005. It covers the determination of the modulus of elasticity of AAC in compression as derived from stress-strain test data. The test specimens are prisms having the dimensions of 100 mm x 100 mm x 200 mm. The specimens are prepared such that the loading is applied to the 100 mm x 100 mm surface and is perpendicular to the direction of rising during manufacturing (Figure 4).
Electric strain gages, or a mechanical compressometer, for determining the strain of the specimen during loading are attached to two opposite longitudinal surfaces of the specimen in the vicinity of the middle portion. A base load is applied, which is equal to 0.33 times the ultimate expected compressive strength, f’aac, for the material, and maintained for 90 seconds. The corresponding strains, eb1 and eb2, are measured during the last 30 seconds of the applied load. If eb1 and eb2 deviate by more than 20 percent, the applied loading is considered eccentric. The specimen should then be unloaded, realigned, reloaded to 0.33 f’aac and the corresponding strains measured. When the strain readings at 0.33 f’aac are within 20 percent, decrease the load gradually until a value of 0.05f’aac is reached (this should take approximately 30 seconds). This load is maintained for 90 seconds, and the corresponding strains, ea1 and ea2, are measured during the last 30 seconds of the applied load. If the difference in readings from the two gages (eb1 - ea1, eb2 - ea2) corresponding to the applied loads of 0.33 f’aac and 0.05 f’aac deviate by more than 20 percent, the applied loading is considered eccentric. The specimen should then be unloaded, realigned, and the test repeated. If the difference in readings from the two gages (eb1 - ea1, eb2 - ea2) corresponding to the applied loads of 0.33 f’aac and 0.05 f’aac are within 20 percent of each other, the loading cycle above is repeated. The loading is taken to 0.33 f’aac; read eb1 and eb2 and calculate the average eb; decrease the load to 0.05 f’aac; read ea1 and ea2 and calculate the average ea. These values will be used to calculate the modulus of elasticity, Eaac. After completion of this second loading cycle the compressometer shall be removed, and the specimen loaded to failure. The complete loading cycle is illustrated in Figure 5.
The modulus of elasticity, Ec, is determined by
where fa = stress recorded at 0.05faac; fb = stress recorded at 0.33faac; ea = average strain calculated at 0.05faac, and eb = average strain calculated at 0.33faac.
ASTM C 1555
Standard C 1555 was developed to specifically address materials, tests, and workmanship of masonry made of AAC units. Direct reference is made to ASTM C 1386 for material specifications and testing. A note in the standard recommends using the modulus of elasticity test for AAC once it is developed by Subcommittee C27.60 (currently ASTM C 1591). The workmanship requirements of the ACI/ASCE Specification for Masonry Structures are adopted and supplemented by additional requirements that are specific to AAC masonry. A liquid permeability test for exterior surface treatments, which are used to protect AAC masonry exposed to weather, is described in detail.
Additional ASTM standards for AAC have been suggested and are currently under development. Test methods for flexural strength, shear strength, and freeze-thaw durability are examples of such standards. These tests are needed as current test methods for conventional concrete or masonry may not be directly applicable to AAC.
AAC is a fairly new construction material in the U.S. and as such technical information is needed to provide guidance on the use as well as testing of the material. The development of AAC standards through ASTM is a forward step to accomplish this goal. As the use of the AAC in the U.S. increases, it is expected that additional standards will be needed to address various performance aspects of the material. //