International Concrete Abstracts Portal

The use of industrial waste in concrete and controlled low-strength mixtures (CLSM) along with the experimental analysis of the fresh and hardened properties was investigated in this research. Four waste materials were used to design 17 mixtures. Fly ash and glass powder were investigated at high rates of replacement for cement, from 60 to 90%. This information is scarce in published literature and can help practitioners and concrete batchers in developing mixtures with a high level of replacement. Additionally, natural sand was substituted by glass sand which, in combination with fly ash and glass powder as cement replacement, provides an entirely new body of knowledge of concrete mixtures that use limited newly produced materials. Adequate strength and flowability was achieved with the use of recycled waste materials for both normal concrete and CLSM. All normal concrete mixtures except one, which had a 90% fly ash replacement, achieved a 28-day compressive strength of at least 29 MPa. Concrete with this compressive strength has multiple applications that represent a significant portion of the concrete produced. Using these mixtures has the potential to significantly reduce the amount of virgin products, especially cement that has a significant carbon footprint. All CLSM mixtures except two had a compressive strength of less than 2 MPa, therefore meeting the walkability and excavability requirements as set out in American Concrete Institute (ACI) guidelines and codes. Finally, an equation was proposed to predict the 28-day compressive strength of concrete with high volumes of fly ash replacement (>60%). As far as the authors are aware, there is no method to calculate the compressive strength of this type of concrete. This equation represents a significant contribution not only to the research body but also to practitioners and concrete batchers.

Crushed charcoal (biochar) was introduced into mortar as lightweight, high-carbon fine aggregate, at eight levels of sand replacement varying from 0 to 100% and up to 275% of cement content by mass. Carbon encapsulated in hardened mortar offset the carbon footprint of cement production and reduced demand for natural sand. Water content was increased to accommodate 125% biochar absorption and maintain workability. Mixture proportions affected water-cement ratio (w/c), fresh density, and compressive and splitting tensile strength of hardened mortar, with significantly diminished strength at increased biochar content. A net carbon benefit accrued when biochar content exceeded approximately 10% of the total aggregate mass or one-third of the cement mass. At this level, compressive strength is less than typically associated with structural concrete, but net sequestration of 800 kg carbon per m3 (1350 lb/yd3) could be realized at strength levels associated with controlled low-strength materials (CLSM). Multiple environmentally effective applications are suggested.

An increase in industrial and construction activities has resulted in the generation of wastes that consume large volumes of landfill spaces. Controlled low-strength materials (CLSMs) are an obvious choice for reuse of large quantities of these waste materials. This paper examines the fresh and hardened properties of CLSM mixtures produced using wastes such as bagasse ash and fly ash as pozzolanic materials, and broken hollow concrete blocks and quarry dust as fine aggregates. Engineering properties such as spread flow, Marsh flow, compressive strength, settlement, and density were investigated. Flow and strength phenomenological models were generated. The predicted values were also compared with Lagrange’s interpolation values and a new set of experimental data. Results indicate that phenomenological models encourage the production of CLSM of required parameters instead of a conventional trial-and-error process. The use of fly ash, bagasse ash, and quarry dust in large quantities increased water demand of the mixtures. Bagasse ash mixtures containing quarry dust resulted in lower strengths when compared to fly ash mixtures containing powdered hollow concrete blocks. All these wastes are encouraged to be reused in CLSM and, hence, reduce the burden on landfills.

Controlled low-strength material (CLSM) mixture design remains a trial-and-error process. A new approach using relative proportioning of the constituent materials instead of prescribed mass contents is proposed. Relative proportions allow for independent adjustments that enable unbiased estimation of their effects on CLSM properties. For the CLSM mixtures studied, a central composite experimental design was defined using three relative proportions: volumetric paste percentage (VPP), water-cementitious material ratio (W/CM), and portland cement-total cementitious materials ratio (OPC/CM). Second-order response models for slump flow, subsidence, and 28-day compressive strength were obtained for different sets of constituents, including virgin and recycled concrete fine aggregates and two fly ash sources. Slump flow and subsidence were most affected by the VPP and W/CM, respectively, whereas strength was explained by the combined effect of the W/CM and OPC/CM. The W/OPC ratio was not a reliable predictor of CLSM strength.

A series of low-cost controlled low-strength materials (CLSMs)mixtures were produced without cement, using only by-products, including Class C fly ash, large quantities of limestone quarry fines, and synthetic gypsum. Flow, setting time, compressive strength, elastic modulus, and freezing-and-thawing resistance of mixtures were evaluated. Results indicated that CLSM mixtures solely comprised of by-products can be designed to provide a wide range of flow, setting time, and strength values. Obtained flow values varied between 200 and 600 mm (8 and 24 in.), setting time varied between a couple of hours and a day, and strength values varied between 237.4 and 9932 kPa (34.4 and 1440.5 psi). The maximum measured mass loss after 12 freezing-and-thawing cycles was 8%. Results showed that the addition of synthetic gypsum significantly improved strength and freezing-and-thawing resistance of mixtures.