Optimizing concrete mixtures with minimum cement content for performance and sustainability

The main purpose of this research is to investigate the minimum cement content required with an appropriate water-to-cement ratio (w/c) to meet given workability, strength, and durability requirements in a concrete pavement; and to reduce carbon dioxide emissions, energy consumption, and costs. An experimental program was conducted to test 16 concrete mixtures with w/c ranging between 0.35, 0.40, 0.45 and 0.50; and cement content ranging from 400, 500, 600 and 700 lb/yd3 (pcy). The fine aggregate-to-total aggregate ratio was fixed as 0.42 and the void content of combined aggregates was maintained the same for all the mixtures. Slump; setting time; 1, 3 and 28-day compressive strength; 28-day chloride penetration; and 1, 3, and 28-day air permeability were determined. The test results showed that strength is a function of w/c and independent of cement content after the required cement content is reached, for a given w/c. Workability is a function of w/c and cement content: increasing w/c or cement content improves workability. Setting time is reduced when cement content is increased for a given w/c. Chloride penetration increases as w/c or cement content increases, when one parameter is fixed. Air permeability increases as cement content increases, for a given w/c. Based on these findings, it is possible to reduce the paste content without sacrificing the desired workability, strength and durability, for a given w/c. When the overall effect of cement content on concrete properties is evaluated, 400 pcy of cement content is not recommended due to its high porosity caused by its low paste content. Furthermore, 700 pcy would also not be appropriate as increasing cement content does not improve the strength, after the required content is reached; and may decrease durability as high cement content both increases air permeability and chloride penetration. Moreover, for a w/c higher than 0.35, cement content of more than 500 pcy adversely affects the concrete performance by decreasing strength (increasing cement content from 500 pcy to 700 pcy approximately reduced the 28-day compressive strength by 15%) and may cause shrinkage related cracking problems. Therefore, for a given w/c and for the aggregate system used in this study, the range of 500 pcy to 600 pcy is found to be the most appropriate cement content range that provides the desired workability, strength, chloride penetration and air permeability. Mixtures with 500 pcy of cement content did not have a high workability (ranging from 0 in. to 3 in. depending on the w/c), but it may be improved by the addition of supplementary cementitious materials, water-reducing agents, or using a different aggregate gradation system. The given cement content range was compared with the values obtained in accordance with the ACI 211 Report (2002): considering the high cement content range of 650 pcy to 930 pcy provided by the ACI 211 Report (2002) for the same given conditions, the recommended cement content range of 500 pcy to 600 pcy will have more significant impact and benefits on the concrete construction industry regarding the reduction of cement content. In addition, to make the findings independent of the selected aggregate system, the relationship between paste volume and concrete properties is established. In order to meet the desired workability, strength and durability requirements; the paste volume should be within the range of 160% to 170% of the volume of voids. Exceeding this range will adversely affect the concrete performance by decreasing strength, and increasing chloride penetration and air permeability