International Concrete Abstracts Portal

An experimental investigation was carried out using the volumetric proportioning approach to achieve printable portland cement concrete mixtures. The types of aggregates investigated were rounded pea gravel and coarse and fine sand. The test matrix of potential concrete mixtures was prepared based on watercement ratios (w/c) of 0.46 to 0.48, sand-to-stone ratios (sa/st) of 1.18 to 1.91, and paste-aggregate ratios (p/a) of 0.74 to 0.81. The workability and early-age strength of fresh concrete were characterized by field-friendly flow-table and unconfined compressive strength (UCS) tests. Test results indicated that the w/c, sa/st, and p/a all significantly affect fresh concrete pumpability and early-age strength. The overall research results revealed that pumpability and buildability can be evaluated with these two tests. The results of these two tests together are used to define a printable region.

A requirement for achieving sustainable concrete structures is to develop a quantitative method for designing concrete mixtures that yields the target rheological properties and compressive strength. Toward this objective, this paper proposes a mathematical model approach to improve the sustainability of the concrete industry. A postulation that packing density, a function of the concrete mixture, provides the link between concrete mixture, rheological properties, and compressive strength was investigated. Rheological models for yield stress and plastic viscosity, and a compressive strength model were adopted with packing density as a central variable. The rheological models employ a cell description that is representative of fresh concrete. The compressive strength model is based on excess paste theory to account for the concrete mixture proportions, gradation of aggregate particles, and porosity. An experimental program was developed to calibrate and test these models. Results revealed that packing density provides a consistent and reliable link, and that the concrete mixture composition can be designed to achieve the target rheological properties and hardened properties and ensure quality control. Consequently, a new mixture proportioning methodology was developed and proposed as an improvement to the ACI 211.1 mixture design method. Furthermore, a case study was conducted to test for the applicability and adequacy of this proposed method. This research outcome, which provides a quantitative approach to design concrete mixtures to meet specific strength requirements and rheology, can also be used to ensure quality control before concrete is cast.

The use of performance-based specifications (PBS) may increase quality and sustainability while lowering project costs through innovations in concrete materials selection and proportioning. A preliminary survey was conducted showing that barriers to implementation for PBS still exist, the main barrier being the enforcement of the specification, followed by cost and time. This study aims to develop guidelines to overcome the identified barriers by presenting a laboratory-scale case study of six concrete mixtures that both conform (one) and do not conform (five) to Georgia Department of Transportation specifications. This case study includes experimental results of mechanical (flexural and compressive strength) and resistivity performance properties, as well as three additional parameters: time, cost, and carbon dioxide (CO2) emissions associated with each mixture design. This study showed that innovation in material use and mixture design can increase durability and performance while reducing the overall project cost and environmental impact.

A new mixture proportioning method is developed for performance-based concrete with supplementary cementitious materials (SCMs). The method is based on the thermodynamic calculations of the properties for concrete and identifying the mixtures that satisfy a predefined set of performance criteria. This new approach considers the chemical composition and reactivity of SCMs while proportioning concrete mixtures. Performance criteria examples are shown for a bridge deck (corrosion and freezing-and-thawing damage), an unreinforced pavement (salt damage), and a foundation (moderate sulfate and alkali-aggregate reaction). The method is used to proportion concrete mixtures satisfying these three performance criteria using four ashes per mixture. Experiments show that these mixtures met the targets. The proposed approach can proportion mixtures that are optimized for predefined performance using a wide range of SCMs, which can be useful in reducing the cost and carbon footprint of concrete.

This paper compiles results from three different laboratory studies and employs multivariate regression analyses to model the effect of mixture parameters and concrete hardened properties on saltscaling performance. The correlations between concrete hardened properties and mixture proportions were also studied. The modeled mixture parameters included water-cementitious materials ratio (w/cm), slag cement, and air content. Concrete performance was evaluated through abrasion resistance, sorptivity, compressive strength, and salt scaling tests. According to the results obtained in this study, concrete scaling performance is affected, in the order of importance, by w/cm, slag-cement replacement, and air content. In addition, concrete hardened properties, especially abrasion resistance, were found useful in making reliable salt-scaling predictions. Based on the results derived from the regression analyses and the discussions provided in the reviewed literature, recommendations are given for proportioning of concrete to obtain adequate performance with respect to compressive strength, abrasion resistance, sorptivity, and salt-scaling resistance. In addition, the relationship between concrete properties, ingredients, and effective mechanisms are investigated.