International Concrete Abstracts Portal

Chloride threshold values for steel rebars in reinforced concrete under the effect of varying temperatures and extended long-term conditions in hot climates are investigated. This investigation covers a gap in the current codes including ACI 318 where the effect of temperature on the chloride threshold is not addressed. A total of 96 concrete specimens reinforced with carbon steel rebars sourced from two manufacturers were cast with different chloride contents and exposed to four temperatures of 20, 35, 50, and 65ºC (68, 95, 122, and 149ºF) for a period of more than 2 years. The chloride threshold values were determined based on corrosion potential, corrosion rate, and mass loss at the end of the exposure period. The results of the three techniques showed a consistent trend of significant dependency of the chloride threshold value on temperature. The average water-soluble chloride threshold values based on mass loss were found to be 0.77, 0.72, 0.47, and 0.12% by weight of cement for temperatures of 20, 35, 50, and 65ºC, respectively. These findings are significant as they showed a dramatic drop in the chloride threshold values at high temperatures. This research highlights the need for reassessment of ACI code limits considering the hot climate.

This paper presents the results of creep rupture tests conducted ontwo different sizes of glass fiber-reinforced polymer (GFRP) bars from two different manufacturers under high alkaline conditions at room temperature (approximately 23 and 60°C [73.4 and 140°F]).Regular tensile tests were also conducted on the bars at the two temperatures to provide insight into the effects of high temperatureon their long-term performance. The results show that the larger bar performed slightly better at room temperature but significantly better at the elevated temperature. The larger-sized bars also lost less tensile strength at the elevated temperature. It was observed that temperature had a greater effect on the long-term performance of GFRP bars than alkalinity. The current design code limits on the allowable stresses were evaluated against the test results and found to be overly conservative.

This paper assesses the suitability of geopolymers (GPs) for use as adhesives for ceramic tile fixing, including their compliance to the relevant EN 12004 specification. Two series prepared with different percentages of metakaolin (MK), blast-furnace slag (BFS), and limestone materials activated by an alkaline NaOH/ Na2SiO3 solution are investigated. Tested properties included the thixotropy, setting, compressive strength, open time, and adhesion bond strength under different exposure conditions (that is, dry, wet, heat, or freezing-and-thawing cycles). Compared to cement-based mortars containing adjusted proportions of cellulose and redispersible polymers, the GPs exhibited higher thixotropy, reflecting additional energy for spreading the material over the substrate, yet better maintenance of the alternating patterns of ripples and grooves at rest. The bond strengths tested under different exposure conditions were remarkably high for the MK-based GP, given the fine MK particle sizes that foster geopolymerization and crosslinking of solid bonds in the hardened structure. The BFS-based GP exhibited relatively lower bond strengths (compared to MK) due to coarser particles. Such results can be of interest to civil engineers and manufacturers of ready-to-use building materials that aim at reducing the portland cement footprint while assuring performance and sustainability of tiling applications.

The use of recycled concrete aggregates (RCAs) in lieu of natural aggregates improves the sustainability of the built environment. Barriers to the use of RCA include its variable composition, including the residual mortar content (RMC), chemical composition, and its potential to contain contaminants, which can negatively affect the properties of concrete or present environmental concerns. In this study, a rapid, economical method to estimate the RMC and provide the chemical characterization of RCA was developed using a portable handheld X-ray fluorescence (PHXRF) device. Models were developed using reference tests (RMC test based on the thermal shock method and chemical composition from whole-rock analysis) to correlate PHXRF results to measured values. The PHXRF shows strong potential for estimating the RMC and chemical composition of RCA. Paired with locally calibrated reference samples, the test method could be used in laboratory or field applications to characterize RCA and increase its use in bound and unbound applications.

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.