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Composite walls consist of two reinforced concrete (RC) walls with or without a filling between them. The use of RC composite walls instead of thick monolithic concrete walls can save material costs significantly. However, it is still unknown if a composite wall system with some granular filling can provide impact resistance comparable to a monolithic RC wall. The objective of this experimental study was to investigate the effect of granular fillings between the two RC walls in improving the impact potential of composite walls and compare the behavior of such walls with that of an equivalent monolithic RC wall. For this purpose, the specimens were prepared to simulate monolithic and composite RC walls. These specimens were tested with a gas gun under single and multiple/repetitive impact loading using hemispherical nose projectiles. The composite walls’ response was then studied and compared with the response of monolithic and no-filling composite walls. Test results and the analysis of the results demonstrate that the idea of using a composite wall system is very promising as the impact response of a composite wall system is much improved than a monolithic wall of the same total thickness and reinforcement. The ballistic limit of the composite wall was also found. In order to predict the ejected mass from a composite wall system, a new analytical model was proposed. There was reasonable agreement between the test results and the prediction.

The concept of microbially induced calcite precipitation (MICP) is in vogue for quite some time; but a multitude of factors that would lead to optimal bacterial activity enabling real time application of the technique in various intended processes are yet to be standardized. This study is an attempt to elucidate some of these factors. The study was conducted in sand columns simulating standard cement mortar conditions of high pH at room temperature. Sporosarcina pasteurii ATCC6453 cultures in broths at variable cell counts of 1 × 103, 105, 107, and 109 ml−1 with 0.2M concentrations of urea and calcium chloride were poured onto sand columns in plastic sheet molds. After variable incubation period of 24, 48, 72, and 96 h, urease activity of the bacterium was assayed calorimetrically. Physical verification of calcite deposition was done with scanning electron microscopy (SEM) and X-ray diffraction (XRD) after 28 days of treatment. Urease activity escalated significantly with increasing cell count and incubation period. SEM and XRD examinations revealed remarkable deposition of calcite in the columns under treatment.

Increasing demand for concrete construction and the consequent increase in the production of conventional Portland cement is thwarting efforts towards sustainable construction practices. This is seen in the carbon dioxide footprint caused by cement manufacturing as well as natural resources consumption. Therefore, the so-called geopolymer or alkali-activated composites using byproduct or natural materials are seen as a potential substitute for cement-based binders. Similarly, recycled rubber recovered from shredded tires has been used in concrete mixes, which in addition to the environmental benefit, offers desirable properties, e.g., improved deformability and energy dissipation capacity. Therefore, this paper investigates the influence of introducing recycled rubber in metakaolin-based geopolymer mixes on the workability, compressive behavior (stress–strain components and failure mode), flexural strength, unit weight, air content, and water absorption percentage. Seven mixes were employed for this purpose with two main variables comprising rubber size (fine, coarse, or combination of both) and replacement percentage (0%, 20%, and 40%). Recycled rubber was observed to reduce the mix workability by 4%–52.5% and 40%–62.5% when fine or coarse rubber particles were added to concrete mixes, respectively. Average compressive strength of 14.3–37.7 MPa was achieved when fine and/or coarse rubber particles replaced 20% or 40% of the conventional fine and/or coarse aggregates. Furthermore, rubberized geopolymer concrete showed more deformability than plain mixes coupled with lightweight characteristics, which are desirable in many construction applications. This indicates the suitability of the rubberized geopolymer concrete for developing low to moderate-strength concrete for non-structural and structural applications.

Geopolymer is emerging as a potential alternative material to the ordinary Portland cement (OPC) owing to its energy efficiency and environmental protection. However, the effect of the mix design parameters on the behavior of geopolymer concrete (GPC) produced using local metakaolin (MK) is not well established. This paper presents findings from a comprehensive experimental program to study the effect of various mix design parameters on the properties of fresh and hardened MK-based GPC. Seventeen mixes were cast in four groups to investigate the effect of four parameters, including sodium silicate to sodium hydroxide ratio, alkaline solids to MK ratio, aggregate content, and water to solids ratio on the properties of GPC. The investigated properties included workability, density (wet and dry), development of compressive strength with age, splitting tensile strength, ultrasonic pulse velocity, water absorption, and horizontal abrasion resistance. Besides, two reference OPC concrete mixes, equivalent to two MK-based geopolymer mixes, were tested for the sake of comparison. The test results were helpful in developing a better understanding of the behavior of MK-based GPC. Some useful models are proposed for predicting workability, splitting tensile strength, water absorption, and weight loss in abrasion for MK-based GPC. The test results were supplemented and confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) analysis performed on some selected GPC mixes.

Ordinary Portland cement (OPC) is commonly adopted as an effective binder in the production of concrete and cementitious mortars. The metakaolin (MK) based geopolymer (GP) binder offers a sustainable alternative to the OPC due to its essential environmental and technical benefits in reducing the carbon footprint. In the present study, two conventional and six GP repair mortars were used. Three GP mortars were plain (without fibers), and the remaining three were produced by mixing Polyvinyl-alcohol (PVA) fibers. The GP mixes were produced by varying sand content, alkaline solids to MK, and sodium silicate to NaOH ratios, resulting in different molar ratios. The test specimens were prepared by overlaying a 5 mm thick layer of repair mortar on a concrete substrate for conducting pull-off strength tests. This research was undertaken to investigate the effect of exposure to elevated temperatures on the performance of bond strength of GP mortar used as a repair material. The test specimens were subjected to ambient temperature and elevated temperatures of 200 and 400 °C for a duration of 3 h before conducting pull-off tests. The test results revealed that the selected GP mixes could be a promising alternative repair material for structures due to their good bond strength performance. The addition of fibers in stable mixes of GP mortars enhanced the bond strength at ambient as well as when exposed to 200 °C, through crack control mechanism.