This investigation aimed to develop hybrid composite lightweight concrete beams with improved shear capacity and strength. A semi-lightweight high-performance engineered cementitious composite (ECC) layer was added to either the compression or tension side of the beam to improve the shear capacity while maintaining low average density of the composite beam. The ECC material was developed with different fiber types, including polyvinyl alcohol (PVA) fibers with 8 mm (0.31 in.) length, and steel fibers (SFs) with 35 mm (1.38 in.) length. The study compared the theoretical predictions of ultimate shear capacity calculated by design code models and proposed a model to the experimental results. The results indicated that the strategy of using a high-performance ECC layer in lightweight concrete beams can successfully alleviate the reduction in the shear strength of lightweight concrete, with a slight increase of no more than 9% in the density. For example, using an ECC layer with PVA fibers in the compression side of the lightweight control beam increased the density from 1727 to 1843 kg/m3 (107.81 to 115 lb/ft3) while it significantly improved the normalized shear strength, reaching a value that exceeded the normalized shear strength of the normalweight concrete beam with a density of 2276 kg/m3 (142.1 lb/ft3). Using an ECC layer in the compression side of the lightweight control beam also showed a noticeably higher post-diagonal-cracking shear resistance and post-cracking shear ductility compared to the control lightweight beam, full-cast ECC beams, and normalweight concrete beam.

A concrete system is identified as highly printable if it can offer minimal resistance to handling while sustaining high load resistance and structural stability. One of the major complexities of three-dimensional (3D) concrete printing lies in its sensitivity to materials and equipment that varies the time among layers, hydration time, and shear history. While nanoclays are effective additives for enhancing structural buildup, methylcellulose is introduced as a secondary additive to significantly amplify the nanoclays’ effect on the static yield stress while prolonging the open time between layers and increasing filament cohesiveness. The compatibility of these two systems at different contents is studied by characterizing rheological properties such as static yield stress, steady-state viscosity, and storage modulus, as well as the heat of hydration through isothermal calorimetry. The hybrid system is found to increase the static yield stress by up to 900% compared to the reference paste at only 3.0 wt.% total content by mass.

This study investigated the structural behavior of lightweight self-consolidating concrete (LWSCC) beams strengthened with engineered cementitious composite (ECC). Four LWSCC beams were strengthened at either the compression or tension zone using two types of ECC developed with polyvinyl alcohol (PVA) fibers or steel fibers (SFs). Three beams were also cast in full depth with LWSCC, ECC with PVA, and ECC with SFs, for comparison. The performance of all tested beams was evaluated based on loaddeflection response, cracking behavior, failure mode, first crack load, ultimate load, ductility, and energy absorption capacity. The flexural ultimate capacity of the tested beams was also estimated theoretically and compared to the experimental results. The results indicated that adding the ECC layer at the compression zone of the beam helped the LWSCC beams to sustain a higher ultimate loading, accompanied with obvious increases in the ductility and energy absorption capacity. Higher increases in the flexural capacity were exhibited by the beams strengthened with the ECC layer at the tension zone. Placing the ECC layer at the tension zone also contributed to controlling the formation of cracks, ensuring better durability for structural members. Using ECC with SFs yielded higher flexural capacity in beams compared to using ECC with PVA fibers. The study also indicated that the flexural capacity of single-layer and/or hybrid composite beams was conservatively estimated by the ACI ultimate strength design method and the Henager and Doherty model. More improvements in the Henager and Doherty model’s estimates were observed when the tensile stress of fibrous concrete was obtained experimentally.

The present study investigates the impact of agro-green waste as an additive to concrete composition. The objective of the investigation is to evaluate the impact of agro-waste material as a thermal insulator for buildings. It helps in the reduction of energy consumption inside buildings. Wheat straw, rice husk, and coconut coir are used for the preparation of the composite concrete mixture. With a decrease in thermal conductivity and an increase in compressive strength, this paper provides a possibility of agro-waste in the concrete industry.

The current study aims to assess the transport and durability properties of alkali-activated concretes made with hybrid aluminosilicate precursors having different proportions of natural pozzolan as a low-calcium precursor and ground-granulated blast furnace slag as a high-calcium precursor, which are activated with different concentrations and combinations of sodium hydroxide and sodium silicate. The studied parameters included precursor combination (natural pozzolan/slag combinations of 30/70, 50/50, and 70/30), sodium hydroxide concentration (1, 1.75, and 2.5 M), and activator combination (sodium hydroxide/sodium silicate combinations of 70/30, 75/25, and 80/20). The resulting concrete mixtures were tested for slump flow, setting time, compressive strength, absorption, rapid chloride penetration, rapid chloride migration, resistance to sulfuric acid attack, chloride-induced corrosion, and frost resistance. Mercury intrusion porosimetry and X-ray diffraction were used to justify the observed behaviors. The performance of alkali-activated natural pozzolan/slag concretes was also compared with that of a reference concrete made with solely portland cement binder. In view of overall performance, an equal proportion of natural pozzolan and slag (50/50) and a 30/70 combination of sodium silicate and sodium hydroxide proved to be the optimum precursor and activator combinations. The optimum sodium hydroxide concentration was dependent on the precursor and activator combinations as well as the expected fresh, strength, transport and durability performance. In terms of the measured transport properties (that is, absorption, chloride penetration depth, and passing charges) and resistance to acid attack and chloride-induced corrosion, all the studied alkali-activated concretes performed considerably superior to the reference portland cement concrete. In the case of frost resistance, only alkali-activated concretes with 50 and 70% slag performed superior to the reference portland cement concrete.