There is constant demand for high-performance materials to build and rehabilitate concrete infrastructure. The current study investigated the properties of nano-modified cementitious composites incorporating emerging basalt fiber pellets (BFP), including their suitability as repair/overlay for concrete. The composites comprised 50% cement replacement with fly ash or slag, 6% nanosilica addition, and two BFP dosages (2.5 and 4.5% by volume). They were assessed in terms of fresh and hardened properties, as well as their compatibility with concrete substrate. Furthermore, microstructural and thermal analyses were performed to evaluate the evolution of microstructure and interpret the bulk trends. The results showed that the composites had high strength, ductility, and resistance to infiltration of fluids. BFP effectively contributed to the dimensional stability of the composites, which had high thermal and elastic compatibility with concrete substrate even after an aggravated exposure. Hence, they may offer an attractive option as high-performance repair/overlay materials for concrete.

The maximum post-cracking tensile strength (σpc) recorded in numerous investigations of ultra-high-performance fiber-reinforced concrete (UHP-FRC) remains mostly below 15 MPa, and the corresponding strain (εpc) below 4/1000. Both values are significantly reduced when the specimen size increases, as is needed for real structural applications. Test data on σpc and εpc from close to 100 series of direct tensile tests carried out in more than 20 investigations are analyzed. Factors influencing the strain capacity are identified. However, independently of the numerous parameters encountered, two observations emerged beyond all others: 1) the higher the post-cracking tensile strength (whichever way it is achieved), the higher the corresponding tensile strain; and 2) fibers mechanically deformed and/or with slip-hardening bond characteristics lead to an increase in strain capacity. A rational explanation for these observations is provided. The authors believe that achieving a large strain (εpc) at maximum stress is paramount for the successful applications of ultra-high-performance concrete in concrete structures not only for strength but, more critically, for ductility and energy absorption capacity improvements.

Three-dimensional (3-D) printed concrete is a new technology for civil engineering. In this paper, 3-D printed concrete was prepared for a study on static and dynamic properties. The best fluidity of the concrete was researched and the optimization mixture ratio for better mechanical performance was discussed. The mechanical performances of the concrete were tested and the anisotropy phenomenon in 3-D printed concrete was found. The computed tomography (CT) scanning and imaging progress methods were used to discuss the reason for the phenomenon. The penetration experiments were carried out to research the dynamic performance of the 3-D printed concrete. The results of the penetration tests were compared with the empirical formulas. The Young formula was improved according to the results.

The main focus of the current research is the development of high-performance fiber-reinforced cementitious composites with large amounts of coarse aggregates without risking deflection-hardening response, and the evaluation of the autogenous self-healing capability of these composites at different scales. The structural performance of cementitious composites exhibiting strain hardening should be known to be used in large-scale specimens. In addition to the studies carried out in small sizes, there is a need to examine the self-healing performances of large-scale specimens. Composite mixtures included different design parameters—namely Class F fly ash-to-portland cement ratio (FA/PC = 0.20, 0.70), aggregate-cementitious materials ratio (A/CM = 1.0, 2.0), addition/type of different fibers (for example, polyvinyl alcohol [P], nylon [N], and hooked-end steel [S] fibers), addition/type of nanomaterials (for example, nanosilica [NS] and nanoalumina [NA]) and inclusion of steel reinforcing bar in tested beams. Small-scale (80 x 75 x 400 mm [3.15 x 2.96 x 15.76 in.]) and large-scale beams (100 x 150 x 1000 mm [3.94 x 5.91 x 39.4 in.]) were produced and considered for performance comparison. Four-point bending tests were performed on different-scale beams loaded by considering different shear span-effective depth ratios (a/d) ranging between 0.67 and 2.00 and 0.67 and 2.96 for small- and large-scale beams, respectively. Autogenous self-healing evaluation was made using different-scale beam specimens subjected to 30-day further cyclic wetting-and-drying curing in terms of changes in microcrack characteristics and recovery in flexural parameters of preloaded beams. Experimental results showed that it is possible to successfully produce concrete with large amounts of coarse aggregates without jeopardizing the deflection-hardening response both at small and large scale. Autogenous self-healing is valid for small- and large-scale beams in terms of crack characteristics/flexural parameters and is found to improve with the increased FA/PC, decreased A/CM, in the presence of nanomaterials, and with the increased fiber amount (regardless of the type). Outcomes of this research are thought to be important because they show the manufacturability of deflection-hardening concrete with large amounts of coarse aggregates at large scale and validate their autogenous self-healing capabilities, which are important for the real-time applicability of such mixtures in actual field conditions.

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.