This study evaluates the performance of two hybrid type geopolymer mortars composed of alkaline-activated fly ash (FA) and metakaolin (MK) as protective coatings against chloride-induced corrosion in ...reinforced concrete. In both cases, the coated, Portland cement (OPC)-based concretes (substrates) were subjected to accelerated techniques such as impressed voltage and wetting/drying (w/d) cycles in the presence of a 3.5% NaCl solution. The corrosion was also monitored by applying techniques involving linear polarization resistance and open circuit potential. The results allowed for the conclusion that the MK-based geopolymer coating exhibited the best performance, reducing the corrosion rate compared to concrete without coating (reference) by 4cycles of w/d. After cycle 5, the corrosion rate was similar for the specimens with or without coating. The application of geopolymer type coatings (mortars) is suggested as a method of protecting structures exposed to marine environments.
•GP coatings applied to concrete subjected to accelerated corrosion were examined.•Impress voltage and wetting/drying cycles were used as chloride induced corrosion.•The MK-based geopolymer coating exhibited the best performance.•Both coatings evaluated had the same protection durability (useful lifetime).
•Silica fume play an important role to improve durability performance.•Silica fume decreased water absorption according to BS and ASTM standards.•Recycled aggregates decreased UPV and ER in SCC ...mixtures.•Silica fume increased significantly ER and chloride ion penetration resistance.
Due to adhered mortar in recycled concrete aggregates, some of the properties of concrete made with these types of aggregates, such as water absorption, porosity, electrical resistivity, and chloride ion penetration, may be affected. In this study, silica fume was used as a part of cementitious materials to improve the properties of self-compacting concrete (SCC), which were made with fine and coarse recycled aggregates. Three series of mixtures were made in the laboratory. In the first and second series, coarse recycled aggregates with replacement of 25%, 50%, 75% and 100% were used with and without silica fume. In the third series, 25% of fine recycled aggregates were replaced with fine natural aggregates. Slump flow and J-ring tests were considered for fresh state of SCC. The compressive strength, water absorption, ultrasonic pulse velocity, electrical resistivity, and chloride ion penetration tests were performed for hardened concrete. The use of silica fume improved the properties of fresh SCC. The results showed that silica fume can reduce water absorption and porosity. The silica fume showed that can resulted in a significant increase in electrical resistivity. On the other hand, the replacement of 25% of recycled aggregates did not have a significant effect on electrical resistivity, the electrical resistivity decreased by increasing the amount of replacement. Silica fume was very effective in controlling chloride ion penetration and reducing total charge passed. Silica fume also controlled the temperature of the solutions used during the test which indicates the control of the movement of ions in the penetration into concrete.
This paper presents an experimental study on how the crack-parallel stress affects the fracture properties of fiber-reinforced concrete (FRC) using the gap test—a new simple fracture test invented ...and used for concrete at Northwestern University in 2020. First, it was conducted for plain concrete and was successfully applied to cross-ply carbon-fiber composite and to aluminum. An advantage of this test is that it is unambiguous because the test setup changes from one statically determinate configuration to another. The gap test, combined with the standard notched three-point-bend test, is now applied to geometrically scaled FRC specimens to determine how the fracture energy, Gf, and the effective size, cf, of the fracture process zone (FPZ), are changed by the crack-parallel stress, σxx. For σxx equal to about 2/3 of the standard uniaxial compression strength, the increase in Gf is 64% and 78% for the two FRCs, respectively, which is large but not as large as the 126% increase observed in tests of plain concrete. This indicates that the fiber reinforcement mitigates the effect of σxx, while introducing some degree of ductility into the fracture process. The compressive σxx also increases the effective size of the FPZ by about 81% and 64% while such increase is 134% in plain concrete. Because crack-parallel stresses are ubiquitous in practice, the implications for design are significant.
In this work, the evolution of strength (compressive, tensile, and flexural) and toughness of steel fiber-reinforced rubberized concrete (FR-RC) with various fiber dosages and rubber contents was ...studied. The toughness of FR-RC was investigated using both four-point bending unnotched beams and three-point bending of notched beams. The toughness characteristics were quantified using toughness indexes proposed in ASTM C1018 and doubleK fracture model. The results show that the compressive strength of FR-RC is dependent on both rubber content and fiber dosage, while the flexural and tensile strengths are dominated by the fiber dosage. Although steel fiber can slightly increase the modulus of elasticity of FR-RC, it is mainly controlled by the rubber content. In addition, the first peak strength of FR-RC is influenced by both rubber and steel fiber inclusion, which is in agreement with its strength development. The steel fiber controls the straining hardening and softening behaviors of FR-RC. According to the double-K fracture model, it is found that the rubber content dominates the initial fracture toughness, while the steel fiber dominates the unstable fracture toughness.
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•A chain-extended polyurea has been coated on concrete substrate.•The efficacy of polyurea towards shock loadinghas been demonstrated.•The effect of coating thicknesstowards blast ...survivability has been studied.•Detailed mechanism for explaining the improved blast resistance has been discussed.
Polyurea is a synthetic high-strength elastomeric coating that can be sprayed over existing structures for protection against weathering effects. In view of the fast kinetics associated with the isocyanate-amine reaction, practical processing of polyurea is performed using spray coating technique. In this paper, we report the effect of varying the type and amount of chain extender on the mechanical properties of spray coated polyurea. An optimal ratio of aromatic: aliphatic chain extender was found to result in optimal H-bonding, which in turn reflected in terms of improved mechanical properties of the coating. In this paper, the efficacy of polyurea towards improving the performance of the substrate towards shock and ballistic loading is demonstrated. Polyurea coated concrete tiles were subjected to controlled blast loadings, and an attempt was made to understand the mechanism responsible for the improved blast survivability. Unreinforced concrete tiles underwent extensive fragmentation at peak pressures <50 psi, while polyurea coated tiles could withstand much higher peak pressures. The extent of mitigation increased with the coating thickness and composites with 6 mm polyurea could withstand substantially higher pressures. Upon further loading, polyurea-concrete debonding was evidenced, however the membrane arrested the fragments formed. Dynamic mechanical studies revealed that polyurea remains as an elastomer under the frequency range associated with the shock tube testing and behaves as a catcher system for the fragments formed due to blast loading.
•The shear properties of SFRC with fibers recycled from scrap tires were studied.•Fresh and hardened concrete properties are reported.•Experimental direct shear tests were carried out on specimens ...varying the type of fibre reinforcements.•An analytical analysis was done aims to evaluate the shear stress versus displacement curves.•The obtained results confirm promising applications of concrete reinforced with RSF.
The sustainability of construction materials is a mandatory issue that started to be strongly felt in view of a global perspective of environmental protection. Wasted materials often may find a new lifecycle if well re-engineered, even in structural applications. In this field short steel fibers obtained from used tyres at the end of their life may find promising applications within a concrete matrix. In the present research the mechanical properties of recycled steel fiber-reinforced concrete in terms of workability, compressive and tensile strength, toughness and shear behaviour are analysed and compared with those of industrial steel fiber-reinforced concrete and ordinary Portland concrete. An experimental campaign is illustrated, and an extensive comparison in terms of shear strength has been studied considering different experimental works available in scientific literature. Moreover, a theoretical analysis aimed at evaluating and comparing the shear modulus of the analysed concrete type was carried out. The results obtained through this study show a satisfactory behaviour of the concrete reinforced with recycled steel fibers compared with industrial new steel fibers reinforced concrete, both in terms of toughness and shear behaviour.
An experimental program designed to determine the compressive strength, permeability, and interfacial properties of reinforced concretes with two modified softwood and an unmodified hardwood Kraft ...pulp fibre. The modified fibres were engineered by extreme refinement followed by proprietary chemical modification. The modification was done to create numerous tiny fibrils on the surface of the fibres to facilitate increasing the bond between the fibres and the matrix. These fibres are called mechanically modified fibre and chemically treated fibre, and these are not yet commercially available. In addition, binary use of five supplementary cementitious materials (SCM) were also included in the production of the concretes. Test results showed an increase in the permeability characteristics for the reinforced concretes. However, two modified fibres performed better than the unmodified fibre in terms of compressive strength, chloride ion permeability and water sorptivity. Moreover, silica fume, metakaolin, and slag blends, significantly improved the strength and permeability characteristics of fibre reinforced concretes. Scanning electron microscopy for specimen with no SCM, showed that fibres de-bonded from the matrix. Hence, this study found that SCM improved the adherence between the fibre and the matrix.
•Long-term behavior of self-compacting concrete is investigated.•Coarse and fine recycled concrete aggregates are used in self-compacting concrete.•Self-compacting characteristics are maintained when ...recycled aggregates are used.•Recycled aggregates of good quality promote high mechanical properties.•Creep behavior is influenced by the content and assortment of recycled aggregates.
This paper investigates the shrinkage and creep of self-compacting concrete prepared with coarse and fine recycled concrete aggregates (up to 40% of total amount of aggregates). Physical properties and porosity measurements are studied and related to the mechanical properties.
Results highlight that self-compacting characteristics are maintained when recycled aggregates are utilized and their good quality promotes high mechanical properties. Creep behavior and pores size distributions are more influenced by the content and assortment of recycled aggregates, although their effect is more limited compared to what occurs in traditional concrete with recycled aggregates.
•Fibers do not have any considerable effect on compressive strength of the concretes.•The drying shrinkage strength was highly dependent on fibers’ modules of elasticity.•The physical properties of ...fibers have direct effects on reducing the cracking width.•The steel fibers showed the best performance due to their hook-shaped tail.•Polypropylene fibers also showed better performance in preventing crack development.
Drying shrinkage cracks mainly start to develop at the exposed surface of the concrete elements due to the shrinkage strain caused by self-desiccation. For the purpose of controlling drying shrinkage cracks additions and fibers are used in fresh concretes in order to provide high early age mechanical capacity for moderating the crack development. The purpose of this study is to evaluate the performance of using different fibers in reducing the drying shrinkage and cracking under restrained conditions. To investigate the effectiveness of using both polymeric and metal fibers in concrete, three different types of fibers, including polypropylene fiber, polyolefin fibers, and steel fiber were used in this study. The maximum drying shrinkage strength was highly dependent on fibers’ module of elasticity. The average length cracking and its pattern in fiber reinforced concretes were different than control concrete. The physical properties of fibers have direct effects on reducing the cracking width.
Recycled crumb rubber is a material created by grinding and commutating used tyres. There is no doubt that the increasing piles of tyres create environmental concerns. The long term goal of this ...paper is to find a means to dispose of the crumb rubber in lightweight self-compacting concrete (SCC) and still provide a final product with good engineering properties. This paper has considered replacement of natural normal-weight aggregates with crumb rubber aggregates and lightweight scoria aggregate together with the addition of macro fibres which provides a sustainable alternative which assists in minimizing the environmental damages associated with the disposal of waste tyres. Also, the purpose of this study is to determine the effect of fibres on fresh and mechanical properties in additional to the performance of the concrete after exposure to elevated temperatures. Investigation has been performed after concrete exposure to both room and elevated temperatures. Fibre reinforcement were added to a control mix containing 80% replacement of traditional coarse aggregate with lightweight scoria aggregate and 20% replacement of traditional fine aggregate with crumb rubber aggregate. Steel and polypropylene (PP) fibres were explored so as to ascertain the benefits each fibre can provide through a range of temperatures. Nine mixes were prepared; a control mix and four mixes per each fibre with increasing fibre addition. Chemical admixture dosages were adjusted so as to achieve the desired slump flow. Experimental program investigated the fresh properties of the SCC through slump flow, slump flow T500 and J-ring tests. Mechanical properties were investigated after 28 days curing, standard 100 × 200 mm cylinder specimens were subjected to compressive and indirect tensile tests after exposure to 25, 100, 300, 600 and 900 °C. Compiled results will be compared to those of the control mix.
