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Nanoparticles have exhibited a promising potential in enhancing the bituminous materials’ performance. As a cost-effective material, hot mix asphalt (HMA) has been employed in pavement structure for a long time. Their viscoelasticity however results in various modes of failure under different temperature profiles and loading stress. Rutting of asphalt pavements is considered as the most prominent distress factor under almost all climate conditions. Moreover, steel slag has been employed for improvement of asphalt mixture properties as it exhibits excellent physical features. In this content the aim of the present research is to analyze improving effects of nano clay and styrene-butadiene-styrene (SBS) on the rutting resistance of modified steel slag asphalt mixture (SSAM). Various tests were employed to investigate the asphalt binder properties and the rheological characteristics of modified bitumen among which penetration grade, softening point, ductility and dynamic shear rheometer (DSR) can be mentioned. Marshal Test was applied to examine the optimum bitumen content in conventional and modified asphalt mixture. The rutting resistance against was then evaluated through two tests: repeated load axial (RLA) test and wheel track test. The results indicated respective improvement of toughness and viscosity by an average of 25% and 101% upon addition of nano-polymer which enhanced the bitumen rheological characteristics while reducing the penetration grade. The asphalt rutting resistance and rutting depth exhibited some improvements as well.

Cement emulsified asphalt mortar (CEAM) is a key component of ballastless rail track systems which is tasked with damping the train movement energy and transferring loads to the concrete roadbed. This mortar consists of asphalt emulsion, sand, cement, and water and enjoys better toughness and flexibility properties than cement mortar thanks to the combined effect of cement and asphalt emulsion. One of the primary causes of failure in this mortar is cracking at low temperatures. This study investigated the fracture toughness of CEAM (CRTS- II) with different volume percentages of mortar constituents, namely cement, sand, water, and asphalt emulsion, at low temperature in the linear elastic fracture mechanics (LEFM) framework. The critical stress intensity factors of the mortar for pure tensile (mode I), pure shear (mode II), and mixed tensile-shear (mixed mode I/II) fractures with Me values of respectively 1.0, 0, and 0. 67 were obtained. Considering the necessity of examining the effect of mix design ratios and their interactions and the need to limit these ratios to ensure acceptable rheological and mechanical properties, the experiments were designed using the D-Optimal mixture design method at 5% significance level. By assuming a quadratic model between the critical stress intensity factors of each fracture mode and the volume percentages of mortar constituents, a total of 20 randomized mix designs were obtained. For each mix design, the semi-circular bending (SCB) test was performed with at least 3 replications. The regression models based on the experimental results were developed. The findings demonstrated the dependence of mode I, mode II, and mixed mode I/II critical stress intensity factors on the mix design ratios. The lowest and highest fracture toughness estimates were related to the mixed tensile-shear and pure shear modes, respectively. Finally, the mixture design method was used to delimit the optimum mix design region for maximizing the fracture toughness of all three modes.

In this paper, three-point bending tests on precast notched beam and multiscale numerical simulation were used to evaluate and predict the effect of different Basalt Fiber (BF) dosages on the fracture resistance performance for Basalt Fiber Reinforced Concrete (BFRC). The three-point bending test was performed on a series of the notched beams in different sizes and volumetric fiber dosage to obtain the Load-Crack Mouth Opening Displacement (P-CMOD) curves during the loading process. The fracture energy and the double-K fracture parameters were calculated from P-CMOD curves to investigate the influences of fiber volume fraction and beam size on fracture parameters of BFRC. The results indicate that the numerical simulation results are in good coherence with the experimental results. The increase of BF dosage could markedly increase the peak load and the fracture energy of concrete. There was no size effect on fracture energy. Based on the two-parameter fracture theory, the initiation toughness had size effect and increased with the rise of specimen height, while size effect was not significant for unstable toughness. With the augment of fiber dosage, the initiation toughness increased linearly. The unstable toughness varied irregularly with the alteration of fiber dosage but higher than that of ordinary concrete. In addition, this paper developed a homogenization algorithm based on Mori-Tanaka and a multi-scale finite element simulation based on continuum progressive damage theory to evaluate and predict the fracture behavior of BFRC with different sizes and fiber volumetric dosages. The calculated P-CMOD curves from this algorithm were in good agreement with those from experiments and revealed the effects of fiber dosage and size effect on fracture behavior for BFRC.

The results of experimental investigation of ultra-high performance concrete (UHPC) by evaluating the flexural response are presented in order to qualitatively evaluate the effects of chloride-induced corrosion. The third-point bending test is carried out using the hydraulic servo-controlled testing machine. The experimental variables are thickness of specimen (10, 25 and 50 mm), immersion duration, and mixture design of UHPC. The specimens are immersed in 10 wt% NaCl solution up to a year to induce corrosion. The effect of chloride-induced corrosion of steel fiber of UHPC is evaluated in terms of compressive strength, flexural strength and flexural toughness. The experimental results indicate that there is no significant loss in flexural strength and toughness for the specimens which were thicker than 25 mm over a period up to 365 days of immersion in chloride solution; whereas, the specimen with thickness of 10 mm showed about 10% decrease in maximum stress and corresponding toughness after 180 days of the immersion. Furthermore, it is revealed by comparing the compressive strength with and without fibers that the presence of steel fibers in UHPC under corrosive environments do not lead to substantial strength decrease.

Accumulation of waste is subsequently increased to hazardous levels. Tire waste is one of them that cause serious environmental issues because of the rapid rise in and numerous variations of modern developments worldwide. Thus, recycling waste tire rubber in the form of aggregates as supplementary construction material is advantageous. This paper reviews the source of waste tire rubbers and rubberized cementitious composites along with their material properties, usages, durability, and serviceability performances. This study also aims to provide a fundamental insight into the integrated applications of rubberized concrete (RuC) composite materials to improve construction methods, including applications to enhance environmental sustainability of concrete structures in the construction industry. Inclusion of recycled rubber aggregate (RA) lightens concrete, increases its fatigue life and toughness, advances its dynamic properties, and improves its ductility. Concrete with recycled RA performs well in hot and cold weather and achieved significant results under critical exposure and various loading conditions. Though RuC possesses low mechanical strength in general, specific treatment and additives inclusion can be a good solution to improve those properties reliably. Investigations of RuC as materials are available significantly, but researches on the structural members of RuC should be enriched.

Structural strengthening comprises modification of the structural elements to enhance their load bearing capability, stiffness, toughness, and ductility. The present study is an experimental investigation on the performance of reinforced concrete (RC) beam specimens strengthened by the side near surface mounted (SNSM) technique with reinforcement strengthening of glass fiber reinforced polymer (GFRP) bars in flexure. Sustainable palm oil fuel ash (POFA) mortar (PM) and normal mortar were used as bonding mediators between the concrete substrate and GFRP bars as a replacement for epoxy adhesive. A total of eight specimens, in which one was a control, one was strengthened by GFRP bars and full epoxy adhesive in grooves, and six specimens were strengthened by GFRP bars and partial replacement of epoxy adhesive by PM and NM, respectively. The specimens were tested in the static condition under four-point bending. During the test, the loads, deflection, and strains of the specimens were stored in a data logger and the failure modes of the specimens were observed. Critical discussions were made based on the flexural capacities, load-deflection, ductility, energy absorption, the influence of epoxy replacement and the type of mortars in the strengthened specimens as compared to the control specimen. ACI 440.2R-08 and ACI 318-11 were applied to predict the ultimate load-carrying capacity and deflection characteristic graphs of the tested specimens. The predicted results of the specimens were in close agreement with the experimental results. The test results also exhibited that the epoxy substituted by sustainable PM had superior flexural performance to the NM strengthened SNSM-GFRP bar specimens, particularly when considering the economic and eco-friendly viewpoint.

Hot-humid geological environments cause cement substrates to bulge and crack, which seriously affects the strength of lining concrete structures. This paper proposed a composite concrete material suitable for hot-humid environments, using three additives of steel fibers, polypropylene fibers and vitrified beads. The compressive properties of composite concrete were studied, and the relationship between the pore structure and compressive properties was described by a microscopic quantitative method. At the same time, the failure characteristics of the composite concrete in a hot-humid environment were studied in combination with the acoustic emission (AE) events. The results show that the combination of steel fibers, polypropylene fibers and vitrified beads renders the highest compressive strength for the concrete at all temperatures. The strength loss rate is the lowest with increasing temperature, and the toughness is also greatly improved, which indicates that the composite concrete can adequately adapt to the hot-humid environment. The microstructure parameters of concrete have a good nonlinear relationship with the compressive strength. The continuous high number of AE events in an adjacent timing event can be used as precursor information for the failure of the composite concrete. The use of the three additives causes the failure mode to change from tensile to shear. The results can provide a theoretical basis for the research and quantitative evaluation of the tunnel lining structure in hot-humid geological environments.

Scrap tire-derived crumb rubber (CR) in concrete, along with recycled coarse aggregate (RCA) and polypropylene fiber, constitute a way to reusing and recycling waste material and providing green and sustainable concrete structures. This study investigates the effect of substituting recycled constituents such as RCA and CR as a partial replacement of coarse aggregate and fine aggregate, respectively, along with polypropylene fiber addition into the concrete mixture. Fifteen different mixes are considered, with 10% and 30% RCA, 5% and 10% CR, and 1% and 2% fiber content. This study focuses on the experimental investigation of concrete combining RCA, CR, and fiber and evaluates its compressive strength, splitting tensile strength, and flexural strength at different ages. The compressive strength, splitting tensile strength, and flexural strength decrease as the CR content increases, but increase with the increasing fiber content. With regard to toughness and ductility, the effect of fiber is greater than that of RCA and CR, with each being found to increase with the incremental addition of fiber. It is also observed that the beams with fiber show failure in a more gradual manner. Finally, as a general recommendation in the interest of sustainability and environmental concern this paper suggests the use of rubberized concrete with RCA and polypropylene fiber for any structural purpose subjected to further investigations.

To obtain an optimum balance in performance and cost, this study aims to investigate the effect of wood flour (WF) content in both shell and highly filled core layers on the mechanical properties, creep behavior, and dimensional stability of the resulting coextruded WF/high density polyethylene composites (Co-WPCs). Scanning electron microscopy showed good interfacial adhesion between the core and shell layers. The flexural strength and modulus of the Co-WPCs increased with increasing WF content in shell layer, while the flexural strain and impact strength decreased but still higher than those of the core-only controls. Finite element analysis showed that the coextruded flexible shell layer acted as a stress transferring medium avoiding stress concentration and endowed the Co-WPCs with higher toughness. Increasing the content of rigid WF in both the core and shell layers resulted in significant reduction in creep strain. Coating the highly filled core layer with less filled shell layer can markedly reduce the water absorption and thickness swelling. For Co-WPCs with 70 wt% WF in core layer and 20 wt% WF in shell layer, the flexural strength, modulus, and creep strain were comparable to those of the core-only control, but the flexural strain, impact strength, and water resistance were much better.

In order to popularize magnesium oxysulfate (MOS) cement in civil engineering, it is very necessary to investigate its carbonation related durability. It was found that carbonization can increase compressive and flexural strength of MOS cement to some extent and compressive strength of MOS cement increases gradually at first, then decreases during carbonation process. The displacement to ultimate compressive force of hardened MOS cement paste carbonated for 28 days is larger than that of non-carbonated one at the same age. Evaporable water testing and X-ray computed tomography (X-CT) were adopted to examine the porous structure of MOS cement paste with and without carbonation. It was found that carbonation refines the meso-pore (≧38 µm meters) volume ratio of hardened MOS cement with 0.5 water-to-cement ratio by about 17%. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were adopted to analyze the carbonation characteristics of MOS cement. The results indicate that magnesium hydroxide and some basic magnesium oxysulfate phase are obviously neutralized with carbon dioxide to form magnesium carbonate phases in the hardened MOS cement, which leads to the refined porous structures and corresponding increased toughness.

Plant fiber reinforced polymer composites are experiencing rapid growth in terms of applications where they may be subject to corrosion and wear. The present work explores the possibility of reinforcing polyvinyl chloride (PVC)/sorghum straw (SS) composites with micro-silica (MS) and poly(acrylonitrile-styrene-acrylate) (ASA) for developing a new corrosion and wear-resistant material. Sea water (salinity 3.5%, temperature 55 °C) and acid rain (pH 2.5, temperature 55 °C) were utilized to simulate extreme cyclic corrosion conditions. The results revealed that the wear and corrosion resistance of the PVC/SS composites was significantly enhanced by the addition of 6 wt% MS (particle size 2.6 μm) and 34% ASA, which was attributed to that the MS and ASA could render the PVC matrix with the improved toughness, strength, and heat resistance.

In this paper, an experimental investigation was carried out to study strength, toughness and microstructural properties of early age cemented tailings backfill (CTB) reinforced with three different types of fiber. Polypropylene, polyacrylonitrile and glass fibers were used in CTB to better understand the effect of fiber addition on the backfill’s strength and toughness properties. Different fiber contents (0 wt%, 0.3 wt%, 0.6 wt% and 0.9 wt%) and curing times (3 and 7 days) were designed for the preparation of CTB samples with two cement-to-tailings ratios (c/t = 1:4 and 1:6). The results indicated that the addition of different type and content of fibers caused a significant change in CTB’s toughness. The fiber content influenced the backfill’s strength performance. The strength gain of fiber-reinforced CTB samples showed an increasing trend as the fiber content rises. Linear relationships could be used to express relations between strength gain and fiber content. The peak strain factor K was defined during the toughness tests. K showed a monotonous increasing trend by the increase of fiber content. The polypropylene fiber was larger than polyacrylonitrile, but less than glass fiber when fiber content was 0.3 wt%. The unreinforced CTB samples were mainly parallel to the axial tensile cracks and a small amount of shear cracks, while the deformation of fiber-reinforced CTB was relatively large, it did not break into small pieces. The crack resistance of fiber-reinforced specimens was notably better than that of unreinforced ones. Failure mechanism and modes of fiber addition in CTB samples were also studied. As a result, this study could provide a theoretical/application basis for mines to reduce cement usage within the CTB matrix, achieve safe mine production, increase ore extraction and reduce ore losses and dilution.

Marble specimens were used to investigate the effects of freezing and thermal cyclic loading on crack initiation and propagation and fracture modes. To this end, four loading types of freezing-cooling (F-C), heating-cooling (H-C), freezing-cooling- heating (F-C-H) and heating-cooling-saturating-freezing (H-C-F (sat.)) were considered. Forty-four toughness tests were carried out, and changes in the fracture mechanism in the marble were analyzed at −30 °C and 160 °C. The fracture toughness parameters, load-displacement curves, crack initiation angle for mode II and the mixed mode (I-II) were measured and then predicted using the conventional criteria. Crack initiation angles for the mixed mode (I-II) and mode II fracture toughness in different loading conditions were calculated to be about 58° and 78°, respectively. The F-C-H loading condition seemed to reduce mode I fracture toughness in the tested samples. The experimental data on fracture toughness for H-C, F-C-H and H-C-F (sat.) loading conditions were well predicted by the modified maximum tangential stress (MMTS), maximum tangential stress (MTS) and G-criterion. The results indicated that very good predictions are obtained by the extended maximum tangential strain (EMTSN) for the fracture toughness of marble envelopes in the entire range of mode mixities from pure mode I to pure mode II. Load-displacement curves for different loading conditions were monitored, indicating that the peak load significantly decreased for F-C-H and H-C-F (sat.) cycles.

Coal gangue concrete can effectively utilise coal gangue, minimise natural stone and has substantial environmental and economic benefits. At high-latitude and high-altitude regions, the durability and bearing capacity of gangue concrete structures are critically threatened by freeze–thaw cycles. Here, the freeze–thaw cycles experiment was performed using the accelerated freeze–thaw method. Various characteristic parameters, such as stress–strain curve and acoustic emission under axial compression, were measured for the frost-damaged coal gangue concrete. The volume replacement rates of coarse aggregate mixtures (C1, C1M2, C1M4 and C1M6) were 0%, 20%, 40% and 60%, respectively. After freeze–thaw cycles, water absorption and square root of time were nonlinearly related, showing a relatively rapid growth in the early stage, a gradual increase in the middle stage and stability and balance in the late stage. Water absorption rate was positively correlated with the gangue content in concrete. The physical and mechanical properties of gangue concrete decreased with prolonged freeze–thaw cycles, and the degradation rate increased with the gangue amount. The damaged layer thickness and peak strain increased, whereas the initial elastic modulus, peak stress and toughness decreased. The parameter alterations increased with the gangue content in concrete. The damage evolution rule of gangue concrete under axial compression was accurately described by acoustic emission.

This paper investigated influence of formwork wall effect on fiber orientation and flexural properties (flexural strength and toughness) of ultra-high performance concrete (UHPC) prepared by a direct cast and flow control method. Fiber orientations in samples with different distance from formwork were quantitatively evaluated using image analysis technique. Based on experimental results, the better fiber orientation and higher flexural performance can be obtained near formwork boundaries due to restriction effect on fiber rotation. Considerable variations of flexural strength and toughness with position were induced by the existence of mold sidewalls for directly cast UHPC specimens, reaching up to 54.9% and 49.6%, respectively. This is related to the significant fiber orientation coefficient variation of 21.1%–30.3%. When adopting the flow control method, the variability of fiber orientation coefficient was decreased to the range of 6.0%–12.3%, inducing the reduction of flexural strength and toughness differential to lower than 19.5% and 18.6%. Therefore, the flow control casting method can significantly inhibit the wall effect of formwork and is favorable for improving the homogeneity of UHPC in real structures.

The objective of this work is to show the effect of the initial moisture content of fiber-cement composites on their carbonation degree (CD), mechanical and physical properties after their exposition to CO2 atmosphere, and after accelerated ageing cycles. Fiber-cement composites produced by Hatschek process and submitted to carbonation at early age for 10 h of exposition to 15% of CO2. Part of the composites was previously submitted to different drying times (20, 40, 60 and 120 min). The CD was determined using thermogravimetric measurements. Mechanical (flexural) and physical properties were determined at 10 days of cure and after 200 soak and dry ageing cycles. As expected, as higher as the time of drying, the higher was the removing of moisture from the composites. Accelerated carbonation caused the decrease of Aft, C-S-H, Afm and Ca(OH)2; and increased the content of CaCO3. The higher time of drying (120 min) led to the lower content of Ca(OH)2 and the higher content of CaCO3. Fiber-cement composites without drying and with 40 min of drying presented moisture between 19% and 21% and led to average CD between 28 and 39%; while those fiber-cements dried for 120 min (14% of moisture) presented an average CD of 44%. Higher CDs led to the significant decrease of water absorption (WA) and apparent porosity (AP) of the composites, while caused the increase of bulk density (BD) of the fiber-cement composites. LOP and MOE values increased with carbonation, while they were less affected by the previous drying before carbonation. 200 ageing cycles caused increase of LOP, MOR, MOE and BD of all the composites, while decreased toughness, WA and AP. Therefore, the control of the initial moisture and free pores of the fiber-cement composites seems to be a strategy to improve the efficiency of the accelerated carbonation of fiber-cement composites.

Slurry Infiltrated Fiber Concrete (SIFCON) is a special kind of steel fiber-reinforced cement composite which has extraordinary toughness values and surpassing mechanical characteristics like compressive, tensile, shear, and flexural strengths. In this study, steel and polypropylene fibers have been used by mixing in different combinations. 1/1, 1/3 and 2/3 of the beam molds have been filled with these fibers. In the study, the physical and mechanical properties and also the unit strength cost analyses of SIFCON have been examined. The use of the fibers with combinations has caused to an increase in the fiber volume. The highest flexural strength values have been reached with the beam samples completely filled with fiber respectively as 44.02 MPa and 41.23 MPa with the steel fiber at the length of 60 mm and 35 mm and with 60 mm steel and 50 mm polypropylene fiber combination. The combined use of polypropylene fiber and steel fiber has increased the flexural strength and fracture toughness. Consequently; it has been observed that the polypropylene fiber provides significant advantages for the use of SIFCON in terms of unit weight and cost due to the fact that its density and cost are lower when compared to those of the steel fiber.

In this paper, the poly-acrylonitrile synthetic fiber concrete (PSC) (the representative of organic fiber concrete) and alkali resistant glass fiber concrete (ARGC) (the representative of inorganic fiber concrete) were studied and compared. The test were carried out by the IMYD-X single side freeze-thaw machine of concrete and the modified wheel impact test machine which was pressurized and accelerated. The dynamic deformation and toughness performance of fiber concrete have been comparatively analyzed, and the effect of fiber on the performance of concrete has been investigated. Three concrete performance indexes were selected to evaluate the anti-wheel impact performance of the airport pavement concrete in freeze-thaw environment, namely relative mass ratio, relative compressive strength ratio, and relative dynamic modulus ratio. The results showed that adding fiber in the concrete can significantly improve these indexes. Four indexes of concretes decreased when the test durations increased, and the relationship between various indexes and test durations can be described as an exponential function. Moreover, all the indexes of fiber concrete increase with the increasing fiber volume content. The optimal content of both organic fiber concrete and inorganic fiber concrete were 1.6 kg/m3. From the overall trend, the relationship among fiber concretes by the anti-wheel impact performance in freeze-thaw environment can be listed as follow: ordinary concrete

This study investigated the effects of steel fiber length and coarse aggregate maximum size on mechanical properties of steel fiber reinforced concrete (SFRC). The experimental results show that the mixture slump, splitting tensile strength, flexural strength, initial flexural toughness ratio, flexural toughness ratio, fracture energy and unstable fracture toughness of SFRC increase as steel fiber length increases. The compressive strength and initial fracture toughness are barely influenced by steel fiber length. With the increase of coarse aggregate maximum size, (a) the mixture slump increases, (b) compressive strength shows slight variations, (c) splitting tensile strength, flexural strength, fracture energy and unstable fracture toughness increase first and then decrease, (d) there are none indicative variations for initial flexural toughness ratio, flexural toughness ratio and initial fracture toughness. The rational range of the ratio of steel fiber length to coarse aggregate maximum size is 1.25–3 for the considerable strengthening effect of steel fiber on splitting tensile strength and flexural performances of concrete and 1–4 for that on fracture characteristics respectively.

The repair of corroded reinforced concrete (RC) structures is necessary to improve their serviceability. Ultra-high toughness cementitious composite (UHTCC), characterised by excellent tensile behaviour, crack control capacity, and durability, shows great potential for the repair and strengthening of corroded members, especially in harsh marine environments. This paper presents an experimental study and a theoretical analysis of the flexural performance of corroded RC beams repaired with UHTCC. The experimental results indicated that macro cracks in concrete were transformed into multiple fine cracks and a slight nonlinear strain distribution was observed along the sectional depth. The replacement of damaged concrete with UHTCC can recover the initial load-carrying capacity at corrosion ratios within 11.2%. After yielding of beams, the development of UHTCC-concrete interface crack in the pure bending region was beneficial to ductility improvement. A theoretical model considering the effect of pitting corrosion was proposed for calculating flexural capacity of repaired beams.

The paper contains quantitative characteristics of changes in the porosity and mechanical properties of RPCs caused by applying different pressures during concrete mixing, i.e. atmospheric pressure and pressure reduced to 500 and 40 mbar. The effect of curing conditions was determined as well: the impact of curing in water (Tmax = 20 °C), steaming (Tmax = 90 °C) and autoclaving (Tmax = 250 °C) on the porosity, compressive strength and flexural tensile strength of the materials tested. In addition, the maximum volumetric proportion of 6 and 14 mm steel micro-fibres which can be introduced in a standard manner, i.e. during the production of the RPC mix, was determined depending on their length. Besides the basic mechanical characteristics, tests of mechanical properties of RPCs containing fibres included the determination of the following parameters: σLOP (limit of proportionality), σMOR (modulus of rapture), Wf (work of fracture) and the toughness indices. Test results clearly confirm the considerable impact of pressure during RPC mixing and curing conditions on the durability and mechanical properties of the hardened composites. The maximum volumetric proportion of micro-fibres introduced during mixing was 8%. Steel fibre reinforcement was found to be more effective when longer fibres were used.

Combination of different kinds of fibre to achieve synergic response in concrete is termed hybrid fibre reinforced concrete (HFRC). Earlier research demonstrates that short and fine fibres enhance the fresh property of concrete by controlling the growth of micro-cracks. Long and coarse fibres enhance the hardening property of concrete by arresting the propagation of macro-cracks. Thus a combination of short-fine fibres and long-coarse fibres improves the overall performance of concrete. In this study an attempt has been made to investigate the effect of fibre hybridization on mechanical behaviour of concrete reinforced with metallic (Steel) and non-metallic (Polyester and Polypropylene) fibres on 30 MPa concrete. Hybridization was done in two stages; the first stage of investigation was to study the effect of polyester (PO)-polypropylene (PP) HFRC, where three hybrid combinations were considered i.e. 75% PO + 25% PP, 50% PO + 50% PP and 25% PO + 75% PP at a total fibre volume of 0.15%. The optimum hybrid combination was achieved at 75% PO + 25% PP. Further, the investigation was carried out to develop a HFRC made with metallic and non-metallic fibres at a total fibre dosage of 1%. Mechanical properties, namely compressive strength, direct tensile strength, flexural strength and stress-stain behaviour of concrete under uni-axial stresses were investigated. The results obtained were compared with mono-fibre reinforced concrete and conventional concrete. Significant improvement in tensile strength and toughness was observed with the fibre hybridization. This may be due to synergic response of different fibres at different scales of cracking at different stress levels in concrete. Superior results were observed at metallic – non-metallic hybridization.

In this study, the impact of partially replacing sand with sustainable and renewable biomass such as ground wood chips on the mechanical properties of mortar is investigated. The effect of thermal treated biomass on strength properties is also studied. Compared to the mortar containing untreated ground wood chips, those containing highly or lightly torrefied ground wood chips were found to have higher flexural strength and compressive strength due to the enhanced bonding between the wood chips and paste matrix. Although all the tested mortars with ground wood chips have lower strength than the mortar without any wood chips, their flexural toughness and toughness indices were found to be comparable or higher than the controlled mortar, indicating better energy absorption capacity and post-peak behavior.

This study investigated the effects of the coarse aggregate and steel fibre contents on the mechanical properties of high performance concrete (HPC) at an ambient curing temperature. HPC bulk materials were formed using four replacement levels by volume of mortar (0%, 16%, 28% and 38%) and four steel fibre volume dosages (0%, 1%, 2% and 3%). The mechanical properties (e.g., compressive strength, splitting tensile strength and flexural behaviour) and the synergistic effect between coarse aggregate and steel fibre were studied. The results indicated that compressive and splitting tensile strength increased as the coarse aggregate replacement level increased from 0% to 28%, but decreased with a further increase in coarse aggregate content up to 38%. The inclusion of coarse aggregate also weakened the flexural behaviour of HPC, especially in terms of the peak flexural load, peak deflection, toughness and post-peak ductility. Furthermore, the steel fibre volume dosage had a limited effect on compressive strength but significantly enhanced the splitting tensile strength and flexural behaviour. Finally, this study discusses the mechanism of synergy between coarse aggregate and steel fibre based on scanning electron microscopic observations of the HPC microstructure. These research outcomes pave a way toward prospective engineering applications of HPC.

This paper experimentally studied the impact behavior of hybrid basalt-macro synthetic polypropylene fiber reinforced concrete under impact loads (101–102 /s) with a split Hopkinson pressure bar (SHPB) device. The fiber content of basalt fiber (BF) was 0.05%, 0.075% and 0.1% and that of macro synthetic polypropylene fiber (SF) was 0.15%, 0.25%, 0.35% and 0.5%. Both static and impact tests were conducted to investigate the effect of strain rate and fiber hybrid ratio on the dynamic performance of the hybrid FRC, i.e. impact strength, dynamic increase factor (DIF), impact strain and toughness. The test results indicate that the hybrid FRC is strain-rate sensitive and a proper fiber hybrid ratio can improve the impact performance of concrete. DIF and l g e are linearly related, whereas the impact strain, toughness and strain rate present quadratic polynomial relationship. Both BF and SF can enhance the impact strength of concrete. However, BF has a better enhancement effect than SF in terms of improving impact strength but not as good as SF in improving impact toughness. Also, appropriate BF and SF hybrid ratio can enhance the impact strength and toughness of concrete but excess fiber content has a weakening effect. In this study, the optimal fiber hybrid ratio was 0.075%–0.35% (BF-SF) which had the best impact resistance. Finally, a damage dynamic constitutive model suitable for the hybrid FRC was proposed to fit the test curves from SHPB test based on Zhu-Wang-Tang (ZWT) constitutive model.

With the increasing shortage of natural materials, using steel slag instead of natural sand to prepare concrete has become a promising technology. This study aims to investigate the utilization of unprocessed steel slag in normal- and high-strength concrete, focusing on the effects of steel slag as fine aggregate on the compressive behaviour of concrete. Two types of concrete (normal- and high-strength concrete) and eight replacement percentages of fine aggregate by steel slag (0%, 10%, 20%, 30%, 40%, 60%, 80%, and 100%) are used as the test parameters. A series of axial compression tests are conducted on cylinders of fine steel slag concrete (SSC). The influence of the replacement percentage of steel slag on the stress-strain relation, compressive strength, inflation ratio, elastic modulus, and toughness of normal- and high-strength concrete is analysed. Moreover, the failure mechanism of SSC under compression is discussed. The results show that the compressive strength of SSC is not monotonous with increasing steel slag content and that the use of steel slag as fine aggregate in concrete mixes has a positive effect on the energy absorption capacity. SSC with an optimal steel slag content can exhibit better compressive behaviour compared with normal concrete.

In recent years, the application of synthetic fibers in concrete industry has surged to take advantages of their impacts on mechanical properties like enhancing toughness and impact resistance, decreasing shrinkage and even benefits from replacing of thermal reinforcing bars and steel fibers with these fibers. Nonetheless, there is paucity of practical studies conducted on the durability aspects of using these types of fibers in concrete structures under stressor medium. This research aims to investigate the impacts of three synthetic fibers with different physical properties and variable volume percentages on water permeability under pressure, electrical resistivity, half-cell test and water absorption. Moreover compressive and flexural strengths are measured. Results manifest that poly-olefin macro fibers had the best short term durability performance among those three fibers have been tested.

A new class of high-performance fiber reinforced cementitious composite, namely Engineered Cementitious Composites (ECC) reinforced with Polyvinyl alcohol (PVA) fibers, is characterized by strain-hardening, multiple-cracking and tight crack width. Due to the high cost of imported PVA fibers, the practical applications of ECC are limited, especially in China. In the present study, different oiling agents are utilized to modify the local PVA fiber surface. A direct single fiber pull-out method was utilized to study the interfacial behavior after fiber surface treatment. The composite flexural performance was studied using a three-point bending test. Different flexural parameters such as first-cracking and post-cracking strength, first-cracking toughness and post-cracking toughness, ultimate load and toughness indices were determined from the load–deflection curves. The oil treatments have proven effective in modifying the fiber-matrix interface with the reduced chemical debonding energy ( G d ) values of 50%–85% in comparison to untreated PVA fiber. Chemical debonding energy as low as 2.8 J/m2 and frictional bond strength ( τ 0 ) 1.8 Mpa have been observed. Flexural strength varies between 4.5 Mpa and 8.2 Mpa depending on the oiling treatment. The oiling treatments have shown increased toughness values and toughness index up to I60 by using 2% volume of fibers. With most of the treated fibers have shown strain-hardening performance, presenting possible potential of utilizing Chinese PVA fiber in the design of cost-effective strain-hardening cementitious material.

A concrete mixture consisting of steel fibers (S) and polyvinyl alcohol fibers (PVA) has been proven to be an effective type of hybrid fiber-reinforced concrete (HyFRC). To further investigate the engineering potential of S-PVA HyFRC, this paper studies the effects of different factors on mixture performance, and establishes uniaxial constitutive models through an orthogonal experiment. A total of fifteen factors were incorporated. Slump test and various mechanical tests were conducted. Then, a methodology to assess the parameter combination considering various performance indicators was proposed and used in this study to maximize concrete performance. Finally, the generalized and the simplified compressive and tensile constitutive prediction models of S-PVA HyFRC were established and the predicted curves were compared with the test curves. The results show that the interaction factors are not negligible in S-PVA HyFRC. Cast process significantly influences mixture workability and strength, but it has no impact on toughness. Parameter combination assessments among five performance indicator categories were performed and the assessment methodology was verified. The generalized constitutive prediction model was also verified by the test curves. Nevertheless, the simplified model may cause large errors and should only be used to roughly estimate the mixture property in the preliminary design stage of an engineering project.

The effects of water pressure in the crack on the fracture parameters of a concrete beam under three-point bending were discussed. A novel sealing device for hydraulic loading was proposed to simulate the hydraulic load in a test beam. Experimental results showed that the proposed novel sealing device had less effect on the crack propagation in concrete. Then, 12 three-point bending tests of concrete beams with different water pressure in the crack were conducted. A method for determining the fracture parameters of concrete within water pressure was introduced. Moreover, an analytical method based on the relationship of the water pressure and crack width was employed to determine the water pressure distribution along the crack surface. The experimental results indicated that the water pressure applied in the crack had little effect on the initiation fracture toughness but decreased the unstable fracture toughness of concrete. Moreover, as water pressure increased from 0.0 MPa to 0.3 MPa, the unstable fracture toughness decreased by 17.5%, and the cohesive fracture toughness decreased by 21.8%.

The bond behavior between deformed bar and ECC has a significant effect on the mechanical performance of reinforced ECC structures when subjected to dynamic loading. In this paper, 12 groups of ECC specimens and one group of concrete specimen as control were prepared to investigate the bond behavior of deformed bar under cyclic loading. Designed parameters included compressive strength, flexural toughness, cover thickness and anchorage length. Experimental results showed that, owing to the fiber bridging effect preventing the opening and propagation of internal cracks, failure modes of ECC specimens demonstrated an obvious ductility whereas the brittle splitting failure occurred for concrete specimen. The bond strength of steel bar in ECC had an extreme enhancement as the compressive strength of matrix increased. In comparison with the monotonic specimen, the cyclic specimen undergoes an obvious degradation in terms of bond strength as well as stiffness, and the degradation coefficient of bond strength ranged from 0.6 to 0.9. It was noticed that the increase in compressive strength and cover thickness can reduce the degradation degree of bond strength. Furthermore, the energy dissipation capacity of cyclic specimens rose first and then fell since the toughness of matrix increased. Finally, a calculating equation of bond strength under cyclic loading was proposed and the predicted results had a great agreement with the test results.

Given the different hydration processes of normal concrete without accelerator, shotcrete with accelerator has not only a short setting time and high early mechanical properties, but also different hydration products and microstructures. In high latitude and altitude areas, shotcrete lining structures are subject to freeze–thaw cycles. Thus, the durability, bearing capacity, and service life of lining shotcrete is critically threatened. In this work, a freezing–thawing durability experiment was conducted using the accelerated frost method. Dynamic elastic modulus, weight, compressive and splitting tensile strength, and the stress–strain relationship of frost–damaged shotcrete were measured for a study of the durability and mechanical property degradation rules. Microstructures, through pore structure analysis and microscopic characterization, were identified for elucidating the deterioration mechanism of the mechanical properties of shotcrete. After frost damage, the volume percentage of harmless pores rapidly decreased, whereas porosity increased. Micro-cracks appeared in interfacial transition zone grew into mortars and connected to form main cracks, which accelerated the microstructure damage. Physical and mechanical properties of shotcrete decreased with prolonged freeze–thaw cycles. However, the peak stress and toughness of shotcrete decreased, whereas peak strain, initial elastic modulus, and ultimate compression strain increased. Therefore, steel fiber reinforced shotcrete had improved frost durability and reduced growth ratio of porosity under the bridging effect.

This paper presents an experimental study of the corrosion behavior of reinforced concrete/ultrahigh toughness cementitious composite (RC/UHTCC) beams under simultaneous loading and shrinkage cracking, and of the flexural performance of these beams after corrosion. During corrosion, a two-stage corrosion test was applied – wet/dry cycles of chloride solution first and then accelerated corrosion using impressed current. Corrosion potential in the first corrosion stage and corrosion electrical resistance and corrosion pattern in the accelerated corrosion stage were measured, and rebar corrosion appearance was observed after bending tests. The results indicated that the corrosion potential decreased rapidly after exposure to chloride solution while the corrosion electrical resistance increased with corrosion time. No corrosion-induced longitudinal cracks occurred at corrosion levels up to 16.32%, but severe pitting corrosion of reinforcement was observed at transverse cracks. The load-carrying capacity, deformation, ductility, and flexural crack pattern of corroded RC/UHTCC beams were degraded greatly due to pitting corrosion effect. Ductile concrete crushing failure mode was transformed to brittle rebar rupture failure mode. The sustained loading action further aggravated the degradation in flexural capacity of corroded RC/UHTCC beams.

Fiber reinforced shotcrete (FRS) is widely used for tunnel construction. However, the systematic control of FRS properties is hampered by the complexities of the experimental procedures used. The experiments are normally based on the load-deflection response obtained from flexural tests with third-point loading performed under displacement control. These types of tests are characterized by instability when the cracking load is reached and, subsequently, errors occur in the deflection measurements, increasing the dispersion of the results. An alternative test, the Barcelona test, has some experimental advantages for FRS control as the use of much smaller specimens, an easy procedure and a lower scatter. Using the mean crack opening, correlations were established between the Barcelona test and the flexure test to estimate the toughness and residual strengths at a deflection of 3.0 mm. Equivalences between the two tests were obtained based on the laboratory results and were validated based on work site results, with differences of less than 5% of the residual strength. These relationships and advantages have allowed the Barcelona test to be proposed to control the properties of the FRSs used in the Chuquicamata Underground (Chuquicamata Subterránea) Project developed by the mining company CODELCO-Chile.

Synergistic effect of different fibers on the improvement of fracture behavior of concrete is a widely accepted phenomenon. However, the fracture mechanisms behind their improvement need to be clearly understood for optimizing the fiber dosage. In this work, the fracture behavior of synthetic fiber-reinforced concrete (SynFRC), steel fiber reinforced concrete (SFRC) and hybrid fiber reinforced concrete (HFRC) under flexural loading is studied using acoustic emission (AE) technique. Three different fiber dosages, by volume of concrete, of 0.50%, 0.75% and 1.0% and their hybrid combinations were considered as study parameters. Test results show that it is possible to retain sufficient workability in the fresh state and obtain good fracture resistance through a hybrid combination of hooked end steel fibers and macro- synthetic polyolefin fibers. AE parameters were investigated to acknowledge the efficiency of the fibers in improving the fracture behavior and toughness of SynFRC, SFRC, and HFRC. AE parameters such as hits, events, AE energy, and 3D-crack source locations were presented to illustrate the role of different fiber dosages on the Mode-I fracture response. The source locations of AE event above the artificial notches created at the mid-span of specimens were investigated and classified into tensile or shear cracks. AE and fracture energy increased with fiber dosage. SFRC demonstrated higher energy dissipation capacity followed by HFRC and SynFRC.

Subsurface micro-cracks in rock, detrimental to long-term structural integrity due to potential moisture or water penetration and stress concentration, should be carefully assessed and repaired in service. In this study, we propose a resin pre-coating (RPC) technique for sealing the sub-surface micro-cracks that normally cannot be accessed by adhesive bonding. The subsurface micro-cracks with narrow openings are penetrated by a specially prepared solution of resin (without hardener) and acetone through capillary action. After evaporation of acetone, the resin is left to fill the micro-cracks. The normal resin and hardener mixture is then applied. The diffusion process will grow the adhesive joint deep into those micro-cracks leading to stronger adhesive bonding. Notched three-point-bending (3-p-b) granite specimens were first tested to fracture, and then bonded together using epoxy adhesive after RPC. Short Aramid fibers (SAF) were also used to strengthen the brittle epoxy adhesive joint. After removing potential premature failure sites by sealing those sub-surface micro-cracks, the peak loads of repaired samples were 35% higher than those measured from the original granite samples, and the total fracture energy consumption was 118% higher than the original value.

Ultra high toughness cementitious composites suitable for spraying (sprayed UHTCCs) is characterized by strain-hardening property under direct tensile stress with tight crack-control ability. It has a great potential in the application of repairing and retrofitting cracked or corroded concrete structures. The utilization efficiency of sprayed UHTCC is significantly affected by the integrity of the repair or strengthening system, which depends on the interfacial bonding behavior between the sprayed UHTCC and concrete substrate. A Type L934 orthogonal experiment and scanning electron microscope (SEM) observation were performed to investigate the bond behavior between sprayed UHTCC and concrete substrate, where three factors, namely sprayed direction, surface roughness of concrete substrate, and application of primer, were taken into consideration with three test levels for each of the former two factors and two levels for the other factor provided. The results demonstrated that the spraying direction had a significant influence on the bond splitting tensile strength and a quite significant influence on the interfacial shear strength between sprayed UHTCC and concrete substrate, and the surface roughness of concrete substrate had significant influence on the interfacial shear strength but not much significant influence on the bond splitting tensile strength. It had little influence on the bond behavior between sprayed UHTCC and concrete substrate whether a thin layer of primer, which contained the same matrix of sprayed UHTCC without polyvinyl alcohol (PVA) fibers, was adopted or not.

In this study, the use of dispersed multi-walled carbon nanotubes (MWCNTs) to enhance the mechanical properties, especially the initial cracking fracture toughness, of the steel-polyvinyl alcohol hybrid fibers ultra high toughness cementitious composite (UHTCC) was reported. The influences of the MWCNTs contents (0.00%, 0.05%, 0.10%, 0.20% and 0.40% by the cement weight) were investigated by the fluidity test, compression test and uniaxial tensile test. An optimal concentration of 0.10 wt% was obtained, at which the compressive strength (to 60.8 MPa), tensile strength (to 6.8 MPa), elastic modulus (to 15.2 GPa), ultimate tensile strain (to 0.960%) were enhanced by about 18%, 42%, 12% and 16%, respectively. The pre-notched three-point bending beam tests were then conducted to study the improvements of the fracture properties at the optimal MWCNTs content, based on the double-K fracture model and the standard toughness index method. The initial cracking fracture toughness KIcini (to 1.8 MPa·m1/2) was improved by about 21%. The defined effective fracture toughness KIcun− (to 8.1 MPa·m1/2) and KIcun+ (to 20.5 MPa·m1/2) were improved by about 14% and 58%, respectively. The residual tensile strengths fR,1-CMOD (to 23.2 MPa), fR,2−CMOD (to 26.1 MPa), fR,3−CMOD (to 24.6 MPa) and fR,4−CMOD (to 20.7 MPa) were increased by about 24%, 27%, 47% and 59%, respectively. The toughening effect at the peak load state may imply that the bonding strengths between the hybrid fibers and the cement matrix were enhanced by MWCNTs addition. The scanning electron micrograph was used to observe the microstructures of the MWCNTs-enhanced hybrid fibers UHTCCs. Observations showed that the MWCNTs can act as nucleation spots to accelerate the hydration reaction, be embedded in the hydrates to form a more compact 3D cross network, and arrest the nano-cracks due to their bridging and load-transfer abilities.

Anchor bolts are often fixed into a concrete soffit of structures and they are used in ambient and cold store locations. The chemical anchor bolt relies purely on the tensile strength of the concrete to carry the imposed load, assuming the bond strength of the resin is greater than the tensile strength of the concrete.\ud \ud The properties of concrete are changed by the addition of both steel and polypropylene fibres. This paper investigates the relative performance of each fibre type with regard to initial and final post crack failure. Anchor bolt pull-out testing was used to determine the maximum load a fixing can hold as well as the residual post crack toughness of a bolt embedded in a concrete block. The concrete used was a C40 design mix and resin anchor bolts were selected for this test for their stress-free conditions prior to loading.\ud \ud The results showed that the addition of both types of fibres when used in concrete improved the maximum load and toughness of the samples, compared to plain concrete. There was not a significant difference between the results obtained for steel and polypropylene fibres. The effects of a reduction in core temperature of the samples was examined. The results show that the strength of concrete is significantly improved when tested at −20 °C, compared to ambient temperature.

To minimise disruption due to repairs of concrete pavements, rapid hardening and tough materials need to be used. This paper investigates the flexural performance of rapid hardening mortar mixes made with two commercial cement types, calcium sulfo-aluminate cement and calcium aluminate cement, for thin concrete repair applications. Three-point bending tests are performed on plain and steel fibre reinforced concrete specimens containing 45 kg/m3 of recycled clean steel fibres to characterise the flexural performance of notched and unnotched prisms at different ages, ranging from one hour up to one year. The recycled fibers are shown to enhance both the flexural strength and toughness of FRC prisms, leading to hardening behaviour. Constitutive equations based on the RILEM and Model Code 2010 recommendations are found to overestimate the loading capacity of the bending tests. FE analyses using multilinear σ-ɛ tensile curves obtained by employing inverse analysis can capture better the post cracking strength and cracking pattern of the tested prisms.

Aiming to improve the flexural and compressive strength of cement, graphene oxide (GO) was incorporated into the production of cement-based composites in this study. The effects of the GO content and water/cement (w/c) ratio on the mechanical properties and microstructure of GO–cement composites were systematically investigated through experimental tests. Testing results show that with a constant w/c ratio, the flexural and compressive strengths of the developed GO–cement composites increased first and then declined with respect to the increase in GO content. Specifically, the increase in flexural strength was more significant than that in compressive strength, indicating that the addition of GO could help improve the toughness of cement composites. Additionally, a combination of a w/c ratio of 0.35 and a GO content of 0.05 wt% led to both the highest flexural and compressive strengths. The investigation of the composition–microstructure–strength relationship shows that the introduction of GO could help improve the macroscopic properties for the cement-based composites via improving the microstructure of cement hydration products, refining the crystal size, and forming a denser and more uniform network structure. The filling effect, hydration effect and nucleation effect of GO could be optimized with an appropriate combination of w/c ratio and GO content.

For further investigating the dynamic size effect of roller compacted concrete (RCC), this paper focuses on the concrete fragmentation under impact loadings, which compensates the one-sidedness of pure strength and deformation analysis. 101 RCC specimens with different size have been prepared and tested under impact loadings. Then, more comprehensive understandings on size-dependence and strain-rate sensitivity of dynamic behaviors for RCC can be obtained in terms of failure process, fragment size distribution, and fractal characteristics. The experimental results indicate that the dynamic fragmentation process is highly related to the multiaxial stress state in the specimens. The fragment size distribution can be used to depicture the dynamic behaviors of RCC, indicating the tight correlation among crack propagation, energy dissipation and failure strength. The suggested exponential growth models, interpreting dynamic compressive strength and toughness with fractal dimension, provide an effective way to evaluate the dynamic behaviors of RCC based on the fragment size distribution.

Nowadays, hybrid fiber reinforced concrete is being considered for structural applications due to its enhanced mechanical properties compared to concrete without fibers/plain concrete. In this work, the mechanical properties of new kind of hybrid fiber reinforced concrete, i.e. CaCO3 whisker-steel fiber-basalt fiber reinforced concrete (CSBFRC) with various basalt fibers percentages are studied. All the fiber-reinforced concretes are compared with that of plain concrete (PC). The CaCO3 whisker, steel and basalt fiber lengths are 20–30 μm, 35 mm and 12 mm, respectively. Steel fibers and CaCO3 whiskers contents are 0.32% and 0.9%, by volume, respectively. Various basalt fiber contents of 0.34%, 0.68%, 1.02% and 1.36%, by volume, are added. For each batch, cylinders and beam-lets are cast and tested under respective compressive, splitting tensile and flexural load as per ASTM standards. Stress-strain curves and load-deflection curves are obtained. Strengths, energy absorptions and toughness indices are determined against for each type of loading. The scanning electron microscopy (SEM) analysis is performed to reveal the behavior (interfacial bonding) of CaCO3 whiskers, basalt fibers and steel fibers. It is concluded that, with increasing content of basalt fibers up to 0.68%, there is an increase in the mechanical properties of hybrid fiber reinforced concrete and CSBFRC4 is found to be an optimum. However, beyond 0.68%, the mechanical properties of CSBFRC decrease with an increase in the basalt fiber content. The resistance against cracking provided by hybrid fibers is observed by SEM images.