Showing papers in "Cement & Concrete Composites in 2020"

Fiber-reinforced geopolymer composites: A review

Abstract: There is a burgeoning interest in the development of geopolymers as sustainable construction materials and incombustible inorganic polymers. However, geopolymers show quasi-brittle behavior. To overcome this weakness, hundreds of research have been focused on development, characterization, and implementation of fiber-reinforced geopolymers for a wide range of applications. This paper discusses the rapidly developing state-of-the-art of fiber-reinforced geopolymer composites, focusing on material and geometrical properties of construction fibers, and underlying mechanisms on fiber-binder interaction at fresh and hardened states, mechanical properties, toughening mechanisms, thermal characteristics, and environmental durability. It is intended to build a strong conceptual and technical background for what is currently understood on fiber-reinforced geopolymers by tying the subject together with knowns for other similar cementitious composites rather than a historical report of literature.

Utilization of CO2 curing to enhance the properties of recycled aggregate and prepared concrete: A review

Abstract: In recent years, the CO2 curing technique has been developed to enhance the properties of recycled aggregate (RA) and recycled aggregate concrete (RAC), materials that can absorb the CO2 gas and are more sustainable for the construction industry. A body of literature on the CO2 curing of RA and RAC is currently available, but a systematic review is lacking. Therefore, this paper reviewed the published literatures on the use of CO2 curing to enhance the properties of the RA and prepared RAC. The studies on CO2 curing technology, the properties of carbonated recycled aggregate (CRA), and the micro-properties, workability, mechanical properties and durability performance of concrete with CRA are respectively reviewed. The results showed that the RA properties, CO2 concentration and pressure, relative humidity and curing time all had a significant impact on the properties of CRA. Carbonation treatment improved the pore structure of RA and reduced its porosity, and the workability, mechanical properties and durability performance of prepared RAC were also improved. In addition, an outlook on the CO2 curing of RA and prepared concrete was presented, and we expected that this work may inform further investigation of the use of CO2 curing to enhance the properties of concrete and cement products.

The utilization of eco-friendly recycled powder from concrete and brick waste in new concrete: A critical review

Abstract: Recycled powder (RP) is the main by-product in the reclamation of construction and demolition (CD in addition, improvement methods and a benefit evaluation of RP concrete are further introduced. Based on statistical data that describe the activity index of RP and the compressive strength of RP concrete, the median diameter and replacement ratio of RP in concrete preparation should be below 30 μm and 30%, respectively. The use of RP improves the durability of concrete when the RP fineness is superior to the cement fineness. Increasing the RP fineness is an effective approach to improving the properties of RP concrete, and a CO2-curing treatment of RCP is an eco-friendly modification method. Furthermore, the use of RP in concrete has good economic and environmental benefits. Therefore, one expects that this review helps the further use of RP in concrete.

Mechanical and fracture properties of ultra-high performance geopolymer concrete: Effects of steel fiber and silica fume

Abstract: This study investigates the effects of steel fiber and silica fume on the mechanical and fracture properties of ultra-high performance geopolymer concrete (UHPGC). Four volume fractions of steel fiber (0%, 1%, 2% and 3%) and four contents of silica fume by the mass of total binders (5%, 10%, 20% and 30%) were used. The mechanical and fracture properties evaluated include the compressive, splitting tensile and ultimate flexural strengths, modulus of elasticity, flexural behavior, fracture energy and stress intensity factor. In addition, the correlations among the compressive and splitting tensile strengths, and compressive strength and elastic modulus were studied. The results indicated the increase of steel fiber dosage resulted in the decrease of the workability, but the continuous improvement of mechanical and fracture performance of UHPGC. The empirical equations for predicting elastic modulus of conventional ultra-high performance concrete overestimated the elastic modulus of UHPGC, however some prediction formulas for the splitting tensile strength of PC-based concretes could be applied for UHPGC. Silica fume had a complicated influence on workability and hardened properties of UHPGC, which is strongly dependent on its amount. The inclusion of 10% silica fume induced the increase of the flowability, but the sharp degradation of the mechanical performance, while the specimens with 20% and 30% silica fume possessed the superior mechanical characteristic to that with 5% silica fume. The steel fiber dosage could be decreased without sacrificing the mechanical and fracture performance of UHPGC, via the increase of silica fume content.

A review on the deterioration and approaches to enhance the durability of concrete in the marine environment

Abstract: This paper presents a review of the deterioration of concrete under seawater attack with particular interests in field exposure. The research reported in the literature has shown that salinity of seawater in different areas varies considerably but the type of ions and their proportion are similar. Because of this variation, laboratory studies should use specific artificial seawater to simulate on field environments. The phase changes induced by chloride, magnesium and sulfate ions contained in seawater are reviewed. The interaction between hydrates and chloride ion can lead to the formation Friedel's and Kuzel's salts. Magnesium ion can replace the calcium in Portlandite, and lowers the alkalinity of pore solution and eventually destabilizes C-S-H gel. The expansive ettringite is inhibited at the presence of chloride ions. At the tidal zone, the phase change mainly occurs on the surface of concrete, which weakens the structure and leads to spalling and delamination under the physical attack of the wave. Based on the existing deterioration mechanisms, the protocols to enhance the durability performance of marine concrete are also reviewed, such as using supplementary cementitious materials (SCMs) to mitigate rate of chloride penetration and, more promisingly, to use alternative binder systems. This paper also proposes a concept of designing a more durable concrete cover system by enhancing the chemical stability of cement hydrates, rapid self-healing and intelligent alkalinity control.

Hardened properties of layered 3D printed concrete with recycled sand

Abstract: 3D concrete printing has received worldwide attention while the development on new cementitious material compositions for 3D printing is inadequate This study employed the recycled sand instead of natural sand to achieve 3D concrete printing and investigated the hardened properties of this extrusion-based material The effect of replacement ratio of recycled sand, curing age, nozzle height and anisotropic behavior were evaluated based on the compressive tests, tensile splitting tests and flexural tests Moreover, the digital image correlation (DIC) technique was adopted to capture the strain behavior and failure pattern of this layered and printed concrete Owing to the high un-hydrated cement paste attached to the recycled sand and the internal curing mechanism, the compressive and flexural strengths of the 3D printed concrete with recycled sand were a little lower than those specimens without recycled sand The compressive, tensile splitting and flexural strength of 3D printed concrete with recycled sand had obvious anisotropy The replacement of recycled sand had limited effect on the anisotropy of compressive and flexural strength, but had certain effect on the tensile splitting strength Since recycled sand is one of those major products derived from the construction and demolition waste, it is believed that the employment of recycled sand to the mix of 3D printed concrete will significantly improve the sustainability of 3D printed concrete structures

Chloride diffusion resistance and chloride binding capacity of fly ash-based geopolymer concrete

Abstract: This study evaluated the chloride diffusion resistance of low-calcium fly ash-based geopolymer concrete through electrical and bulk diffusion techniques. The geopolymer concretes were prepared using 12 different heat curing conditions; three temperatures of 60, 75 and 90 °C and four curing durations of 8, 12, 18 and 24 h, as well as ambient curing. The mechanical and transport properties and microstructural characteristics of the geopolymer concretes were examined. NT BUILD 492 chloride migration and ASTM C1556 bulk diffusion tests were carried out. Results showed that the chloride diffusion resistance and the chloride binding capacity of fly ash-based geopolymer concrete is very low. The fly ash-based geopolymer concrete appears to be suitable for applications where there are little or no chloride-related durability concerns.

Development of ultra-high performance geopolymer concrete (UHPGC): Influence of steel fiber on mechanical properties

Abstract: This study reports the development of ultra-high performance geopolymer concrete (UHPGC) and overcoming the brittleness feature of geopolymer matrix by using different steel fibers. Four straight steel fibers with different aspect ratios and two different deformed steel fibers were investigated. Flowability, compressive strength and flexural behavior including strengths and deflection, and energy absorption capacity of UHPGC, were systematically evaluated. A deformation ratio of steel fiber was introduced to quantitatively correlate the steel fiber shape and the mechanical performance. The flowability of fresh UHPGC mixtures decreased when the fiber content and length increased, as expected, and was inconspicuously influenced by fiber shape. The increase in fiber content and the decrease of fiber diameter contributed to the improvement of the mechanical strengths of UHPGC. The flexural behaviors of UHPGC improved as the fiber volume and length increased, while the compressive and first crack strengths were affected by both curing conditions and fiber dosages as well. Different from Portland cement-based composites, the corrugated fibers with a higher deformation ratio added in UHPGC, had an inferior strengthening and toughening efficiency, while for straight fibers, those longer and smaller in diameter were more preferred. Finally, based on the previous research, a new one with adjustment and simplification was proposed for that of newly-developed UHPGC, and the fitted results had higher correlation coefficients (r2).

Spalling behavior of metakaolin-fly ash based geopolymer concrete under elevated temperature exposure

Abstract: Fire-induced spalling is a serious risk to concrete structures, especially for high strength concrete structures. This paper presents results from high temperature spalling tests on geopolymer concrete. The effect of moisture content, concrete strength, heating rate and temperature level on the spalling behavior of geopolymer concrete is studied. The temperature-induced spalling mechanism in geopolymer concrete is investigated through the measurement of residual compressive and splitting tensile strength, variation in permeability (by sorptivity test) and chemical composition (by X-ray diffraction test) of geopolymer concrete after elevated temperature exposure up to 700 °C. The test results indicate that geopolymer concrete exhibit a good spalling resistance as compared to that of OPC concrete. The lower spalling risk in geopolymer concrete under high temperature exposure is facilitated from the highly connected pore structures and lower strength degradation with temperatures. Further results indicate that the pore structure (permeability) of geopolymer concrete gets a significant evolution with the exposure temperature, especially above 500 °C range. This is related to the sintering reaction in geopolymer binders at high temperatures.

Interlayer bonding improvement of 3D printed concrete with polymer modified mortar: Experiments and molecular dynamics studies

Abstract: The inherent weak interfaces between adjacent layers by the laminate stacking process of 3D concrete printing hinder this advanced technique for general engineering applications. In this study, polymer-modified mortars were fabricated to improve the weak interlayer bonding performances. The effects of the setting time of the printed concrete and changes of the interlayer moisture were tested. Both epoxy resin- and chloroprene latex-modified mortars were prepared and used as interlayer interface enhancement materials according to different interlayer intervals. The direct tensile and shear strengths of the polymer mortar-strengthened interfaces were tested and evaluated by the crossover test. Moreover, the effects of the moisture released from the surface of the cement-based composite materials on the interlaminar bonding properties and the enhancing mechanism of the epoxy resin- and chloroprene latex-modified mortars were simulated through molecular dynamics and density functional theory. The results demonstrate that the electrostatic interaction (Coulomb force) between the epoxy resin and calcium ions from hydrated calcium silicate counteracts the weakening effect of water molecules (surface moisture) on the interlaminar bonding, thus improving the interface bonding of printed concrete. The optimal polymer mortar that strengthens the interfaces most effectively was identified.

Mechanical properties and water absorption of cement composites with various fineness and contents of waste brick powder from C&D waste

Abstract: The use of waste brick powder (WBP) as supplementary cementitious material (SCM) provides an effective approach to reclaiming construction and demolition (CD when the WBP fineness is higher than the cement fineness, the compression strength with WBP contents up to 15% is superior to that without WBP, while the compression strength decreases with WBP incorporation when the WBP fineness is close to or lower than the cement fineness. Incorporating an appropriate WBP content decreases the water absorption of mortar, and the water absorption further decreases with increasing WBP fineness; for example, when the median diameter of WBP is 6 μm and 42 μm, the capillary absorption coefficient of mortar with 30% WBP is 34.1% lower and 10.3% higher than that of plain mortar, respectively. In addition, a similar conclusion is observed for the water distribution in WBP mortar.

Performance of recycled aggregate concrete with rice husk ash as cement binder

Abstract: This research aims to utilize rice husk ash as a cementitious materials in recycled aggregate concrete (RAC). Rice husk ash was ground until the particles remained on a No. 325 sieve were 4.6%wt. Then, the ash was used to partially replace cement at 20 to 50%wt of binder to cast concrete. The compressive strength, steel corrosion, and chloride penetration depth by the impressed voltage method of RAC were examined. The results revealed that the replacement of 20% of ordinary Portland cement (OPC) by ground rice husk ash (GRHA) enhances the compressive strength of the RAC to be greater than the RAC without GRHA at 60 days. Concrete with GRHA at 20 to 50%wt of binder significantly improved the steel corrosion and chloride resistance of the RAC. The utilization of GRHA at 50% to replace OPC gave the highest chloride penetration resistance and produced the lowest steel corrosion of the RAC. Although, the RAC with GRHA had less compressive strength than CT concrete, the concrete provided a positive effect of increasing the resistance of chloride penetration and lowering steel corrosion.

Evaluating the printability of concretes containing lightweight coarse aggregates

Abstract: This paper examines the printability of concrete formulations made using lightweight expanded clay aggregates (LECA) of maximum size 10 mm. The effect of coarse aggregate addition on the two essential aspects of printing, i.e., extrudability and buildability are examined using a piston-pump-based 3D printer system. The extrudability is assessed by measuring the desorptivity, a fundamental material parameter for assessing the phase separation tendency during the extrusion process. It is found that increasing the coarse aggregate content increases the desorptivity, thereby decreasing the water retention capacity. Mixes with up to 30% volume substitution with coarse aggregate were extruded successfully. However, for mixes with volume substitution higher than 30%, a significant amount of phase separation is found to occur during the extrusion process, resulting in blockages. To assess the effect of coarse aggregate on buildability, uniaxial compression tests were carried out on concretes after different ages of casting. For both the mixes with and without coarse aggregates, the material behaviour changes from an elasto-plastic type response at early ages to a strain softening type response (typical of hardened concrete) at later ages. At any given age, it is found that the addition of LECA, increases both the strength and elastic modulus of the concrete. This may be attributed to the increase in the internal friction angle as the size of the aggregate increases, as well to the higher amount of dewatering that occurs during the extrusion of mixes containing coarse aggregates resulting in a relatively stiffer mix after extrusion. Finally, discussions are provided on the effect of using low-density aggregates like LECA on improving the resistance to both plastic and buckling type collapse that may occur during printing.

Accelerated carbonation of reactive MgO and Portland cement blends under flowing CO2 gas

Abstract: The use of MgO-based materials for sequestration of CO2 offers technical advantages and environmental incentives. However, the understanding of accelerated carbonation of MgO-based materials in flowing CO2 gas is limited. This study elucidates the carbonation behaviour of reactive MgO cement (MC) and MgO-Portland binary cement (BC) in a simulated CO2-rich industrial exhaust. Quantitative X-ray diffraction and thermogravimetric analyses showed that nesquehonite (MgCO3·3H2O) was the major carbonation product in MC pastes, whereas CaCO3 was preferentially generated in BC pastes. The relative humidity of exhaust gas influenced CO2 diffusion and the carbonation rate; 98% humidity facilitated MC carbonation whereas 50% was favourable for BC carbonation. Although CO2 concentration determined the carbonation rate, 10% CO2 gas in the exhaust was sufficient to accelerate carbonation. The carbonation degree and compressive strength of samples cured for 7 days at 10% CO2 were comparable to the values of samples cured for 1 day at 100% CO2. The presence of acid gases during CO2 curing inhibited the carbonation and hydration processes, but the presence of Portland cement in the BC system gave good compatibility with acids and relieved the inhibitory effect. Desulphurization and denitrification of industrial exhaust gas are nonetheless desirable before CO2 curing. This study builds the foundation for utilising industrial CO2 exhaust to accelerate the carbonation of Mg-based materials.

Investigation on expansion effect of the expansive agents in ultra-high performance concrete

Abstract: In this study, the expansion mechanisms of calcium-sulfoaluminates -CaO based expansive agent (CSA-CaO EA) on the volume stability of ultra-high performance concrete (UHPC) were systematically studied. The test results show that the incorporation of CSA-CaO EA is beneficial to reduce the autogenous shrinkage of UHPC effectively due to the formation of additional Ca(OH)2 and ettringite. However, the reduction of early autogenous shrinkage is limited and the cracking risk is still very high when the addition of CSA-CaO EA reaches 15% of binders. According to the results, the factors hindering the expansion effect of CSA-CaO EA include the low w/b ratio, compact microstructure, out-sync of time “window” of expansion and shrinkage and the potential exchange of water between solids. Strategies and experimental validations on improving the shrinkage compensation are discussed. It indicates that the expansive UHPC can be prepared by using an CaO based EA with high reactivity and low water consumption via controlling the time “window” of expansion.

Comparative durability of GFRP composite reinforcing bars in concrete and in simulated concrete environments

Abstract: Many studies suggest that the durability of glass-fiber-reinforced-polymer (GFRP) bars in a simulated concrete pore solution is very different than the same bars in an actual concrete environment. This study conducted a comparative evaluation of the durability of GFRP bars in concrete and in simulated concrete environments by investigating their interlaminar shear strength. It focused on evaluating the physical, mechanical, and microstructural properties of GFRP bars subjected to high moisture, saline, and alkaline environments. Bare GFRP bars and cement-embedded GFRP bars were immersed in solutions at different temperatures (23 °C, 60 °C, and 80 °C) and for different exposure times (28, 56, and 112 days). The results show that the percentage water uptake and the apparent diffusivity of the GFRP bars were strongly dependent on the type and temperature of the immersion solution. The interlaminar shear strength of the GFRP bars directly immersed in a solution degraded more significantly than those embedded in concrete and immersed. Moreover, the alkaline solution was more aggressive to the GFRP bars than tap water or saline solution, affecting bar fiber, matrix interface, and chemical structure. As a result of this study, master curves and time-shift factors were developed to correlate the retention of interlaminar shear strength from the accelerated aging test to the service life of GFRP bars in an actual concrete environment.

Engineered Cementitious Composites (ECC) with limestone calcined clay cement (LC3)

Abstract: Recent research have recognized that coupled use of calcined clay, limestone and cement clinker in concrete is viable to reduce environmental footprints at manufacture and to enhance material durability. In this study, a novel application of the limestone calcined clay cement (LC3) is demonstrated by substituting the Ordinary Portland Cement (OPC) in Engineered Cementitious Composites (ECC). The composite mechanical properties including σ-δ and σ-e relationships and residual crack widths were evaluated to 28 days under uniaxial tension. Matrix chemistry was characterized using thermogravimetric analysis and X-ray diffraction, while the pore structure of matrices and composites was analyzed using mercury intrusion porosimetry. The LC3-based ECC showed more rapid early strength development but lower 28-day strength (~32 MPa) due to a 20% higher water-to-solid ratio for attaining adequate workability and fiber dispersion. Nevertheless, the tensile strain capacity of LC3-based ECC achieved over 6% with an average residual crack width less than 50 μm. Additionally, the composite pore structure exhibited a decreasing volume fraction of large pores and voids (>100 nm) after substituting LC3 for OPC. The use of LC3 marginally decreased the embodied material energy and cost, but led to about 32% and 28% reductions in CO2 emissions compared to traditional OPC-based ECC and concrete, respectively. As a preliminary study, LC3-based ECC shows promise as a greener ductile concrete compared with OPC-based ECC.

Rheological and mechanical properties of cemented foam backfill: Effect of mineral admixture type and dosage

Abstract: In this work, the effect of mineral admixture type and dosage on the rheological and mechanical behaviors of a newly developed backfilling material, cemented foam backfill (CFB), is comprehensively investigated. Fly ash (FA), ground granulated blast-furnace slag (GGBS) and quicklime are selected as mineral admixtures, while H2O2 and CaCO3 whisker are used as foaming agent and foaming stabilizer respectively. CFB samples are prepared with solid content between 70% and 76%, replacement rates of cement with FA or GGBS between 0% and 30% and cured at 7, 14 and 28 days. In addition, 5 wt % of FA or GGBS is replaced by an equal amount of quicklime to accelerate pozzolanic reaction in a portion of CFB samples. Then, fresh CFB is subjected to the rheological property (i.e., apparent viscosity and equivalent Bingham yield stress) test, while hardened CFB is subjected to porosity and mechanical tests. Additionally, thermogravimetric and X-ray diffractogram analyses are conducted on 7- and 28-day hydrated binder pastes (pure cement, cement + FA/GGBS, cement + FA/GGBS + quicklime) to reveal the influence of mineral admixture type and dosage on the hydration product of CFB.

Improvement in corrosion resistance of recycled aggregate concrete by nano silica suspension modification on recycled aggregates

Abstract: This work modified recycled aggregates (RAs) by soaking them in a nano-silica (NS) suspension and evaluated the modification effects on recycled aggregate concrete (RAC). Firstly, different soaking times were studied based on the surface microhardness and the penetration depth of NS particles in the RA. Secondly, mechanical properties (e.g., compressive strength and microhardness of interfacial transition zones) and durability properties (e.g., steel corrosion and corrosion-induced cracking) were tested. The 1-h soaking modification was selected as the optimal soaking time. The nano-modification improved the protection of steel and the resistance to corrosion-induced cracking, which was believed to be related to the improvement of the ITZ properties according to the microhardness test results.

Energy dissipation characteristics of all-grade polyethylene fiber-reinforced engineered cementitious composites (PE-ECC)

Abstract: Systematical research was implemented to explore the impacts of fiber reinforcement index VfLf/df (i.e., the product of fiber volume Vf and fiber aspect ratio Lf/df) on the energy dissipation characteristics including strain energy density and fracture energy, of polyethylene fiber-reinforced engineered cementitious composites (PE-ECC) with compressive strength varying from normal to high. Six VfLf/df values (i.e., 5, 7.5, 9, 10, 15, and 18) and five water/binder ratios (i.e., 0.14, 0.16, 0.18, 0.22, and 0.32) were considered in total. Strain energy density and fracture energy reflect the crack resistance capacity during the strain-hardening and softening processes of PE-ECC, respectively. The strain energy density of PE-ECC increased significantly with the increase of VfLf/df and the decrease of water/binder ratio. The fracture energy increased noticeably with the growth of VfLf/df, while it attained the maximum value at the water/binder ratio of 0.16. For ECC including both the strain-hardening and softening processes, neither the strain energy density nor fracture energy alone can reflect its crack resistance capacity and energy dissipation capacity comprehensively. Thus, in the design and simulation processes of ECC, both energy parameters need to be considered in the constitutive model.

Strength development and microstructural investigation of lead-zinc mill tailings based paste backfill with fly ash as alternative binder

Abstract: The increasing popularity of paste backfilling demands alternative binder optimisation and bulk waste disposal. This study investigates the efficacy of fly ash (FA) as partial replacement of ordinary portland cement (OPC) for paste backfill application in underground mines. The effect of FA addition on uniaxial compressive strength (UCS) and cohesion of paste backfill are demonstrated. The strength development was correlated with microstructural evolution using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS). The study revealed that the rate of strength development in paste backfill slowed down when OPC was substituted by FA. However, the targeted 28 days' UCS of 1.1 MPa for the backfilling stope of lead-zinc mine was achieved with binder dosages of 8 wt% OPC, 7 wt% OPC, 6 wt% OPC, 7 wt% OPC +1 wt% FA and 6 wt% OPC +2 wt% FA. Thus, FA is a suitable binder substitute and up to 25% of OPC (8 wt%) can be replaced with FA. Analysis of microstructural evolution in paste backfill revealed that calcium silicate hydrate (C–S–H) did not develop in OPC-FA binder based paste backfill at its early days’ of curing and gypsum was found only in samples with OPC as sole binder. The multiple linear regression analysis on interaction effects of OPC, curing time, FA replacement percentage for 8 wt% and 5 wt% binder groups indicated that the UCS development is more sensitive towards FA replacement. The obtained results would help in better understanding and design of paste backfill for lead-zinc underground mines.

Mechanical and durability performance of mortars with fine recycled concrete aggregates and reactive magnesium oxide as partial cement replacement

Abstract: In this paper, mortar specimens were produced using two types of commercially-available reactive MgO as partial cement replacement (10%, 15% and 20%, by weight) and fine recycled concrete aggregate as siliceous sand substitute (50% and 100%, by volume). The specimens were subjected to thermogravimetric, energy dispersive X-ray, differential thermal and powder X-ray diffraction analyses. The mechanical and durability performance of all specimens was evaluated in terms of their flexural and compressive strength, dynamic modulus of elasticity, water absorption by capillary action, carbonation, and shrinkage. The main results indicate an overall decline in mechanical and durability-related performance with the use of both MgO and fine recycled concrete aggregates, but enhanced shrinkage behaviour was observed in all MgO-containing specimens.

The role of limestone and calcined clay on the rheological properties of LC3

Abstract: Understanding the rheological properties of cementitious materials is important for controlling their fresh properties and for improving the practical applications. In this study, the rheological properties of pastes made from ordinary Portland cement blended with different amounts of calcined clay and limestone were investigated in order to understand the combined and independent effects of limestone and calcined clay on the rheological properties of limestone calcined clay cement (LC3). Large Amplitude Oscillation Strain was applied and the subsequent harmonic distortion was used to evaluate nonlinear response for the first time for cementitious materials. The results showed that calcined clay leads to increased static and dynamic yield stress, initial thixotropic index, plastic viscosity and cohesion, as well as decreased harmonic distortion, while limestone has an opposite effect. It is believed that these results will aid in understanding the viscoelasticity of this blended cement.

High-strength seawater sea-sand Engineered Cementitious Composites (SS-ECC): Mechanical performance and probabilistic modeling

Abstract: Engineered Cementitious Composite (ECC) is an advanced fiber-reinforced concrete exhibiting multiple-cracking and strain-hardening under tension. This study aims to explore the feasibility of producing high-strength seawater sea-sand Engineered Cementitious Composites (SS-ECC) for marine and coastal applications facing the shortage of freshwater and river/manufactured sand. The effects of key composition parameters including the sea-sand size (1.18/2.36/4.75 mm), the polyethylene fiber length (6/12/18 mm), and the fiber volume dosage (1.0/1.5/2.0%) on the mechanical performance of SS-ECC were comprehensively investigated. SS-ECC with tensile strength over 8 MPa, ultimate tensile strain about 5%, and compressive strength over 130 MPa were achieved. Using seawater and sea-sand had almost no negative effects on the 28-day mechanical properties of high-strength ECC. For SS-ECC, increasing fiber length and dosage enhanced the tensile strain capacity, and sea-sand size had limited effects on the tensile performance; these phenomena were interpreted by the micromechanical analysis. A probabilistic-based method was proposed to analyze the reliability of the tensile strain capacity of SS-ECC, and it showed good agreement with the experimental results. The findings provide new insights into the design and applications of ECC in marine and coastal infrastructures for improving safety, durability, sustainability, and reliability.

Classification and quantification of cracks in concrete structures using deep learning image-based techniques

Abstract: Visual inspection has been the most widely used technique for monitoring concrete structures in service. Inspectors visually evaluate defects based on experience, skill, and engineering judgment. However, this process is subjective, laborious, time-consuming, and hampered by demanding access to numerous parts of complex structures. Accordingly, the present study proposes a nearly automated inspection model based on image processing and deep learning for detecting defects in typically inaccessible areas of concrete structures. Results indicate that using the Keras classifier combined with Otsu image processing can achieve superior classification accuracy of 97.63%, 96.5%, and 96.17% for training, validation, and testing data, respectively, along with low quantification error of 1.5%, 5% and 2% for the crack length, width, and angle of orientation, respectively. The type of structural damage and its severity are identified based on the allowed range of concrete crack width for different structures, including buildings and bridges based on different international standards and codes. The proposed method can deploy unmanned aerial vehicle image acquisition to offer a nearly automated inspection platform for the colossal backlog of aging concrete structures.

Time-dependent drying shrinkage model for concrete with coarse and fine recycled aggregate

Abstract: This study aims to develop a drying shrinkage model for recycled aggregate concrete (RAC) using both fine and coarse recycled aggregate (termed FRA and CRA, respectively). An experiment was conducted to measure the drying shrinkage of RAC with varying replacement ratios of CRA (0%, 50%, and 100%) and two types of FRAs (0%, 50%, and 100%). Test results show that: (1) both FRA and CRA have significant influence on the development of drying shrinkage, (2) the influence of FRA on the development of drying shrinkage decreases with increasing content of CRA, and (3) the influence of FRA on the final drying shrinkage is almost independent of the CRA. For example, the use of 100% FRA results in an increase of 23%–41% for concrete with 0% CRA. Similarly, 100% FRA results in 29%–38% increase for concrete with 100% CRA. Based on the results, a new model with two influence factors is proposed accounting for the influence of recycled aggregate on the development of drying shrinkage and on the final value of drying shrinkage of RAC; the expression was validated using a database of RAC shrinkage test data.

Effect of different grade levels of calcined clays on fresh and hardened properties of ternary-blended cementitious materials for 3D printing

Abstract: This study aims to investigate the influences of different grades of calcined clay on 3D printability, compressive strength (7 days), and hydration of limestone and calcined clay-based cementitious materials. Calcined clays that contained various amounts of metakaolin were achieved by blending low-grade calcined clay (LGCC) and high-grade calcined clay (HGCC) in three different proportions. The results revealed that increasing the HGCC% ranging from 0 wt% to 50 wt% in calcined clay could: (1) increase the flow consistency; (2) impressively improve the buildability, and reduce the printability window of the fresh mixtures; (3) enhance and accelerate the cement hydration. The reduction of mean interparticle distance induced by increasing HGCC% may be the primary reason for the enhancement of buildability and very early-age hydration. However, increasing HGCC% led to an increase of air void content in the interface region of the printed sample, which weakened the compressive strength of the printed sample at 7 days. Besides, it confirmed that the cold-joint/weak interface was easily formed by using the fresh mixture with a high structuration rate.

Fractal characteristics of pore structure of hybrid Basalt–Polypropylene fibre-reinforced concrete

Abstract: The pore characteristics of hybrid basalt–polypropylene fibre-reinforced concrete (HBPRC) are investigated using mercury intrusion porosimetry. The research results indicate that the cumulative pore volume of concrete increases with fibre addition. The pore surface fractal dimension (DS) of HBPRC in gel, capillary, and large pore regions decreases sequentially although it has no physical characteristics in a transition pore region. The incorporation of fibres has an insignificant effect on DS in gel and capillary pore regions; however, it has a reducing effect on DS in the large pore region. Furthermore, the greater the concrete strength, the larger DS becomes and the greater the reducing effect of fibres on DS in the large pore region. Through microscopic and mesoscopic analyses, it has been suggested that bubbles introduced by fibres and the weak dispersion of such fibres are the main reasons for the deterioration of the large pore structure of HBPRC.

A new design approach of steel fibre reinforced ultra-high performance concrete composites: Experiments and modeling

Abstract: This paper addresses a new design of steel fibre reinforced ultra-high performance concrete (UHPC) composites by implanting the fibres into modified Andreasen and Andersen (MAA) particle packing model. A novel method for determining the equivalent spherical diameter of steel fibres is proposed, which preserves the effect on the wet particle packing of an UHPC system, when there are equal volume fractions of steel fibers or these extra spherical particles. Then, the employed steel fibres are treated as spheroidal particles, and implanted into the MAA model for the design of a new UHPC. To demonstrate that the newly designed UHPC has superior performance, its macro and micro properties are analyzed in detail. The obtained experimental results reveal that the utilized steel fibre (L = 13 mm, d = 0.2 mm) can be treated as a spheroidal particle with a diameter of 5.65 mm in the MAA model. Moreover, based on the method proposed in this study, the negative effect of steel fibres on the packing system of UHPC can be minimized, which can enrich the basic design theory of UHPC composites.

Influence of water to cement ratio on CO2 uptake capacity of belite-rich cement upon exposure to carbonation curing

Abstract: The present study investigates the effect of water to cement ratio on the physiochemical and microstructural properties of carbonation cured belite-rich cement. Five varying water to cement ratio for belite-rich cement were carbonation cured for 28 days. It is observed that distinct form of behavior is exhibited on variation of water to cement ratio and its relation to the strength development in carbonation curing. The results demonstrated that higher water to cement ratio led to greater uptake of CO2 which enhanced the consumption of belite phase leading to increased production of calcite. The microstructural analysis showed that the varying water to cement ratio allowed for refinement of pore structure for belite-rich cement with reduced interconnection between pores. In particular, the relationship between water to cement ratio and formation of carbonation product is kept in focus along with the porosity distribution.