Showing papers in "Cement & Concrete Composites in 2022"

Characterization of sustainable mortar containing high-quality recycled manufactured sand crushed from recycled coarse aggregate

Abstract: The current utilization of low-quality recycled fine aggregate sourced from concrete waste acting as river sand replacement cannot achieve the preparation required for high-quality recycled mortar, due to abundant existence of hardened old mortar in recycled fine aggregate. In this work, an innovative and viable approach is proposed to crush recycled coarse aggregate into recycled manufactured sand, which has high-quality properties similar to the manufactured sand crushed from natural stone, for preparing high-quality recycled mortar with high strength and good durability. The results showed that recycled manufactured sand constituted by large proportion of natural stone particles and small proportion of old mortar, owned higher apparent density and lower water absorption than recycled fine aggregate. Incorporating recycled fine aggregate increased the drying shrinkage and decreased the mechanical strength; however, the substitution of natural river sand by recycled manufactured sand in mortar decreased the drying shrinkage as a result of its irregular shape and coarse surface, meanwhile, first increased and then declined the mechanical strength following the increase of the replacing percentage, amongst which the mortar containing 50% recycled manufactured sand showed the best mechanical strength. The tested mortars with recycled manufactured sand showed a general increment in the water and chloride ingress yet exhibited the water absorption and chloride ingress much lower than the mortar with recycled fine aggregate. Particularly, decreasing the content of hardened old mortar in recycled manufactured sand further improved the properties of recycled mortar.

Effects of carbon nanotube dosage and aggregate size distribution on mechanical property and microstructure of cemented rockfill

Abstract: The mechanical and structural properties of cemented rockfill produced by mineral wastes as recycled aggregates limit its application. In this paper, the macroscopic mechanical and microscopic scanning tests were carried out to study the mechanical property and microstructure of cemented rockfill reinforced by carbon nanotubes (CNTs) and fractal aggregates. The strengthening and deterioration mechanisms of CNT dosage on cemented rockfill were revealed for systematically evaluating its engineering availability. The influencing mechanism of aggregate size distribution on cemented rockfill was clarified to analyze the contribution of optimizing aggregate size distribution. The results show that the strengthening of cemented rockfill by appropriate CNTs includes bridging microcracks, compacting micropores and reinforcing cemented matrix, while the deterioration by excessive CNTs is attributed to form the agglomerative clusters and hollow cemented matrices. The cemented rockfill with superior aggregate size distribution has uniform and dense microstructure with relatively few defects, which is conducive to CNT reinforcement.

Interpretable Ensemble-Machine-Learning models for predicting creep behavior of concrete

Abstract: This study aims to provide an efficient and accurate machine learning (ML) approach for predicting the creep behavior of concrete. Three ensemble machine learning (EML) models are selected in this study: Random Forest (RF), Extreme Gradient Boosting Machine (XGBoost) and Light Gradient Boosting Machine (LGBM). Firstly, the creep data in Northwestern University (NU) database is preprocessed by a prebuilt XGBoost model and then split into a training set and a testing set. Then, by Bayesian Optimization and 5-fold cross validation, the 3 EML models are tuned to achieve high accuracy (R2 = 0.953, 0.947 and 0.946 for LGBM, XGBoost and RF, respectively). In the testing set, the EML models show significantly higher accuracy than the equation proposed by the fib Model Code 2010 (R2 = 0.377). Finally, the SHapley Additive exPlanations (SHAP), based on the cooperative game theories, are calculated to interpretate the predictions of the EML model. Five most influential parameters for concrete creep compliance are identified by the SHAP values of EML models as follows: time since loading, compressive strength, age when loads are applied, relative humidity during the test and temperature during the test. The patterns captured by the three EML models are consistent with theoretical understanding of factors that influence concrete creep, which proves that the proposed EML models show reasonable predictions.

Influence of reactivity and dosage of MgO expansive agent on shrinkage and crack resistance of face slab concrete

Abstract: Enhancing cracking resistance of face slab concrete is essential for the structural integrity and normal operation of concrete-faced rockfill dams (CFRDs). In this study, the effects of MgO with three reactivities and three dosages on the shrinkage and crack resistance of face slab concrete were systematically investigated by slab test, restrained drying shrinkage test and temperature stress test machine (TSTM). The results indicate that: (1) 7 %–25% of the total drying shrinkage and all (or most of) the autogenous shrinkage can be compensated by adding 5 %–10% MgO. The reactive MgO (M60) starts to compensate the drying and autogenous shrinkage at 1 day and compensates more shrinkage at early age than the moderate reactive MgO (M150) and the weak reactive MgO (M300), while M300 begins to compensate the shrinkage at about 50 days and produces larger compensation than M60 and M150 at late age. (2) M60 performs better than M150 in improving the crack resistance of face slab concrete to constraint and thermal stress. The increase in dosage of M60 and M150 from 0 to 10% prolongs the initial cracking time of face slab concrete by 10.0–27.5 h, increases the cracking strain by 9.2 %–25.7%, enhances the cracking tensile stress (σ) by 4.7 %–18.8% and lowers the cracking temperature (Tc) by 2.6–7.4 °C. Conversely, M300 weakens the cracking resistance. (3) The reactive MgO with relatively high dosage is suggested to eliminate the early shrinkage and to improve the cracking resistance of face slab concrete.

Interpretable Ensemble-Machine-Learning models for predicting creep behavior of concrete

Abstract: This study aims to provide an efficient and accurate machine learning (ML) approach for predicting the creep behavior of concrete. Three ensemble machine learning (EML) models are selected in this study: Random Forest (RF), Extreme Gradient Boosting Machine (XGBoost) and Light Gradient Boosting Machine (LGBM). Firstly, the creep data in Northwestern University (NU) database is preprocessed by a prebuilt XGBoost model and then split into a training set and a testing set. Then, by Bayesian Optimization and 5-fold cross validation, the 3 EML models are tuned to achieve high accuracy (R2 = 0.953, 0.947 and 0.946 for LGBM, XGBoost and RF, respectively). In the testing set, the EML models show significantly higher accuracy than the equation proposed by the fib Model Code 2010 (R2 = 0.377). Finally, the SHapley Additive exPlanations (SHAP), based on the cooperative game theories, are calculated to interpretate the predictions of the EML model. Five most influential parameters for concrete creep compliance are identified by the SHAP values of EML models as follows: time since loading, compressive strength, age when loads are applied, relative humidity during the test and temperature during the test. The patterns captured by the three EML models are consistent with theoretical understanding of factors that influence concrete creep, which proves that the proposed EML models show reasonable predictions.

High-strength high-ductility Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates

Abstract: In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates were successfully developed with high strength and high ductility. A multi-scale investigation was conducted to gain an in-depth understanding of the microstructure and ductility enhancement mechanism of geopolymer aggregate ECC (GPA-ECC). The use of geopolymer fine aggregates enabled the high-strength ECC to achieve higher tensile ductility and finer crack width compared to existing ones with similar compressive strength in the literature. It was found that the GPA reacted with the cementitious matrix, and the width of the GPA/matrix interfacial transition zone (ITZ) was larger than that of the silica sand/matrix ITZ. Moreover, the GPA achieved a strong bond with the cementitious matrix and could behave as “additional flaws” in high-strength matrix, resulting in saturated multiple cracking and excellent tensile ductility of ECC. This study provides a new avenue for developing high-performance fiber-reinforced cementitious composites based on artificial geopolymer aggregates.

High-strength high-ductility Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates

Abstract: In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates were successfully developed with high strength and high ductility. A multi-scale investigation was conducted to gain an in-depth understanding of the microstructure and ductility enhancement mechanism of geopolymer aggregate ECC (GPA-ECC). The use of geopolymer fine aggregates enabled the high-strength ECC to achieve higher tensile ductility and finer crack width compared to existing ones with similar compressive strength in the literature. It was found that the GPA reacted with the cementitious matrix, and the width of the GPA/matrix interfacial transition zone (ITZ) was larger than that of the silica sand/matrix ITZ. Moreover, the GPA achieved a strong bond with the cementitious matrix and could behave as “additional flaws” in high-strength matrix, resulting in saturated multiple cracking and excellent tensile ductility of ECC. This study provides a new avenue for developing high-performance fiber-reinforced cementitious composites based on artificial geopolymer aggregates. • Geopolymer fine aggregates were successfully applied to develop high-strength high-ductility ECC with fine crack width. • Geopolymer fine aggregates reacted with cementitious paste, resulting in a strong interfacial bond. • Geopolymer fine aggregates acted as “additional flaws” in high-strength ECC matrix, leading to saturated multiple cracking. • Compared with existing ambient-cured high-strength ECC, geopolymer aggregate ECC exhibited superior tensile ductility.

Long-term shrinkage and mechanical properties of fully recycled aggregate concrete: Testing and modelling

Abstract: To advance the research on recycled concrete (RC), recycled coarse aggregate (RCA), recycled fine aggregate (RFA) and recycled powder (RP) were used in preparing fully recycled aggregate concrete (FRAC), which lead to more comprehensive usage in the recycling of concrete waste. Recycled aggregates were studied as types of aggregate combinations instead of partial substitutes, and four aggregate combinations were designed with the use of natural and recycled aggregates in this study. The effects of different aggregate combinations on the long-term mechanical and shrinkage properties of concrete were examined. Furthermore, the effects of using RP on the properties of FRAC were studied. The results showed that using fully recycled aggregate (FRA) combinations had negative effects on the long-term mechanical and shrinkage properties of concrete, which were consistent with previous research on recycled concrete. However, the effect caused by FRA combination were found to be higher than the sum of that caused by fully recycled coarse aggregate and fully recycled fine aggregate combination. Moreover, RP decreased the mechanical properties of FRAC by 5%–17% whereas the shrinkage strain of FRAC in 180 days were decreased by 3%–13% with the increase of RP content from 10% to 30%. A long-term shrinkage model suitable for FRAC was derived by adding modification coefficients for aggregate combinations and RP contents to the current models. • Fully recycled concrete (FRC) is developed with recycled aggregate and powder. • Long-term mechanical and shrinkage properties of FRC are experimental studied. • Models for long-term shrinkage of FRC are developed and evaluated.

Ultra-high-strength engineered/strain-hardening cementitious composites (ECC/SHCC): Material design and effect of fiber hybridization

Abstract: Abstract It is well known that an increase in the compressive strength of cementitious composites is usually accompanied by a loss of tensile ductility. Designing and developing ultra-high-strength cementitious composites (e.g., ≥200 MPa) with high tensile strain capacity (e.g., ≥3%) and excellent crack resistance (e.g., crack width ≤100 μm) remain challenging. In this study, a series of ultra-high-strength Engineered Cementitious Composites (UHS-ECC) with a compressive strength over 210 MPa, a tensile strain capacity of 3–6% (i.e., 300–600 times that of ordinary concrete), and a fine crack width of 67–81 μm (at the ultimate tensile strain) were achieved. Hybrid design of fiber reinforcement and matrix for UHS-ECC was adopted by combining the ECC and ultra-high-performance concrete (UHPC) design concepts, and the effect of fiber hybridization and aspect ratio on the mechanical behavior of UHS-ECC was comprehensively investigated. The overall performance of UHS-ECC was assessed and compared with the existing high-strength ECC and strain-hardening UHPC, and it was found that the currently designed UHS-ECC recorded the best overall performance among the existing materials. Finally, the multiple cracking behavior of UHS-ECC was analyzed and modeled based on a probabilistic approach to evaluate its critical tensile strain for durability control in practical applications. The results of this study have pushed the performance envelope of both ECC and UHPC materials and provided a basis for developing cementitious composites with simultaneously ultra-high compressive strength, ultra-high tensile ductility, and excellent crack resistance. • Hybrid design of fiber reinforcement and matrix for UHS-ECC was introduced by combining the UHPC and ECC design concepts. • UHS-ECC with a compressive strength over 210 MPa and a tensile ductility over 6% was developed. • UHS-ECC with 2% 18-mm PE fiber and 1% 13-mm steel fiber recorded the best overall performance. • Cracking behavior was modeled by a probabilistic approach to estimate the critical tensile strain of UHS-ECC.

Unloading and reloading stress-strain relationship of recycled aggregate concrete reinforced with steel/polypropylene fibers under uniaxial low-cycle loadings

Abstract: To solve the key scientific problems such as low toughness and easy cracking of recycled aggregate concrete (RAC), A new sustainable material (fiber-reinforced recycled aggregate concrete - FRAC) was obtained by adding steel fiber (SF) and polypropylene fiber (PPF) into RAC matrix. Few experimental results were reported for the material under uniaxial low-cycle loading, and consequently, there is a gap in constitutive propositions to predict its behavior. Thus, one purpose of this research is to provide experimental results for FRAC characterizing damage growth and residual strain during cyclic compression in low-cycle tests with increasing strain amplitudes. Another purpose is to present a constitutive model to predict this behavior of FRAC accounting for fiber content. It is worth noting that the present results are relevant to displacement-controlled tests. The development law of residual strain (permanent strain) is explored, and the relationships between residual strain and unloading strain/reloading strain are proposed. The unloading stress-strain/reloading stress-strain equations are given. The damage evolution law of FRAC is revealed. Also, a new damage model represented by residual strain is suggested accounting for the fiber content. Furthermore, a stress-strain constitutive model coupling damage for FRAC is proposed. The model predicted with accuracy the unloading path, reloading path, residual strain development, and damage evolution for the composite accounting for fiber content.

Passivation and depassivation properties of Cr–Mo alloyed corrosion-resistant steel in simulated concrete pore solution

Abstract: This paper is aimed to investigate the passivation and depassivation properties of the Cr–Mo corrosion resistance (CR) steel in the simulated concrete pore solution (SCPS). Electrochemical methods, XPS and AFM tests were used to investigate the composition and morphology characterizations of passive film. As the results shown, passivation of CR steel will start and complete in the SCPS in one or two days, while that of LC steel is in three days. The passive film is mainly composed of metal oxides or hydroxides, and the metal oxidation degree varies with depth of the film. When exposed to Cl− with a concentration high enough, the passive film will be gradually dissolved. The CR steel shows higher corrosion resistance than the LC steel. By theoretical analysis, the growth of passive film can be divided into four steps. The theoretical thickness of the CR steel is thinner than that of the LC steel.

High-ferrite Portland cement with slag: Hydration, microstructure, and resistance to sulfate attack at elevated temperature

Abstract: Among the four mineral components in cement, C4AF has been demonstrated to have satisfactory corrosion resistance to sulfate attack. Here, high-ferrite Portland cement (HFPC) is used to prepare high corrosion-resistance cement-based materials. The hydration mechanisms and pore structures of HFPC pastes containing different slag dosages are studied at 20 °C, 40 °C, and 60 °C. The results indicate that the elevated temperature greatly stimulates the slag activity and significantly promotes the hydration of HFPC/slag blends. As the temperature increases, the heat curve of the cement pastes with high content slag shows a third or fourth exothermic peak. The hydration of pastes with more than 40% slag is controlled by the nucleation of hydrates in the acceleration stage and by the diffusion of ions in the deceleration stage above 40 °C. In addition, the HFPC pastes show excellent mechanical properties in a high-temperature environment. The HFPC pastes exhibit better sulfate attack resistance performance compared with pure ordinary Portland cement (OPC) ones. Therefore, the excellent performance of HFPC-based materials can be applied in the construction of concrete in mass under the environment of the South Sea in China, to solve the problems of high environmental temperature, large temperature differences, and sulfate attack.

Quantitative evaluation of the characteristics of air voids and their relationship with the permeability and salt freeze–thaw resistance of hybrid steel-polypropylene fiber–reinforced concrete composites

Abstract: Hybrid fiber affects the structure of air voids of hardened concrete and further affects the performance of concrete. A laboratory test quantitatively analyzes the characteristics of air voids and their relationship with the permeability and salt freeze-thaw (F-T) resistance of hybrid steel fiber (ST)–polypropylene fiber (PP) reinforced concrete. Quantitative parameters such as the fractal dimension, number, tortuosity, and shape factor of the air voids are used to characterize their structure. In addition, the relationship between the structural characteristics of air voids and the permeability and salt F-T resistance of concrete is analyzed using the rough set theory. The hybrid ST–PP fiber can reduce the total air-void content, spacing factor, average chord length, and total chord length of the air voids. However, the number and specific surface area of the air voids increased by 113.9–380.2% and 185.3–422.7%, respectively. In addition, the tortuosity of air voids with a diameter of 180–500 μm is more complex than that of other apertures. Moreover, hybrid ST–PP fibers can change the shape of air voids in concrete and make the shape of air voids in concrete simple and round. The compressive strength and splitting tensile strength of concrete after salt F-T cycles are enhanced by the hybrid ST–PP fiber. Nevertheless, this hybrid fiber has a negative effect on the permeability of the concrete. The total air–void content, number of air voids and tortuosity are the main factors affecting the permeability and F-T resistance of concrete.

Quantitative evaluation of the characteristics of air voids and their relationship with the permeability and salt freeze–thaw resistance of hybrid steel-polypropylene fiber–reinforced concrete composites

Abstract: Hybrid fiber affects the structure of air voids of hardened concrete and further affects the performance of concrete. A laboratory test quantitatively analyzes the characteristics of air voids and their relationship with the permeability and salt freeze-thaw (F-T) resistance of hybrid steel fiber (ST)–polypropylene fiber (PP) reinforced concrete. Quantitative parameters such as the fractal dimension, number, tortuosity, and shape factor of the air voids are used to characterize their structure. In addition, the relationship between the structural characteristics of air voids and the permeability and salt F-T resistance of concrete is analyzed using the rough set theory. The hybrid ST–PP fiber can reduce the total air-void content, spacing factor, average chord length, and total chord length of the air voids. However, the number and specific surface area of the air voids increased by 113.9–380.2% and 185.3–422.7%, respectively. In addition, the tortuosity of air voids with a diameter of 180–500 μm is more complex than that of other apertures. Moreover, hybrid ST–PP fibers can change the shape of air voids in concrete and make the shape of air voids in concrete simple and round. The compressive strength and splitting tensile strength of concrete after salt F-T cycles are enhanced by the hybrid ST–PP fiber. Nevertheless, this hybrid fiber has a negative effect on the permeability of the concrete. The total air–void content, number of air voids and tortuosity are the main factors affecting the permeability and F-T resistance of concrete.

3D printable concrete with natural and recycled coarse aggregates: Rheological, mechanical and shrinkage behaviour

Abstract: In the current study, we examine the effect of using natural and recycled coarse aggregates in designing 3D printable concrete. We assessed the rheological behaviourusing a dynamic shear rheometer, and it was observed that the addition of coarse aggregates significantly decreased the yield stress and marginally lowered the plastic viscosity. This was attributed to the increase in the paste and water film thickness with the addition of larger aggregates. Therefore, a reduction in the superplasticizer dosage is required to obtain coarse aggregate mixtures with similar yield stress and buildability to the control mixture. The mechanical properties were evaluated by using beam and cube samples cut out from printed wall elements. A marginal decrease in compression and flexural strength was observed for both the mixtures with natural and recycled coarse aggregates. The total and autogenous shrinkage assessment was performed using mould cast prismatic specimens, while the shrinkage cracking potential was evaluated using the restrained ring test. The coarse aggregate mixtures showed lower total and autogenous shrinkage. Notably, the addition of the saturated recycled aggregates significantly lowered the autogenous shrinkage, possibly due to internal curing. As a result, a relatively lower strain rate factor and slower development of tensile stresses occurred in the restrained shrinkage test, increasing the cracking age for the coarse aggregate mixtures. The current study, therefore, shows good potential for using natural and recycled coarse aggregates in 3D printable concrete.

Mechanical anisotropy of ultra-high performance fibre-reinforced concrete for 3D printing

Abstract: In this study, a novel 3D-printing ultra-high performance fibre-reinforced concrete (3DP-UHPFRC) was developed. The effect of fibre content, fibre type and printing direction on the mechanical properties of 3DP-UHPFRC was evaluated through compressive, flexural, splitting tensile and uniaxial tensile tests, and the anisotropic properties of 3DP-UHPFRC were investigated. The experiment results indicated that 3DP-UHPFRC prepared with 1 vol% 6 mm steel fibre was more suitable for construction than 3DP-UHPFRC prepared with 1 vol% 10 mm steel fibre under the printing conditions in this test. The maximum flexural strength of 3DP-UHPFRC with 1 vol% 6 mm steel fibre reached 45.21 MPa in the Z-direction (printing direction), which was substantially higher than those obtained in previous studies. The flexural and splitting tensile failures of 3DP-UHPFRC could be either ductile or brittle in different directions; thus, the printing mode could be flexibly adjusted according to different engineering requirements. The latest test results indicated that the compressive elastic modulus was anisotropic, but there was little difference in the tensile elastic modulus in each direction.

3D printable concrete with natural and recycled coarse aggregates: Rheological, mechanical and shrinkage behaviour

Abstract: In the current study, we examine the effect of using natural and recycled coarse aggregates in designing 3D printable concrete. We assessed the rheological behaviourusing a dynamic shear rheometer , and it was observed that the addition of coarse aggregates significantly decreased the yield stress and marginally lowered the plastic viscosity . This was attributed to the increase in the paste and water film thickness with the addition of larger aggregates. Therefore, a reduction in the superplasticizer dosage is required to obtain coarse aggregate mixtures with similar yield stress and buildability to the control mixture. The mechanical properties were evaluated by using beam and cube samples cut out from printed wall elements. A marginal decrease in compression and flexural strength was observed for both the mixtures with natural and recycled coarse aggregates. The total and autogenous shrinkage assessment was performed using mould cast prismatic specimens, while the shrinkage cracking potential was evaluated using the restrained ring test. The coarse aggregate mixtures showed lower total and autogenous shrinkage. Notably, the addition of the saturated recycled aggregates significantly lowered the autogenous shrinkage, possibly due to internal curing. As a result, a relatively lower strain rate factor and slower development of tensile stresses occurred in the restrained shrinkage test, increasing the cracking age for the coarse aggregate mixtures. The current study, therefore, shows good potential for using natural and recycled coarse aggregates in 3D printable concrete.

Mechanical anisotropy of ultra-high performance fibre-reinforced concrete for 3D printing

Abstract: In this study, a novel 3D-printing ultra-high performance fibre-reinforced concrete (3DP-UHPFRC) was developed. The effect of fibre content, fibre type and printing direction on the mechanical properties of 3DP-UHPFRC was evaluated through compressive, flexural, splitting tensile and uniaxial tensile tests, and the anisotropic properties of 3DP-UHPFRC were investigated. The experiment results indicated that 3DP-UHPFRC prepared with 1 vol% 6 mm steel fibre was more suitable for construction than 3DP-UHPFRC prepared with 1 vol% 10 mm steel fibre under the printing conditions in this test. The maximum flexural strength of 3DP-UHPFRC with 1 vol% 6 mm steel fibre reached 45.21 MPa in the Z-direction (printing direction), which was substantially higher than those obtained in previous studies. The flexural and splitting tensile failures of 3DP-UHPFRC could be either ductile or brittle in different directions; thus, the printing mode could be flexibly adjusted according to different engineering requirements. The latest test results indicated that the compressive elastic modulus was anisotropic, but there was little difference in the tensile elastic modulus in each direction.

Comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC)

Abstract: This paper presents a comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC). Firstly, based on the modified Andreasen & Andersen packing model, a UHPC with ultra-low cement content is produced. Then, the steel fibers and polyoxymethylene (POM) fibers are added into this UHPC matrix separately and together. The obtained experimental results show that the addition of POM fiber has limited contribution to the compressive strength of UHPC, while it has a positive influence on its flexural strength and high temperature resistance. The results of nanoindentation indicate that POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Based on the nanoindentation results, it can be found that the interfacial transition zone (ITZ) is about 50 μm in the case of UHPC with only steel fibers, while this value gradually increase to about 60 μm when POM fiber is added individually. This results further demonstrate that the POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Therefore, to efficiently apply the POM fiber in UHPC, it is suggested to combine it with steel fibers and its optimal content should be around 1% (vol.). It can be concluded that it is difficult for POM fiber to fully replace steel fiber in referencing UHPC, while the hybrid fibers should be a good choice to produce excellent UHPC composite.

A mix design methodology of slag and fly ash-based alkali-activated paste

Abstract: In this paper, a series of experiments were conducted to systematically and quantitively explore the effects of control factors on the early age properties, i.e., workability and strength of slag and fly ash-based alkali-activated paste (BFS/FA-AAP). The control factors on the workability (flowability and setting time) of BFS/FA-AAP include the Na2O/b ratio, the SiO2/Na2O ratio, the w/b ratio, and the BFS/b ratio. The control factors on strength (compressive strength and flexural strength) of BFS/FA-AAP further also include the curing condition and the curing age. The results show that a higher BFS/b ratio and Na2O/b ratio could increase the strength while decreasing the workability. A higher w/b ratio could increase the workability while slightly decreasing the strength. Higher SiO2/Na2O ratio increases both strength and workability. Despite that, higher Na2O/b ratio and SiO2/Na2O ratio could hinder the strength development. Sealed curing condition is proved to be a simple but efficient way to assure the steady strength development of BFS/FA-AAP. Although the strength of BFS/FA-AAP could generally stabilize after 90 days, the strength development rate varies with different mix proportions. In addition, a general methodology have been proposed to predict the compressive strength of BFS/FA-AAP and verified with experiments. Finally, a mix design table is proposed for the preliminary design of BFS/FA-AAP according to the principle of satisfying early age requirements.

Experimental and theoretical investigation of chloride ingress into concrete exposed to real marine environment

Abstract: Chloride-induced corrosion is a critical issue that influences the durability and mechanical performance of reinforced concrete (RC) structures exposed to marine tidal and splash environments. This paper presents an experimental and theoretical investigation on the chloride transport behavior in concrete exposed to an aggressive marine environment for up to 720 days. Field exposure tests were performed in marine tidal and splash zones, and free chloride concentration profiles for different types of concrete after several exposure times, i.e. 90, 180, 360 and 720 days, were obtained. The effects of the water-to-cement ratio, binder type, exposure environment and exposure time on the time dependency of the chloride diffusion coefficient (D) and surface chloride concentration (Cs) were analyzed and discussed. Based on the obtained results, a modified time-dependent prediction model of chloride ingress into concrete was established and validated by considering the time-dependency effect of material and environmental factors. The results indicate that the comparisons of the predicted chloride profiles using the developed time-dependent prediction model were in good agreement with the results of the real marine exposure test.

Nanomaterials in ultra-high-performance concrete (UHPC) – A review

Abstract: This paper presents a state-of-the-art review of the utilization of nanomaterials in ultra-high-performance concrete (UHPC). First, various types of nanomaterials currently available for use in UHPC are summarized and their geometric and physical properties are analyzed. Then, the effects of nanomaterials on the packing density and fresh properties of UHPC are addressed. The hydration kinetics and mechanical properties of UHPC, such as the interfacial bonding , compressive strength , tensile and flexural responses, and nano-mechanical properties, are analyzed according to the nanomaterial type and dosage. Based on the data acquisition and analysis, the optimal dosages of nanomaterials for strength enhancements of UHPC are suggested. The influence of nanomaterials on the durability-related properties, such as the porosity, water sorptivity and permeability, sulfate attack , abrasion resistance , corrosion resistance , freeze–thaw resistance, fatigue performance, and shrinkage, are also investigated. Finally, nanomaterial-based functional UHPCs with electrical, self-sensing, and electromagnetic shielding properties are reviewed and discussed.

Development of fly ash and slag based high-strength alkali-activated foam concrete

Abstract: Foam concrete is a lightweight construction material with excellent thermal and acoustic insulation ability. It is widely used in building envelopes, partitioning walls, sandwich components, etc. However, the foam concrete applied most is made by Portland cement that consumes much energy and emits a large amount of CO2 during its production process. As a green binding material without clinker, the alkali-activated material fully utilizes fly ash, slag and other industrial solid wastes or by-products, which largely reduces CO2 emissions. In this paper, the fly ash and slag based alkali-activated foam concrete with a design density ranging from 200 kg/m3 to 1200 kg/m3 was developed. Based on the bubble dynamics theory, the mechanical behavior and the stability of bubbles in the mixture were analyzed, and a calculation method for judging the stability of bubbles in the mixture was proposed. According to the analysis and performance of prepared bubbles, Sodium dodecyl sulfate was selected as the foaming agent to prepare the foam concrete. The compressive strength of the developed foam concrete ranged from 0.50 MPa to 44.98 MPa while the flexural strength ranged from 0.22 MPa to 13.86 MPa, respectively, with respect to different densities. The results show that the mechanical strengths of alkali-activated foam concrete are higher than that of ordinary Portland cement based foam concrete with the same density. In addition, alkali-resistant glass fibers were used to improve the problem of the brittleness of low-density foam concrete. It was observed that the 0.5% volume fraction of fibers was optimal to improve the mechanical properties.

3D printing geopolymers: A review

Abstract: Geopolymers have been considered as a promising alternative to cementitious materials for 3D printing to enhance sustainability of the construction industry. This paper presents a critical review of the state-of-the-art of 3D printing geopolymers from the perspectives of production process, printability requirement, mix design, early-age material properties and sustainability, with a special focus on the effects of different factors such as matrix composition, reinforcement type, curing regime and printing configuration on the fresh and hardened properties of 3D printed geopolymers. The relationship between key fresh properties and printability of geopolymers is discussed, based on which the potential optimal mix proportions are obtained, containing the blended precursors of fly ash, ground granulated blast-furnace slag and silica fume, liquid or solid activator, river sand with a maximum size of 2 mm, thixotropic additives (e.g., nano clay), and retarder (e.g., sucrose). This paper aims to summarise the recent advances in the development of 3D printing techniques suitable for geopolymers and geopolymers feasible for 3D printing, and to identify the knowledge gap, remaining challenges, and opportunities for future research.

Influence of carbonation treatment on the properties of multiple interface transition zones and recycled aggregate concrete

Abstract: Because of the carbonation treatment of recycled coarse aggregates (RCAs), the properties of the weak multiple interface transition zones (ITZs) in recycled aggregate concrete (RAC) can be improved, contributing to the enhanced behaviour of the RAC. In this study, different carbonation conditions (carbonation pressure, initial moisture contents of the RCAs and carbonation duration) were considered. A model of the RACs (MIRAC) was prepared to accurately locate multiple ITZs. Nanoindentation tests , scanning electron microscopy (SEM) tests were conducted to study the effect of carbonation treatment on the microproperties of RAC. At the same time, the influence of the carbonation treatment on the compressive strength and chloride ion penetration resistance of the RAC was evaluated. Moreover, the relationship between the microproperties and macroproperties of the RAC was discussed. The results showed that the modulus of the ITZs, the old mortar matrix and the new mortar matrix increased while the thickness of the ITZs decreased when the carbonation pressure or carbonation duration were increased. However, there was no further significant change after the carbonation pressure exceeded 1 bar, and the carbonation rate began to decrease significantly after 3 h of carbonation. The carbonation efficiency was superior when the moisture content was 1.81%. The effects of different carbonation parameters on the microproperties and macroproperties of the RAC were consistent. The compressive strength and chloride ion penetration resistance of the RAC had a linear correlation with the modulus of the ITZ between the aggregate and old mortar.

Prediction of concrete strengths enabled by missing data imputation and interpretable machine learning

Abstract: Machine learning (ML)-based prediction of non-linear composition-strength relationship in concretes requires a large, complete, and consistent dataset. However, the availability of such datasets is limited as the datasets often suffer from incompleteness because of missing data corresponding to different input features, which makes the development of robust ML-based predictive models challenging. Besides, as the degree of complexity in these ML models increases, the interpretation of the results becomes challenging. These interpretations of results are critical towards the development of efficient materials design strategies for enhanced materials performance. To address these challenges, this paper implements different data imputation approaches for enhanced dataset completeness. The imputed dataset is leveraged to predict the compressive and tensile strength of concrete using various hyperparameter-optimized ML approaches. Among all the approaches, Extreme Gradient Boosted Decision Trees (XGBoost) showed the highest prediction efficacy when the dataset is imputed using k-nearest neighbors (kNN) with a 10-neighbor configuration. To interpret the predicted results, SHapley Additive exPlanations (SHAP) is employed. Overall, by implementing efficient combinations of data imputation approach, machine learning, and data interpretation, this paper develops an efficient approach to evaluate the composition-strength relationship in concrete. This work, in turn, can be used as a starting point toward the design and development of various performance-enhanced and sustainable concretes.

A review on the modelling of carbonation of hardened and fresh cement-based materials

Abstract: Different models have been proposed to investigate and predict the carbonation process of cement-based materials. This paper firstly introduces the carbonation process occurred in cement-based materials and those influential factors. Then the proposed empirical, thermodynamic, kinetic and numerical models to characterize the carbonation process of cement-based materials are reviewed. The theoretical basis, input parameters and major applications of different models are summarized and discussed. The advantages and disadvantages of those models are comparatively reviewed. Finally, modelling of early CO2 curing of fresh cement-based materials is also reviewed and compared with the carbonation of hardened cement-based materials.

Synthesis of pyrolytic carbonized bagasse to immobilize Bacillus subtilis; application in healing micro-cracks and fracture properties of concrete

Abstract: Growing amount of agricultural waste and its burning in open environments is contributing towards the carbon emissions and triggering serious health hazards. It can be reduced by beneficially employing the wastes, as carrier media of calcite (CaCO3) precipitating microbes, into the concrete. Instant formation of CaCO3 in micro-cracks using bio-inspired concrete prevents aggressive ions to penetrate into the inside concrete, hence boosting the durability. In the present study, Bacillus subtilis (BS) were immobilized with nano-micro sized carbonaceous solid material, bagasse ground biochar (GBC), to enhance the CaCO3 precipitation. The mechanical behavior of the samples was investigated in terms of bending and compression. Biochar immobilized BS concrete (BSCM) exhibited promising flexural behavior, higher strain energy storing capability, and higher modulus of fracture toughness. Moreover, 23.18% enhancement in compressive strength was achieved after 56 days of curing in comparison to the control samples. Furthermore, autonomous cracks closure mechanism was monitored as the function of time, the BSCM showed effective crack healing with maximum 100% and 68% sealing of 500 μm and 800 μm wider cracks respectively, in the selected time frame. The BSCM samples revealed higher ultrasonic pulse velocities and lesser sorptivity due to the densified microstructure of concrete by bacterial precipitated CaCO3. The x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy and thermal gravimetric analysis confirmed the existence of CaCO3 inside the cracks. Consequently, immobilizing BS with bagasse GBC could be considered as a promising solution for prompt cracks repairing and enhancing the mechanical properties of concrete.

Microstructure and macro properties of sustainable alkali-activated fly ash mortar with various construction waste fines as binder replacement up to 100%

Abstract: Utilizing construction waste fines as eco-friendly binder for sustainable alkali-activated materials provides new approach to recycling construction waste. This work investigated the characterization of alkali-activated fly ash mortar (AAM) incorporating construction waste fines at varied types and replacement levels. The results showed that incorporating waste paste powder containing abundant hydrated products refined the pore size of AAM, but mixing waste mortar powder and waste concrete powder that contained massive inert quartz and calcite adversely affected the alkali-activated reaction and pore structure of AAM. The incorporation of construction waste fines was found to reduce the fluidity but increased the drying shrinkage of AAM. At identical replacement level of construction waste fines, the waste paste powder incorporated AMM has lower fluidity and greater drying shrinkage than the waste mortar powder or waste concrete powder incorporated AAM. Substituting waste paste powder for fly ash improved the mechanical strength of AAM, but the strength decreased as waste mortar powder or waste concrete powder was incorporated. Particularly, the AAM blended with 100% construction waste fines still owned good strength. Mixing construction waste powder reduced the chloride diffusion coefficient and chloride ingress depth of AAM, and the waste paste powder mixed AAM had better chloride resistance than the waste mortar powder or waste concrete powder mixed AAM. Therefore, the hydrated products in construction waste fines was revealed to benefit the micro-characteristics of alkali-activated materials, and optimizing the type and replacement level of construction waste fines can achieve sustainable AAM with good mechanical strength and chloride ingress resistance.

A review on the modelling of carbonation of hardened and fresh cement-based materials

Abstract: Different models have been proposed to investigate and predict the carbonation process of cement-based materials. This paper firstly introduces the carbonation process occurred in cement-based materials and those influential factors. Then the proposed empirical, thermodynamic, kinetic and numerical models to characterize the carbonation process of cement-based materials are reviewed. The theoretical basis, input parameters and major applications of different models are summarized and discussed. The advantages and disadvantages of those models are comparatively reviewed. Finally, modelling of early CO2 curing of fresh cement-based materials is also reviewed and compared with the carbonation of hardened cement-based materials.