Compressive Strength Of Ultra-high Performance Concrete Research Articles

Mechanical Strength and Microstructure of Ultrahigh-Performance Concrete under Long-Term Autoclaving

Solar thermal energy is a technology that collects radiant energy from the sun to generate heat or electricity. Thermal energy storage is required to compensate for the solar thermal energy variations across time scales, and a popular strategy is storing thermal energy in hot water tanks. Due to superior strength and durability, ultrahigh-performance concrete (UHPC) is exploited to construct such water tanks for applications at 200°C. Therefore, the mechanical strength and microstructure of UHPC after long-term temperature-pressure load (autoclaving) is studied. The compressive strength of UHPC can stay robust due to the accelerated formation of hydrates, while the flexural strength is vulnerable to long-term autoclaving due to the transformation of amorphous calcium silicate hydrate (C-S-H) to more ordered phases. The main hydrates in autoclaved samples are poorly crystallized C-S-H, hydroxylellestadite, and hydrogarnet. The porosity of autoclaved samples is not strictly related to the mechanical strength, and the influence of hydrate assemblage outweighs that of porosity on mechanical strength after long-term autoclaving. The partial replacement of cement by limestone powder can decrease crystalline hydrates and increase poorly crystallized C-S-H, which densifies the microstructure and enhances the mechanical strength. However, excessive poorly crystallized C-S-H aggravates the thermal mismatch between the matrix and quartz aggregates after autoclaved samples cool to room temperature, leading to interstice in matrix-quartz interfaces and, thus, reduced mechanical strength. Therefore, an appropriate addition of limestone powder is required to induce the hydrate assemblage with appropriate poorly crystallized C-S-H, ensuring durable UHPC structures under autoclaving.

Advanced machine learning algorithms to evaluate the effects of the raw ingredients on flowability and compressive strength of ultra-high-performance concrete.

The estimation of concrete characteristics through artificial intelligence techniques is come out to be an effective way in the construction sector in terms of time and cost conservation. The manufacturing of Ultra-High-Performance Concrete (UHPC) is based on combining numerous ingredients, resulting in a very complex composite in fresh and hardened form. The more ingredients, along with more possible combinations, properties and relative mix proportioning, results in difficult prediction of UHPC behavior. The main aim of this research is the development of Machine Learning (ML) models to predict UHPC flowability and compressive strength. Accordingly, sophisticated and effective artificial intelligence approaches are employed in the current study. For this purpose, an individual ML model named Decision Tree (DT) and ensembled ML algorithms called Bootstrap Aggregating (BA) and Gradient Boosting (GB) are applied. Statistical analyses like; Determination Coefficient (R2), Root Mean Square Error (RMSE), and Mean Absolute Error (MAE) are also employed to evaluate algorithms' performance. It is concluded that the GB approach appropriately forecasts the UHPC flowability and compressive strength. The higher R2 value, i.e., 0.94 and 0.95 for compressive and flowability, respectively, of the DT technique and lesser error values, have higher precision than other considered algorithms with lower R2 values. SHAP analysis reveals that limestone powder content and curing time have the highest SHAP values for UHPC flowability and compressive strength, respectively. The outcomes of this research study would benefit the scholars of the construction industry to quickly and effectively determine the flowability and compressive strength of UHPC.

Effect and mechanism of composite early-strength agents on sulfoaluminate cement-based UHPC

Some major construction projects require materials that can quickly and easily obtain high mechanical properties. Nowadays sulfoaluminate cement (SAC) has attracted attention for such projects due to its fast hardening rate, high early strength, and low alkalinity. Added early strength agents are commonly used to improve the early strength of Ultra-high performance concrete (UHPC). Most early-strength agent research has focused on Portland cement systems. Due to the different components of SAC and Portland cement, the compatibility between different early strength agents and SAC needs to be further explored.This paper selected three inorganic early-strength agents, calcium formate (Ca(HCOO)2), aluminum sulfate (Al2(SO4)3), and lithium carbonate (Li2CO3), and an organic early-strength agent, triethanolamine (TEA), which are commonly used. Experimental studies were carried out for single and compound early-strength agent systems in a SAC-based Ultra-high performance concrete (UHPC) system with a low water-cement ratio of 0.23. The results showed three inorganic early-strength agents reduced the setting time and improved the strength of the UHPC. The 4 h compressive strength of UHPC with 0.4 % Ca(HCOO)2 increased from 22.1 MPa to 41.2 MPa due to the improved microstructure and denser matrix.The composite mixture of Ca(HCOO)2 and Li2CO3 had a good synergistic effect in the early and middle stages, the compressive strength can reach 88.7 when curing 3 days. Different from Portland cement system, TEA had an adverse effect in the both single and compound systems.

Prediction of compressive strength and elastic modulus for ultra-high-performance concrete

This study conducted compression tests to assess the behavior of ultra-high-performance concrete (UHPC) subjected to uniaxial compression. The primary test parameter is the fiber volume fraction in the 0 % to 2 % range. According to the test results, the presence of steel fibers in the matrix significantly affected the failure mode of UHPC. However, the compressive behavior of the test specimens appeared to be very similar, regardless of the fiber volume fraction. The compressive strength of UHPC was improved by approximately 10 % by the addition of the steel fibers. However, the fiber volume fraction and transverse strain had a strong association. The steel fibers significantly affected the volume change of the UHPC. Based on the test results and numerous test data sets, this study suggests that the compressive strength of the UHPC with steel fibers is a function of the UHPC compressive strength without steel fibers and design compressive strength. In addition, the elastic modulus for design is suggested. The proposed equations are applicable for the design and analysis of the UHPC with a fiber volume fraction of up to 6 % and a compressive strength between 120 and 255.9 MPa.

Influence of sand grain size distribution and supplementary cementitious materials on the compressive strength of ultrahigh-performance concrete

ultrahigh-performance concrete is a type of advanced concrete that can improve the resilience and durability of concrete structures. The utilization of available supplementary cementitious materials is a critical step in saving energy and materials, as well as lowering the cost of concrete. This research investigated the impact of sand grain size distribution, supplementary cementitious materials, and curing regimes (CRs) on the compressive strength of ultrahigh-performance concrete. The findings showed that a 90-d strength of 175 MPa could be obtained with a microsilica volume of 5 % and without heat-curing. The highest strength was obtained after curing for 48 h at 60 °C, followed by 72 h at 90 °C. Moreover, when compared to natural grain size distribution sand, the usage of finer sand enhances compressive strength. When microsilica is added, however, this impact is minimized.

Sustainable use of recycled autoclaved aerated concrete waste as internal curing materials in ultra-high performance concrete

Great autogenous shrinkage has become a common problem in cement-based materials with a low water-to-binder ratio (w/b). To limit the autogenous shrinkage in ultra-high performance concrete (UHPC) during the process of hydration and curing, sustainable use of recycled micropowder (RMP) made from autoclaved aerated concrete waste (AACW) to cure UHPC internally is proposed in this paper. The influence of RMP (at the levels 5%, 10%, and 15% by mass) with three particle size classes ranging from 0 to 600 μm on the autogenous shrinkage, hydration reaction, internal relative humidity, and compressive strength of UHPC were investigated. The results show that mixtures with different dosages and particle sizes improve the fluidity of UHPC, and its internal curing effect can effectively inhibit the early-age autogenous shrinkage of UHPC and maintain a higher internal humidity compared with the control group. The incorporation of RMP can advance the appearance of a hydration peak and increase the cumulative heat of hydration. Microscale analyses show that the curing water released from RMP promotes the hydration of the unhydrated cement around RMP. The generated hydration product filled the surface pores of RMP and weakened the negative effect of the pores introduced by RMP on the compressive strength of UHPC. Thus, RMP can be utilized as internal curing materials for UHPC to reduce autogenous shrinkage.

Experimental Study on Seismic Performance of Precast Pretensioned Prestressed Concrete Beam-Column Interior Joints Using UHPC for Connection.

The traditional connections and reinforcement details of precast RC frames are complex and cause difficulty in construction. Ultra-high-performance concrete (UHPC) exhibits outstanding compressive strength and bond strength with rebars and strands; thus, the usage of UHPC in the joint core area will reduce the amount of transverse reinforcement and shorten the anchoring length of beam rebars as well as strands significantly. Moreover, the lap splice connections of precast columns can be placed in the UHPC joint zone and the construction process will be simplified. This paper presented a novel joint consisting of a precast pretensioned prestressed concrete beam, an ordinary precast reinforced concrete (RC) column, and a UHPC joint zone. To study the seismic performance of the proposed joints, six novel interior joints and one monolithic RC joint were tested under low-cyclic loads. Variables such as the axial force, the compressive strength of UHPC, the stirrup ratio were considered in the tests. The test results indicate that the proposed joints exhibit comparable seismic performance of the monolithic RC joint. An anchorage length of 40 times the strands-diameter and a lap splice length of 16 times the rebar-diameter are adequate for prestressed strands and precast column rebars, respectively. A minimum column depth is suggested as 13 times the diameter of the beam-top continuous rebars passing through the joint. In addition, a nine-time rebar diameter is sufficient for the anchorage of beam bottom rebars. The shear strength of UHPC in the joint core area is suggested as 0.8 times the square root of the UHPC compressive strength.

XRD INVESTIGATIONS ON COMPRESSIVE STRENGTH GAIN OF UHPC SUBJECTED TO VARIOUS CURING TECHNIQUES

Ultra High Performance Concrete (UHPC) is a relatively novel structural material which can be regarded as an advanced cementitious material with great stiffness, very high mechanical strength, and, extraordinary toughness in a broad sense. Even after cracking, the existence of tiny steel fibers confers the capacity to withstand deformations and, resist flexural and tensile stresses. Curing procedure is a key characteristic to enrich the qualities of the toughened concrete due to the inclusion of the mineral admixtures and a small water to binder ratio. If the process of curing is flawlessly completed, properties such as durability, volume stability, water tightness, strength, abrasion resistance, and resistance to freezing and thawing is highly enhanced. The impact of conventional water curing, hot air curing and, steam curing on the compressive strength of UHPC is explored in this study. UHPC cubes of side 70 mm are cast and the samples are subjected to water curing at 25°C, steam curing at 70°C temperature and, 75% RH and, hot air curing at 200°C. The cured samples are tested for compressive strength at different ages. The powders acquired after crushing the cubes are used for X-ray diffraction (XRD) studies. The variation in the compressive strength when subjected to different types of curing is analyzed using XRD studies, based on the products formed during hydration.

Study of nonlinear relationships between dosage mixture design and the compressive strength of UHPC

With advances in building materials research, new materials more suitable for specific applications due to their superior durability and mechanical properties are emerging to meet the requirements of infrastructure construction and maintenance. In this sense, ultra-high-performance concrete (UHPC) is considered one of the most promising materials for concrete construction. However, a traditional UHPC mixture contains large amounts of cement, silica fume, superplasticizer, and other expensive and high carbon footprint components. Hence, several researchers have focused on developing alternative UHPC dosages using locally available materials, including several mineral admixtures, to obtain a more affordable and sustainable UHPC. Therefore, deep knowledge about the relationships between the UHPC dosage and its resulting properties could help to improve these developments. However, these relationships are nonlinear and complex. This paper aims to address this gap by using a random forest model trained with a 931 UHPC mixture collection with 17 input variables and a unique response, namely the UHPC compressive strength. After adjusting a regression model, different tools, such as input variable importance analysis and partial dependence plots, were used to assess the nonlinear relationships between the dosage of UHPC and its resulting compressive strength. In most cases, a proper alignment was observed when the findings provided by these analyses were contrasted with the results provided by other researchers. The results indicated that the inputs with the highest significance on UHPC compressive strength are those related to water content, the packing density, and the quartz powder and superplasticizer dosages, followed by the cement content. Furthermore, mineral admixtures with high amorphous SiO2 content, such as RHA and SF, presented a relevant contribution to the compressive strength, while the other admixtures would seem to have a more modest contribution to it. Finally, it is also worth noting that the machine learning model created might help to create novel UHPC dosages by decreasing the duration and expenses of the experimental campaign by permitting the pre-selection of those components with a superior model response.

XRD INVESTIGATIONS ON COMPRESSIVE STRENGTH GAIN OF UHPC SUBJECTED TO VARIOUS CURING TECHNIQUES

Ultra High Performance Concrete (UHPC) is a relatively novel structural material which can be regarded as an advanced cementitious material with great stiffness, very high mechanical strength, and, extraordinary toughness in a broad sense. Even after cracking, the existence of tiny steel fibers confers the capacity to withstand deformations and, resist flexural and tensile stresses. Curing procedure is a key characteristic to enrich the qualities of the toughened concrete due to the inclusion of the mineral admixtures and a small water to binder ratio. If the process of curing is flawlessly completed, properties such as durability, volume stability, water tightness, strength, abrasion resistance, and resistance to freezing and thawing is highly enhanced. The impact of conventional water curing, hot air curing and, steam curing on the compressive strength of UHPC is explored in this study. UHPC cubes of side 70 mm are cast and the samples are subjected to water curing at 25°C, steam curing at 70°C temperature and, 75% RH and, hot air curing at 200°C. The cured samples are tested for compressive strength at different ages. The powders acquired after crushing the cubes are used for X-ray diffraction (XRD) studies. The variation in the compressive strength when subjected to different types of curing is analyzed using XRD studies, based on the products formed during hydration.

Efficient estimating compressive strength of ultra-high performance concrete using XGBoost model

An ensemble technique using XGBoost model is employed to predict the compressive strength of ultra-high performance concrete (UHPC). A 931 UHPC mixture collection with 17 input variables is employed, in which 230 were from laboratory experiments and the rest from scientific literature. The best results are obtained by tuning the hyper-parameters using Pareto multi-objective optimisation to find the optimum values of R2 for both train and test dataset. The obtained solution with R2 = 0.8922, RMSE = 7.860 MPa, MAE = 5.930 MPa show better performance with those from previous study. Partial dependence plots are illustrated to investigate the effects of some important input variables on the compressive strength of UHPC. Graphical User Interface (GUI) is written in Python and provided freely for users to support the design and interpretation the results of UHPC. This model could be benefit in the development of new dosages of UHPC by reducing the time and cost of the experimental campaign, which allows the preselection of components with better response in the model.

Prediction of Early Compressive Strength of Ultrahigh-Performance Concrete Using Machine Learning Methods

In this study, a new prediction model is proposed to predict the 7-day compressive strength of ultrahigh-performance concrete (UHPC) with different mix proportions using artificial neural network (ANN) and support vector machine (SVM). The predicted results are compared with the experimental results to verify the proposed model. Then, the importance of each component and the sensitivity of parameters are investigated. The research proves that the proposed model can estimate the 7-day compressive strength of UHPC based on the mix proportions.

Effect of incorporating hematite on the properties of ultra-high performance concrete including nuclear radiation resistance

Radiation is one of the most important problems for nuclear power plants. To fulfill the urgent need for building materials with high radiation resistance and improved mechanical and durability properties, a novel antiradiation ultra-high-performance concrete (UHPC) was designed. This was accomplished by using hematite powder to partially replace natural river sand at different replacement ratios. A series of tests were conducted, addressing fresh concrete’s fluidity, hardened concrete’s compressive strengths at room and elevated temperatures, impact strength, chloride resistance, radiation resistance, micropore structure, and hydration products. Experimental results show that the addition of hematite slightly decreased the work performance and compressive strength of UHPC, but substantially increased its flexural and impact strength and showed satisfactory high-temperature performance. The gamma-ray shielding performance of hematite UHPC was substantially enhanced by increasing the hematite replacement ratio. Compared with ordinary concrete, the linear attenuation coefficient of the UHPC with a 40% hematite replacement ratio increased by 43%, and the half-value layer thickness was reduced by 30%. The addition of hematite powder did not change the types of cement hydration products, but did improve the internal micropore structure, and cause the UHPC to exhibit excellent chloride ion penetration resistance.

Study on the mechanical and rheological properties of ultra-high performance concrete

The ultra-high performance concrete (UHPC) was prepared by using the limestone coarse aggregate and ordinary Portland cement. The effects of sand ratio, water to cement ratio (W/C), cementitious materials, type and content of fiber, dosage of water reducer and defoamer on the mechanical and rheological properties of the UHPC were investigated. The correlations among influence factors and performances of UHPC were discussed. Moreover, the internal relationship between loading rate and elasticity modulus of UHPC was also studied. The results indicate that the compressive strength of UHPC first increases and then decreases with the increase of sand ratio, and a maximum compressive strength of UHPC was found at the sand ratio of 41%. The water reducer content of fresh UHPC firstly decreases and then increases with the increase of sand ratio. The compressive strength of UHPC firstly increases and then decreases with the increase of W/C, which also increases with the increase of cementitious materials. The elasticity modulus of UHPC decreases with increase of sand ratio and cementitious materials content, and the compressive strength and elasticity modulus of UHPC cured for 28 d increase with the increase of defoamer dosage. The slump flow of fresh UHPC first increases and then decreases with the increase of water reducer dosage. An optimum fiber content of UHPC can enhance the tensile strength and adhesive property, which can play the restraining deformation and toughening effect.

Mechanical Performance and Microstructure of Ultra-High-Performance Concrete Modified by Calcium Sulfoaluminate Cement

High autogenous shrinkage property is one of the disadvantages of ultra-high-performance concrete (UHPC), which may induce early age cracking and threaten the safety of concrete structure. In the present study, different dosages of calcium sulfoaluminate (CSA) cement were added in UHPC as an effective expansive binder. Hydration mechanism, autogenous shrinkage property, and compressive strength of UHPC were carried out to investigate the effect of CSA addition on the mechanical properties of UHPC. Scanning electron microscopy was also employed to characterize the intrinsic microstructural reasons relating to the changes in macroproperties. Based on the XRD diagram, increasing formation of ettringite and Ca(OH)2 can be found with increasing CSA content up to 15%. In the heat flow results of UHPC with 10% CSA addition, the maximum heat release increases to 2.6 mW/g, which is 8.3% higher than the reference UHPC, suggesting a higher degree of hydration with CSA addition. The results in autogenous shrinkage show that CSA expansion agent plays a significantly beneficial role in improving the autogenous shrinkage of UHPC. The corresponding autogenous shrinkage of UHPC is −59.66 μ ε , −131.11 μ ε , and −182.31 μ ε , respectively, at 7 d with 5%, 10%, and 15% addition, which is 108%, 117%, and 123% reduction compared to the reference specimen without CSA. In terms of compressive strength, UHPC with 5%, 10%, 15%, and 20% CSA addition has 10.5%, 17.4%, 30.2%, and 22.1% higher compressive strength than that for the reference UHPC at 28 d. Microstructural study shows that there is an extremely dense microstructure in both the bulk matrix and interfacial transition zone of UHPC with 10% CSA addition, which can be attributed to the higher autogenous shrinkage property and can therefore result in higher mechanical performance.

Effect of Elevated Temperature on the Bond Strength of Prestressing Reinforcement in UHPC.

The study explores the effect of elevated temperatures on the bond strength between prestressing reinforcement and ultra-high performance concrete (UHPC). Laboratory investigations reveal that the changes in bond strength correspond well with the changes in compressive strength of UHPC and their correlation can be mathematically described. Exposition of specimens to temperatures up to 200 °C does not reduce bond strength as a negative effect of increasing temperature is outweighed by the positive effect of thermal increase on the reactivity of silica fume in UHPC mixture. Above 200 °C, bond strength significantly reduces; for instance, a decrease by about 70% is observed at 800 °C. The decreases in compressive and bond strengths for temperatures above 400 °C are related to the changes of phase composition of UHPC matrix (as revealed by X-ray powder diffraction) and the changes in microstructure including the increase of porosity (verified by mercury intrusion porosimetry and observation of confocal microscopy) and development cracks detected by scanning electron microscopy. Future research should investigate the effect of relaxation of prestressing reinforcement with increasing temperature on bond strength reduction by numerical modelling.

Effects of lightweight sand and steel fiber contents on the corrosion performance of steel rebar embedded in UHPC

Ultra-high-performance concrete (UHPC) is able to provide better protection for embedded steel rebar from corrosion due to its superior impermeability characteristics. Lightweight sand (LWS) and steel fibers are utilized in mixing UHPC to promote internal curing and improve flexural strength, respectively. But the effects of different contents of LWS and steel fibers on the corrosion performance of steel rebar and resistance of UHPC matrix are rarely investigated. In this study, steel bar reinforced UHPC specimens with different LWS and steel fiber contents were prepared for electrochemical tests, involving open circuit potential (OCP), Tafel polarization, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS). The experiment results indicated OCPs values based on ASTM C876 could not be applied to decide the corrosion state of steel rebar since lower content of oxygen in UHPC reduced the cathodic reaction, which led to lower OCP values. Chloride ions did not penetrate into UHPC matrix and steel rebar were in passive state in the whole test period of 147 days (d). Corrosion resistance of steel rebar, and resistance of UHPC matrix and passive film displayed an increasing trend with time. Generally, the resistance decreased when the LWS and steel fiber contents were increased. This is because LWS increased the permeability of UHPC and steel fibers led to shortcut of current. For better electrical resistance and compressive strength of UHPC, it is recommended to limit the LWS replacement ratio to 25%. It is also concluded that, steel fiber with the volume content up to 3% will not lead to corrosion of steel rebar and can be safely used in UHPC.

Flexural capacity prediction of composite RC members strengthened with UHPC based on existing design models

Ultra-high-performance concrete (UHPC), a new generation of cementitious materials with very high strength, ductility, and durability, has been used for enhancing reinforced concrete (RC) structures as a strengthening material. The structural performance of RC members strengthened by UHPC or UHPC-concrete composite members is significantly improved. However, there are few methods for predicting the capacity of UHPC-concrete composite members. The present study proposes a simple method based on existing design models for the prediction of the flexural strength of composite RC members strengthened with UHPC at the tension zone. Rectangular stress block diagrams are used for the compression and tension zones of the conventional concrete and UHPC layers, respectively, in a composite section. The prediction results show good agreement with experimental results. The failure modes are predicted and compared with those observed in an actual test. Furthermore, a parametric study is carried out by varying some parameters, including the thickness of the UHPC layer and the compressive strength of UHPC. The results show that the proposed method can predict the flexural strength of UHPC-concrete composite members.

Effects of key parameters on fluidity and compressive strength of ultra‐high performance concrete

AbstractThe compositions and curing regimes have significant effects on the compressive strength and the fluidity of ultra‐high performance concrete (UHPC). Fifty‐eight mixtures were examined to determine the effects of key factors on the fluidity and the compressive strength of UHPC. These parameters include water‐binder ratio, cement strength, steel fiber content, fly ash content, nanoparticles content, silica fume grade, and curing regimes. The fluidity of UHPC was in the ranges 122–237 mm, while the 28d compressive strength on 70.7 mm cube specimens of UHPC were in the ranges 77–211 MPa. The results showed that fly ash and nanoparticles had significant effects on the fluidity and slight effects on the compressive strength; silica fume grade had slight effects on both the compressive strength and the fluidity; the water to binder ratio, curing regimes, and steel fiber had noticeable effects on the compressive strength. Based on results, the influence coefficients of the investigated key parameters were obtained by regression analysis. Furthermore, the prediction models of the fluidity and the compressive strength of UHPC were proposed by combining the influence coefficients of the investigated key parameters. Therefore, the prediction models can be utilized to optimize the proportions of UHPC mixtures within the ranges of the investigated parameters.

Finite element analysis of headed studs embedded in thin UHPC

With an increasing use of ultra-high performance concrete (UHPC) in civil engineering, steel-concrete composite structures continue to progress. However, studies focusing on headed studs embedded in UHPC are limited, which hinders the design of steel-UHPC composite structures. This paper investigates the shear-resisting behavior of headed studs embedded in thin UHPC by using the finite-element (FE) analysis technique. A FE model was established for a push-out test, aiming to reflect the plastic behavior of the specimens and to evaluate the damage process in the headed studs and UHPC. The analysis results were compared to test results. It was revealed that the load-slip curve, shear strength, and failure mode all agreed well with the experimental observations. A parametric analysis was performed based on the validated FE model. The influence of stud diameter, stud height, and compressive strength of UHPC on the shear behavior of the headed studs was revealed. Further, the shear strengths obtained in the FE analysis were compared to the results predicted through theoretical equations. The comparison indicates that for headed studs embedded in UHPC, the contribution of the weld collar should be considered to evaluate the shear strength.