Ultra-high Strength Concrete Research Articles

Intrinsic self-sensing piezoresistive behaviors of ultra-high strength alkali-activated concrete

In this work, the self-sensing piezoresistive behaviors of carbon fiber (CF)-reinforced ultra-high strength concrete (UHSC) are investigated using alternating current impedance spectroscopy (ACIS) technique. The effects of different fiber contents, fiber lengths, and curing ages are considered. The electrical behaviors of alkali-activated slag (AAS)-based UHSC (AAS-UHSC) and ordinary Portland cement (OPC)-based UHSC (OPC-UHSC) at similar strength from 80 to 110 MPa are compared. The findings reveal that the intrinsic electrical resistivity of AAS-UHSC is significantly lower than that of OPC-UHSC, approximately 1/30 of the latter. The addition of CF results in a substantial reduction in electrical resistivity for both UHSC types, with reductions of up to 96 %, and shows better compatibility with the AAS-UHSC matrix. The piezoresistive properties of CF-reinforced UHSC are investigated using ACIS at a fixed frequency and further evaluated by parameters including sensitivity, linearity, repeatability, and hysteresis. The results indicate that plain OPC-UHSC does not exhibit piezoresistivity, whereas the addition of CF enhances both the sensitivity and linearity of piezoresistivity. In contrast, AAS-UHSC demonstrates inherent piezoresistive behaviors even without CF. The introduction of 0.5 wt% CF improves the sensitivity (by up to 50 %) and linearity (R2 increased from 0.76 to above 0.90) of piezoresistivity in AAS-UHSC. However, increasing the CF content to 1.0 wt% diminishes the sensitivity (by up to 62.5 %) due to decreased fiber dispersion uniformity. Moreover, the addition of 1.0 wt% 8-mm CF reduces the hysteresis of UHSC regardless of the binder type, with the maximum reduction reaching 62.2 %.

Seismic Behavior of Steel Reinforced Ultra-High Strength Concrete Composite Frame: Experimental and Numerical Study

The seismic behavior of steel reinforced ultra-high strength concrete (SRUHSC) composite frame was investigated through finite element analysis (FEA) modeling. A FEA model for the seismic analysis of the SRUHSC frame was first established and verified with test results. The numerical model was subsequently used to study the seismic performance of the SRUHSC frame, including the P-Δ skeleton curves, the stiffness degradation, the failure mode, the subsequence mechanisms of plastic hinges and the stress–strain distribution. Finally, a parametric study was carried out to investigate the effect of salient parameters on the behavior of the SRUHSC frame. It was found that with the increment of the concrete strength, yield strength of steel, and linear stiffness ratio of beam to column, the horizontal load-bearing capacity and the elastic stiffness of the structure were improved, but there was no significant effect on the ductility. With the increment of the volume stirrup ratio and structural steel ratio, the horizontal load-bearing capacity and the ductility of the structure were both improved. However, with the increment of the axial-load ratio, there was no obvious change in the elastic stiffness of the structure, but the horizontal load bearing capacity and the ductility of the structure decreased obviously. In addition, the accuracy of a concrete constitutive model in the different degrees of constraint for the SRUHSC frame proposed by the authors was verified with the FEA model.

Experimental Study on Properties of Graphene and Hollow Glass Powder-Added Ultra-High Strength Concrete

A new ultra-high strength concrete, in which oxidized graphene nanoplatelet (GO) and hollow glass powder (HGP) are added, has been developed by authors. This paper presents the material properties of the concrete such as workability, compressive and tensile strengths, internal micro structure (SEM and MIP) as well as air-tightness which was tested using an equipment developed in this study. Test results show that workability and tensile strength significantly increase by a small addition of HGP, and that cGO (GO product of company c) and HGP are well dispersed without agglomeration effect, resulting in more than 20% of reduction in porosity. It is also observed that air-tightness increases by 40% compared with conventional ultra-high strength concrete due to reduction in porosity; thus, new ultra-high strength concrete is anticipated to be effectively used for structures that requires air-tightness such as hyperloop tube. Consequently, it was observed that the workability and mechanical properties of UHSC were increased when cGO and HGP were used instead of silica fume (SF), and authors believe that utilization of new material would contribute to the change in manufacturing method and increase in mechanical properties of concrete.

Interaction and compatibility between steel and concrete of circular CFST stub columns with high-strength materials

To promote the application of high-strength-materials in concrete-filled steel tubular (CFST) columns, finite element modeling, and parametric analysis were conducted on circular CFST stub columns filled with normal-strength concrete (NSC) and ultra-high strength concrete (UHSC). For simulating properties of concrete under passive confinement, a three-dimensional constitutive model for NSC and UHSC was established, of which the stress-strain relation and evolution of dilation angle are both related to confining pressure and axial plastic strain. The model was verified by tests of CFHSST (concrete-filled high-strength steel tube) and UHSCFST (ultra-high-strength concrete-filled steel tube), demonstrating that the interaction between concrete and steel tube can be revealed correctly. Parameter analysis of CFST stub columns filled with NSC and UHSC was carried out, with the grade of steel tube covering from Q235 to Q890 and the D/t ratio ranging from 8.9 to 75. With the decrease of D/t ratio, the deformation capacity and post-peak performance of columns are improved. A recommended range of steel contribution ratio of UHSCFST was proposed for the consideration of cost and safety, which is 25% ∼ 63%. Based on the analysis of interaction between concrete and steel tubes, the compatibility of the two materials is recommended, of which the yield of steel tube occurs after contact with concrete and before the concrete reaches its peak strength. Besides, the formula in EC4 overestimates the load-bearing capacity of CFST stub columns with high-strength materials. Therefore, a modified formula was proposed that reduces the enhancement factor of concrete strength and enables a safer prediction of load-bearing capacity.

Strain-rate-dependent performance of alkali-activated binder-based ultra-high strength concrete

Ultra-high performance concrete(UHPC) is a superior protective material in impact and blast resistance, but negatively influences the environment due to the large amount of cement consumption and CO2 emissions during the preparation process. Alkali-activated binders were invented as a promising alternative to Portland cement for preparing UHPC in a method that saves energy and emits less CO2. However, the strain-rate-dependent performance of alkali-activated binder-based ultra-high strength concrete (AAB-UHSC) is still unknown. Therefore, this work focused on the strain-rate-dependent performance, e.g., dynamic mechanical properties, energy absorption and failure patterns, of AAB-UHSC materials incorporated with different steel fiber contents, as well as the microdamage at different strain rates. The quasi-static compression test, split Hopkinson pressure bar test, the sieving test and the scanning electron microscope test were carried out. The results indicated that the AAB-UHSC material is a strain rate-dependent material, the different stress-strain curves corresponding to different failure patterns vary with the strain rate, and an obvious strain rate transition zone existed for different failure patterns, as did the energy for different failure patterns. Additionally, the addition of steel fibers could effectively enhance the dynamic macromechanical performance and reduce the microdamage. An impact toughness index suitable for AAB-UHSC was defined. Eventually, a modified ZWT dynamic constitutive model with a new damage evolution equation for the AAB-UHSC material was proposed, and results show that this model can well describe the stress-strain relationship of the AAB-UHSC under impact load.

A Modified Reactive Powder Concrete Made with Fly Ash and River Sand: An Assessment on Engineering Properties and Microstructure

Containing a high quantity of both fine powders and steel fiber makes reactive powder concrete (RPC) a unique kind of ultra-high strength concrete. However, the cost of manufacture, shrinkage, and hydration heat are increased when silica fume and cement are used in significant amounts. To mitigate these negative consequences and the environmental impact, this study assessed the use of fly ash (FA) with high volume combined with natural-fine river sand (NFRS) in the manufacturing of RPC. FA was utilized to partially substitute cement at 0, 20, 40, and 60 wt% in RPC mixtures that had a set water/binder ratio of 0.2. Thermal conductivity, porosity, water absorption, and compressive strength tests were performed. Furthermore, RPC's microstructure was examined using a scanning electron microscope (SEM). This study also included a cost and global warming potential analysis of RPC production. Test results indicated that a modified RPC with a 60 MPa compressive strength value could be created by using NFRS and a large amount of FA. In comparison to the reference mixture, a higher compressive strength, reduced water absorption, and lesser porosity were observed in RPC when the FA replacement amount was less than 40%. Many FA particles did not engage in the hydration reaction when the FA replacement level was more than 40%, which had a detrimental impact on the RPC's characteristics. In general, using FA to produce RPC has certain benefits for the economy and the environment. It is recommended that 40% of FA be used in actual practice.

Creep and fracture of UHSC — A microindentation study

Creep and fracture behaviour of normal strength concrete was investigated by various researchers whereas, research on creep and fracture of Ultra-High-Strength Concrete (UHSC), i.e., concrete with 28-day compressive strength greater than 100MPa is scanty. Also, a detailed investigation of the creep–fracture interaction of UHSC has not been conducted.This study aims at examining creep and fracture of UHSC cement paste mixes (w/b = 0.20) with varying levels of silica fume (SF) replacement (0%, 20%, and 40%) at different ages (3d, 7d, 28d, 90d) using microindentation technique. Two different holding durations, namely 5 s and 180 s, were considered in the indentation test. The findings revealed a noteworthy variation in estimated fracture toughness (Kc) between the two dwelling periods. Longer sustained loading times (180s) indicated a decrease in fracture toughness for all ages considered, suggesting progressive internal cracking during creep phase. Attempt has been made to investigate the interaction effect.

Axial compressive behavior and failure mechanism of CFRP partially confined ultra-high performance concrete (UHPC)

With comparable mechanical properties, higher cost-effectiveness and ease of column joints rehabilitation, partial fiber-reinforced polymer (FRP) confinement holds promise as an alternative to full FRP confinement in ultra-high performance concrete (UHPC). Nevertheless, the compressive behavior and underlying failure mechanism of FRP partially confined UHPC are not fully understood. To address this issue, axial compression tests on ultra-high strength concrete (UHSC, removing fibers in UHPC) and UHPC cylinders fully and partially confined by FRP are conducted in this study. The stress-strain relationship, strain distribution and crack development of specimens subjected to axial compression are profoundly analyzed using a combination of strain gauges and digital image correlation (DIC). Experimental results indicate that partial FRP confinement has a comparable effect on compressive performance compared to full FRP confinement. FRP confinement and fiber inclusion can effectively overcome the brittle nature of UHSC in compressive failure and consequently extend the transition stage, raising the stress/strain retention ratio from 0.91% to 26.45% and improving the enhancement ratio of ultimate strain from 1.97% to 34.4%. Furthermore, a more reliable and accurate prediction model is proposed in this study for evaluating the stress-strain relationship of partial FRP confinement under ultimate conditions.

Confinement-based direct design of axially-compressed octagonal concrete-filled double skin steel tubes

Prior research has highlighted distinctions in the confinement mechanism for concrete-filled double-skin steel tube (CFDSST) columns with octagonal sections compared to circular and square sections. Current design codes primarily address the models for the design of square and circular sections in columns made of concrete-filled steel tube (CFST). This study addresses this gap by presenting a novel confinement-based direct design model specifically developed for octagonal CFDSST columns, encompassing both unstiffened and stiffened outer steel tubes. Initial data collection involved experimental and numerical results of axially-compressed octagonal CFST and CFDSST columns sourced from literature. Subsequently, ultimate compressive resistance, as determined by international design codes (EC4, ACI-318, AISC-360, GB-50936, T/CCES-7, AS/NZS-2327, CSA-S16, AIJ) and existing analytical models, underwent comparison with test results. The results reveal that existing codes and models generally offered accurate predictions, but their consistency across the entire side width-to-thickness (w/to) ratio range varied. The study proposes a new confinement design model for predicting the compressive resistance of octagonal CFDSST columns. Although the new model does not outperform existing ones, it demonstrates satisfactory prediction scatter throughout the w/to ratio and compressive strength (fc′) range, particularly excelling in high-strength concrete (HSC) and ultra-high-strength concrete (UHSC) scenarios. Notably, the proposed formula incorporates the influence of stiffening ribs on the confinement of the octagonal steel tube on concrete, making it applicable for practical design considerations for both unstiffened and stiffened octagonal CFDSST columns.

Nonlinear analysis and design of high-strength concrete filled steel tubular columns under nonuniform fires

The main objective of this study is to investigate the behaviour of concrete-filled steel tubular (CFST) columns with ultra-high strength concrete (UHSC) and high strength steel (HSS) under fires through finite element method (FEM). High-strength materials can be used in CFST columns to reduce member dimensions, lowering material consumption and foundation loads. However, their fire behaviour has been insufficiently examined, especially under nonuniform fire exposure. In this study, thermal-structural models of the composite columns are developed using the SAFIR software. The numerical results are compared with published test data on temperature distribution, axial displacement, lateral displacement, failure modes and failure time. Then, parametric analyses are conducted to investigate the influence of various factors such as concrete strength, steel strength, different fire exposures, column length and load ratio on the fire performance of the CFST columns. In addition, the tabular design approach in European code EN1994–1-2 and Australian/New Zealand code AS/NZS 2327:2017 for designing uniformly exposed CFST columns is evaluated. It is observed that the prescribed design values in the table for R30 and R60 fire ratings under load ratio lower than 0.47 appear insufficient for the allowable slenderer columns. The applicability of the tabulated data for a load ratio up to 0.47 is further checked for the columns with UHSC of 120 MPa and HSS of 690 MPa, which is the strength limits recommended in AS/NZS 2327:2017.

Corrigendum: Influence of ternary hybrid fibers on the mechanical properties of ultrahigh-strength concrete

Corrigendum: Influence of ternary hybrid fibers on the mechanical properties of ultrahigh-strength concrete

Mechanical and Micro-structural Properties of Ultra-High Strength Concrete: A Review

Mechanical and Micro-structural Properties of Ultra-High Strength Concrete: A Review

Effect of cementitious material composition on the mechanical properties of ultra-high strength concrete

Effect of cementitious material composition on the mechanical properties of ultra-high strength concrete

Effect of retarders on the properties of ultra-high strength alkali-activated concrete

Alkali-activated slag-fly ash-silica fume cement (AA-SFSC) is suitable for the production of clinkerless alkali-activated binder-based ultra-high strength concrete (AAB-UHSC). However, using a relatively low water-to-binder ratio and a high alkali dosage to prepare AAB-UHSC leads to rapid setting and a loss of flowability. Thus, the setting time of the AA-SFSC needs to be controlled. In our study, the effect of various retarders (single retarder borax and hybrid retarder a combination of zinc nitrate hexahydrate and sodium gluconate) on the AA-SFSC was investigated. The setting characteristics, reaction kinetics, mechanical performance, pore structure alteration, mineralogical and molecular transformations, and microstructure of the AA-SFSC were systematically examined. The results showed that both borax and a combination of zinc nitrate hexahydrate and sodium gluconate had a remarkable retarding effect on AA-SFSC with a low water-to-binder ratio and high alkali dosage. However, borax had a detrimental effect on the compressive strength, and the effect of adding zinc nitrate hexahydrate and sodium gluconate as the hybrid retarders on the compressive strength depended on the dosage; and a dosage of 1% zinc nitrate hexahydrate and 2% sodium gluconate was recommended for prolonging the setting time of AA-SFSC without compromising the mechanical performance.

Behaviour of ultra-high strength concrete-filled dual-stiffened steel tubular slender columns

This paper is concerned with the behaviour of square concrete-filled dual-stiffened steel tubular (CFDSST) slender columns with a concentrically-placed inner circular steel tube. Previous studies have illustrated that these columns have greater structural performance in terms of load-carrying capacity compared with conventional concrete-filled stiffened steel tubular (CFSST) columns. However, the behaviour of CFDSST slender columns filled with ultra-high strength concrete (UHSC) has not been investigated and current design codes do not include provisions for UHSC, although it is increasingly popular owing to demands for structures to be lighter and more sustainable. Accordingly, the current paper fills that gap in existing knowledge and explores the behaviour of CFDSST slender columns using finite element (FE) analysis. The available test results from previous studies were collated and are employed to validate the numerical model. The validated FE model is then employed to investigate the axial load versus deflection responses for a wide variety of UHS-CFDSST slender columns. The behaviour of both intermediate-length and long columns is assessed through parametric analyses. The results of these studies show that the strength of the concrete sandwiched between the two steel sections, the yield strength of outer steel tube, and the outer tube slenderness ratio have a significant effect on the axial resistance of UHS-CFDSST intermediate-length columns, while the capacity of long columns is most affected by the sandwiched concrete strength. The ultimate resistances are compared with different available design methods, and AISC 360–16 code is recommended for predicting the ultimate resistance of UHS-CFDSST slender columns with modifications proposed to account for the different components forming this innovative cross-section.

Application of response surface methodology for optimizing the compressive strength of green ultra high strength concrete using silica fume

Application of response surface methodology for optimizing the compressive strength of green ultra high strength concrete using silica fume

An improved constitutive model for steel tube-confined ultra-high-strength concrete

This study aims to accurately simulate the compressive behavior of ultra-high-strength concrete (UHSC) under various confinement conditions through conventional triaxial and equi-biaxial compressive tests. The triaxial compressive tests revealed that the stress and dilation angle of the concrete varies with changes in axial plastic strain and confining pressure, which is not well-considered in existing mechanical models for UHSC material. To address this limitation, an improved constitutive model based on the concrete plastic damage (CDP) model is proposed in this paper. The improved model incorporates the confining pressure in the flow rule and the hardening/softening rule, in addition to the axial plastic strain. The tensile and compressive meridians of the yield surface are calibrated based on the triaxial and biaxial compressive test results. Furthermore, the study proposes formulas for expressing the stress–strain relation and the lateral-to-axial plastic strain relation of UHSC under different confining pressures. These formulas help generate the hardening/softening rule and the dilation angle evolution of the flow rule, respectively. To verify the proposed constitutive model, the study simulated tests on UHSC-filled steel tube (UHSCFST) stub columns under compression using the finite element method. The simulated results of the peak bearing capacity of the tested UHSCFST stub column specimens are satisfactory, and the interaction between the steel tube and the concrete is reasonably predicted. The proposed improved constitutive model has the potential to accurately simulate the compressive behavior of UHSC under different confinement conditions, which can aid in the design of composite columns made with UHSC materials.

An experimental and numerical study of ultra-high strength concrete filled circular tube column post-fire

Extensive experimental research was conducted on 10 ultra-high strength concrete filled tubular (UCFCT) columns to investigate their performance after exposure to the ISO-834 standard fire and external load. Two different diameters of steel tubes (219.1 mm and 273 mm) with concrete strength ranging from 161 MPa to 181 MPa were included in the study. The primary parameter examined was temperature, ranging from 94 °C to 618 °C. The study obtained data on the residual capacity, working mechanism, and ductility of the columns under compressive load after the fire. The compression experiments revealed that the load capacity of the UCFCT columns remained high. As the columns deformed beyond the inflection point, the load capacity increased gradually without decline. This indicates that the UCFCT columns exhibited notable ductility even after the fire and cooling process. The residual capacity of the columns was influenced by the highest temperature experienced during the experiment. The study also observed the splitting of the ultra-high strength concrete (UHSC) inside the specimens and described the failure mode. Local wall buckling did not occur prior to yielding, and the specimens demonstrated strain-hardening after reaching the plastic point. The mechanical properties of the steel and UHSC were investigated and incorporated into a numerical model that satisfactorily described the post-fire behavior of the composite columns compared to the test data. Based on this theoretical model, the influence of changing material strength (steel and concrete), fire duration time, specimen diameter, and steel ratio on the residual capacity ratio and residual rigidity ratio was discussed. It was generally observed that sectional dimensions and fire duration time had a significant impact on the residual capacity ratio. Finally, formulas suitable for calculating the residual strength of ultra-high strength concrete-filled circular tube columns after exposure to the ISO-834 standard fire were developed based on the results of the parametric analysis.

Probabilistic capacity models and fragility estimate for NRC and UHSC panels subjected to contact blast

A Performance-based Design (PBD) framework for protective structural walls/slabs susceptible to contact blast has been developed. Performance-based capacity models are predicted for three performance levels namely, operation with minimum damage (P1), medium damage (P2) and collapse prevention (P3) allied to four damage states. The current study examines multi-level PBD instead of single-level conventional collapse-based design. All these probabilistic models are developed by combining mechanical models with dimensionless explanatory functions. Bayesian inference based on numerical data is used to evaluate the unknown model parameters. The constructed models take into account all inherent aleatoric and epistemic uncertainties of material and geometry. For Normal Reinforced Concrete (NRC) and Ultra-high Strength Concrete (UHSC) panels subjected to contact blast, finite element (FE) modelling (LS-DYNA) is used to obtain the necessary data. The validated models have a high level of agreement with chosen experimentation. In addition, local damage formulations, such as crater diameter, crater depth, spall diameter, spall depth, and perforation diameter, are also developed. The developed capacity models are used to estimate fragility, and the obtained plots demonstrate the consistency of the evaluated equations. When subjected to a contact blast, these probabilistic models can be used to construct protective structures based on the expected demand.

Behavior of hybrid FRP-concrete-steel double-skin tubular beams with ultra-high strength concrete and PBL shear connectors under bending

Hybrid double-skin tubular beams (DSTBs) with the external fiber-reinforced polymer (FRP) tube-annular concrete-inner steel tube configuration have excellent mechanical performance and corrosion resistance, but are prone to slip problems. In this study, perfobond shear connectors (PBLs) are proposed to reduce the slip between infilled concrete and inner steel tube. Moreover, using ultra-high strength concrete (UHSC) as the annular material could further improve the structural performance and magnify the lightweight merit of hybrid DSTBs, while hybrid DSTBs with PBLs and UHSC have not been investigated yet. To this end, three-point bending experiments were conducted to hybrid DSTBs with PBLs and UHSC to study the flexural behavior. The mid-span load–deflection curves revealed a three-branched behavior: (i) a first linear ascending branch up to the first yielding at the extreme tension fiber of inner steel tube; (ii) a transition branch to the peak load with a significant slope deterioration when infilled concrete in the compression side reached the unconfined failure strain; (iii) a post-peak residual branch characterized by progressive load reductions over a number of load plateaus. The adoption of UHSC could significantly delay the plastic hinge formation in inner steel tube, and meanwhile improve the bond with PBLs. Thus, the slips between infilled concrete and inner steel tube were minimal (less than 0.2 mm) for most of the specimens, except for those with foamy sheet insertions between PBLs (at around 1 mm). In the end, the design loads of the hybrid DSTBs were predicted by a sectional analysis with high level of accuracy (average error at 6%).