Ultra-high Strength Concrete Research Articles

Mechanical properties and microcosmic mechanism of multi-walled carbon nanotubes reinforced ultra-high strength concrete

There have been much research on carbon nanotubes (CNTs) modified cement-based materials. However, few studies utilized multi-walled CNTs (MWCNTs) to prepare the ultra-high strength concrete (UHSC). This study aims to discover a new UHSC reinforced by MWCNTs and reveal the enhancement mechanism of MWCNTs. In this study, the mechanical properties of UHSC with different contents of MWCNTs (both dispersed and undispersed) were tested. The element distribution and microstructure of UHSC were observed by energy-dispersive spectrum (EDS) and scanning electron microscopy (SEM). In addition, the pore structures of UHSC were exhibited through X-ray CT images. The result shows that the compressive strength and flexural strength of UHSC reached the maximum value with the addition of 0.1 wt% dispersed MWCNTs. The dispersed aqueous solution of 0.1 wt% MWCNTs can be uniformly distributed in the UHSC matrix, and MWCNTs can fill the initial defects and bridge the microcracks inside the UHSC. Moreover, with the addition of 0.1 wt% dispersed MWCNTs, the number of pores decreased by 59.98%, and the porosity of UHSC decreased by 14.41%. The expectation of this study is to provide a reference for the better preparation of UHSC.

Reliability analysis & performance-based code calibration for slabs/walls of protective structures subject to air blast loading

A multilevel performance-based design (PBD) framework is developed for panels of Protective structures subjected to air blast. These provisions improve the existing design approach of blast resistance structures for ultimate limit state. Present study develops probabilistic deflection-based capacity and demand models for three performance levels associated with four damage states occurring during an air blast. The chosen protective structures for current study are reinforced Normal Strength Concrete (NRC) and reinforced Ultra-High Strength Concrete (UHSC) panels. The probabilistic models are developed using Bayesian inference (Posterior Statistics). The database is generated through numerical experimental design using finite element (FE) based software, LS-DYNA. The influence of air blast on panel is validated on a panel and obtained results shows the trustworthiness of numerical model. Grounded on these models the fragility estimation is carried out and attained plots demonstrates the consistency. Using a reliability based approach, the Load and Resistance Factors for Design (LRFD) are developed associated with the proposed three performance levels. Hazard curves and total probability are estimated using established dataset from literature available. The estimated LRFD factors can be used for the design of NRC & UHSC panel of slab/walls of protective structure subjected to air blast.

Autogenous shrinkage reduction and strength improvement of ultra-high-strength concrete using belite-rich Portland cement

Autogenous shrinkage is a non-trivial problem affecting the use of ultra-high-strength concrete (UHSC). Hydration is an important cause of autogenous shrinkage. This study proposes a method to reduce autogenous shrinkage and increase strength by controlling the hydration reaction of the binder. Belite-rich Portland cement (BPC) was used to replace a fraction of the type I ordinary Portland cement (OPC) in the binder (replacement ratios of 33%, 66%, and 100%), and the hydration heat, autogenous shrinkage, compressive strength, hydration products, ultrasonic pulse velocity, electrical resistivity, and CO2 emissions were simultaneously determined. The results are summarized as follows: 1) With increasing BPC substitution ratio, the autogenous shrinkage of the specimen decreased because the addition of BPC lowered the hydration rate. 2) The addition of BPC reduced the early compressive strength (3 and 7 d) and promoted the development of long-term compressive strength (28 and 56 d). 3) The microstructural analysis showed that compared with the control group, the low reactivity of BPC leads to less Ca(OH)2, AFt, and AFm production. 4) The ultrasonic pulse velocity and strength had a strong linear relationship for various specimens at different ages. 5) The electrical resistivity gradually decreased as the amount of BPC substitution increased. 6) The addition of BPC reduced the normalized CO2 emissions per MPa of strength. In summary, upon replacing OPC with BPC, UHSC exhibited lower autogenous shrinkage, lower cumulative hydration heat, lower CO2 emissions, and higher long-term strength.

Comprehensive study on the mechanical property and fracture behavior of ultra-high strength concrete

In order to figure out the relationship between mechanical property and fracture behavior of UHSC and investigate its influencing factors, a series of UHSC with different mix proportion have been prepared. The mechanical properties (including compressive strength, flexural strength, and splitting tensile strength) were tested on a hydraulic servo universal testing machine. Three-point bend loading method was used to measure the fracture parameters (including fracture toughness and characteristic length) and a crack was prefabricated in the sample. The results show that the strength of UHSC decreases with w/b ratio and the maximum compressive strength and flexural strength can reach 132 MPa and 21.5 MPa, respectively. UHSC displays significant brittle fracture behavior and its brittleness increases with strength. The main reason is that the micro-defects of UHSC are reduced and the homogeneity of the whole cement matrix is enhanced, thus the crack initiation strength is improved but crack resistance ability is impaired. High temperature curing increases the strength as well as brittleness of UHSC. Adding fly ash will improve its characteristic length but reduce the facture toughness. Our work may provide guidance for reducing the brittleness of ultra-high strength concrete.

Experimental Research on Flexural Mechanical Properties of Ultrahigh Strength Concrete Filled Steel Tubes

Based on the project of the Guansheng Qujiang Bridge, the flexural mechanical properties of an ultrahigh strength concrete filled steel tube (UHSCFST) were discussed. A total of six UHSCFST beam specimens were tested, and the cube strength (fcu) of the core concrete reached 80.3–115.2 MPa. The effects of concrete strength on flexural bearing capacity, deformation characteristics, and failure modes of UHSCFST specimens were discussed. Test results showed that the bending failure modes of UHSCFST specimens were the same as those of ordinary ones. The failure of UHSCFST specimens was attributed to excessive deflection, and local buckling occurred in the compression zone. Moreover, the bending capacity of the specimens did not decrease, even if they had yielded. Although ultrahigh strength concrete was poured, all of the specimens displayed outstanding bending ductility. The main function of core concrete was to provide radial restraint for the steel tube to avoid premature buckling. When the steel content of the specimen section was constant, the strength increases of core concrete had a slight impact on the bending failure mode, bearing capacity and ductility of UHSCFST specimen. The research results can deepen the understanding of the mechanical behaviors of the UHSCFST composite truss structure.

Effect of Aggregate Type on Properties of Ultra-High-Strength Concrete.

In this work, we present an analysis of natural fine aggregates’ influence on the properties of ultra-high-strength concrete. The reference concrete mix was made of natural sand with the addition of fly ash and microsilica. It was assumed to obtain concrete with a very high strength without the addition of fibers and without special curing conditions, ensuring the required workability of the concrete mix corresponding to the consistency of class S3. The reference concrete mix was modified by replacing sand with granite and basalt aggregate in the same fractions. Five series of concrete mixes made with CEM I 52.5R cement were tested. Experimental investigations were carried out regarding the consistency of the concrete mix, the compressive strength, the flexural strength and the water absorption by hardened concrete. A comparative analysis of the obtained results indicated significant improvement in the concrete strength after the use of basalt aggregate. The strength of the concrete series based on basalt aggregate, BC1, allowed it to be classified as ultra-high-performance concrete. Concrete based on sand, SC1, was characterized by the lowest compressive and flexural strength but obtained the best workability of the mix and the lowest water absorption. The results presented in the paper, show a significant influence of the type of aggregate used on the mechanical and physical properties of ultra-high strength concrete.

THE USE OF ULTRA-HIGH STRENGTH CONCRETE (UHSC) WITH LONGITUDINAL GFRP REINFORCEMENT UNDER FLEXURE

This research investigates the behavior of ultra-high strength concrete (UHSC) beams with GFRP reinforcement under flexure. The design parameters included the type of longitudinal reinforcement (GFRP or steel), as well as the reinforcement ratio (0.26%, 0.41%, and 0.67%). The beam specimens had a width of 185 mm, a depth of 250 mm, and a total span length of 2200 mm. All the beams were developed with 140 MPa concrete and tested in a four-point bending setup. The test results revealed that increasing the GFRP ratio improved the flexural capacity of the tested beams. By increasing the ratio from 0.26% to 0.41%, the moment capacity increased by 47.5%, whereas increasing the ratio from 0.26% to 0.67% increased the capacity by 76.4%. Moreover, the steel-reinforced beam reported a smaller moment capacity and mid-span deflection values as compared to its GFRP counterpart. Overall, the ACI 440 code provided a good description of the behavior of FRP reinforced beams developed with UHSC.

STUDY ON FLEXURAL BEHAVIOR OF ULTRA HIGH STRENGTH CONCRETE USING L-BOX

Ultra-high strength concrete (UHSC) a new technology to strengthen precast industry business with new products and solutions. The purpose of this research is to gauge the performance of ultra-high strength concrete in flexure, as incremental compressive strength of concrete will increase its brittleness. In the present investigation, the cement is partially replaced with scm’s like ultra-fine GGBS. M125 mix has been designed with W/C ratio 0.15 with high end water reducing agent with normal curing in ambient temperature of 27°C +/-2°C and concrete has been placed in the prisms with and without using L-Box. The maximum flexural strength has been achieved 22.86 N/mm2 for M125 when used L-Box (as concrete flow through L-Box made the steel fibers orientation in the uniform direction and parallel to the neutral axis in tension zone) during the concreting. With this study we can conclude that the flexural strength of ultra-high strength concrete is purely depending upon the placing of the concrete during the application and the orientation of fibers in tension zone of the concrete. The flexural strength of UHSC can be improved easily by making the concrete flow through L-Box.

Mitigation on the shrinkage properties of ultra-high strength concrete via using porous coral sand and shrinkage reducing agent

This work used porous coral sand aggregate (PCSA) and shrinkage reducing agent (SRA) in hybrid systems in order to compensate high shrinkage of ultra-high strength concrete (UHSC). Accordingly, the flowability, setting times, mechanical properties autogenous shrinkage and drying shrinkage of UHSC incorporating PCSA and SRA were studied. Moreover, Scanning electron microscopy (SEM) and Mercury intrusion porosimeter (MIP) were used to characteristic the microstructure development. The results showed that pretreated PCSA improved the fluidity and compensate the autogenous and drying shrinkage, while it decreased the compressive strength of UHSC. The addition of SRA exhibited similar trend. The hybrid system of PCSA and SRA were beneficial to the fluidity and compensated the autogenous and drying shrinkage significantly, whereas it had slightly adverse effect on the compressive strength of UHSC. It was also found that, due to the synergistic effects of internal curing by PCSA and reduction effect by SRA, the dense microstructure and increased of the distribution of large capillary pores in UHSC were effectively mitigation on the autogenous shrinkage. Overall, the specimens with 45% PCSA and 1.0% SRA had the minimum autogenous and drying shrinkage.

A systematic review on CFST members under impulsive loading

Concrete-filled steel tubes (CFST) have been increasingly used in the construction industry due to their favourable structural performance. With the increase of accidental events in the past two decades, researchers paid more attention to the response of CFST members under impulsive (lateral impact/blast) loading. Research projects conducted aim to better understand the structural performance of CFST members through experimental, numerical, and analytical approaches. This paper aims to systematically characterise the state-of-the-art research on the structural behaviour of CFST subjected to lateral impulsive loading. The reviewed studies include circular and rectangular/square cross-sections, conventional members and those strengthened with fibre reinforced polymer (FRP). Members with normal strength concrete (NSC), high strength concrete (HSC), as well as ultra-high-strength concrete (UHSC) infill are all included. Following the literature review, the current research trends are identified, and potential future research directions are discussed based on the identified knowledge gaps. A consensus of the results shows that CFST members have superior resistance to impulsive loading compared to their hollow counterparts and reinforced concrete (RC) members.

Artificial intelligence-based estimation of ultra-high-strength concrete's flexural property

Advancement in Artificial Intelligence (AI) techniques and their applications in the construction industry, particularly for predicting mechanical properties of concrete, leads to conservation of efforts, time and cost. However, insufficient research has been done on the Ultra-high-strength concrete (UHSC). For this reason, this study aims to predict the UHSC flexural strength by applying sophisticated AI approaches. Ensembled machine learning techniques performed well compared to the individual decision tree (DT) model. In the current research, UHSC flexural strength is predicted by employing supervised Machine Learning (ML) approaches, i.e., DT-Bagging, DT-Gradient Boosting, DT-AdaBoost, and DT- XG Boost. Moreover, the model performance is assessed with the help of R2, Mean Absolute Error (MAE), and Root Mean Square Error (RMSE). In addition to that, the validation of model performance is also done by using the k-fold cross-validation technique. Higher R2 of 0.95 and lesser error (i.e., RMSE and MAE) values, in the case of the DT-Bagging model, depict improved model performance with respect to other applied ensemble methods. The assessment output shows that anticipated outcomes from proposed models, i.e., DT-Bagging, are much closer to actual results from experiments, which indicates the enhanced prediction of flexural strength for UHSC. Further, the SHapley Additive exPlanations (SHAP) analysis shows that steel fiber content has the highest positive influence on UHSC flexural strength.

Effect of different burning degrees of sugarcane leaf ash on the properties of ultrahigh-strength concrete

The global expansion of agricultural production increases agricultural waste ash (AWA). Accordingly, AWA should be disposed to preserve the environment. This study focuses on using AWA as a partial substitute for cement to produce ultrahigh-strength concrete (UHSC). This research investigated the effect of using sugarcane leaf ash (SLA) as a pozzolanic material on the properties of UHSC. The cement replacement rates by SLA were 10%, 20% and 30% by weight. SLA was heat-treated at 400, 500, 600, 700 and 800 °C for 2 h to improve its physical and chemical properties. The effects of the heat-treated SLA on UHSC mechanical properties such as, compressive strength, split tensile strength, flexural strength and modulus of elasticity, were studied. The effects of heat-treated SLA on UHSC durability such as, water permeability, chloride penetration and sorptivity of UHSC were also investigated. In addition to, microstructure analysis of several UHSC mixtures was presented. Results showed the efficiency of SLA as a partial substitute of the 20% of cement weight with mechanical properties and durability higher than the mechanical properties and durability of the reference mixture. Compared with the reference mixture, the heat-treated SLA used as a partial substitute generally improved all properties. The UHSC containing SLA heat-treated at 700 °C and a 20% substitution rate achieved the best results of 162.5, 17.78, 24.05 and 55,820 MPa for compressive strength, tensile strength, bending strength and modulus of elasticity at test age of 28 days.

THE USE OF ULTRA-HIGH STRENGTH CONCRETE (UHSC) WITH LONGITUDINAL GFRP REINFORCEMENT UNDER FLEXURE

This research investigates the behavior of ultra-high strength concrete (UHSC) beams with GFRP reinforcement under flexure. The design parameters included the type of longitudinal reinforcement (GFRP or steel), as well as the reinforcement ratio (0.26%, 0.41%, and 0.67%). The beam specimens had a width of 185 mm, a depth of 250 mm, and a total span length of 2200 mm. All the beams were developed with 140 MPa concrete and tested in a four-point bending setup. The test results revealed that increasing the GFRP ratio improved the flexural capacity of the tested beams. By increasing the ratio from 0.26% to 0.41%, the moment capacity increased by 47.5%, whereas increasing the ratio from 0.26% to 0.67% increased the capacity by 76.4%. Moreover, the steel-reinforced beam reported a smaller moment capacity and mid-span deflection values as compared to its GFRP counterpart. Overall, the ACI 440 code provided a good description of the behavior of FRP reinforced beams developed with UHSC.

STUDY ON FLEXURAL BEHAVIOR OF ULTRA HIGH STRENGTH CONCRETE USING L-BOX

Ultra-high strength concrete (UHSC) a new technology to strengthen precast industry business with new products and solutions. The purpose of this research is to gauge the performance of ultra-high strength concrete in flexure, as incremental compressive strength of concrete will increase its brittleness. In the present investigation, the cement is partially replaced with scm’s like ultra-fine GGBS. M125 mix has been designed with W/C ratio 0.15 with high end water reducing agent with normal curing in ambient temperature of 27°C +/-2°C and concrete has been placed in the prisms with and without using L-Box. The maximum flexural strength has been achieved 22.86 N/mm2 for M125 when used L-Box (as concrete flow through L-Box made the steel fibers orientation in the uniform direction and parallel to the neutral axis in tension zone) during the concreting. With this study we can conclude that the flexural strength of ultra-high strength concrete is purely depending upon the placing of the concrete during the application and the orientation of fibers in tension zone of the concrete. The flexural strength of UHSC can be improved easily by making the concrete flow through L-Box.

Compressive Strength Evaluation of Ultra-High-Strength Concrete by Machine Learning

In civil engineering, ultra-high-strength concrete (UHSC) is a useful and efficient building material. To save money and time in the construction sector, soft computing approaches have been used to estimate concrete properties. As a result, the current work used sophisticated soft computing techniques to estimate the compressive strength of UHSC. In this study, XGBoost, AdaBoost, and Bagging were the employed soft computing techniques. The variables taken into account included cement content, fly ash, silica fume and silicate content, sand and water content, superplasticizer content, steel fiber, steel fiber aspect ratio, and curing time. The algorithm performance was evaluated using statistical metrics, such as the mean absolute error (MAE), root mean square error (RMSE), and coefficient of determination (R2). The model’s performance was then evaluated statistically. The XGBoost soft computing technique, with a higher R2 (0.90) and low errors, was more accurate than the other algorithms, which had a lower R2. The compressive strength of UHSC can be predicted using the XGBoost soft computing technique. The SHapley Additive exPlanations (SHAP) analysis showed that curing time had the highest positive influence on UHSC compressive strength. Thus, scholars will be able to quickly and effectively determine the compressive strength of UHSC using this study’s findings.

Practical ultra-high-strength concrete for precast concrete applications

This paper focuses on developing ultra-high-strength concrete (UHSC) with mechanical properties compara­ble to those of ultra-high-performance concrete (UHPC) and with production procedures similar to those used for high-strength concrete. A concrete mixture was developed to achieve a compressive strength of 17 ksi (117 MPa) at 28 days without using silica fume and with a slump flow as high as 35 in. (889 mm). Another concrete mixture with compressive strength of 20 ksi (138 MPa) was designed using only 130 lb/yd3 (77.1 kg/m3) of silica fume. In addition to these two mixtures, a UHPC mixture was also designed with a compressive strength of 24 ksi (165 MPa). Laboratory tests were conducted to investi­gate predictions of modulus of elasticity, flexural strength, creep, and shrinkage for the developed mixtures. This paper demonstrates that using UHSC and 0.7 in. (18 mm) diameter strands will produce adequate flexural strength to design 300 ft (91.4 m) long prestressed concrete I-girders that have service stresses within safe limits

Simulation of the behavior of ultra high strength concrete (UHSC) filled in steel tube columns under axial compression

Concrete filled steel tube columns (CFST) using UHSC (Ultra-high performance concrete) has been a new trend in civil engineering. This paper is aimed at presenting the finite element method (FEM) using ABAQUS software to verify some previous test results of circular UHSC filled steel tube columns under axial compression. The material models of UHSC and steel tube were suggested to modify the conventional models in ABAQUS software. The research finding from the paper indicates that there is a good agreement between the FEM results and the experiments with regard to the failure modes and the curves of load versus strain. Furthermore, the distribution of total load on the steel tube and the UHSC core of each load increment can be predicted by the FEM.

Multiscale modelling framework for elasticity of ultra high strength concrete using nano/microscale characterization and finite element representative volume element analysis

Ultra-High Strength Concrete (UHSC) (greater than 100 MPa) is a mechanically superior material compared with the Normal Strength Concrete (NSC) due to its inherent performance characteristics. Improved modulus of elasticity is one of the key target characteristics in the development of UHSC. Macroscopic response of UHSC is a result of a multitude of phases in different spatial length scales such as mesoscale, microscale, nanoscale etc. and investigating these spatial scales can yield a better understanding about contribution of each heterogenous phases to the macroscopic behaviour of UHSC. In this paper, a new multiscale modelling method including procedures to obtain micro/nano scale properties is proposed to predict the macroscopic elastic modulus using nanoindentation experiments, hydration simulations, Scanning Electron Microscopy (SEM) and Finite Element Representative Volume Element (FE-RVE) modelling.Characterization of nanomechanical properties of the cementitious composite was carried out using nanoindentation, microstructural characterization was performed using scanning electron microscopy, and hydration simulation of the cementitious paste was carried out using Virtual Cement and Concrete Testing Laboratory (VCCTL) software. A five-level multiscale framework is proposed for UHSC and results from these experimental testing and simulations were used as inputs in the proposed framework.A novel algorithm which can model any volume fraction of different phases was developed to generate geometries for RVEs to be used in FE-RVE simulations. Upscaling of elastic modulus using FE-RVE was found to be very accurate, and this method can generate detailed variation of microfields inside the RVE. A parametric study was carried out on how varying inhomogeneities in the RVE, boundary conditions, and the shape of the inhomogeneities would affect the homogenized elastic modulus.Continuum micromechanics models such as Mori-Tanaka method and Self Consistent Scheme were used for the analytical homogenization at each scale for comparison with FE-RVE method. The results of the proposed FE-RVE analysis, the Mean Field Homogenization (MFH) method, and experiment were compared and found to be a very good fit.

Effect of silicate-modified calcium oxide-based expansive agent on engineering properties and self-healing of ultra-high-strength concrete

At an early age, ultra-high-strength concrete (UHSC) exhibits a large autogenous shrinkage, which easily leads to matrix shrinkage and cracking. This poses a significant threat to the service life of a building structure. In this study, the effects of silicate-modified calcium oxide-based expansive agents (SCEAs) on the engineering performance and self-healing of UHSC were systematically investigated. SCEA was used as a self-healing promoter for early cracked UHSC, and the compressive strength, ultrasonic pulse velocity, and surface resistivity were determined. Moreover, the water permeability of the cracked samples and the healing degree of the surface cracks were tested. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy, and other microscopic analyses were used to characterize the chemical composition of the healing compounds. The experimental results showed that the compressive strength, ultrasonic pulse velocity, and surface resistivity of UHSC showed decreasing trends with an increase in SCEA doping. Furthermore, inductively coupled plasma optical emission spectroscopy tests showed that the addition of SCEA increased the amount of calcium ion penetration into the UHSC during the healing process. The UHSC doped with SCEA showed more visible crack width healing than the control group, and the reductions in permeability were especially obvious. The results of X-ray diffraction, FTIR, and other microscopic analyses indicated that the main composition of the UHSC healing compounds was a mixture of calcium (aluminate) silicate hydrate, calcium hydroxide, and calcite.

Bond behavior between GFRP bars and seawater sea-sand fiber-reinforced ultra-high strength concrete

To tackle the challenge of steel corrosion in conventional reinforced concrete (RC) structures, particularly in maritime environment, the implementation of glass fiber reinforced polymer (GFRP) bars in seawater sea-sand concrete (SSC) structures becomes increasingly popular due to their excellence corrosion resistance. However, the bond characteristics of GFRP bars in fiber reinforced ultra-high strength SSC have not been explored. To this end, a series of pull-out tests on 63 specimens were performed in this study to investigate the influences of different GFRP bar diameters of (i.e., 6 mm, 10 mm and 16 mm), anchorage lengths (i.e., 2.5 and 5 times of the bar diameter), concrete types (SSC and conventional concrete), and polyethylene (PE) fiber contents (i.e., 0, 0.5% and 1% volume fraction) on bond characteristics. The results show that similar to FRP-conventional concrete bond joints, larger bar diameter and longer anchorage length would lead to bond strength reductions for FRP-SSC bond joints. A 0.5% PE fiber addition in SSC would lead to a 3–10% increase in bond strength. However, further increase in PE content to 1% may not necessarily result in further increase in bond strength. Furthermore, the assessment of different design codes revealed that, although ACI440.1R-15 yielded relatively more accurate estimations for bond strength than other design codes, the bond strength could be overestimated if the bar diameter is greater than 16 mm or the anchorage length is less than 5 times the bar diameter.