Strength Of Ultra-high Performance Concrete Research Articles

Pull-out behavior of cluster stud-post pouring UHPC connectors in prefabricated steel-concrete composite beam

The use of ultra-high performance concrete (UHPC) as a connection material for cluster studs of prefabricated steel-concrete composite beams can achieve higher connection strength and less space for reserved holes. However, in addition to bearing the shear force, the studs need to bear the pull-out force to counteract the vertical relative separation between the steel and concrete. The pull-out behavior for cluster stud-post pouring UHPC connectors is complex and has not been clarified completely. Thus, a total of 30 cluster stud pull-out tests in 10 groups were carried out in this study, and the interface type, length-to-diameter ratio of the stud, transverse spacing of stud group, dimensions of the reserved holes, and type of post pouring concrete were considered. The results showed that the pull-out behavior of the cluster studs was different from that of studs embedded in cast-in-place intact UHPC slabs due to the UHPC conical integrity having been reduced significantly by the old-to-new concrete interface. The rough groove interface and UHPC as a post pouring material significantly improved the pull-out capacity and stiffness of the cluster studs. The stud fracture belongs to ductile failure, whereas UHPC and normal concrete (NC) cone failures are brittle. The bearing capacity of the UHPC cone is observed to increase with the effective embedment depth of the stud, the transverse spacing, and the dimensions of the reserved holes. The angle of the UHPC conical failure surface is 70° based on 3D laser scanning. Finally, the theoretical equations for predicting the pull-out capacity of stud-post pouring UHPC connectors with a rough groove interface were proposed, and the critical length-to-diameter ratio of the stud was calculated for different combinations of stud and UHPC strength.

A New Insight into the Design Compressive Strength of Ultra-High Performance Concrete

Compressive strength is one of the most critical mechanical properties of various types of concrete and is the main input variable for structural concrete design. Recently, with the advances in concrete technology, applications of fiber-reinforced concrete (FRC), including ultra-high performance concrete (UHPC), have grown rapidly. These new types of concrete are well known to exhibit superior mechanical characteristics such as compressive strength, fracture toughness, and durability compared to conventional concrete and thus are popularly used in urgent repair jobs where compressive strength is an important parameter to determine the required curing time until open to the public. Considering the importance of compressive strength in practice, this study aims to evaluate the effect of age and maturity on the compressive strength characteristics of three different types of concrete, namely UHPC with micro and macro steel fibers, FRC, and plain concrete, and to propose a new design strength criterion for UHPC. To this end, 180 concrete cube specimens were tested at 12 different ages between 3 and 126 days. The results indicated that irrespective of the type and presence of fibers, UHPC gained more than 90% of their ultimate compressive strength after only 21 days, while FRC and plain concrete specimens required a longer time (i.e., 28 days) to achieve 90% of their ultimate strength. Therefore, UHPC may adopt a 21-day compressive strength as a design input instead of a 28-day compressive strength commonly required for structural concrete specified by many codes of practice. Moreover, the obtained experimental results were compared with existing compressive strength predictive models in the codes of practice.

Influence of vibrating compaction time on the strength and microstructure of ultra-high performance concrete

During the forming process of ultra-high performance concrete (UHPC), the duration of vibrating compaction plays a pivotal role in influencing its performance. Our study delved into the impact of various factors on the macroscopic mechanical properties, considering the influence of vibrating compaction time on the pore size distribution in the matrix, fibre distribution characteristics, and fibre–matrix interface. We employed SPSS software to analyse the correlation between different factors and UHPC strength. Based on our experimental findings, the influence of vibrating compaction time on the flexural and compressive strengths of UHPC varied within the 0–15 s vibration range. Notably, vibration time had a significant effect on the distribution of pores and fibres within the UHPC matrix. The alterations in pore size distribution and fibre distribution induced by vibrating compaction time were primarily responsible for its impact on UHPC strength. The study provides a solid theoretical foundation for optimising the vibrating compaction time in UHPC mixtures.

Predict the compressive strength of ultra high-performance concrete by a hybrid method of machine learning

Ultra-high performance concrete (UHPC) benefits the construction industry due to its improved flexibility, high workability, durability, and performance compared to normal concrete. Some investigators have conducted observed papers on the UHPC’s mechanical properties for establishing a reliable analytical approach for calculating the compressive strength, tensile strength, slump, etc. However, most of these studies were performed with limited samples because of the UHPC’s high cost. This study aims to predict the compressive strength (CS) of UHPC through hybrid machine-learning approaches. The model is included Adaptive-Network Fuzzy Inference System (ANFIS). Moreover, three meta-heuristic algorithms were employed to improve the developed model's accuracy, including the Generalized Normal Distribution Optimization, the COOT optimization algorithm, and the Honey Badger Algorithm. Several metrics were used to compare and assess the performance of the hybrid models in the framework of ANGN, ANCO, and ANHB. A comparison of the predicted and measured results generally shows that the proposed developed models can reasonably estimate the mechanical properties of UHPC. The results indicated that the ANHB model could estimate the CS of UHPC with the most suitable accuracy.

From machine learning to semi-empirical formulas for estimating compressive strength of Ultra-High Performance Concrete

Although some machine learning (ML) models have successfully developed for ultra-high-performance concrete (UHPC), they do not provide insights and explicit relations between all input variables and its compressive strength. This paper will address these ambiguities and provide a tool to predict the compressive strength by explainable and interpretable equations and plots. Explicit semi-empirical formulas are derived from a multivariate polynomial regression (Lasso) and automated feature engineering and selection (Autofeat) using a 810 dataset of UHPC with 15 input variables, which are collected from literature. Coefficient of determination R2 = 0.8223 is the same for the first-order degree (linear regression) of both models, however, the Autofeat achieves a better result than Lasso for the third-order degree with R2 = 0.9616 vs. R2 = 0.9503. A comprehensive parametric study is carried out via relative feature importance and partial dependence plots to explain and gain profound insights into the effects of some important input variables on the compressive strength of UHPC. Some details discussions related to these effects with previous studies are also presented. The proposed models not only show better performance with those from reference in terms of R2, especially for Autofeat model but also have explicit relations of compressive strength with 15 input variables. Hence, they can be used as a reliable tool in mixture design optimization of the UHPC.

Shear behavior of short studs in steel-thin ultrahigh-performance concrete composite structures

Steel-concrete composite structures gradually tend to be thinner and lighter in modern bridge engineering. Ultrahigh-performance concrete (UHPC) as an innovative solution has been used to upgrade the behavior of composite structures and accelerate construction, and short stud connectors are the key elements to guarantee the effective connection of steel and concrete components. This study conducted push-out test to explore the failure modes and load-slip relationships of short studs in steel-thin UHPC composite structures (STUCs). The experimental findings revealed that the fracture of the stud shank and local concrete crushing dominated the failure modes of all specimens. Increasing stud diameter could enhance shear strength, while arranging short studs densely and decreasing stud height could result in the reduction of shear capacity of a single stud. The experiment data were employed for the construction and verification of the finite element models. The impact of several parameters on the shear strength was investigated via the validated numerical models. The shear strength increased approximately linearly with stud diameter for short studs in thin UHPC layers, but the cover thickness and UHPC strength had a slight impact. The shear capacity of short studs couldn't be negatively impacted by reducing the aspect ratio to 3.16. In addition, the ultimate shear capacity significantly decreased due to the grouped stud effect for grouped short studs when the spacing between the studs was less than 70 mm. The shear capacity was conservatively predicted by these specifications for short studs in thin UHPC layer, according to the comparison of the simulated data with the existing construction specifications. Finally, a new equation considering multiple parameters was proposed and validated by the test data, which could precisely predict the shear strength of short studs in STUCs.

Retracted: Experimental Study on Compressive Strength of Ultrahigh Performance Concrete Prefabricated Wall Structure

Retracted: Experimental Study on Compressive Strength of Ultrahigh Performance Concrete Prefabricated Wall Structure

A new technique based on the gorilla troop optimization coupled with artificial neural network for predicting the compressive strength of ultrahigh performance concrete

A new technique based on the gorilla troop optimization coupled with artificial neural network for predicting the compressive strength of ultrahigh performance concrete

Influence of silica fume and thermal curing on long-term hydration, microstructure and compressive strength of ultra-high performance concrete (UHPC)

In order to enhance the hydration rate and degree of ultra-high performance concrete (UHPC) originated from the extremely low water to binder ratio, thermal curing is widely used to improve the hydration and mechanical properties of UHPC. In this study, the effect of thermal curing temperature on hydration process, microstructure evolution and long-term compressive strength of UHPC is investigated and the effect of silica fume is also studied. The results show that when silica fume is 20%, the maximum compressive strength of UHPC by 170 MPa can be obtained after 90 °C thermal curing for 3 d, which is increased by 26.9% comparing with 20 °C curing for 1 year. Correspondingly both the degree of hydration (DOH) of cement and degree of pozzolanic reaction (DOPR) of silica fume are separately increased by 5.8% and 20.6%. After thermal curing for 1 year, the DOH of cement varies within a maximum increase range of 3%, while the DOPR of silica fume increases slightly, up to 7%. In addition, the total porosity can be reduced by 64.0% under 90 °C thermal curing. Porosity characterized with 10 nm to 50 nm has a decisive impact on the compressive strength whose correlation coefficient can reach 0.90.

Experimental Study on Shear Behavior of Non-Stirrup Ultra-High Performance Concrete Beams.

Due to the high tensile strength of ultra-high performance concrete (UHPC), the shear stirrups in UHPC beams could potentially be removed. The aim of this study is to assess the shear performance of non-stirrup UHPC beams. Six UHPC beams were tested and compared with three stirrup-reinforced normal concrete (NC) beams, taking into consideration the testing parameters of steel fiber volume content and shear span-to-depth ratio. The findings demonstrated that incorporating steel fibers can efficiently strengthen the ductility, cracking strength, and shear strength of non-stirrup UHPC beams and alter their failure mode. Additionally, the shear span-to-depth ratio had a significant impact on the shear strength of beams, as it was negatively related to it. This study revealed that the French Standard and PCI-2021 formulae were suitable for designing UHPC beams with 2% steel fibers and no stirrups. When applying Xu's formulae for non-stirrup UHPC beams, taking into account a reduction factor was necessary.

Study on mechanical and shrinkage properties of ES-UHPC

Some road and bridge projects to be repaired require materials that can quickly and easily obtain high mechanical properties. Added early strength agents are commonly used to improve the early strength of Ultra-high performance concrete (UHPC). Previous studies focused primarily on improving the early strength of UHPC, and few studies on how to reduce early age shrinkage of UHPC. This paper selected two inorganic early-strength agents, lithium carbonate (Li2CO3), nano-calcium carbonate (NC), and two organic early-strength agents, calcium formate(CF), triethanolamine(TEA),and an CaO expansion agent, which are commonly used. Experimental studies were carried out for single and compound early-strength agent systems in a cement-based UHPC system. The results showed three early-strength agents (NC, Li2CO3,CF) reduced the setting time and improved the early-strength of the UHPC. TEA had an adverse effect in the both single and compound systems. The 1-d compressive strength of the group(L0.15N3P10) reaches 84.5 MPa, with good workability and lower shrinkage. It can be used to prepare early strength ultra-high performance concrete (ES-UHPC) at room temperature. In terms of strength, shrinkage, and workability, it is suggested that the combination content of Li2CO3 is 0.15%, NC is 3%, and the expansion agent is 10% for fast repair and reinforcement projects.

Assessing the Results of Compressive Strength of Ultra High-Performance Concrete Using Soft Computing

Ultra-High-Performance Concrete (UHPC) is a new type of concrete that has gained popularity in recent decades due to its high strength and durability. In this study results of M5P and random forest are compared to predict the compressive strength of ultra-high-performance concrete for 28 days. A total of 236 readings are taken in this investigation. Out of 236 readings 70% that is 157 readings are used in training period and the remaining 79 are used for testing period. The results of Random Forest and M5P are then compare to find which is more effective in predicting the compressive strength of Ultra High-Performance concrete. The accuracy of the model depends upon Performance evaluation parameters which are Corelation Coefficient (CC), Root mean square error (RMSE) and mean absolute error (MAE). After getting the value of coefficient of correlation, root mean square error, mean absolute error it is observed that that Random Forest is better than M5P as the values of CC, RMSE, MAE are 0.8568, 16.005 and 12.03 for testing stage respectively.

Influence of Silica Fume and Thermal Curing on Long-Term Hydration, Microstructure and Compressive Strength of Ultra-High Performance Concrete (Uhpc)

Influence of Silica Fume and Thermal Curing on Long-Term Hydration, Microstructure and Compressive Strength of Ultra-High Performance Concrete (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.

Effects of pre-corroded steel fibers on mechanical properties and interface bond behavior of ultra-high performance concrete-normal concrete

The effects of pre-corroded steel fibers on mechanical strength of ultra-high performance concrete (UHPC) and interface bond performance between UHPC and normal concrete (NC) were investigated. To obtain corrosion degree of 2.5%, 5%, 7.5% and 10% by weight, steel fibers were pre-corroded in 3.5 wt% NaCl solution and then used to fabricate UHPC. The bond strength of UHPC-NC interface was evaluated by double-sided direct shear test and three-point bending test. The observation of corroded steel fibers by optical microscope (OM) shows that the surface roughness increases with the corrosion degree increases. Overall, the compressive strength, flexural strength and tensile strength of UHPC gradually decrease with increasing the corrosion degree of steel fibers. By contrast, the bond strength of UHPC-NC interface increases as the corrosion degree increases, both the NC substrate with rough surface and smooth surface. Additionally, rough surface on NC substrate effectively enhances the adhesion of UHPC-NC interface.

Effects of Curing Temperature and Chemical Admixture Type on Fresh Properties and Compressive Strength of Ultra High-performance Concrete

The types of post-setting curing and high range water reducing (HRWR) admixture used in Ultra-High-Performance Concrete (UHPC) mixtures play significant roles in determining their rheological and mechanical properties. This study compares the performance of three types of HRWR admixtures commercially available when added to UHPC mixtures under three different curing regimes. Mixtures made with the different superplasticizers were evaluated for their flow, 45mins flow retention, and setting time as fresh mix properties. Compressive strength was also tested for each mixture after 3, 7, and 28 days of curing at the investigated various curing regimes. Sika Viscocrete 180GS produced the highest mixture flow and flow retention levels with a 241 mm flow and 93.7% flow retention. Sika Viscocrete 168-1 produced the best results of setting time with 3 hours as compared to 12 hours with Sika Viscocrete 180GS. Using Hyperplast PC-202, the required 150 MPa compressive strength was secured as early as 3 days of curing with a 48hrs-90ºC curing regime. Using the same HRWR admixture, compressive strength values slightly lower than 150 MPa were reached after 7-28 days when the 72hrs-60ºC regime was adopted. The last curing regime was recommended for producing architectural UHPC units to minimize the delayed formation of ettringite.

Enhancement of local concrete compression performance by incorporating ultra-high performance concrete (UHPC) tube

PurposeThe application of ultra-high performance concrete (UHPC) in anchorage zones can significantly improve the local compression performance of structures. However, the high cost and complex preparation of UHPC make UHPC difficult to be widely used in practice. This study proposes a method to strengthen the local compression zone of structures built by normal strength concrete (NSC) by incorporating UHPC cores.Design/methodology/approachIn this study, a Finite Element Model (FEM) of local compression specimens was established by ABAQUS, and the accuracy of FEM was verified by comparing the FEM calculation results with experimental results. The verified FEM was adapted to the research on the influences of affecting factors on local compression performance of structures, including NSC strength, UHPC strength, spiral steel bar strength, and UHPC core diameter.FindingsThe results show that the peak load of the strengthened specimen SC1-U + N increases by 210.2% compared to that of the SC1-NSC. Furthermore, compared to SC1, the strengthened specimen SC1-U + N can save 64.7% amount of UHPC while the peak load decreases by only 34.4%. The peak load of the strengthened specimens increases with the axial compressive strength and the diameter of UHPC cores increasing, crack load increases with increasing the compressive strength of NSC, the spiral steel bar with high strength can prevent the sharp drop of load-deflection curve and the residual bearing capacity increases accordingly. All findings indicate that increasing the diameter of UHPC cores can improve the overall performance of the specimens. Under loading, all specimens fail by following a similar pattern. The effectiveness of this new strengthen method is also verified by FEM analytical calculations.Originality/valueBased on the experimental study, this study extrapolates the influence of different parameters on the local bearing capacity of the strengthened specimens by finite element simulation. This method not only ensures the accuracy of bearing capacity assessment, but also does not require many samples, which ensures the economy of the reinforcement process. The research results provide a reference for the reinforcement design of anchorage zone.

Roles of carbon black or carbon fibre on improving mechanical and electromagnetic properties of ultra-high performance concrete in K-band frequency range

To prevent electromagnetic wave pollution, the electromagnetic wave absorption capability and compressive strength of ultra-high performance concrete (UHPC) mixed carbon black (CB) and carbon fibre (CF) are methodically examined in K-band frequency range (i.e. 18–26.5 GHz). The mechanical properties, loss tangent, reflection loss, and microstructure of UHPC with the addition of CF and CB contents are determined via pressure tests, probe method, reflectivity tests, and scanning electron microscopy. The test results reveal that the compressive strength, dielectric loss tangent and reflection loss of UHPC are enhanced by the addition of CB and CF. When 0.5 vol.% CF is added, the reflection loss less than −10 dB in the K-band frequency range, and less than –15 dB in the frequency range of 21.68–23.38 GHz. By comparison, the reflection loss of UHPC with 0.5 vol.% CF is more than 1.8 times that of plain UHPC in the K-band frequency range.

Effect of post-fire curing on compressive strength of ultra-high performance concrete and mortar

• Compressive strength of both UHPC and mortar increases at 300 ℃ but decreases at higher temperature. • Post-fire water curing is more effective on recover the compressive strength of the UHPC mix and Mortar than the post-fire air curing. • Strength recovery of the Mortar is greater than that of the UHPC. • Precipitation of portlandite heals the coarsened microstructure and recovers compressive strength. This paper investigates effects of post-fire curing on strength recovery and changes in microstructure of ultra-high performance concrete (UHPC) and mortar. Specimens were heated to elevated temperatures 300, 600, and 900 °C with a heating rate of 1 °C/min. After cooling, the specimens were subjected to post-fire curing in either air or in water for 56 days. Compressive strength, X-ray diffraction (XRD), mercury intrusion porosimeter (MIP), and field emission scanning electron microscope (FESEM) tests were conducted to examine the changes at both macro and micro scale and to reveal deterioration mechanisms due to fire effect and healing mechanisms due to post-fire curing. The test results showed that compressive strength of both UHPC and mortar increased slightly after 300 °C exposure but decreased significantly at higher temperature due to the coarsening of microstructure. Post-fire curing in air showed very limited strength recovery since new curing products did not heal thermal cracks larger than 1 μm. On the other hand, curing in water led to substantial strength recovery of the mortar mix due to formation of portlandite which compactly filled the microcracks in fire-damaged samples. However, the loose ettringite phase formed in the UHPC mix did not contribute to regaining the initial compressive strength.

Experimental study on lap behavior of CFRP indented bars in UHPC

The lap behavior of Carbon Fiber Reinforced Polymer (CFRP) indented bars in Ultra-High Performance Concrete (UHPC) were investigated based on pullout test under direct tension on 27 specimens in total. The test variables include splice length, UHPC strength and stirrup ratio. The lap failure mechanism of CFRP bar embedded in UHPC and the influence of test parameters on lap performance were analyzed. Test results showed that the failure modes, pull-out and rupture of CFRP bar, in those lap-spliced specimens were governed only by the splice length of the CFRP bar embedded in UHPC. The pullout failure was caused by the shearing off of indented ribs on the surface of the bars and the fracture failure was governed by the tensile strength of the bars. The bond strength of lap-spliced indented CFRP bars in UHPC is mostly dependent on the shear strength of indented ribs on CFRP bars, little on the UHPC strength and stirrup ratio. For CFRP bars with a 2480 MPa tensile strength and a 9.8 mm diameter (d), a splice length of 30d in UHPC can fully develop the tensile strength of the bar. A simplified equation for predicting the critical splice length of CFRP bars in UHPC was proposed. Compared with the results on anchorage of CFRP bars in UHPC, the critical splice length of CFRP bar in UHPC was 1.45 times the corresponding anchorage length.