Strength Grade Research Articles

Seismic behavior of irregular shaped beam-column connections

This paper reports test results and numerical analysis of special-shaped beam-column connections based on a frame-shear wall structure. Four connections of 1/5 scale ratio were tested under constant axial force and reversed lateral load on the top of column. The experiment investigated the effect of steel reinforced concrete (SRC) beam, in-plane and out of-plane reinforced concrete (RC) wall on the connections’ seismic behavior. The failure modes, hysteresis curves, ductility, strength degradation and energy-dissipating capacity of test specimens are reported. The results indicated that test joints were susceptible to suffer more brittle failure caused by formation of plastic hinge on column, which is considered unfavorable compared to conventional joint with regular shape in earthquake resisting. Adding in-plane shear walls to the connection switched column failure to beam failure mode, and significantly improved the connection’s strength, stiffness and ductility. Finite element models were detailed established and validated through experiment data. Influence of parameters including axial load ratio, concrete strength grade, section area and steel ratio of upper column, transverse hoops volumetric, RC wall of two orthogonal directions on connection’s seismic behavior were studied through modeling results.

Study on the Shear Resistance Performance of Grouped Stud Connectors.

In order to further investigate the grouped stud effect on the force properties of stud connectors, based on the premise that the correctness of the finite element simulation method, in this paper, a finite element model of grouped stud connectors was developed, and the grouped stud effect and its sensitivity factors were analyzed in order to validate the recommended formula for calculating the shear capacity of grouped stud connectors. Results show that the number of grouped stud rows and stud row spacing have a significant influence on the grouped stud effect, and the unevenness coefficient of grouped stud force is negatively correlated with the number of grouped stud rows as well as the grouped stud row spacing. Grouped stud connectors with commonly used concrete grades greater than C50 and height-to-diameter ratios of greater than 4 in steel-concrete composite structural bridges are insensitive to changes in the concrete strength grades and the length of the studs. The direction of force transmission for grouped stud changes with the change in loading angle and the unevenness coefficient of force for the grouped stud will therefore be reduced. By comparing the results of the 62 existing groups of grouped stud connectors push-out tests, the mean of the tested to calculated value ratio was found to be 1.12, the variance was 0.023, the dispersion was small, and it was shown that the recommended formula has a high degree of accuracy. The results of this paper can be used as a theoretical basis for the study of the shear stress performance of grouped stud connectors.

Flexural performance tests and machine learning analysis of prestressed high strength steel reinforced concrete beams

ABSTRACT This paper reports a study on the flexural performance of prestressed high strength steel reinforced concrete (PHSSRC) beams and proposes machine learning (ML) models for predicting the flexural bearing capacity of steel reinforced concrete (SRC) beams. Seven prestressed steel reinforced concrete (PSRC) beams were tested under a four-point bending load to investigate the effect of steel strength grade, steel ratio, and longitudinal reinforcement ratio on the flexural load capacity of beams. The test results indicated that increasing the steel strength grade and steel ratio significantly enhanced the bearing capacity of PSRC beams. A database consisting of 112 sets of experimental data on the flexural performance of SRC beams, including the test data in this study, was established for training and validating the ML models. Artificial Neural Network (ANN) model and eXtreme Gradient Boosting (XGBoost) model were respectively developed to predict the flexural load capacity of SRC beams, and the accuracy of the flexural load capacity values predicted by the design specification, the ANN model and the XGBoost model were compared. The results show that the XGBoost model has the best prediction performance. Finally, the sensitivity analysis of the parameters was performed using the Shapley Additivity (SHAP) interpretation method.

Construction Material Selection Criteria for Timber Gridshell Application: A Literature Review

Timber gridshells have the potential to be a sustainable and cost-effective solution for long-span applications and free-form architecture. Despite this, their overall use has been limited, and there is a lack of research focusing on their construction materials. This literature review aims to investigate timber gridshells and their construction materials to identify the criteria used in selecting suitable materials for gridshell applications. A review of peerreviewed scientific articles, books, and theses was conducted to gather information on timber gridshells, construction materials used in gridshells, timber used in active bending structures, and building standards. The research findings identified six main factors that are important when selecting a suitable material for timber gridshell application: structural strength and strength grading, bending strength and behaviour, bending strength/bending elasticity ratio, durability, commercial availability, and cost. These findings are also discussed to identify the characteristics that make a material suitable for timber gridshell applications, depending on the gridshell’s context, whether in a tropical or international context. This review serves as a necessary reference for architects and engineers when selecting materials for their timber gridshell projects, providing insight into the selection criteria for construction materials and sharing information on the material properties of suitable timber gridshell materials.

On the critical role of martensite hardening behavior in the paradox of local and global ductility in dual-phase steels

The ductility of sheet metal is typically limited by either localized necking or by damage and fracture. A recent ductility classification refers to these failure modes respectively as “global” and “local” formability. The forming limit curve (FLC) and the uniaxial tensile test assess global formability, whereas the fracture forming limit (FFL), the true thickness fracture strain, hole expansion ratios (HER), etc. are indicators of local formability. Experimental hole expansion data in the literature for different dual-phase (DP) steel grades of similar strength and composition presents a paradox: grades which are found to be ductile in a tensile test and/or FLC show a low ductility in hole expansion, whereas other grades with a low ductility in conventional tests perform surprisingly well at cut edges. In this work, an in-depth systematic statistical analysis of idealized artificial two-phase microstructures is carried out to unravel the underlying mechanisms of the observed paradoxical trends. This is done by scaling the hardness of martensite and its volume fraction to generate virtual DP steels of the same strength but different strain hardening and mechanical phase contrast (hardness difference). The proposed micromechanical model adequately reproduces the experimentally observed trends. The results show that in DP steels, a higher global (necking-driven) ductility is obtained upon delaying martensite plasticity by increasing the mechanical contrast of the two phases. Consequently, the stress–strain distributions becomes more heterogeneous, resulting in a lower fracture limit of one of the phases or interfaces in shear loading, thereby, reducing the local ductility. Global ductility is improved by higher mechanical phase contrast and lower martensite volume fraction, whereas local ductility is improved by low phase contrast and higher martensite volume fraction. The hardening behavior of martensite is the key to avoiding the above trade-off between local and global ductility. It is shown that if the strain hardening capacity of martensite in the later stages of deformation (in high strains) can be increased, this would result in removing the observed paradoxical trends of local and global ductility in DP steels.

Research on the Distributive Relationship between Bond Force and Bearing Pressure for Anchorage Force by Headed Bars

Anchorage force comprises bond force and bearing pressure when headed bars are used. Pull-out tests were conducted on 120 concrete specimens to derive a method for calculating the bond force for reinforcement in straight anchor sections at the yield moment. The parameters include the diameter d, embedded length lae, and yield strength fy of the reinforcement, as well as the strength grade of the concrete fcg. The experimental results indicated that the specimens underwent three failure modes depending primarily on the embedded length lae. The nominal average bond stress τu concept was proposed and the difference between τu and the actual average bond stress τ caused by the headed bars was observed. To reduce the difference between the τu and τ values, the correction coefficient γ was proposed. Analysis indicated that γ increased with an increase in the lae/d (on average, 146% higher than the initial value) and decreased with an increase in the fy/ft (on average, 33% less than the initial value). A formula was developed to calculate γ, and the bond force in the straight anchor sections at the yield moment for the reinforcement was determined. Thus, a distributive relationship was established for the anchorage force, the bond force, and the bearing pressure.

Performance and strength analysis of shear walls with vertical steel encased profiles under tensile-shear load

This study investigates the behavior of composite steel reinforced concrete (SRC) walls under tension-shear (TS) coupled stress state during earthquakes. Quasi-static tests on five SRC shear walls with a shear span ratio of 1.42 were conducted. The experimental results demonstrated that the SRC shear wall retained its high shear capacity and good ductility in shear failure mode. The stiffness analysis revealed that the concrete provided most axial stiffness before cracking but gradually withdrew from the work as the cracks developed and extended rapidly in the tension loading phase. And the effective lateral stiffness was approximately 10%–15% of the elastic lateral stiffness. Parametric analysis was conducted on the shear capacity, including the axial tensile ratio, steel profiles content, concrete strength grade, and horizontal distributed bar diameter. Additionally, the predictive formulas for the shear capacity in the codes of various countries were compared. It was found that the European and American code inadequately predict the shear capacity of specimens, and the Chinese code has higher safety. These findings can provide references for the practical engineering applications of SRC shear walls in earthquake-prone regions.

Moisture Diffusion Coefficient of Concrete under Different Conditions

Humidity change in concrete is the leading cause of concrete shrinkage. Moreover, the moisture diffusion coefficient of concrete is an essential parameter for assessing and predicting the internal moisture content of concrete. However, there is a lack of theoretical construction and experimental studies on the effect of different conditions, especially different constraints, on the moisture diffusion coefficient of concrete. Therefore, the internal humidity, pore structure parameters, and basic mechanical properties of concrete under different strength grades C30, C40, C50, and C60 (C stands for concrete and numbers indicate the strength class of the concrete), curing environments (dry and sealed curing conditions), and constraints were tested in this study. In addition, a calculation model of concrete’s internal humidity and humidity diffusion coefficient was established. The research findings show that the internal humidity of concrete decreased with age due to hydration and drying. External humidity had a significant effect on the moisture change of concrete, and the lower the external humidity, the larger the humidity diffusion coefficient and the faster the internal humidity of concrete decreases. Reinforcement (confinement) changes the pore structure parameters of the concrete, which in turn affects the transport of moisture within the concrete. The higher the reinforcement rate, the larger the pore structure parameters of the concrete, the larger the humidity diffusion coefficient, and the faster the concrete humidity decreases. The method proposed in the study can accurately predict the internal humidity of concrete using the humidity diffusion coefficient. The research results are a reference for preventing concrete shrinkage and cracking in construction.

Temperature effect on strength and chloride migration in nuclear power plant concrete

In this study, strength and chloride migration tests were conducted considering nuclear power plant concrete exposed to high temperature and chloride ingress. Two temperature grades (20 ℃ and 50 ℃) were considered for curing and diffusion testing. In the curing at high temperature, cross-over behavior appeared for strength development from 7 to 14 days, however lower migration coefficients were evaluated under high temperature curing conditions. This was because even if cement hydrates were not completely stable, the absorption of chloride was still effective and the pore structure improved due to accelerated hydration. Migration test referred to NT-BUILD 492 method was performed on 40 MPa and 27 MPa concrete of 91 days curing. Activation energy for two strength grades was individually obtained during migration test with chaining temperature to 50 °C. The obtained results were compared with the previous results considering test method (diffusion and migration) and mix proportions.

Axial compressive behavior of corroded double-steel-plate composite shear wall with binding bars

Steel structure is susceptible to corrosion, and corrosion will seriously affect its own load-bearing capacity and service life. In this paper, the axial compressive behavior of double-steel-plate composite shear wall (DSCW) with binding bars under different corrosion degrees is experimentally studied. The typical failure modes include the steel plate local buckling, binding bars fracture, concrete crushing, and steel plate weld tearing. The increase of corrosion rate has unfavorable influence on the axial compressive bearing capacity, initial stiffness, and ductility of DSCW. Finite element (FE) models are established to further investigate the axial compressive behavior of DSCW. It is concluded that the concrete and steel plate strength grade, the concrete section area, the steel plate thickness, and the binding bars diameter and spacing significantly affect the axial compressive bearing capacity of DSCW. A formula calculating the axial compressive bearing capacity of corroded DSCW with binding bars is proposed. The formula agrees well with the test results and can provide a reference for the engineering application.

Bond-slip model for rebar mounted to high-strength grouting material

The anchorage performance of reinforcement in high-strength grouting materials plays a decisive role in the safety of prefabricated reinforced concrete structures. At present, the design value of anchorage length of reinforcement in grouting materials or concrete in the codes of various countries is conservative, generally greater than 20 times the diameter of reinforcement. Moreover, the anchorage length is the empirical value obtained from the test of connecting the reinforcement with the grouting sleeve and is not based on the bond-slip performance of the reinforcement and the grouting material. Therefore, based on the interface bond-slip performance test of two media, this paper comprehensively considers the influence of reinforcement diameter, strength grade, anchorage length, and high-strength grouting material strength on the ultimate bond strength, gives the calculation formula of anchorage length and establishes a nonlinear three-segment bond-slip constitutive model. The research in this paper shows that the anchorage length between high-strength grouting material and reinforcement to ensure bonding safety is 7.1 d–10.8 d, which is far less than the recommended anchorage length in the specification. It can provide a reference for the design of prefabricated structures based on grout-anchored reinforcement connections.

Experimental Investigations on load‐bearing Capacity and characteristic Preload of Lockstud Systems

AbstractThe joining technology Lockstud System combines the advantages of a bolt thread for making a tapped thread joint and the advantages of a lockbolt. On the tapped thread joining side, the bolt has a metric ISO thread. On the assembly side, the bolt has a grooved geometry like lockbolts of type A, B and C. This article publishes results on the mechanical‐technological properties, the assembly process with the initial preload, the preload losses due to embedding under long period of time and the load‐bearing capacity subjected to static and fatigue normal stress for Lockstud systems of nominal diameters M12, M16 and M20 (strength grade 10.9) compared to conventional bolt assemblies and lockbolt systems.

Classification of compressive strength grades for lightweight aggregate concrete with palm oil fuel ash (POFA) using k-Nearest Neighbour (k-NN)

Annually, a massive number of agricultural by-products of the palm oil extraction process including palm oil fuel ash (POFA) were generated which contributes towards ammonia pollution and emission of nitrogen compounds. Fortunately, both by-products can be utilised as mixing additives in lightweight aggregate concrete manufacturing. The utilisation leads to a more sustainable green environment. Traditional methods for classifying concrete grades in civil engineering are difficult due to the non-linear relationship between the composition of concrete and its strength and require a significant amount of time, material resources, and labour. To address these shortcomings, a technique to classify the compressive strength grades for lightweight aggregate concrete containing POFA using a machine learning algorithm has been developed. In terms of method, concrete mixtures consisting of POFA, cement, sand, superplasticizer and water were prepared and tested to determine the compressive strength. The data from this process were first transformed using min-max normalization and then, analysed using exploratory and descriptive analysis to discover patterns between input variables and concrete grades. Next, the grades of concrete were classified using a machine learning algorithm named k-Nearest Neighbour (k-NN). Lastly, a confusion matrix was used to assess the performance of the k-NN classifier. The results showed that k-NN can classify the grades of concrete with accuracies between 71% and 95% using five nearest neighbours. The accuracies are inversely proportional to the number of nearest neighbours. To conclude, the study succeeds in classifying the compressive strength grades for lightweight aggregate concrete with POFA using k-Nearest Neighbour. It can cut down a significant amount of time, material resources, and labour in determining the grades of compressive strength for POFA-based lightweight concrete.

Optimization design for alkali-activated slag-fly ash geopolymer concrete based on artificial intelligence considering compressive strength, cost, and carbon emission

Traditional mix proportion design methods are difficult to consider the multiple objectives, while the design of economical and environmentally friendly alkali-activated slag-fly ash geopolymer concrete on the premise of guaranteeing compressive strength is an essential and meaningful task. The novelty of this paper is that the optimization design model for alkali-activated slag-fly ash geopolymer concrete considering 28 days compressive strength, cost, and carbon emission was developed based on machine learning and Particle Swarm Optimization (PSO) algorithm. The results show that Random Forest (RF), GB, and Back Propagation Neural Network (BPNN) can achieve a good prediction effect for compressive strength with R2 over 0.85 and 0.70 for training and testing set. Slag and sodium hydroxide (NaOH) contents have remarkable effects on the compressive strength of alkali-activated slag-fly ash geopolymer concrete. The addition of slag is beneficial to enhancement of compressive strength, and the relatively optimal coarse and fine aggregate contents are 1200 kg/m3 and 750 kg/m3. It is noted that alkali-activated geopolymer is suitable for preparation of high-strength concrete. Production cost is need to be paid more attention in the optimization process. The alkali-activated slag-fly ash geopolymer concretes with production cost reduction of 7.6%∼10.6% and carbon emission reduction of 77.3%∼80.7% at strength grade of C30, and with production cost reduction of 22.5%∼27.0% and carbon emission reduction of 76.9%∼81.3% at strength grade of C50 are achieved.

Effects of temperature and bleeding on rheology of cement paste

The bleeding and rheology of a cement paste affect the grouting reinforcement effect in a high-temperature stratum. However, the effect of the slurry bleeding on the rheology at high temperatures is not clear. This research examined the bleeding characteristics of slurries in a high-temperature environment and changes in the density and flow pattern during bleeding. The material was ordinary Portland cement with a strength grade of 42.5. The water-cement ratio (W/C) ranged from 0.6 to 2.0. An environment of 20 °C–90 °C was simulated in a laboratory. The bleeding process was divided into six stages according to the total bleeding rate and rheological tests were conducted at each stage. The results showed that the total bleeding rate of the slurry increased with the increase of the W/C and decreased approximately linearly with the increase of temperature. The density change rate before and after bleeding decreased with the increase of temperature and increased with the increase of the W/C. The final density change rate of the cement paste at different temperatures was determined by the W/C. Newtonian and power law models were used to describe the rheological characteristics of slurries with W/Cs ranging from 1.5 to 2.0 and 0.8 to 1.2, respectively, during the bleeding period. As the bleeding rate increased, the R2 of the Newtonian model decreased from 0.98 to 0.9. A constitutive equation considering the temperature and bleeding characteristics was proposed. The relationship between the flow characteristic and consistency indexes at different bleeding stages changed from non-linear to linear with increasing temperature.

Local buckling behaviour of high strength steel and hybrid I-sections under axial compression: Numerical modelling and design

This paper presents a numerical investigation on the local buckling behaviour of high strength steel (HSS) and hybrid I-sections under axial compression, which has not been comprehensively discussed in previous literature. Three sectional steel combinations, featuring different web strength grades (Q690, Q460, and Q355) and using HSS Q690 flange plates, were studied. Through the validated numerical method, parametric studies on the effect of web strength grade, boundary condition and plate slenderness were carried out. The design specifications in European, Australian, and American codes were evaluated using the results of 243 numerical models and collated test data. In addition, the continuous strength method (CSM), direct strength method (DSM), and Kato’s method, which can account for element interaction were extended to design for the local buckling behaviour of HSS and hybrid I-sections subjected to axial compression. Assessment results showed that ANSI/AISC 360-16 provides more accurate results than Eurocode 3 and AS 4100. Statistical and reliability results demonstrated the satisfactory reliability level of the proposed design expressions for CSM, DSM, and Kato’s design methods. This paper provides insight into the local buckling behaviour and mechanism behind plate interaction of hybrid I-sections.

Effect of ceramic fiber on mechanical properties of concrete with different strength grades and its strengthening and toughening behaviors under impact load

The mechanical properties of ceramic fiber reinforced concrete (CRFRC) with different strength grades (C30, C50 and C80) under impact load were studied, and the strengthening and toughening behaviors of ceramic fiber on plain concrete were analyzed and compared with that of carbon fiber. The results show that ceramic fiber can improve the mechanical properties of concrete with different strength grades under impact load, and the optimal content of ceramic fiber is 0.2%. Ceramic fiber has strengthening and toughening effect on concrete, and the strengthening and toughening behaviors of ceramic fiber are better than that of carbon fiber. Ceramic fiber has the best strengthening effect on C50 concrete and the best toughening effect on C80 concrete. The dynamic compressive strength (fcd), energy dissipation density (Edd) and dynamic increase factor (DIF) of CRFRC all have strain rate effect. As the ceramic fiber content increases, the strain rate sensitivities of fcd and Edd of CRFRC increase first and then decrease. As the strength grade increases, the strain rate sensitivities of fcd and Edd decrease. The strain rate effect of DIF of CRFRC has a transition strain rate. The transition strain rate of DIF of CRFRC decreases with strength grade and it decreases compared with plain concrete.

Axial compressive property of square and rectangular UHPC-filled duplex stainless steel tube stub columns

This study presents experimental and numerical investigations of the axial compression performance of square and rectangular ultrahigh performance concrete (UHPC)-filled duplex stainless steel tube (UFSST) stub columns. A total of twelve UFSST stub columns, six ordinary concrete filled duplex stainless steel tube (CFSST) stub columns, and six hollow duplex stainless steel tube (HSST) stub columns were tested under axial compression. The test parameters include the section type, three kinds of concrete strength grades, and three kinds of duplex stainless steel tube thicknesses. The test results showed that UFSST stub columns behaved with high bearing capacity and favourable ductility. In the UFSST stub columns with ξ ≥ 1.87, the square and rectangular duplex stainless steel tubes could well confine the shear failure of UHPC. The developed finite element model could effectively reflect the compressive behavior of the HSST, CFSST and UFSST stub columns. The available experimental database was employed to evaluate the feasibility of current design codes and existing calculation models in the literature to design UFSST stub columns. Finally, a modified computational model was proposed that takes into account the nonuniform confinement effect induced by square and rectangular section tubes on concrete and showed a higher prediction accuracy.

Experimental study on crushing of concrete slabs by high-voltage pulse discharge

To investigate the effectiveness of high voltage pulse discharge (HVPD) in crushing concrete slabs, HVPD in-liquid tests were conducted in pre-drilled holes in three double-layer bi-directionally reinforced and twelve single-layer bi-directionally reinforced concrete slabs. The parameters that were varied in the tests included the concrete strength, discharge hole diameter and spacing, discharge voltage and number of copper wires discharged. The results showed that the explosion caused by HVPD using copper wires increased the crack width by more than 34.8% compared to that caused by the electrohydraulic effect. After 10 explosions caused by HVPD using copper wires, the single-layer reinforced concrete slab was split into several slabs by cracks to achieve the crushing effect. The single discharge energy, i.e., discharge voltage, had a significant effect on the crushing of concrete slabs, and the width of concrete cracks produced by a 100 kV discharge was more than twice that of a 40 kV discharge. The increase in concrete strength acted as a crack arrestor for the slabs, and the crack width of C20 concrete slabs increased by approximately 10% to 25% compared to C40 concrete slabs. The increase in crack width with increasing number of discharged copper wires decreased with increasing discharge voltage. The width of concrete cracks produced by the explosion of 60 copper wires compared to 30 copper wires increased by approximately 10% at 100 kV and by approximately 20% from 60 to 80 kV. The crack width of concrete slabs with discharge holes of diameter 40 mm increased by approximately 5% to 15% compared to concrete slabs with holes of 50 mm. Expressions for the relationships between the damage degree of the concrete slab (i.e., the width of the concrete cracks around the holes on the outside of the slab) and the key parameters such as the output voltage, the number of discharged copper wires, and the strength grade of the concrete were established based on the experimental results.

Axial compressive behaviour and physical characteristics of high strength self-compacting geopolymer concrete (HSGC) columns exposed to elevated temperature

The growing demand in the utilization of the cement has increased carbon emissions, necessitating the adoption of new technologies that are eco-friendly. Geopolymer concrete (GC) is a viable alternative to traditional cement-based concrete, with superior mechanical properties and sustainable performance. However, research on the elevated temperature exposure of geopolymer paste, mortar, and concrete is limited, particularly with respect to high-strength and self-compacting geopolymer concrete. This study aims to investigate the performance of high-strength self-compacting geopolymer concrete (HSGC) at room temperature and elevated temperatures up to 1029 ℃. The investigation is carried out consisting of two strength grades: normal strength GC and HSGC. Tests are conducted on both HSGC cube and column specimens to examine the residual compressive strength (CS) and axial compression performance of the concrete before and after exposure to elevated temperature. The fresh physical characteristics of HSGC are examined following EFNARC guidelines. Post-fire performance is analysed through physical observation, surface modification, crack pattern, weight loss (WL), and residual CS of HSGC samples. Load-Moment (P-M) interaction curves are developed to predict the ultimate load and ultimate moment capacity of the HSGC columns before and after exposure to elevated temperature. The ductility of heated and unheated HSGC columns is found to be almost same. Microstructural analysis, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) are performed to examine the internal morphology of the HSGC samples before and after exposure to elevated temperature. Furthermore, a life-cycle assessment (LCA) is conducted to compare the cost of HSGC production with conventional cement-based concrete. The analysis indicates that HSGC emits 6.15% less CO2-e than its counterpart cement-based concrete.