High-strength Reinforced Concrete Research Articles

Combined nomograms for calculating the bearing capacity and reinforcement of high-strength and normal concrete RC columns

PurposeThis paper aims to obtain a calculation method by hand without iteration.Design/methodology/approachThis paper adopts strains as known quantities to solve the internal forces and deformations of the section, simplifies the deflection curve of the column and obtains nomograms that can calculate the bearing capacity and reinforcement of circular reinforced concrete (RC) columns by hand.FindingsNomograms include five variables: mechanical reinforcement ratio, relative normal force, dimensionless bending moment, slenderness ratio and ultimate dimensionless curvature. Nomograms corresponding to all classes of concrete have been drawn, and their dimensionless form makes them widely applicable. The calculation results of nomograms are compared and analysed with numerical calculation results, and the difference is within 5%, meeting the engineering requirements.Originality/valueCalculating the bearing capacity of compression bending components requires considering second-order effects. Therefore, the calculation of the bearing capacity of circular RC columns requires iterative calculation, as it includes dual nonlinearity of material and geometry, and the two are coupled with each other. To calculate the bearing capacity of the section adopting ordinary concrete, it is necessary to solve the transcendental equation iteratively. For high-strength concrete, it can only be solved by numerical integration. A fast calculation method by hand is proposed in this paper.

Experimental and numerical simulation study on flexural performance of high-strength reinforced concrete beams under static loading

In recent years, high-strength steel bars (HSSB), such as hot-rolled ribbed bars (HRB) and heat-treated ribbed bars (HTRB), have been promoted and used in reinforced concrete (RC) structures worldwide. There is still some controversy in existing research regarding the crack development process, failure modes, and the applicability of current code-specified calculation formulas for high-strength RC beams. The study of the mechanical properties of high-strength RC beams is of great significance in promoting the development of construction engineering science. In order to investigate the flexural behavior of high-strength RC beams, four-point bending static loading tests were carried out on 11 groups of RC beams with different parameters (reinforcement strength, concrete strength, and reinforcement ratio). The test results indicate that the crack development process and failure mode of high-strength RC beams exhibit obvious bending failure characteristics. Increasing reinforcement strength and reinforcement ratio significantly improves the bearing capacity of the beams, while the impact of increasing concrete strength on the beam's capacity is relatively small. The applicability of the calculation formulas in the current codes was analyzed, it was found that the yield moment formula in the current code accurately predicts the yield moment of high-strength RC beams, and the formula for calculating the maximum crack width was modified by proposing the maximum crack width adjustment factor kω. Subsequently, the failure process of high-strength RC beams was simulated using explicit dynamic finite element software LS-DYNA. Parameterized analyses were conducted to broaden the research scope. The ultimate moment prediction factor ku was proposed and a formula that can accurately predict the ultimate moment of high-strength RC beams was established.

Low-cycle fatigue design for reinforced high-strength concrete under high compressive stress

As engineering endeavors push the boundaries of material and design capabilities, the significance of understanding and mitigating fatigue in construction materials becomes paramount. This study specifically investigates the low-cycle fatigue performance of reinforced high-strength concrete (RHSC). Using rigorous data collection, we established a clear link between interpretable machine learning analysis and the fatigue properties of RHSC. A trained model was developed, yielding a straightforward formula tailored to low-cycle fatigue design considerations for RHSC. This model stands as a testament to the potential for marrying traditional engineering practices with advanced statistical techniques. Our results emphasize that, when appropriately applied, regression analysis can be instrumental in enhancing the durability and longevity of RHSC structures exposed to dynamic loadings. This research not only underscores the pivotal role of statistical methods in fatigue design but also champions the broader adoption of such techniques in evolving engineering landscapes.

Experimental investigation of behavior of reinforced high-strength concrete walls under standard fire

Reinforced high-strength concrete (RHSC) walls are widely used in high-rise buildings. A greater likelihood of spalling and smaller member sizes poses a high risk to structural fire safety. In this study, five RHSC wall specimens with various concrete strengths, axial load levels, thicknesses and reinforcement configurations were tested under one-sided bottom-up standard fire. The time-dependent thermal and deformation evolutions and the spalling characteristics were measured and analyzed. The results indicate severe spalling of RHSC walls which can lead to catastrophically early buckling failure at about half an hour of fire. Distinguishing spalling types highly related to concrete strength are identified, and the axial load is revealed to have significantly two-fold effects in increasing spalling which will be much more critical for RHSC walls with higher strength's concrete. Besides, it is discovered that reduction of wall thickness leads to considerable decrease of spalling and can effectively increase the fire resistance of thin RHSC walls with relatively lower concrete strength. The spalling-eliminating effect of flexural cracks will become limited in thicker walls or when concrete is of higher strength. Finally, it is found that applying double-layer reinforcement configuration has little effect in reducing spalling but can still significantly increase the fire resistance of RHSC walls.

Identifying the influence of split tensile strength to crack width of high-strength reinforced concrete beam with polypropylene fiber from medical mask waste

Cracks in concrete structures due to tensile weakness can be repaired by using fiber concrete, including medical mask waste as an additive in high-strength concrete mixes. This research the influence of polypropylene fiber from medical mask waste on the mechanical characteristics, flexural behavior, and crack width of high-strength reinforced concrete (RC) beams. The initial stage involved examining the properties of the concrete constituent materials. The testing process was based on a high-strength concrete mix design using the Aitchin method mix design sheet. Compressive strength and split tensile strength were tested at fiber content (0 %, 0.15 %, 0.20 %, and 0.25 %). The three-point flexural testing procedure was carried out at 28 days on 1200×100×150 (mm) RC beams. The use of LVDT, strain gauge, and other measuring devices supported the acquisition of the required data. The results showed that the split tensile strength reached the optimum value of 66.19 MPa at 0.24 % fiber content. Polypropylene fiber from medical mask waste in RC beams showed a positive impact on reducing crack width at increased split tensile strength. Waste mask fiber content of 0.15 %, 0.20 % and 0.25 % gave stable and better results compared to 0 % content (no fiber). With high steel stress (fs), and high strain, it offers the potential to improve the mechanical properties of high-strength concrete, thereby reducing the width of cracks that occur. This improves the tensile weakness of the concrete. The effect of split tensile strength on the crack width (w) of beams with the formula approach: wexp=3.74ftf-1.513, wan=0.187ftf-0.022 shows that the experimental results have a significant effect on decreasing the crack width that occurs in high-strength RC beams, thereby improving the quality of concrete

Influence of Aging Time on Compressive Strength of Silica Fume Reinforced High Strength Concrete Composite

The goal of this study is to gather a High Strength Concrete (HSC) composite reinforced by silica fume and superplasticizers. Specimens are Cured by water and tested after aging time of 3, 7, 28 days. Specimens are cubes of 150 and 200 mm and 150x300mm cylinder. The aim is to get mean strength of 80- 85 MPa, and determination of physical properties of the materials used, which are basic components of the High Strength Concrete (HSC) composites. After designing mix proportions, according to design requirements some adjustments were done. Both fresh and hardened properties of the specimens were tested under standard condition (curing schemes, setting time, comp.str. etc). Compressive strength test results were gathered and compared with the requirements.

Effect of Aging Time on Split Strength of Silica Fume Reinforced High Strength Concrete Composite

Effect of Aging Time on Split Strength of Silica Fume Reinforced High Strength Concrete Composite

Numerical analysis of high-strength centrifugal precast RC hollow pipe columns using grouted corrugated duct connection: Confinement effect and ductility evaluation

High-strength centrifugal precast (HSCP) reinforced concrete (RC) hollow pipe columns using grouted corrugated duct connection (GCDC) is a novel structure form in accelerated bridge construction (ABC). Nevertheless, the confinement effect of the HSCP hollow column core concrete provided by single-layer transverse rebar and its impact on ductility is not clear. Therefore, in this paper, the confinement effect and the ductility of hollow columns is studied using finite element analysis (FEA) method. A modified model is established to evaluate the constitutive relation of the core concrete confined by single layer transverse rebar. Based on the modified model, a simplified fiber model for predicting the hysteric response of HSCP hollow pipe column using GCDC, which considers bond-slip effect in GCDC joint was developed. The results indicate that the confinement effect of HSCP hollow pipe column is sensitive to the wall thickness ratio. When the wall thickness ratio decreases, the ultimate strain of the core concrete significantly decreases, while the effect on strength is not significant. The simplified model which considers confinement effect is more accurate in predicting the ductility of HSCP hollow columns. It proves that the weak confinement effects can lead to early cracking of the inner wall of hollow columns, which leads a significant impact on the ductility of HSCP hollow columns. Based on the parametric analysis, the peak load of the hollow column increases by 16.7% when the wall thickness ratio rises from 0.1 to 0.2, but only slightly changes when the wall thickness ratio is increased beyond 0.2. The wall thickness ratio of HSCP hollow pipe columns should be set above 0.2 to ensure the strength and ductility. In addition, a ductility evaluation method and a design example were developed to guide preliminary ductile design of HSCP hollow columns.

EXPERIMENTAL RESPONSES OF HIGH-STRENGTH KC COLUMN WITH LARGE AXIAL LOAD

A kernel-confined (KC) column, which a reinforced-concrete (RC) column moves partial longitudinal reinforcements from conventionally placed on outer periphery of section to inner kernel-confined region (KC bar), was concerned in this paper. This paper mainly intended to investigate seismic behaviors of the KC column with large axial loads. There are six sets of high-strength RC column specimens with 600 mm × 600 mm in section to be conducted. Their design strength of concrete was 70 MPa, and steel grades of SD550W and SD790 were designed as the longitudinal and transverse reinforcements, respectively. The number and location of the KC bar and the number and strength of transverse reinforcement are key study parameters. Test results indicated that for all specimens, including KC columns, the testing maximum moments were larger than the computed moment from ACI model and their average value of overstrength ratio of the testing to the computed was 1.25. In addition, the strength obtained from the XTRACT software had a better prediction with an average error of 0.99. Based on test deformation results for all specimens, the amount of the confined reinforcement per ACI 318-19 with an axial load of 0.5fc’Ag, can provide 3% radian of inter-story drift angle capacity for the high-strength RC column.

Shear strengthening of reinforced concrete T-beams with anchored fiber-reinforced polymer composites

T-shaped beams are widely used in reinforced concrete (RC) structures, both buildings and bridges. Under-designed shear reinforcement and long-term aging make these structural elements susceptible to shear failure during earthquakes. The currenlty availalbe experimental work investigating the retrofit of these beams against shear failure is very limited. In this work, the external strengthening of RC T-beams with splay anchored strips of fiber reinforced polymer (FRP) composites under cyclic shear loading for the cases of normal and high strength concrete was studied. Six beams with 3.05 m length and 0.46 m that were conservatively reinforced against flexural failure were designed. The shear reinforcement of the beams was sufficiently high to ensure the dominance of shear failure. Three of the beams were built using a normal strength concrete and the other three were built using a high strength concrete with 28-day strengths of 30.5 MPa and 44.0 MPa, respectively. From each group of normal and high strength RC beams, one beam served as the reference. The other two beams were retrofitted with external strips of FRP composites that were bonded on the web and anchored under the flange of the beams. The beams were tested under cyclic four-point loading to evaluate the performance of the FRP retrofitting system against shear failure under simulated seismic loading. The deflection and the strain in the FRP composites were recorded while monitoring the strain of the longitudinal reinforcement to ensure an elastic response in flexure. The reference beams failed by shear cracking and the retrofitted beams failed by shear cracking and FRP delamination. The retrofitting technique increased the shear capacity of the beams by up to 37% and 20% for the normal and high strength beams, respectively, and promoted the beam ductility prior to failure.

Fatigue behavior and calculation methods of high strength steel fiber reinforced concrete beam

Adding steel fibers into concrete was considered as one of the most effective ways to restrain the crack development and improve the stiffness for reinforced concrete (RC) structures. To explore the reinforcement mechanism of steel fibers on the fatigue behavior of high-strength RC beam, eight high-strength steel fiber reinforced concrete (HSSFRC) beams subjected to fatigue loading were tested in this study. The main design parameters considered in this work were stress level and steel fiber content. The failure mode, crack patterns, fatigue life, crack width, and stiffness degradation of HSSFRC beams under fatigue loading were discussed. The results showed that steel fibers could significantly increase the fatigue life, restrain crack development, and improve crack patterns of HSSFRC beams under fatigue loading compared to ordinary RC beams. Both the crack width and stiffness degradation rate of beams decrease with increasing steel fiber content. Besides, the empirical formulas for calculating the maximum crack width and midspan deflection of HSSFRC beam under fatigue loading were proposed and validated using experimental results.

Torsional Capacity Prediction of Reinforced Concrete Beams Using Machine Learning Techniques Based on Ensembles of Trees

For the design or assessment of framed concrete structures under high eccentric loadings, the accurate prediction of the torsional capacity of reinforced concrete (RC) beams can be critical. Unfortunately, traditional semi-empirical equations still fail to accurately estimate the torsional capacity of RC beams, namely for over-reinforced and high-strength RC beams. This drawback can be solved by developing accurate Machine Learning (ML) based models as an alternative to other more complex and computationally demanding models. This goal has been herein addressed by employing several ML techniques and by validating their predictions. The novelty of the present article lies in the successful implementation of ML methods based on Ensembles of Trees (ET) for the prediction of the torsional capacity of RC beams. A dataset incorporating 202 reference RC beams with varying design attributes was divided into testing and training sets. Only three input features were considered, namely the concrete area (area enclosed within the outer perimeter of the cross-section), the concrete compressive strength and the reinforcement factor (which accounts for the ratio between the yielding forces of both the longitudinal and transverse reinforcements). The predictions from the used models were statistically compared to the experimental data to evaluate their performances. The results showed that ET reach higher accuracies than a simple Decision Tree (DT). In particular, The Bagging Meta-Estimator (BME), the Forests of Randomized Trees (FRT), the AdaBoost (AB) and the Gradient Tree Boosting (GTB) reached good performances. For instance, they reached values of R2 (coefficient of determination) in the range between 0.982 and 0.990, and values of cvRMSE (coefficient of variation of the root mean squared error) in the range between 10.04% and 13.92%. From the obtained results, it is shown that these ML techniques provide a high capability for the prediction of the torsional capacity of RC beams, at the same level of other more complicated ML techniques and with much fewer input features.

Development of Prediction Models for the Torsion Capacity of Reinforced Concrete Beams Using M5P and Nonlinear Regression Models

Torsional strength is related with one of the most critical failure types for the design and assessment of reinforced concrete (RC) members due to the complexity of the associated stress state and low ductility. Previous studies have shown that reliable methods to predict the torsional strength of RC beams are still needed, namely for over-reinforced and high-strength RC beams. This research aims to offer a novel set of models to predict the torsional strength of RC beams with a wide range of design attributes and geometries by using advanced M5P tree and nonlinear regression models. For this, a broad database with 202 experimental tests is used to generate highly reliable and resilient models. To build the models, three independent variables related with the properties of the RC beams are considered: concrete cross-section area (area enclosed within the outer perimeter of the cross-section), concrete compressive strength, and torsional reinforcement factor (which accounts for the type—longitudinal or transverse—amount, and yielding strength of the torsional reinforcement). In contrast to multiple nonlinear regression approaches, the findings show that the M5P tree approach has the best estimation in terms of both accuracy and safety. Furthermore, M5P model predictions are far more accurate and safer than the most prevalent design equations. Finally, sensitivity and parametric studies are used to confirm the robustness of the presented models.

Analytical and numerical analyses of RC beams exposed to fire adopting a LITS trilinear constitutive law for concrete

A trilinear compressive constitutive law for concretes exposed to fire is proposed. Concrete stress-strain relation as a function of temperature is characterized by means of only two parameters (residual values of concrete compressive strength and modulus of elasticity). The modulus of elasticity is determined through a Load Induced Thermal Strain (LITS) semi empirical model. The compressive strength is established through the design specifications. The constitutive law is implemented in finite element numerical models and in a simplified cross-sectional procedure. In the latter case, a simple methodology is proposed to estimate the residual moment capacity and the midspan deflection at any given time. The uncoupled two-step solution includes a thermal homogenization procedure followed by a sectional analysis. The deflection is estimated considering a constant equivalent stiffness for the RC beam. The case studies included the structural response under transient (hot state) condition of both a reinforced-high strength concrete (R-HSC) beam and a reinforced-ultra-high performance fiber reinforced concrete (R-UHPFRC) beam. The results are compared with experimental tests carried out in literature, showing a good approximation. Moreover, a steady state (after cooling) cross-sectional analysis of a R-HSC beam is performed. The load bearing capacity, the failure type and the midspan deflections are estimated. The analytical results approximate the experimental values, demonstrating the reliability of the proposed simplified procedures for the cases investigated.

Damage-controlled allowable maximum stress of the stirrup for HSRC beam members based on lower-bound theory

The main purpose of this work is to investigate the damage-controlled allowable maximum stress of the stirrup of a high-strength reinforced concrete (HSRC) beam member in the general design using the lower-bound theory. Since several works have noted that the allowable stress of the stirrup of an RC beam in the general design is limited by the shear crack width, the relationship between shear crack width and applied shear force is obtained herein using available experimental data for HSRC beam members. Moreover, based on the fact that the allowable maximum shear crack widths that have been suggested elsewhere are related to the damage state, formulas for estimating the allowable maximum stress of the stirrup of an HSRC beam member with various specified compressive strengths of concrete and stirrup ratios are provided herein. Restated, the allowable maximum stress of the stirrup can be determined from the specified compressive strength of the concrete and the stirrup ratio.

Crack-controlled allowable forces for high-strength reinforced concrete columns under cyclic loading using finite element analysis

High-strength reinforced concrete (HSRC) columns have been gradually promoted in use. However, the crack-controlled design method for HSRC columns is still insufficient to ensure serviceability and reparability. Therefore, the main purpose of this work is to provide a simplified method for assessing crack damage to HSRC columns. For this purpose, Finite Element Analysis (FEA) is developed to simulate crack development in selected specimens. Additionally, the FEA results are used to identify the reduction factors that are related to the stiffness and the unloading stiffness and the ratio of shear deformation to total deformation, which are required in the force-crack quantification model. Finally, for practical use in the design, a simplified formula for building the relationship between the shear force and maximum residual crack width for an HSRC column is also proposed. For practical use in the design, this work conducts a case study to build the relationship between the shear force and damage level for an HSRC column. Restated, an engineer can use this relationship to determine the allowable shear forces for ensuring serviceability and reparability.

Explainable machine learning using real, synthetic and augmented fire tests to predict fire resistance and spalling of RC columns

This paper presents the development of systematic machine learning (ML) approach to enable explainable and rapid assessment of fire resistance and fire-induced spalling of reinforced concrete (RC) columns. The developed approach comprises an ensemble of three novel ML algorithms namely; random forest (RF), extreme gradient boosted trees (ExGBT), and deep learning (DL). These algorithms are trained to account for a wide collection of geometric characteristics and material properties, as well as loading conditions to examine fire performance of normal and high strength RC columns by analyzing a comprehensive database of fire tests comprising of over 494 observations. The developed ensemble is also capable of presenting quantifiable insights to ML predictions; thus, breaking free from the notion of “black-box” ML and establishing a solid step towards transparent and explainable ML. Most importantly, this work tackles the scarcity of available fire tests by proposing new techniques to leverage the use of real, synthetic, and augmented fire test observations. The developed ML ensemble has been calibrated and validated for standard and design fire exposures and one-, two-, three- and four-sided fire exposures thus; covering a wide range of practical scenarios present during fire incidents. When fully deployed, the developed ensemble can analyze over 5,000 RC columns in under 60 s; thus, providing an attractive solution for researchers and practitioners. The presented approach can also be easily extended for evaluating fire resistance and spalling of other structural members under varying fire scenarios and loading conditions and hence paves the way to modernize the state of this research area and practice.

Displacement-Based Simplified Calculation for Pile-Soil Interaction under Reciprocating Low-Cycle Pseudo-Static Loads

Abstract A reciprocating low-cycle pseudo-static loading test on three reinforced concrete (RC) piles was performed in the Structural Laboratory of Fuzhou University to study their failure modes and seismic behaviors. The modified calculations of lateral deformation and soil pressure along the piles were further verified based on the test results. In this article, a displacement-based simplified calculation for pile-soil interaction of piles was proposed based on the shear equilibrium and test results, and then it was compared with the traditional m method and p-y curve method. The test results show that m method overestimated the bearing capacity of piles, leading to a lower safety factor for the piles. Therefore, it was not suitable to calculate the internal force and lateral displacement under large displacement. Moreover, the ductility of prestress high-strength concrete pipe piles or RC piles would be overestimated by the p-y curve method. Although the p-y curve method took into account the nonlinearity of soil, the calculated force-lateral displacement curve still showed a significant difference with the test results under large deformation. The test results further show that the modified calculation of lateral displacement and soil pressure can be preferably validated by test results, which indicates that the displacement law of laterally loaded pile can be accurately assessed. Furthermore, the proposed displacement-based simplified calculation showed a good agreement with the test values in terms of internal forces and force-lateral displacement curve. Therefore, it was recommended for analyzing the cyclic behavior of laterally loaded pile foundations considering soil-pile interaction in integral abutments jointless bridges (IAJBs).

ASSESSMENT OF NOMINAL SHEAR STRENGTH OF REINFORCED CONCRETE COLUMN

: SNI 2847-2019 and ACI 318-19 have regulated the use of materials in the design for shear strength. This study presents an evaluation of nominal shear strength in Reinforced Concrete (RC) column from SNI 2847-2019 and ACI 318-19 by variants of the RC columns experimental database. RC column variants consist of normal strength and high strength RC column with a total of 162 specimens. Furthermore, the database also provides a variety of shear span to depth ratio and axial-load ratio. In this study, the calculation was carried out the mean and the coefficient of variation of measured to calculated strength ratio which is according to SNI, ACI, and other experimental models. Both SNI 2847-2019 and ACI 318-19 have 2 different (concrete nominal shear strength) equations. The calculation results show that the equation is more conservative in both SNI and ACI compared to other nominal shear strength equations. Overall, the combination of both normal strength and high strength RC columns, the calculation using ACI 318-19 with the first nominal shear strength equation can be said to be more conservative than any other calculations models. This study also shows that by using the ACI 318-19 model in the second equation which is used the size effect modification factor , the results do not have a significant effect because the value is close to 1.

Effect of plastic rotation on the concrete contribution to shear strength of RC beams

This paper provides an analytical model to predict the concrete contribution to shear strength of reinforced concrete (RC) beams that fail in flexure. In the RC members subjected to cyclic loading, the stiffness and hysteretic energy dissipation decreases with diagonal web cracking in the plastic hinge region. The proposed method takes into account plastic rotation in the plastic hinge region by means of critical shear crack theory. To verify the concrete contribution to shear strength predicted by the proposed method, six normal- and high-strength RC beams having various shear span-to-effective depth ratios are tested under cyclic loading. The predictions by the proposed equation and various researchers' equations are compared to the test results. It is found that the proposed method is in good agreement with the test results.