Variation Of Concrete Strength Research Articles

A Sustainable Steel-GFRP Composite Bars Reinforced Concrete Structure: Investigation of the Bonding Performance

Steel-fiber reinforced polymer (FRP) composite bars (SFCBs) can enhance the controllability of damage in concrete structures; thus, studying the interfacial bonding between them is fundamental and a prerequisite for achieving deformation coordination and collaboration. However, research on the interfacial bonding performance between SFCBs and concrete remains inadequate. This study conducted central pullout tests on SFCB-concrete specimens with different concrete strengths (C30, C50, and C70), bar diameters (12, 16 and 20 mm), and hoop reinforcement constraints, analyzing variations in failure modes, bond-slip curves, bond strength, etc. Additionally, finite element simulations were performed using ABAQUS software to further validate the bonding mechanism of SFCB-concrete. The results showed that the failure mode of the specimens was related to the confinement effect on the bars. Insufficient concrete cover and lack of hoop restraint led to splitting failure, whereas pullout failure occurred otherwise. For the specimens with pullout failure, the interfacial damage between the SFCB and concrete was mainly caused by the surface fibers wear of the bar and the shear of the concrete lugs, which indicated that the bond of the SFCB-concrete interface consisted mainly of mechanical interlocking forces. In addition, the variation of concrete strength as well as bar diameter did not affect the bond-slip relationship of SFCB-concrete. However, the bond strength of SFCB-concrete increased with the increase of concrete strength. For example, compared with C30 concrete, when the concrete strength was increased to C70, the bond strength of the specimens under the same conditions was increased to 50–101.6%. In contrast, the bond strength of the specimens decreased by 13.29–28.71% when the bar diameter was increased from 12 to 14 mm. These discoveries serve as valuable references for the implementation of sustainable SFCB-reinforced concrete structures.

Analytic Investigation of Hooked Stirrups on Seismic Behavior of Reinforced Concrete 3D Frame Buildings

Ensuring the safety and stability of buildings during earthquakes is of utmost importance. This can be achieved by assessing the seismic performance of reinforced concrete structures with consideration of design details. This study focused on the seismic behavior of reinforced concrete buildings by comparing the effects of two different types of stirrups, namely those with a 135° angled end-hook shape and straight hooks, with variation of concrete strength. Pushover analysis of a sample building was performed to determine the effect of hook shape on stirrup reinforcement with a constant volumetric ratio for various concrete strength classes. The results of the analysis indicated significant differences in concrete strength and seismic behavior between the two stirrup configurations. The hooked stirrups demonstrated superior energy dissipation capability and ductility, which led to better seismic performance compared to unhooked stirrups across varying levels of concrete strength. To extend the investigation, the study compared the Mander et al., Kent–Scott–Park, and Kappos–Konstantinidis concrete models with different concrete classes (C50-C25-C20-C16-C10). The findings emphasized the importance of stirrup configuration in the design of earthquake-resistant structures. The study concluded that RC structural performance with the 135-degree hooked concrete members exhibited much better behavior of the 90-degree members for the various concrete strength. In this way, it has been revealed the arrangement and detailing of reinforcement in the construction beams and columns improves the governing effect on seismic structural performance.

Progressive Collapse Resistance Mechanism of RC Frame Structure Considering Reinforcement Corrosion

Corrosion causes reduction in cross-sectional area of reinforcement, deterioration of mechanical properties, and degradation of bonding properties between reinforced concrete, which are the most important factors leading to the degradation of structural service performance. In order to investigate the progressive collapse mechanism of a corroded reinforced concrete frame structure, the failure modes, characteristics of the vertical displacement, and load capacity are studied using the finite element method. Based on existing experimental research, the established model is verified, and the influence of different influencing factors on the progressive collapse mechanism is analyzed. The results show that the corrosion of the reinforcement affects the yield load, peak load, and ultimate load of the reinforced concrete substructure. As the corrosion rate increases, the tensile arch action shows a particularly severe deterioration. The variation of concrete strength and the height–span ratio affects the substructure’s load-bearing capacity much more significantly than the stirrup spacing.

Application of concrete produced from reused ready-mixed concrete wastewater filtration residue

During the production of ready-mixed concrete, waste slurry was filtered under pressure to obtain Wastewater Filtration Residue (WFR). Physical and chemical analysis of the WFR was conducted using methods such as XRD, SEM. The flowability and activity index of the cement mortar with composite admixtures, prepared by replacing fly ash (FA) with WFR powder at weight ratios of 0%, 25%, 50%, 75%, and 100%, were tested. Different strength grade concretes were prepared by replacing FA at weight ratios of 0%, 25%, and 50%, and replacing fine aggregate at weight ratios of 0%, 5%, and 10%, in order to analyze the variation of concrete strength. The results indicate the following: 1) The WFR was primarily composed of fine particles with various shapes and loose surfaces, making it suitable for concrete preparation. 2) WFR had poor flowability (59.83%) and hydration activity (51.7%), but FA addition improved both. Increasing FA content enhanced flowability ratio and hydration activity. At 1:1 FA-to-WFR ratio, flowability reached 81.44% and hydration activity reached 75.73%. 3) When WFR replaced FA (25%, 50%) or manufactured sand (5%, 10%) in concrete preparation, the strength of the concrete decreased with an increase in the WFR content. In low-strength grade concrete, the strength reduction was significant when WFR replaced FA. However, the strength reduction was less influenced by the design strength of the concrete when WFR replaced manufactured sand. 4) It was recommended to replace fine aggregate with WFR for concrete preparation, with the replacement ratio not exceeding 10% by weight.

Spatial modeling of concrete strength based on data

Structural verification of concrete structures relies on an underlying probabilistic model of the concrete strength. This concrete strength exhibits a spatial variability, which is of particular relevance in existing concrete structures, for which the strength is assessed based on samples. To accurately account for the spatial variability of the concrete material, a random field modeling approach can be adopted, which includes a spatial correlation function. Unfortunately, the available literature on spatial variability of concrete strength is not sufficient to make an educated choice of this correlation function. In this paper, we propose a hierarchical Bayesian random field model, that enables learning the parameters of a selected correlation function with in-situ spatially distributed measurements of the concrete strength. We propose a correlation function that accounts for the composite nature of the material through distinguishing micro-scale and meso-scale variability. The predictive spatial distribution of the proposed random field model given the spatial data is then obtained through an analytical random field update, resulting in a non-homogeneous random field model with log-Student’s t-marginal distribution. The proposed approach provides an effective means to employ in-situ measurements for updating verification predictions of concrete structures. We apply our approach to two case studies on chamber walls of ship locks, where measurements of the concrete strength are available from core samples.

Chemical Anchor Pullout Force Modeling with Variation of Anchor Embedment Length in Concrete and Concrete Strength

The embedment length influences the adhesion between the cast iron material and the concrete. The concrete's compression strength also contributes to an increase in bond strength. Therefore, this research aims to determine the maximum pullout force on each variation of the anchor and the optimal embedment length. A gauge is modeled as a rod-type with a diameter of 16 mm, and the embedment lengths used are 5D, 10D, and 15D, while the compressive strengths include fc’ 20, 30, 40, 50, and 60 MPa. Furthermore, a finite element-based application was utilized with the ANSYS Workbench student version. The result showed that the concrete with strengths of 20, 30, 40, 50, and 60 MPa has maximum pullout forces of 27.011, 53.536, 68.657, 68.970, and 84.407 kN, respectively at an embedment length of 15D. It was observed that the failure pattern obtained starts from the defect in the concrete cone and ends with the breakage of reinforcement or steel failure at each variation of concrete strength. A combination of two non-parametric techniques was used in this research, which includes Kruskal Wallis and U-Mann Whitney test. The first technique revealed that the chi-square value for strengths 20, 40, 50, and 60 MPa is 9.486, while that of 30 MPa is 9.881. The second test employed showed a significance value below 0.05. In conclusion, the embedment length affected the value of pullout force, and 15D was the optimum embedment length for each variation of concrete strength. The enhancement in tensile strength increases with the strength of the concrete.

The Performance of Empirical Laws for Rebound Hammer Tests on Concrete Structures

The assessment of concrete compressive strength plays a key role in the analysis of the seismic vulnerability of existing buildings. However, the adoption of classical destructive tests is usually limited by their invasiveness, cost and time needed for the execution. Thus, in order to overcome these limits and allow investigations to be extended to a large number of points, the use of the rebound hammer test is investigated here with a detailed analysis of the effects on the accuracy of the strength assessment related to the choice of the conversion model relating rebound index to compressive strength. The analysis has been performed by comparing several empirical laws calibrated with data acquired in an experimental investigation of an existing concrete building. The relationships between the coefficients of the examined conversion models are then established, with the aim of reducing the unknowns in the calibration procedure. Furthermore, the influence of the coefficients of variation of concrete strength and rebound index on the results of the calibration procedure has been analyzed, thereby supporting the assessment of the accuracy of the concrete strength.

Optimal allocation of material and slenderness limits for the rectangular concrete-filled columns

The current American Specification AISC 360–16 for concrete-filled steel tubular (CFT) columns exhibits particular limits in terms of section slenderness and material strengths. This paper proposes a decision tree algorithm, together with a comprehensive compiled experimental database of rectangular CFT columns under concentric loading and reliability analysis, to establish new optimal limits for section slenderness and material strength. The decision tree analysis yielded “data-driven classification rules” indicating that a relative section slenderness of 5, concrete compressive strength of 175 MPa, and steel yield strength of 850 MPa are considered optimal limits for the design equations in the AISC 360–16 in terms of achieving the minimum reliability requirements. The reliability analysis results are used to run a sensitivity analysis on the CFT columns to identify the dominant parameters that affect their axial loading capacity. Based on the sensitivity results, special attention should be given to the variability of concrete strength. The last part of this study demonstrates that the design equation of the high-strength CFT member in the new AISC Specification (AISC 360–22; obtained from a public review draft) can be extended to the conventional materials using the same proposed optimal limits.

Effect of the core sample in low aspect ratio on concrete compressive strength

AbstractCores with diameters of 100, 75, and 50 mm and with varied length‐to‐diameter (l/d) ratios of 1.0, 0.9, 0.8, 0.7, and 0.6 were extracted from concrete panels of 11 different concrete mixtures and prepared in the lab. Compressive strength tests of concrete cores and cubic samples were performed at 7, 28, 60, and 120 days, respectively. Strengths of cores with l/d ratios ranging from 0.6 to 1.0 could be converted by the strength correction coefficient into that of the core with the l/d ratio of 1.0. The evidence for the reliability of compressive strength of cores with low l/d ratios is very seldom in literatures. The test results indicated that correction factors gradually decreased with the decrease in l/d ratio, the strength of the core increased and the effect was more pronounced for cores with smaller diameter and the maximum aggregate size. Correction factors of cores drilled from river gravel aggregate concrete are slightly higher than that from crushed stone aggregate concrete. The longer the age of concrete is, the lower the correction coefficient becomes. The proposed correction factor values for core test samples with the l/d ratios ranging from 0.6 to 1.0 differ from the prior researches. For compliance with the requirement of the test data variability, there is no reason to limit the ratio of aggregate sizes (da) to the diameter of the core (d) to a minimum of 3.0 (d/da). The results show that the concrete core with a diameter less than the standard size and l/d less than 1.0 can effectively reflect the variation of concrete strength with acceptable reliability. Drilling‐core with low l/d ratios will facilitate the reduction of the damage to the structures, so the members with thickness less than 150 mm (e.g., slab and shearwall) can be drilled and tested.

Combining the bi-objective approach and conditional coring for a reliable estimation of on-site concrete strength variability

Combining the bi-objective approach and conditional coring for a reliable estimation of on-site concrete strength variability

Experimental Study on the Correlation between Crack Width and Crack Depth of RC Beams.

The depth of cracks propagating inside reinforcement concrete (RC) components is barely able to be detected by visual inspection. Without any help from facilities, crack width can provide us with a proper way to explore the depth of cracks developing. Therefore, this paper tried to explore the correlation between crack width on the surface and crack depth. A static loading test was conducted on eight RC beams, considering the variation of concrete strength, cover, and reinforcement ratio. The test results indicate that concrete strength has a certain impact on cracking load and the propagation of cracks is mainly related to reinforcement ratio. The linear changes in load and crack width can be found. Originally, crack depth markedly increased with loading, but when restricted by compression zone of concrete and the height of beams, crack depth stopped extending finally. The correlation between crack width and crack depth was analyzed by studying work phases of a cross-section and experimental data. The fitting function achieved in this paper was determined to be a good agreement between the theoretical and the experimental relationship.

Non-linear finite element analysis of prestressed T-beams strengthened with FRP laminates and patch anchors

Although fiber reinforced polymers (FRPs) have become the most widely used material to strengthened existing concrete structures in shear, premature debonding of externally bonded FRP remains a significant cause of material under-utilisation. In an effort to suppress debonding failure, research has shown that the use of ±45° bidirectional fabric patch anchors can delay the occurrence of FRP debonding. However, most research to date on patch anchors has been based on joint-level tests and the data available on full-scale beam applications has been very limited. One means by which additional data can be generated on the performance of patch anchors when applied to full-scale beams is using finite element simulations. This paper presents a 2D and 3D non-linear finite element model calibrated using available data on prestressed T-beams strengthened in shear with FRP laminates and patch anchors. The models captured the failure modes, strains in the FRP and crack patterns of the specimens very well and predicted the failure load for the control, strengthened and anchored specimens within a margin of difference of 6–9%, 4–13% and 1–2%, respectively. Parametric studies involving variation of concrete strength and FRP thickness were conducted to generate additional data and the research findings are presented.

Performance of continuous aerated iron electrocoagulation process for arsenite removal from simulated groundwater and management of arsenic-iron sludge

Removal of arsenite from the simulated arsenite contaminated groundwater by using continuous aerated iron electrocoagulation process (CAIEC) (iron electrodes as both anode and cathode) was examined in the present study. The experiments to treat 1 mg L−1 arsenite concentration at two distinctive flow rates i.e. 5 L h−1 and 10 L h−1 were performed. During the process, it was observed that electrode passivation can lead to a reduction in arsenic removal efficiency. The layer formation/passivation of the electrode during the process cannot be controlled but it can be lowered up to some extent by changing the polarity after each run. The average arsenic-iron sludge generated from the process was 35.8 to 70.64 mL L−1 per cycle. Highlighting the proper arsenic-iron sludge management, reusability of this sludge as a binding material in concrete was investigated through solidification and stabilization techniques. A slight variation in the compressive strength of concrete was observed with the addition of arsenic-iron sludge. The presence of arsenic in the concrete mix was confirmed by XRD analysis. The arsenic leachability from the sludge samples as well as stabilized concrete cubes was analyzed with toxicity characteristic leaching procedure (TCLP) (both static and dynamic) and observed insignificant arsenic leaching.

Stochastic assessment of concrete core strength in fire exposed specimens simulating non-engineered RC structures in Turkey

The spatial variability of concrete compressive strength over the specific region is simulated by the stochastic approach. The compressive strength of cores taken from fire-exposed beams simulating non-engineered structures in Turkey is obtained. Therefore, improper mix-design and low strength concrete are targeted. Then, the uneven distribution of the concrete compressive strength along the fire exposed specimens is handled by the random fields approach. Thus, the adverse effect of the fire is not only established by the compressive test results but also the scatter in the core results is estimated by the stochastic method. The stochastic model accounting for the scatter in the core results is generated from the given distribution and mean cylindrical compressive strength values. The concrete compressive strength is not distributed evenly but established stronger and weaker regions over the specimen in the stochastic models. The statistical samples are generated by the Monte Carlo-type stratified sampling method, which is Latin Hypercube Sampling (LHS). A total of 500 random samples closely predicts the scatter in the compressive strength at each temperature level with a range of 25–700 °C. The variability at different geometrical positions on the fire-exposed beam specimens with low strength concrete is characterized as well. Overall, the experimentally observed uncertainties arising from uneven distribution of concrete compressive strength over the fire-exposed beams simulating non-engineered structures in Turkey are accurately reproduced by the random fields approach.

НАДЕЖНОСТЬ ЖЕЛЕЗОБЕТОННЫХ КОНСТРУКЦИЙ С УЧЕТОМ ФАКТОРА ПОЛЗУЧЕСТИ БЕТОНА

In the process of inspection of reinforced concrete structures being in long-term service, deflections of beams and slabs beyond the standard values are often detected, which is attributable to concrete creep occurring in the case of the early dismantling of shuttering. At the same time, any visual signs of their reduced load-bearing capacity are absent. Taking into account the high degree of uncertainty of the factors influencing the long-term strains of the concrete, the safe service life of such structures can vary within a rather wide range, and its actual value can be assessed only through probabilistic methods of the reliability theory. The paper presents the results of the investigation of the influence of concrete creep caused by the early dismantling of the shuttering on the reliability of prefabricated reinforced concrete floor beams of a three-storey building. The data obtained through the instrumental verification of the mechanical characteristics of the beam materials and their deformed state were used for design modelling. The authors carried out the probabilistic creep analysis of the beam through the method of statistical modelling taking into account the variability of concrete strength for various values of the relative humidity of the ambient air and the age of concrete at the moment of the load application. The statistical characteristics of the stress-strain modulus and the beam deflection values with various levels of probability were obtained. Due to con-crete creep, the safety analysis showed a 2,4 times reduction in the beam reliability index at the service life of a structure of 70 years.

Probabilistic Model for Field Concrete Strength.(Dept.C)

The paper deals with the evaluation of field concrete strength placed on construction sites in Dakahilia, Egypt in the years 1994 – 2000. The main purpose of this research was to study the variability of concrete strength produced in different location on sites. Using the information of cube compressive strength data, probabilistic models have been developed to describe the variability of concrete compressive strength between locations. A total 859 concrete samples were randomly collected from construction sites for strength testing at strength of material laboratory, Mansoura University. The result indicate that concrete is well modeled by the normal distributions. Also, the results of the analysis showed the coefficient of variation are higher for this study than normal, The mean-to-nominal strength ratio varies between 0.99 and 1.05, whereas the coefficient of variation is in the range of 27.47% 35.81%, depending on the location of concrete. The models are verified by ꭕ2 and Kolmogorov - Smirnov goodness of fit tests at the 5% significance level. The models developed in this paper are useful for predicting the performance of structural element and assessing their reliability levels.

Fuzzy seismic fragility analysis of gravity dams considering spatial variability of material parameters

Due to the heterogeneity of massive concrete and unevenness of pouring process, concrete strengths in gravity dams show obvious spatial variability. In addition, since the evolution of damage states for gravity dams is a gradually transitional process, the threshold between adjacent damage states is ambiguous. A mathematical model for describing the fuzziness of damage states threshold is presented. And a methodology for fuzzy seismic fragility analysis of gravity dams considering spatial variability of material parameters is proposed by combining nonlinear dynamic analysis of dam-reservoir-foundation system, Hariri-Ardebili's damage quantification method and an approach for simulation of random field of material parameters. The seismic fragility of gravity dams was investigated to illustrate the proposed methodology. It is found that the spatial variability of concrete tensile strength leads the seismic damage of the dam to be spatially varying as well. The damage of dam body intensifies and the probability of exceeding for each damage state increases. The fuzzy seismic fragility curves are insensitive to the types of membership function (linear, mountain and polynomial). Due to the influence of frequency distribution of damage index, the effects of variation in the size of the fuzzy interval of threshold on seismic fragility curves are different. The probability of exceeding of each damage state for gravity dams obviously increases when the spatial variability of tensile strength and fuzziness of damage states threshold are considered simultaneously.

ANSYS Numerical Modeling of Confined Deep Beam with High Strength Concrete

In the numerical modeling presented, the deep beam is accounted for by means of a three dimensional FEA ANSYS 13.0 approach, in which the structure is modeled with load-displacement-based solid finite elements, whereas the internal work structure interacted by high strength concrete and reinforcement in full scale. Most deep beam collapse is dominated by the shear brittle collapse. Recently, confinement is the most effective way to improve the ductility of the reinforced concrete, whereas required to anticipate the occurrence of direct crack on the beam caused by quite large shear forces. Numerical models of deep beam with high strength concrete is done gradually by giving a variation of concrete strength, confinement and stirrup spacing. The two point load are applied with the ultimate load on all models. Then, analyzing of the deep beam is to be done with and without side reinforcement and also an analysis of the crack pattern of the beam to calculate the brittle area under the ratio of crack volume occurred. Concrete is modeled by SOLID65 and steel reinforcement using SOLID45 element. The collapses occurred are all ultimate flexural compressive collapse on the loading plate area, due to brittle shear collapse. The condition occurred on deep beam first crack was already considered a collapse, the density of reinforcement and the additional distance comprehensive reinforcement bar is most effective in terms of adding high ductility of high strength concrete beams. By using the stirrup reinforcement with confinement mode can significantly enhance the resistance of ultimate capacity, cracking ratio and reduced deflection.

Early hydration properties and performance evolution of phosphoaluminate cement concrete

As a novel cementitious material, phosphoaluminate cement (PAC) has the characteristics of early strength and high strength. The early hydration properties of PAC were investigated, and the influence of curing age on concrete strength, anti-permeability, surface ventilation and micro structure was analyzed. Moreover, the inherent correlation between hydration mechanism and process of PAC was explored deeply by different test methods. The research results indicate that the curing age has significant effects on the performance development and micro structure of the concrete. The variation of concrete strength with curing age can be expressed by a Boltzmann function. Besides, the changes of relative permeability coefficient of anti-water permeability and chloride diffusion coefficient with curing age meet an exponential function. There exists a good linear relationship between natural logarithm of permeability pressure of concrete surface and time. The early hydration process of PAC is consisted of four stages, i.e., dissolution, induction, acceleration and deceleration.

Analysis of a Large Database of Concrete Core Tests with Emphasis on Within-Structure Variability.

In reinforced concrete (RC) structures, the compressive strength of concrete can play a crucial role in seismic performance and is usually difficult to estimate. Major seismic codes prescribe that concrete strength must be determined essentially from in situ and laboratory tests. Mean values obtained from such tests are the reference design values when assessing existing structures under seismic actions. The variability of concrete strength can also play an important role, generally requiring that various homogeneous domains are identified in a single structure, in each of which a specific mean value should be assumed as representative. This study analyzes the inter- and intra-variability of the concrete strength of existing buildings using a very large database made up of approximately 1600 core tests extracted from RC buildings located in the Basilicata region (Southern Italy). The analysis highlighted that concrete strength variability was dependent on the structures’ dimensions as well as on the number of storeys. Moreover, the concrete strength of cores extracted from columns was found to be, on average, lower than that from beams, thus justifying the usual practice to extract cores mainly from columns, which results in a conservative approach as well as a more feasible one. Finally, some case studies were analyzed, specifically focusing on the effects of the within-storey variability. Conservative strength values, to be used especially in the case of vertical members subjected to high axial loads, are suggested.