Strength Grade Research Articles

Experimental Study on Secondary Anchorage Bond Performance of Residual Stress after Corrosion Fracture at Ends of Prestressed Steel Strands

In order to explore the secondary bond anchorage performance between prestressed tendons and concrete after the fracture of steel strands in post-tensioned, prestressed concrete (PPC) beams, a total of seven post-tensioned, prestressed concrete specimens with a size of 3 × 7ϕ15.2 mm were constructed firstly, and the steel strands at the anchorage end were subjected to corrosion fracture. Then, the pull-out test of the specimens was conducted to explore the secondary anchorage bond mechanism of the residual stress of prestressed tendons experiencing local fracture. Moreover, the influences of factors such as the embedded length, release-tensioning speed, concrete strength, and stirrup configuration on anchorage bond performance were analyzed. Finally, the test results were further verified via finite element analysis. The results show that the failure of pull-out specimens under different parameters can be divided into two types: bond anchorage failure induced by the entire pull-out of steel strands and material failure triggered by the rupture of steel strands. The bond anchorage failure mechanism between steel strands and the concrete was revealed by combining the failure characteristics and pull-out load–slippage relation curves. The bond strength between prestressed steel strands and concrete can be enhanced by increasing the embedded length of steel strands, elevating the concrete strength grade, and enlarging the diameter of stirrups so that the specimens are turned from bond anchorage failure into material failure.

Flexural performance of rectangular CFST composite T-beams: Experimental, numerical, and theoretical investigation

Steel-concrete composite beams find extensive application in bridge construction, offering adaptability to complex terrains and structural advantages. Nevertheless, a persistent challenge lies in local instability, especially in steel beams with large aspect ratios. This study proposes stirrup-confined concrete-filled steel tube (CFST) composite beams as a solution to enhance structural performance by mitigating local instability. The research comprises comparative tests, numerical simulations, and theoretical examinations of rectangular CFST composite beams. Results demonstrate a significant improvement, with a 40% increase in flexural stiffness and a 10% increase in ultimate bearing capacity compared to traditional composite beams. Stirrup confinement effectively limits cross-section slip between the steel tube and infilled concrete, resulting in slip measurements of 0.72 mm at the end of loading. Examination of aspect ratios reveals substantial enhancements, showing a 65.6% increase in flexural stiffness and a 54.7% increase in ultimate bearing capacity as aspect ratios increase. Concrete strength grade exhibits minor effects, with slight improvements as the grade increases. Steel yield strength significantly influences ultimate bearing capacity, with a maximum 24.4% increase in higher-strength steel. Moreover, the steel content ratio enhances both flexural stiffness (up to 40.2%) and ultimate bearing capacity (up to 34.6%) with an increased steel ratio. Increased concrete slab thickness leads to improvements of up to 18.3% in flexural stiffness and up to 10.1% in ultimate bearing capacity. Formulas for predicting the flexural stiffness and bearing capacity of rectangular CFST composite T-beams are developed, offering practical guidance for optimizing their performance in structural engineering applications.

Uniaxial tensile properties of steel fiber-reinforced recycled coarse aggregate shotcrete: Test and constitutive relationship

This paper presents the uniaxial tensile characteristics and stress-strain curves of steel fiber-reinforced recycled coarse aggregate shotcrete (SFRCAS). The study variables include concrete strength grade (C20, C30, and C40), recycled aggregate (RA) replacement ratio rg (0%, 30%, 50%, 70%, and 100%), and steel fiber (SF) volume fraction Vf (0%, 0.5%, 1%, 1.5%, and 2%). The experimental results demonstrate that the uniaxial tensile failure process of SFRCAS can be divided into three stages. The cohesiveness of concrete is improved with an increase in strength grade. However, the ratio of tensile to prismatic compressive strength stays constant at approximately 0.07. Adding Vf from 0% to 2% enhances the restraint of micro-cracking within the concrete by the SF. This leads to a significant increase of up to 120% in the uniaxial tensile strength. Furthermore, the stress-strain curve exhibits improved ductility, enhancing the toughness of SFRCAS. Based on the experimental results, empirical segmented constitutive models of Guo and single-parameter constitutive models of Sargin are fitted for SFRCAS. These constitutive models effectively predict the stress-strain behavior of SFRCAS, and the recommended formulas for model parameter calculation exhibit high accuracy. The research findings are essential for the design and detailed analysis of SFRCAS structures.

Mechanical behavior of longitudinally continuous slab tracks reinforced by adhesive anchors under nonlinear temperature load

Adhesive anchors have been widely used as a post-reinforcement measure to strengthen longitudinally continuous slab tracks. However, the mechanical behavior of the longitudinally continuous slab tracks reinforced by adhesive anchors under nonlinear temperature load has not been thoroughly investigated. Therefore, this paper was motivated to conduct experimental and numerical investigation on the temperature-induced deformation and damage of the reinforced track. Firstly, experimental tests were carried out to investigate the influence of the borehole conditions on the bond properties of post-installed adhesive anchors. Next, the bond-slip relationships between anchors and concrete were established for anchors in various borehole conditions. Then, a novel finite element model of the reinforced track incorporating a nonlinear cohesive zone model of structural interfaces was developed and validated by filed monitoring data. Finally, the effects of borehole conditions and number of invalid anchors on the deformation and damage behavior of the reinforced track under nonlinear temperature load were systematically studied. Results show: (1) The cubic compressive strength grade of concrete has a significant effect on the bond-slip curves between concrete and anchors of the longitudinally continuous slab track. (2) Wet boreholes and uncleaned boreholes would reduce elastic stiffness and ultimate bond strength between anchors and concrete by 11–37% respectively, compared with dry and cleaned boreholes. (3) The maximum vertical displacement of slabs could increase by 7% and 4% if the boreholes are wet and uncleaned respectively. (4) The interface at the center of slab end near damaged joint will not fail for the track with 1 or 2 invalid anchors when the temperature reaches its highest. However, interface debonding will occur with a temperature rise of 34.6 ℃ if there are 3 invalid anchors.

Experimental investigation on the cold-forming effect of high strength cold-formed steel sections

Many researches were carried out on the cold-forming effect of cold-formed steel sections while research on that of high strength cold-formed steel sections was quite few. Thus, an experimental investigation on the material property and residual stress of high strength cold-formed steel sections was carried out and the effect of residual stress on the overall stability bearing capacity was studied. It was found that: (1) the increase of yield strength of high strength cold-formed steel was similar to that of Q235 and Q345 cold-formed steel; (2) the residual stress at corner parts was much higher than that at flat parts and the bending residual stress of high strength cold-formed steel was much lower than that of Q235 and Q345 cold-formed steel; (3) residual stress would lead to a decline of the overall stability bearing capacity of axially loaded high strength cold-formed C-shaped steel sections and the effect became lower with the increase of strength grade.

The role of eigen-stresses on apparent strength and stiffness of normal, high strength, and ultra-high performance fibre reinforced concrete

Concrete is characterized in terms of its engineering properties, mainly strength and stiffness, which are subsequently used in structural design. However, the apparent (i.e., measured) concrete properties are not intrinsic but dependent on the conditions under which the measurement is performed. Herein a combined experimental and numerical study is performed to clarify the effects of hygral gradients and resulting eigen-stresses in self-restrained concrete on its apparent strength. Compressive strength, splitting tensile strength and Young's modulus are tested. Three mixes of varying strength grades are studied: normal strength, high strength, and ultra-high performance fibre reinforced concrete. Samples are subjected to two different curing conditions. To investigate the effect of size on the apparent properties, 50 mm, 100 mm and 150 mm cubes are tested. Depending on the size of the specimen there can be an underestimation or overestimation of up to 25% of the real concrete (e.g., splitting, direct tensile) strength.

Mechanical and durability properties of structural grade heavy weight concrete with fly ash and slag

The paper investigates the fundamental mechanical and durability properties of a M50 strength grade heavy weight concrete (HWC), with replacement of cement by fly ash at 15 %, 25 % and 35 % levels and slag at 40 %, 50 % and 60 % levels. The mechanical properties studied include compressive strength, elastic modulus, flexural and split tensile strengths, and the durability properties include water permeability, drying shrinkage and carbonation behavior. Although extended curing improves mechanical strength and water permeability properties of fly ash and slag blended HWCs, fly ash replacement reduces the modulus of elasticity of HWC to some extent. A simple relationship was used to estimate the elastic modulus of fly ash and slag based HWCs from their density and compressive strength, and the estimated values agreed well with the experimental results. Experimental investigation shows that the long term drying shrinkage strain for all fly ash based HWCs (15 %–35 %) is lesser than that for the 100 % ordinary portland cement (OPC) and all slag based HWCs. The resultant hardened density variations of fly ash and slag blended concretes exposed to long term controlled environment discussed in this paper can be used for estimating shielding thickness of HWC members against ionizing radiations. The carbonation resistance of fly ash and slag HWCs exposed to 3 % CO2 condition for 146 days is also evaluated. Accelerated carbonation coefficients (kacc) estimated for all types of HWCs show that the carbonation resistance of HWC prepared with 15 % fly ash and 40 % slag is comparable. The rate of accelerated carbonation of HWCs increases for the increase in replacement level of fly ash and slag. In addition to the higher rate of carbonation, increase in calcium carbonate content estimated through micro analytical studies also suggest that a higher level of slag replacement reduces the carbonation resistance of HWC significantly.

Mechanical properties of 2100 MPa parallel wire strands under and after elevated temperature

In this study, steady-state tensile tests were conducted at elevated temperatures and after cooling from elevated temperatures on 2100 MPa galvanized parallel wire strands (19 × 5.4 mm) used in long-span suspension bridges. Data collection was done using a charge-coupled device camera (CCDC) system. The results from tests at elevated temperatures indicate that the mechanical properties of parallel wire strands deteriorated gradually with increasing temperature. This degradation trend was well described by logistic curves. The results show that the mechanical properties after elevated temperature decreased significantly when the temperature exceeded 350 °C. By comparing with existing research, it was found that the higher the strength grade, the more severe the strength degradation after elevated temperature. During heating and cooling stages, moderate tension led to beneficial effects on residual strength of the specimens. This applies to temperatures below 400 °C. A linear-exponential-linear (L-E-L) stress-strain model was proposed to establish stress-strain equations for parallel wire strands at and after elevated temperatures. Degradation equations were also obtained for elastic modulus, yield strength, ultimate strength and proportional limit. These findings can be utilized to analyze fire resistance performance of cable systems in large-span bridges and assess post-fire damage.

The Application of the Underwater Repair of Concrete Lining Slabs in the South-to-North Water Diversion Project

The cracking of concrete linings in the channel of the Yuzhou section of the South-to-North Water Diversion Project in Henan Province poses a threat to the structural safety of the project and the water quality environment. To solve this problem, the mixing ratio of non-dispersible underwater concrete (NUC) was optimized, the bond strength of new and old concrete was measured, and an underwater repair methodology of the linings was proposed using NUC. The results showed that adding 2.5% of UWB-Ⅱ-type anti-dispersant resulted in NUC with a 28-day underwater compressive strength of 25.1 MPa and a strength ratio of 0.9 between land and water. The effects of water–cement ratio, anti-dispersant dosage, and fly ash dosage on the performance of the NUC were revealed through experiments, and the mix ratio of NUC was optimized. Bond strength measurement at the interface between the NUC and old concrete was tested using the straight shear test. The test results showed that the bond strength between non-dispersible concrete and ordinary concrete was higher than that between ordinary concrete of the same strength grade. Through an analysis of the ionic composition of the water, it was verified that the NUC did not affect the water quality. Therefore, NUC can provide a reference for the underwater repair of the lining panel of the South-to-North Water Diversion Project.

Research on bond behavior of CFRP-concrete interface under quasi-static cyclic loading

In order to investigate the bond behavior of CFRP-concrete interface under quasi-static cyclic loading, static and quasi-static interface shear tests were conducted on three different adhesive layer thicknesses and two concrete strength grades. From these results, it can be seen that the load-slip envelope curves of the quasi-static cyclic loading specimen is similar to the quasi-static monotonic loading curves. As the adhesive thickness increases, the interface bearing capacity increases, the maximum strain of CFRP plate increases, and the effective bond length slightly increases. The interface failure mode is related to concrete strength. The failure mode of C30 concrete bonded with CFRP plate is cohesive failure of concrete surface layer, while the failure mode of C50 interface specimen is adhesive failure between CFRP and adhesive layer interface. As the thickness of the adhesive layer and the strength of the concrete increase, the interfacial bond strength and maximum strain of CFRP plate increase. It can be seen that the bond-slip envelope curves of the quasi-static cyclic loading specimen coincide well with the curves of the quasi-static monotonic loading specimen. As the magnitude of the cyclic loading increases, the unloading/reloading stiffness degradation, residual slip at zero load increases, and the dissipated energy increases. The fitting resulted in a degradation model of interfacial stiffness during quasi-static cyclic loading.

Compression behavior of the concrete-filled double skin steel tube columns under hydrostatic pressure: Experimental and modeling study

This paper conducted an experimental study focusing on the axial compressive behavior of circular concrete-encased concrete-filled steel tubular (CFDST) columns composed of a circular inner steel tube under active confinement. The investigation aimed to explore the impact of concrete compressive strength grades and diameter-to-thickness ratio of steel tubes in the presence of hydrostatic pressure. The effects of varying outer diameter-to-thickness ratios (44, 45.6, 85.84, and 86) and concrete compressive strength grades (10 and 20 MPa) on the performance of CFDST columns were investigated in this paper. Additionally, the study considered the impact of two inner steel tube diameter-to-thickness ratios (20.5 and 39). The experimental results demonstrated that external pressure can significantly enhance both the load-carrying capacity and ductility of the specimens. The ultimate strength of the columns was improved by a minimum of 34% and a maximum of 92%. Finally, empirical formulas for predicting the axial compression capacity of CFDST under active confinement using gene expression programming were presented. The accuracy of the proposed model was then assessed by the artificial neural network, and the results indicate that both models perform well when compared to the empirical equations that take into account the hydrostatic pressure of wet concrete.

Square concrete-filled steel tube strengthened with carbon fiber-reinforced polymer grid-reinforced engineered cementitious composite under axial loading

This paper presents an investigation on axial compression behavior of square concrete-filled steel tube (CFST) columns strengthened with carbon fiber-reinforced polymer (CFRP) gird-reinforced engineered cementitious composite (CFGRE). A total of 19 columns, including 3 un-strengthened CFST columns, 1 CFST column strengthened with CFRP sheets, and 15 CFST columns strengthened with CFGRE, were prepared and tested. The main test parameters involved the strengthening strategy, number of CFRP grid layers, width-to-thickness ratio of the steel tube, and concrete strength grade. The CFST columns strengthened with CFGRE showed a ductile failure, with the CFRP grid rupturing at the corner. The CFGRE provided confinement to the original column in the transverse direction and, along with the bearing load contribution of engineered cementitious composite in the longitudinal direction. This resulted in a significantly greater enhancement in load-bearing capacity compared to the CFST columns strengthened with CFRP sheets. The CFST columns strengthened with CFGRE demonstrated a similar developing trend in load–deformation curves as the CFST columns during the entire loading process, indicating its strong adaptability to the original structural system. A finite element model was established to evaluate the failure process of the strengthened columns. A calculation formula was proposed to predict the load-bearing capacity of the CFST columns strengthened with CFGRE, taking into account the effect of the uneven confining pressure on the confinement provided by CFGRE.

Flexural study on U-shaped steel–concrete composite beam cross joints with L-shaped connection longitudinal transition components

The paper focuses on the U-shaped steel–concrete composite bottom beam cross joints of U-shaped Steel-concrete Composite Open-web Sandwich Plate (USCOSP), presenting a new type of cross joints transmitting the internal force of steel web by L-shaped connection Longitudinal Transition Component (LLTC). LLTC consists of sleeves, L-shaped connection plates and threaded rebars installed on the steel web using sleeves and L-shaped connection plates. Compared with the conventional cross joints which transmit internal force of steel web by Welded Longitudinal Transition Rebar (WLTR), the LLTC cross joints transfer eccentric internal force within the steel web. Through simple-point loading experiments with simply-supported at four ends, the research experimentally investigated the flexural behavior of the WLTR cross joints and the LLTC cross joints. The five specimens were varied to differ in the type of longitudinal transition components, the diameter of WLTR, the number of threaded rebars of LLTC and the strength grades of threaded rebars of LLTC. Experimental results showed that the failure mode of U-shaped steel–concrete cross joint was the buckling of steel web, the buckling of top longitudinal rebar and crush on the concrete under compression near the central region. The LLTC cross joints experienced higher load capacity and better ductility than WLTR cross joints because of little steel web buckling. The ultimate load capacity calculated according to JGJ 138–2016 and the deflection in the elastic stage calculated according to the displacement method agree well with the experiment results.

A machine learning approach for assessing the compressive strength of cementitious composites reinforced by graphene derivatives

The potential reinforcement effect of graphene derivatives (GDs) on cementitious composites (CCs) has attracted significant attention. Previous studies, however, have produced varied results regarding the impact of GDs on CCs. This can be attributed to differences in the properties of GDs and the fabrication details of CCs reinforced by GDs. Experiments to explore these factors are both time-consuming and cost-ineffective. Additionally, no predictive model currently exists for assessing the influence of GDs on the compressive strength of CCs. In terms of Machine Learning (ML), most existing models focus on continuous parameters, including mixture design properties of CCs and reinforcing filler content, but ignore discontinuous parameters such as dispersion technique of GDs in CCs, curing type, and type of GDs.Compiling a unique dataset, this study tailors ML models to comprehensively explore the effect of GDs inclusion on the compressive strength of CCs, considering continuous and discontinuous parameters, including GD properties, fabrication details, and mixture design properties. The most used dispersion techniques and types of GDs were divided into different categories in this study. Moreover, the dataset included cement strength grade and fineness modules to distinguish between the effect of cement types' variety and GDs. Finally, the backwards elimination technique confirmed the necessity of such a customized dataset for trustworthy predictions.Artificial neural networks (ANN), decision trees, and support vector regressors could successfully investigate the impact of GDs' inclusion on the compressive strength of CCs, with ANN demonstrating superior prediction performance. Among the GDs properties, sensitivity analysis revealed that lateral size had the highest effect. Among the fabrication conditions, the dispersion technique had the greatest effect. Considering all investigated parameters, the water-to-cement ratio was considered the most influential, followed by lateral size and curing time. A low w/c significantly reduces the strength growth rate due to poor dispersion of GDs.

Study on Mechanical Properties of Steel Frame Structure with PEC Columns

PEC column has a wide prospect for strengthening existing steel structure buildings because of their high strength, stiffness, and bearing capacity. The seismic performance of the original steel structure could be significantly improved by filling the trough of section steel with concrete to form a partially encased composite column. Especially for multi-story buildings, the PEC column is an ideal edge restriction structural element, so it is of great significance to study its seismic performance. PEC column was introduced into steel frame structure, and six models of 8-story steel frame structures with PEC columns were established by using OpenSees finite element software. The displacement load curve, the maximum axial force in the column, the maximum bending moment at the column base, and the maximum inter-story shear force were obtained by Pushover analysis. The influence of concrete grade and the width-thickness ratio of the steel sheet of the PEC column on the mechanical properties of the whole structure was studied. Results show that lateral stiffness, ductility, and shear load-bearing capacity of the structure are improved with the addition of concrete in section steel, but ductility of the structure decreases with an increase of concrete strength grade. The influence of variation of PEC column parameters on the bottom of the structure is better than that on top. The width-thickness ratio of the section steel flange plays a major role in controlling the mechanical properties of the structure. The elastic stiffness of the structure, axial force of the column, and bending moment of the column base increase with the increase of flange thickness as section height is the same. An increase of the width-thickness ratio of the flange can improve the shear force of the story, and the lower the number of stories, the more significant the effect is.

Brachial plexus block using only 1% lidocaine to reduce pain during the endovascular treatment of dysfunctional arteriovenous access.

Interventional endovascular treatments of dysfunctional arteriovenous (AV) access for hemodialysis can cause pain and discomfort to the patients. Ultrasound-guided brachial plexus block (BPB) is an alternative regional anesthesia method, but conventional BPB using ropivacaine or bupivacaine may cause long-lasting motor power loss, significantly reducing patient satisfaction. This study aimed to introduce BPB using only 1% lidocaine, which induces sensory loss while minimizing motor block, and evaluate the efficacy and safety of this procedure. This retrospective study was conducted on 277 consecutive patients with dysfunctional AV access requiring percutaneous transluminal angioplasty (PTA). Of these, 174 patients underwent the BPB procedure using 1% lidocaine. Time data were recorded, and the motor strength grade (MRC scale, grade 0-5) was evaluated. Numeric rating pain score (NRPS, grade 0-10) was asked during every PTA, and overall NRPS and satisfaction scores (scale 1-3) were asked after the procedure was completed. Of the 174 patients who received BPB, the success rate was 100%, and there were no significant complications related to BPB. The MRC scale measured at the time when the complete sensory loss was achieved was 1.99 ± 0.63, and that at the point of sensory recovery when the block effect expired was 3.93 ± 0.62, indicating a good grade of motor strength. The average NRPS during PTA in the BPB group was significantly lower than that of the control group without BPB (1.04 ± 2.04vs 6.30 ± 2.71, p < 0.001). The overall satisfaction score was significantly higher in the BPB group than in the control group (2.79 ± 0.50vs 2.00 ± 0.81, p < 0.001). BPB using only 1% lidocaine can induce a sensory block while minimizing the effect on motor function. It can be applied safely in an outpatient clinic setting with relatively higher satisfaction.

Research on section dimension optimization of high-rise steel–concrete composite buildings based on improved dividing rectangle algorithm and combined response surface model

Optimizing the design of high-rise steel–concrete composite buildings poses significant challenges due to the diverse range of components and complex arrangement. To optimize this type of structure while considering both optimization efficiency and structural reliability, an optimization method based on an improved DIviding RECTangle algorithm and combined response surface models is proposed herein. Firstly, the standardized section number of components with the same initial section dimension and material strength grade is treated as a variable. Each variable represents a dimension of the initial hyperrectangle, and the subdivision sequence is determined based on the design priority index. Subsequently, after each subdivision, potential optimal hyperrectangles are identified by evaluating the objective function value with constraint penalties at hyperrectangle center points and the distance from the points to the vertices. By continuously subdividing the potential optimal hyperrectangles, initial sampling points are obtained. Furthermore, combined response surface models for the objective and constraint functions are constructed. Finally, by continuously solving the sub-optimization problem to update the response surface models until the algorithm converges, the optimization of structural cost can be achieved. The case studies demonstrate that this method is capable of satisfying the structural constraints while efficiently optimizing the section dimension of high-rise building components.

Research on crystalline admixtures for low carbon buildings based on the self-healing properties of concrete

The increasing fascination surrounding low carbon construction materials has propelled self-healing concrete into the realm of vital technologies for attaining low carbon objectives. This investigation through artificial cracking and precompression experiments demonstrates the efficacy of two specific crystalline admixtures (CAs), namely L-aspartic acid (LAA) and Ethylenediaminetetraacetic acid (EDTA), in augmenting the self-healing characteristics of concrete. The flexural and compressive strength as well as chloride ion diffusion coefficient tests, were used to evaluate the mechanical properties and durability, and the microstructures was analyzed by X-ray diffraction (XRD), differential thermogravimetric analysis (TG-DTG), scanning electron microscopy (SEM), Bruno-Emmett-Teller (BET) and mercury intrusion pore size measurement (MIP). The findings indicate that the optimum dosage for both LAA and EDTA is determined to be 0.5%. At this dosage level, these admixtures effectively fill the pores by stimulating the production of hydrated calcium silicate gel (CSH) and calcium carbonate, leading to a reduction in porosity, enhanced concrete compactness, and successful activation of the self-healing mechanism. Moreover, the incorporation of these admixtures results in improved strength grade and enhanced resistance against chloride ion penetration.

Effects of Concrete Strength and CFRP Cloth Ratio on the Shear Performance of CFRP Cloth Strengthened RC Beams

A three-dimensional meso-scale numerical model, which considers (1) the concrete meso-structures, (2) the bond slip between concrete internal structures and steel bars, and (3) the stripping behavior of carbon fiber reinforced polymer (CFRP) cloth from the surface of the concrete component, is established to investigate the effects of concrete strength and the CFRP cloth ratio on the shear performance of reinforced concrete (RC) beams. On the basis of verifying the rationality of the shear failure model and the feasibility of the CFRP reinforcement simulation method, 30 orthogonally designed numerical models of six kinds of concrete strength and five kinds of CFRP cloth ratios were designed and established. Based on the numerical simulation results, this paper analyzes the damage evolution of the CFRP–concrete interface, the variation trend of CFRP strain along the height direction of beam, the failure mode, and the load-displacement curve. Results show the following: (1) With the increase of concrete strength grade and CFRP cloth ratio, the shear strength of RC beams strengthened with increases in CFRP cloth ratio, the influence range of concrete strength grade is 24–50%, and the influence range of the CFRP cloth ratio is 10–25%; (2) The improvement range decreases, and the improvement range of the concrete strength grade decreases, and the reduction range is about 20%; (3) Based on the simulation results, influences of concrete strength and CFRP cloth ratio on the shear strength of CFRP cloth-strengthened RC beams are quantitatively considered.

Some Technological Parameters of Construction Joints Formation Using the Self-Compacting Concrete Mixtures

Introduction. The important condition ensuring the monolithicity of the erected massive monolithic reinforced concrete structures, divided in compliance with the standards into the heat-shrinkable blocks, is the quality of construction joints in terms of the new-to-old concrete bonding. The research on the bonding strength dependence on the technological parameters of the construction joints formation using the widely spread in recent years self-compacting concretes is a relevant task, because the number of publications on the respective topic is insufficient. The aim of the present paper is to develop a scientific concept regarding the influence of the certain technological parameters on the new-to-old concrete bonding strength during the construction joints formation.Materials and Methods. The research was carried out using the self-compacting and ordinary heavy vibratory-compacted concretes with the compressive strength ranged from 34.5 to 69.2 MPa. The object of the study was the dependence of the new-to-old concrete bonding strength on the method of standardised surface pre-treatment of the “old” concrete in a joint and the compressive strength of the grade of the “new” self-compacting concrete. The composite Mörsch specimens in which the “old” concrete was cured for up to 3 days were used as the main samples. The construction joint relative strength was considered to be the joint quality evaluation criterion equalling to the ratio of the shear strength of concrete in a composite Mörsch specimen to the minimum axial tension strength values of the “new” and “old” concrete. The quality indicators of the concrete were determined by applying the standard methodology. The specimens were cured under conditions simulating the production of works in the spring-summer season.Results. During the research, the construction joint quality criteria in terms of the “old” to “new” concrete bonding were analysed along with the changes they undergo depending on the studied recipe (concrete grade) and technological (length of the technological break, surface treatment method) factors. The dependences of the proposed construction joint quality criterion on the “new” concrete compressive strength (grade) were obtained and the inefficiency of the surface treatment with a metal brush was shown, especially if accompanied with the technological break extension. The required bonding strength values for B25 ... B35 grade concretes can be for sure provided by the surface pre-milling, whereas the extension of the technological break has negative affect on the bonding strength.Discussion and Conclusions. Due to the revealed low bonding strength of the “old” to “new” concretes, regardless the technological factors, it is expedient to look into the ways of ensuring the construction joint quality by implementing structural solutions such as keyed joints and (or) additional reinforcement.