Initial Cracking Load Research Articles

Experimental study on settlement and punching behavior of full-scale RC and SFRC precast tunnel lining segments

The overall stability of a segmental tunnel lining ring is highly dependent on the structural integrity of its individual precast segments, which provide protection against the surrounding exposure conditions. In this study, the settlement and punching behavior of full-scale conventional reinforced concrete (RC) and steel fiber-reinforced concrete (SFRC) precast tunnel lining segments fabricated for a subway tunnel in Canada were investigated. Precast concrete tunnel lining (PCTL) segment specimens were subjected to mid-span load on their intrados faces. Such loading can be induced as a result of vehicular accidents inside the tunnel and/or soil settlement underneath the PCTL segments. Moreover, a punching test was conducted to replicate the expansion of rocks or other geotechnical surrounding conditions above or underneath the tunnel segments. The experimental results showed that the peak load carrying capacity of the RC segments was higher than that of the SFRC segments for both the settlement and punching behavior. However, SFRC segments exhibited higher initial cracking load, more stable post-peak cracking pattern compared to that of the RC segments and significant reduction in crack width. This makes SFRC attractive for the fabrication of segmental tunnel linings where the initial cracking is a governing criterion. Furthermore, it was confirmed that both the tested RC and SFRC PCTL segments satisfied common settlement and punching loads design criteria.

Creep behavior of epoxy-bonded anchor system

The mechanism of epoxy-bonded anchor system (EBAS) has been studied by many researchers in the past several decades with different models. However its long-term performance is seldom mentioned and characterized in the literature, which has led to the fatal accident in the Ted Williams Tunnel, Boston in July 2006. This paper suggests a theoretical model to characterize the long-term performance of EBAS considering the viscoelastic effect of the adhesive layer. First, the theoretical model is compared with a numerical analysis in ABAQUS and the close matched results were achieved. Furthermore, based on the proposed theoretical model, the time dependent behavior of tensile and shear stresses along the anchor and epoxy interface are derived. The closed-form long-term displacement is obtained as well. Moreover, the initial cracking load is recovered and related to the loading time. It was found that EBAS is not going to crack if it is not cracked at the initial loading time. Finally, a parametric study is conducted to analyze the distinct effect of composition factors of EBAS on its long-term behavior.

BEHAVIOR OF R.C.C. BEAM WITH CIRCULAR OPENING STRENGTHENED BY CFRP AND GFRP SHEETS

In the construction of multistory buildings the ope ning in beams are provided for utility ducts and pi pes. Providing an opening in beam develops cracks around the opening due to stre ss concentration. In this paper the behavior of R.C .C. beam with rectangular opening strengthened by CFRP and GFRP sheets were s tudied. This paper presents the behavior of R.C.C. beam with rectangular opening strengthened by CFRP and GFRP sheets with dtechniques. In this experimental study tot al ten beams were casted, one beam without opening (i.e. solid beam) and one beam with rectangular post opening and these two co nsidered as a control beams for comparison. The remaining eight beams were exte rnally strengthened by Carbon fiber reinforced poly mer (CFRP) and Glass fiber reinforced polymer (GFRP) sheets with different str engthening techniques i.e. around the opening, insi de the opening, inside and around the opening and double layer around the open ing. These beams were tested under two point loadin g in the loading frame. The effect of CFRP and GFRP sheets with different stren gthening schemes on such beams were studied interns of initial crack load, ultimate failure load, cracking pattern and deflect ion, From the experimental results it is concluded that the ultimate load carrying capacity of the R.C.C. beam with opening strengthen ed with GFRP sheets of different schemes were incre ased in the range of 3.74 to 37.41% and beams strengthened with CFRP sheets incr eased in the range of 9.35% to 50.50%. Among all th ese techniques, the strengthening with CFRP around and inside the openi ng was found more effective in improving the ultima te load carrying capacity of beam. This investigation helps the practicing engin eers to provide an opening in the beams without red ucing its load carrying capacity.

Behavior and strength of beams cast with ultra high strength concrete containing different types of fibers

Ultra-high performance concrete (UHPC) is a special type of concrete with extraordinary potentials in terms of strength and durability performance. Its production and application implement the most up-to-date knowledge and technology of concrete manufacturing. Sophisticated structural designs in bridges and high-rise buildings, repair works and special structures like nuclear facilities are currently the main fields of applications of UHPC. This paper aimed to evaluate the behavior of ultra-high strength concrete beams. This paper also aimed to determine the effect of adding fibers and explore their effect upon the behavior and strength of the reinforced concrete beams. A total of twelve simple concrete beams with and without shear reinforcements were tested in flexure. The main variables taken into consideration in this research were the type of fibers and the percentage of longitudinal reinforcement as well as the existence or absence of the web reinforcement. Two types of fibers were used including steel and polypropylene fibers. The behavior of the tested beams was investigated with special attention to the deflection under different stages of loading, initial cracking, cracking pattern, and ultimate load. Increased number of cracks was observed at the end of loading due to the use of fibers, which led to the reduced width of cracks. This led to increased stiffness and higher values of maximum loads.

Load capacity evaluated from fracture initiation and onset of rapid propagation for cast iron by digital image correlation

Digital image correlation was used to measure the fracture process of the castiron in this research. The crack opening displacements, crack mouth opening displacements and neutral layer positions were obtained by correlation calculating. The initial crack load was determined by analyzing results of the strain-rate of the area around the tip of the pre-notch. The predicted fracture initiation and crack propagation patterns are consistent with the observed cracking behavior. The load capacity was evaluated by the method combined digital image correlation with the double-K fracture criterion and the simplified double-K fracture model. It indicates that the data determined by the combined method well agree with those of the experiment and better than those from the simplified double-K model.

Experimental Study on Flexural Behavior of Damaged Concrete Beams Strengthened with NSM-CFRP Strips

In order to investigate the strengthen effect of different embedment lengths of the NSM strip on different damage levels. A series of tests were conducted on damaged reinforced concrete (RC) beams in flexure strengthened with near surface mounted (NSM) carbon- fiber-reinforced polymer (CFRP) strips, and initial cracking load, ultimate capacity, loading-deflection curves, and failure modes are examined and analyzed in the paper. The results showed that not only the initial cracking loads and ultimate capacities of the beams are significantly increased，but also the flexural stiffness of the beams in the yield and ultimate behavior stages are improved by using NSM-CFRP strips. The strengthen effect on lower damage level RC beams has no obvious difference with that on non-damaged RC beams. Anchoring of the strip end can increase the ultimate load capacities and decrease the ductility of RC beams. Debonding was found to be the primary failure mode in all cases.

RC Slabs Repaired and Strengthened by Alumina/Polymer Mortar and Prestressing Strands in the Tension Zone: Experimental Investigation Under Static and Fatigue Loadings

While the extent of repair and rehabilitation of existing old concrete structures is rapidly increasing, a vast number of repaired and rehabilitated structures do not function properly during their remaining service life. Especially in the case of using heterogeneous repair materials, it is very important to maintain the bonding performance between materials and to prevent the interface failure under static and fatigue loads. This paper focuses on the experimental investigation of reinforced concrete (RC) slabs, repaired and reinforced with an alumina/polymer (AP) mortar and a prestressing (PS) strand in the tension zone, under static and fatigue loadings. The variables in this experimental study were the space of strengthening, the number of strands, and AP mortar thickness. Attention is concentrated on the overall load-carrying capacity, deflection, strains of reinforcing bars, and the efficiency of repaired and reinforced RC slabs. Test results showed that the deflection of the repaired and reinforced RC slabs was approximately 40% lower than that of control RC slabs. The initial and horizontal cracking loads of a RC slab with an AP mortar of thickness 30 mm in the static test were approximately the same as those of a RC slab with a 20-mm-thick one. In the fatigue test, the deflection, the strain of reinforcing bars at the midspan, and the maximum shear stress of the repaired and strengthened RC slab were about 40~70% lower than those of the control RC slab. Therefore, it can be concluded that RC slabs with an AP mortar and PS strands have a good strengthening efficiency under both static and fatigue loadings, thanks to the high bonding capacity of the AP mortar.

Experimental Research on Shear Property of Steel Bar/Wire Mesh Mortar Strengthening Concrete Beams

In order to enhance the shear capacity of reinforced concrete (RC) beams more efficiently, a thought to strengthen concrete beams by using steel bar/wire mesh mortar was proposed. A comparison experiment study on shear behavior of two kinds of strengthening RC beams(namely steel bar mat(S)mortar strengthening, and steel bar / wire mesh(SW) mortar strengthening) was carried out. The experimental results show that SW strengthening can keep the advantages of a better dispersion of reinforcement due to the addition of wire mesh, leading to a much higher initial diagonal crack load, and a better control to the crack propagation of the strengthened beams. The initial diagonal cracking load and the shear capacity of SW strengthened beams are 83% and more than 30% higher than those of S strengthened beams respectively, while the reinforcement ratio of the formers is only 22% higher than that of the laters. The working mechanism of SM mortar strengthening was discussed.

Tension Stiffening and Cracking Behavior of Ultra High Strength Strain-Hardening Cement Composite (UHS-SHCC) Ties in Monotonic and Cyclic Tension

This study investigates the tensile response of reinforced ultra high strength strain- hardening cement composite (UHS-SHCC) ties in directly monotonic and cyclic tension. The UHS-SHCC exhibits valuable material properties such as high compressive strength, tensile strain-hardening and ductility. However, UHS-SHCC requires high volume of cement, which leads to more shrinkage than conventional concrete. Authors have considered replacing a part of cement by the expansive admixture (EXA) for compensating the shrinkage of UHS-SHCC. Specifically, this paper explores the structural application of a shrinkage-compensating UHS-SHCC to improve tension stiffening in structural members. The cement composite type and EXA replacement were taken as experimental parameters. All specimens had a square cross-section dimension of 100 x 100mm and length of 1,500mm. The test results indicate that the shrinkage compensating UHS-SHCC is very effective to improve tension stiffening behavior and initial crack load of reinforced cement composite ties.

The Effects of Applied Axial Stress on Lateral Bearing-Load Capacity for Fully Grouted Reinforced Concrete Masonry Shear Walls

Ten full-scale fully grouted reinforced concrete masonry shear walls were tested under force-displacement controlled reversed cyclic lateral loads simulating seismic effects. Relevant experimental phenomena and data indicated four walls failing in shear failure and the other six walls failing in flexure to evaluate seismic performance under compress, flexure and shear combined action. The paper mainly examinates lateral bearing-load capacity through two parameters: initial crack and ultimate strength under different failure modes. Through experimental analysis it can be concluded that the shear strength at the initial crack and ultimate load improved with the increased applied axial stress, and the increment of the shear strength was different in shear failure and flexural failure modes. Meanwhile, the ratio of the initial crack load to ultimate load on shear mechanism was from 0.57 to0.59,which was more stable than that(from 0.52 to 0.66)on flexural mechanism. In other words, the applied axial stress have more significant influence on flexural failure walls than shear ones.

Fatigue sensitivity analysis using complex variable methods

The sensitivity of the computed cycles-to-failure and other lifing estimates to the various input parameters is a valuable, yet largely unexploited, aspect of a fatigue lifing analysis. Two complex variable sensitivity methods, complex Taylor series expansion (CTSE) and Fourier differentiation (FD), are adapted and applied to fatigue analysis through the development of a complex variable fatigue analysis software (CVGROW). The software computes the cycles-to-failure and the sensitivities of the computed cycles-to-failure with respect to parameters of interest such as the initial crack size, material properties, geometry, and loading. The complex variable methods are shown to have advantages over traditional numerical differentiation in that more accurate and stable first and second order derivatives are obtained using CTSE and more accurate and stable higher order derivatives are obtained using FD.

Mechanical Behavior of Composite Girder with Perfobond Shear Connector under Hogging Moment

The mechanical behavior of the support regions for continuous composite girders with tensile stresses in the concrete slab and compressive stresses in the lower steel profile becomes strongly nonlinear under negative bending moments. A static test on four half-scale model of a steel and concrete composite girders with different shear connectors including studs and PBL under hogging moments was conducted and observed to evaluate the influence of shear connector on inelastic behavior such as flexural stiffness reduction, crack initiation and development in concrete slab. From the test results, the flexural stiffness and loading capacity of the composite girders were improved by PBL shear connector. Higher initial cracking load and crack resistance stiffness of composite girders with PBL shear connector under serviceability limit state was obtained during crack development process. The test specimen could be assumed as full composite section until the ultimate state from load-slip relationship of shear connector. Analytical and experimental studies can serve as a basis for continuous composite bridges design.

리브플러스 PC슬래브의 수평전단강도 평가

본 연구는 PC와 RC부분이 만나는 접합면에 대한 구조적 거동과 수평 전단강도 평가를 위한 실험적 연구이다. 접합면에서의 수평전단 내력은 접합면의 거칠기나 전단보강근 유무에 따라 결정되어진다. 본 연구에서는 전단 보강근의 형상이 루프형과 래티스형로 구분하고, 보강근 간격 등에 따라 총 4개의 수평전단강도 실험체를 제작하여 실험을 수행하였다. 실험결과, 접합면에서 수평전단 강도는 수직방향 변형에 의해 지배됨을 알 수 있었다. 보강근 형상에 따른 비교 결과, 루프형의 실험체가 평균 초기균열하중, 평균 최대하중 및 접합면의 평균 초기강성 측면에서 각각 33.7%, 45.9%와 55.2%정도 큰 것으로 나타났다. 현행의 국내 전단강도 평가식과 비교한 결과, 루프형 전단보강 실험체는 2.32~4.23배, 래티스형 전단보강 실험체는 1.65~3.06배 상회하는 것으로 나타났다. 따라서 접합면의 거동이나 구조설계기준에 의한 내력이 안전측으로 평가되어 현장에 적용하는 데에는 별다른 문제는 없는 것으로 판단된다. This study investigates the horizontal shear behavior of an interface between precast concrete (PC) and topping concrete(RC), and evaluates the horizontal based on the investigations by the experimental. Horizontal shear strength in connected surface is determined by the roughness an interface and the shear reinforcement or not. In this study, the main experimental parameters are the shear reinforcement types in the shape of loop-type and lattice-type, rebar spacing. A total of four specimens were shear strength tested and manufactured. As a result, the horizontal shear strength of reinforced connected surface was found to be controlled by deformation in vertical direction. Comparison of reinforcement shape, the mean initial crack load loop type specimens, the average maximum load and the junction of the average in terms of initial stiffness, respectively 33.7%, 45.9% and 55.2% were large enough. Evaluation results for shear strength equation of existing standard domestic, the loop-type reinforced 2.32 to 4.23 times, lattice-type reinforced 1.65 to 3.06 times appears to be higher. Behavior of interface or strength of structural design criteria was fairly safe side. It does not have any problems in the applied field is considered.

Crack-damage mitigation of RC one-way slabs with a strain-hardening cement-based composite layer

This paper describes a method to replace part of the concrete at the tensile bottom of one-way slabs with strain-hardening cement-based composite (SHCC) in order to mitigate cracking and improve the durability of the reinforced concrete (RC) members. Specifically, this paper explores the structural application of an expansive SHCC to mitigate crack damage in RC flexural members. The results of tests on eight simply supported RC slabs are described. The effects of the type of SHCC used and the SHCC layer thickness on the first-crack load, ultimate flexural load, crack width and spacing, and the load–deflection relationship of one-way slabs are investigated. The results indicate that the use of SHCC layers at the bottom of the slabs significantly increases the initial crack load, yield load and ultimate load, and the use of an expansive admixture in the SHCC material significantly affects the initial crack load of one-way slabs with an SHCC layer. Considerable reduction in the crack width and spacing was observed for slabs with an SHCC layer, and this tendency was particularly striking for slabs with an expansive and thick SHCC layer. It is concluded that the application of SHCC layers to RC members may prevent the penetration of aggressive substances into the concrete or reinforcement, and could greatly improve the durability of RC members.

Finite Element Analysis of Bending Resistance for RC Beams Strengthened with AFRP Sheets

In order to reveal the failure properties of AFRP sheet-reinforced beams under 4-point bending, a 3-D finite element model of RC beam, including rebar, interface and sheet elements, was established by using MIDAS FEA finite element software. The stress and displacement of each of components in beam were solved by cycling calculations. In terms of Code for Design of Concrete Structures, the initial cracking loads of RC beams strengthened with prestressed and non-prestressed AFRP were further obtained, which are relatively conformed to the existing experiment. The results show that, compared with the beams strengthened with prestressed and non-prestressed AFRP, the initial cracking load of the former has considerable increase.

Behavior of steel-concrete composite cantilever beams with web openings under negative moment

The effect of web openings on the mechanical behavior of composite beams under negative moment was studied through monotonically loading tests. Nonlinear finite element method based analysis was also conducted for cantilever composite beams. The test and the finite element analysis results indicated that the initial cracking loads of composite beams with web opening are lower than that without web opening. The first crack initiated from the concrete slab on the top of the opening. The beams with web opening failed due to shear failure of concrete slab upon the opening. It was also found that the load carrying capacity of beam decreases with the increase of the moment-to-shear ratio at the central line of the opening and the mechanical behavior of beam can be improved significantly by applying stiffening steel plates around the opening. To quantify the reduction of load carrying capacity, a method for calculating the load carrying capacity of beams with web openings under negative moment was derived with consideration of the interaction between moment and shear. Good consistency was obtained between the proposed method, the finite element method and the test results.

Effect of compression point load on the path and life of fatigue crack growth in mixed mode loading

In this study, fatigue crack growth of edge crack in plate made from 5083 aluminum alloy under mixed mode fracture was numerically investigated. Effect of various parameters include initial crack direction angle (30°, 60°, 90°), initial crack length (20 and 25 mm) and load ratio (R=0 and 0.25) were considered. We also consider the effect of point compression load on crack growth life with 20 mm initial crack length under 30°, 60°, 90° at various parts of crack growth path. Mentioned models were simulated using the finite element softwares, ABAQUS and FRANC2D and their results were compared together. For validation we compare the numerical results for a specimen with its experimental data. Numerical results show good agreement with the experimental data for fracture path. It is observed that the point compression load can affect the path and life of crack growth in fatigue loading. If this load was applied in the first cycles of crack growth, its effect is more than another cases. For instance in the crack growth with initial angle 30o applying point load in 5, 10 and 15 mm of crack growth length, increase the life 79.4%, 18.9% and 6.2% respectively.

Ultimate strength of a steel–concrete composite bridge deck slab with profiled sheeting

The ultimate behaviour of a steel–concrete composite deck slab system with profiled steel sheeting and perfobond rib shear connectors was experimentally investigated. The main objectives of this study are to develop a composite deck slab for girder bridges that spans longer but weighs less than the conventional reinforced concrete (RC) deck slab and to experimentally validate the proposed deck slab system. Eight deck specimens were tested with different shear span lengths to evaluate the horizontal shear capacity of the proposed deck system by using the empirical m – k method. To examine the ultimate load-carrying capacity of the proposed deck slab system under sagging and hogging bending actions, two full-scale deck slab specimens supported by a set of steel box blocks were fabricated and tested. The ultimate strength and initial concrete cracking load of the proposed deck system under hogging bending action are approximately 2.5 and 7.1 times greater than those of an RC deck, respectively, while the deck weighs about 25% less than RC deck systems.

Experimental determination of fracture parameters for crack propagation in hardening cement paste and mortar

For all cement-based composite materials, hardening cement paste and mortar are the matrix materials. Their fracture properties are the basis for studying the fracture characteristic of cement-based composite materials. Therefore, three-point bending beams with three different sizes and three different strengths were tested to study the basic fracture properties of hardening cement paste and mortar. Corresponding values of load (P), crack mouth opening displacement (CMOD), and loading point displacement (δ) were simultaneously recorded. The initial cracking load P ini was determined by using resistance strain gauges making up a full-bridge circuit, and the crack propagation process was observed during the test. It was found that the fracture behavior of both hardening cement paste and mortar are not brittle fracture and show a steady crack propagation process before the peak load was reached. This means that the fracture behaviors of all cementitious composites are nonlinear, even hardening cement paste and mortar. Therefore, corresponding fracture parameters of the hardening cement paste and mortar were determined using the double-K fracture model. The results indicate that this model is applicable to hardening cement paste and mortar. In addition, according to the principle of work of fracture, fracture energy values of the tested hardening cement paste and mortar were calculated after taking into account the contribution of the tail of the P–δ curve on fracture energy.

Analytical Method for Pullout of Anchor from Anchor–Mortar–Concrete Anchorage System due to Shear Failure of Mortar

Depending on the relevant material properties, failure of grouted anchors can take forms of pullout of concrete cones, debonding at either anchor–grout or grout–concrete interface, fracture of anchor and combination of some of these failure modes. Further, if the thickness of the grout layer is thin enough, the shear strength of the grout is relatively low or the anchor is in the form of a steel bar with ribs or spirals, the grout would be sheared off so that the anchor is pulled out. The present study presents an analytical method for the last scenario, i.e., anchor pullout from an anchor–mortar–concrete anchorage due to shear failure of mortar. Two different boundary conditions are considered: fixed bottom surface of concrete as Boundary 1, and top surface of concrete with uniform distributed force as Boundary 2. A shear-lag model was introduced to analyze the behaviors of the mortar and the interfacial properties of both the anchor–mortar and the mortar–concrete interfaces were also considered. Based on the deformation compatibilities of the interfaces and the mortar layer, the distributions of the tensile stresses in the anchor and shear stresses in the mortar along the embedment length were obtained analytically during different loading stages for both Boundaries 1 and 2. Moreover, the probabilities and sequences of shear cracks induced by the mortar failure were determined according to the boundary conditions and the comparison between the shear stresses at the loading and nonloading ends. Double shear crack propagation from both ends with different crack lengths was then investigated. Besides, the pullout load was expressed as a function of the shear crack lengths. Then the maximum load and the corresponding critical crack lengths were obtained by using the theories of extremum. Finally, a series of material, structural, and interfacial parameters were adopted to study their influences on the calculated results using the proposed method, including the critical crack lengths, initial cracking load and maximum pullout load. It was found that the initial cracking and maximum loads in Boundary 1 are larger than those in Boundary 2. However, as the longitudinal rigidity of the concrete increases, the values of the maximum pullout loads in both of the boundary conditions approach each other. It was also found that there exists an effective bonding length, beyond which the critical crack length and maximum pullout load are no longer increased.