Constant Moment Zone Research Articles

Bond behaviour of staggered/non-staggered lap-spliced GFRP bars in concrete

This paper reports on the results of an experimental and analytical investigation into the bond behaviour of glass fibre-reinforced polymer (GFRP) splices in concrete. Eleven full-scale beam specimens in which GFRP bars were spliced within the constant-moment zone are examined. The study incorporates several key test variables, encompassing the ratio of bars lapped (33%, 50%, and 100%), surface characteristics (including sand-coated, ribbed, and wrapped bars), bar diameter (12 mm and 16 mm), and splice length (ranging from 30 to 40 times the bar diameter). The findings reveal a reduction in bond strength ranging from 12 to 24% for staggered bars when compared to their non-staggered counterparts. Furthermore, an increase in bar diameter is shown to decrease bond strength by 16%, whilst surface treatment also influences both bond strength and the pattern of cracking in the splice region. Increasing the splice length from 30 to 40 times the bar diameter enhances bar failure stress by 26%, but it also reduces the average bond strength by 9%. A new predictive equation for the design bond strength of fibre-reinforced polymer (FRP) bars is developed by utilizing a data base of 151 test results. The predicted bond strengths demonstrate greater consistency with experimental outcomes in comparison to previous models.

FATIGUE BOND LIFE OF REINFORCED CONCRETE BEAMS WITH LOW REINFORCEMENT CORROSION

Corrosion in reinforced concrete elements can result in a decrease in cross-sectional area and loss of bond between steel and concrete. The cumulative effects of fatigue loading can exacerbate the adverse impact on the bond characteristics between steel and concrete. In this study, the impact of reinforcement corrosion on the fatigue bond life of reinforced concrete members using lap-splice specimens is investigated. The beams are designed such that the failure of the control specimen occurs due to bond failure in the lap-splice region. Stirrups are provided in the lap-splice region to consider the influence of stirrup confinement on bond behavior. The longitudinal rebars and stirrups in the lap-splice region are subject to accelerated corrosion using the impressed current technique, respectively achieving approximately 5% and 15% mass loss. Subsequently, the beam specimens are tested under four-point loading, such that the lap-splice region falls under a constant moment zone. Under monotonic loading, the control specimen experiences abrupt failure due to bond failure. In fatigue loading, the corroded specimen exhibits a significant reduction of 98% in its fatigue life.

FATIGUE BOND LIFE OF REINFORCED CONCRETE BEAMS WITH LOW REINFORCEMENT CORROSION

Corrosion in reinforced concrete elements can result in a decrease in cross-sectional area and loss of bond between steel and concrete. The cumulative effects of fatigue loading can exacerbate the adverse impact on the bond characteristics between steel and concrete. In this study, the impact of reinforcement corrosion on the fatigue bond life of reinforced concrete members using lap-splice specimens is investigated. The beams are designed such that the failure of the control specimen occurs due to bond failure in the lap-splice region. Stirrups are provided in the lap-splice region to consider the influence of stirrup confinement on bond behavior. The longitudinal rebars and stirrups in the lap-splice region are subject to accelerated corrosion using the impressed current technique, respectively achieving approximately 5% and 15% mass loss. Subsequently, the beam specimens are tested under four-point loading, such that the lap-splice region falls under a constant moment zone. Under monotonic loading, the control specimen experiences abrupt failure due to bond failure. In fatigue loading, the corroded specimen exhibits a significant reduction of 98% in its fatigue life.

Retrofitting of severely damaged RC beams using novel bio-composite-based mat-matrix system

Due to the rapid urbanization and boom in the infrastructure sector, the frequency of repair and retrofitting in major structures has significantly increased in recent times. Hence, it is essential to adopt a repair/retrofit strategy that not only improves the load-carrying capacity but is also ductile, durable, and cost-effective. In this study, efforts are put forth to develop a novel bio-composite-based mat-matrix strengthening system for restoring the overall performance of severely damaged reinforced concrete (RC) beams. In total, eight full-scale RC beams of span 2.2 m were tested under two different shear span (av) to effective depth (d) ratios of 5.0 and 3.9 to understand the efficiency of proposed strengthening schemes under flexure and shear load combinations. The initial loading of RC beams (i.e. damage phase) showed intense opening of tension cracks in the constant moment zone and crushing at the compression face of the beam. After failure, all the beams were surface prepared and retrofitted with the novel bio-composite mat-matrix system using different combinations of bi-directional flax/hemp fiber mat and discrete flax/hemp fiber based UHTCC matrix. The parameters investigated include (i) type of bio-fiber mat, (ii) type of discrete bio-fiber in UHTCC matrix, and (iii) shear span (av) to effective depth (d) ratio. Results revealed that the proposed composite system was able to completely restore the overall performance of severely damaged RC beams at low av/d ratios. However, in the case of beams with flexure-dominant failure (high av/d), the strengthening system was able to restore about 90% of the ultimate load.

Lap splice assessment of GFRP rebars in reinforced concrete beams under flexure

To increase the knowledge and experience gained from the existing literature regarding the bond behavior of glass fiber-reinforced polymer (GFRP) rebars, a total of sixteen full-scale GFRP-reinforced concrete beams were subjected to a four-point bending test. The GFRP reinforcement comprised a single M16 (No.5) GFRP sand-coated bar without confinement reinforcement in the constant moment region. The research parameters encompassed three different lap-spliced lengths (i.e., 40-, 60- and 80-db) and two distinct concrete clear covers (i.e., 19 mm and 38 mm) to assess their impact on bond strength and failure modes. Strain gauges along the GFRP rebar and small potentiometers positioned at the end of the splice within the constant moment zone allowed detailed measurements. Rebar slippage in lap-spliced specimens was observed to have a limited impact on bond stress transfer capacity and member flexural stiffness until reaching maximum slippage. Concrete cover significantly influenced the behavior of lap-spliced GFRP-RC beams, with 38 mm-cc specimens exhibiting an average 21% higher capacity compared to 19 mm-cc specimens. The ACI expression exhibited an average overestimation of 24% for 19 mm-cc specimens, whereas for 38 mm-cc specimens, it offered a more accurate estimation, with only a 10% over-prediction. However, the computed lap splice length to achieve the required tensile stress was reduced from the one required by ACI due to extra factors implemented in the development length equation. This aligns with prior research findings that identified unconservative values of bond stress when employing the ACI provisions for specific combinations of parameters, highlighting the necessity for refinements in predictive models.

Flexural performance of high-strength lightweight concrete beams made with hybrid fibers

This paper reports on the flexural response of lightweight high strength concrete beams, (LWHSC) reinforced with different combinations of discrete, discontinuous and randomly distributed steel, crimped polypropylene and recycled plastic fibers. All the LWHSC beams tested failed in a typical flexural failure mode, in which flexural vertical cracks appeared within the constant moment zone. Analysis of the test results indicated that the fiber reinforced LWHSC beams exhibited significant increase in load carrying capacity, flexural toughness and ductility indices compared to control LWHSC beams without fibers. Among all the beams tested, the ones reinforced with 0.5% of 60 mm long steel fibers and 0.25% of crimped polypropylene fibers exhibited the largest increase, about 18% in load carrying capacity and flexural toughness compared to control beam specimens without fibers. While, the beams reinforced with 0.5% (50/50 blend) of short steel fibers (30 mm and 60 mm long), and 0.25% of recycled plastic fibers outperformed the flexural toughness of similar beams containing equal content of steel fibers and 0.25% of crimped polypropylene fibers by about 6%. Furthermore, test results indicated that an eco-friendly locally produced recycled plastic fibers could be successfully used for enhancing the flexural performance of LWC beams without compromising their load carrying capacity. Moreover, finite element software ABAQUS was used for predicting numerically the flexural response and crack progression of the beams investigated. The numerical predictions compare fairly well with the experimental test results

Influence of short glass fibres and spatial features on the mechanical behaviour of weft-knitted textile reinforced concrete elements in bending

Weft-knitted textiles made from high-strength fibrous materials offer great potential for use as a flexible stay-in-place formwork and reinforcement system since they allow creating complex geometries (i.e. doubly curved or folded) and introducing spatial features such as ribs within the fabric. However, the closed surface of the textile and its placement at the bottom edge of the concrete element present major challenges regarding bond, which may lead to premature failure due to delamination of the reinforcement initiated by a substantial opening of a governing crack. This study investigates the influence of short integral glass fibres and spatial bond ribs on the mechanical behaviour of weft-knitted textile reinforced concrete elements subjected to bending and their potential to increase the shear resistance. To this end, an experimental campaign consisting of 14 four-point-bending tests was conducted, where the specimens were examined regarding their load-deformation behaviour, crack kinematics and failure modes. The contribution of the short glass fibres to the load-bearing mechanism was estimated with Pfyl’s fibre engagement model, based on the material characterisation results from prism tests on fibre reinforced concrete members. The Tension Chord Model was used to predict the stress-strain relationship of the reinforcement and the crack widths in the constant moment zone, which yielded a good correlation between the predictions and the experiments. The short fibres mostly contributed to the flexural response in the serviceability limit state, but only slightly increased the shear resistance due to their low fibre effectiveness. The introduction of bond shear connectors was shown to be essential to prevent the premature delamination of the reinforcement. The use of tall connectors allowed doubling the bearing capacity with respect to the flat reinforcement without any spatial features, effectively bridging the governing crack and reaching the tensile capacity of the longitudinal textile reinforcement.

Behavior of reinforced concrete solid and hollow beams that have additional reinforcement in the constant moment zone

This paper presents the effect of non-uniform reinforcement ratio along the beam length on the flexural behaviour experimentally and numerically. Within the experiment, four reinforced concrete beams each had a different reinforcement ratio. However, three of four beams had a similar reinforcement ratio in the constant moment zone (0.012). Cracking load, load carrying and deflection were monitored through the test. A nonlinear finite element software was implemented to simulate the experimental behaviour. Followed up by a parametric study. It was found that, in reinforced concrete beams, the tension stiffening depends on the concrete area in the tension zone not the reinforcement ratio. FEA predicts the reinforced concrete beams behaviour within a good agreement. Finally, the findings show that, determining variable amount of reinforcement ratio along the beam length will not sacrifice the flexural behaviour, but it will reduce the quantity of the steel reinforcement and the overall cost.

Quantification of cracking localization in fibre-reinforced concrete beams

The paper presents an analysis of cracking localization in beams made of fibre-reinforced concrete with conventional reinforcement. This phenomenon has been quantified by the ratio m/n between the number of the significantly wide cracks and the total number of flexural cracks that developed within the constant moment zone of the beams. It is shown that beams with larger reinforcement ratio had larger values of m/n, that is, lower cracking localization. For the given concrete mix, when the reinforcement ratios ρ were larger than ∼2% cracking localization was diminished. Furthermore, for low values of ρ, m/n ratios of the specimens with lower fibre contents are somewhat larger than those of the specimens with larger content. It is also shown that as the reinforcement ratio decreases below ∼0.5% the normalized mid-span ultimate deflections decreases as well, which corresponds with the observation of the cracking localization phenomenon for low reinforcement ratios. For larger reinforcement ratio, when m/n increases above ∼0.6, there is no effect of cracking localization on the ultimate deflection. Moreover, in this range of r the values of d are ≳ 1.0, as commonly expected for RFC structural elements. It is noted that the ultimate deflection of the beams corresponds to their flexural ductility.

Behavior of RC beams with tension lap splices confined with transverse reinforcement using different types of concrete under pure bending

Many research studies have been carried out on the different parameters affecting the bond strength of tension lap splices in RC beams but limited attention has been given to the effect of using transverse reinforcement along the splice length. The first part of this research is an experimental program which consisted of sixteen reinforced concrete beams tested at the concrete laboratory of the Faculty of Engineering, Cairo University. The parameters under study herein are the diameter of the transverse reinforcement as well as its shape and distribution while using three different types of concrete. The beams were all simply supported with 1800mmspan and 150×250mm cross section. The tensile steel was spliced in the constant moment zone. The second part of the research consisted of an analytical study to enhance the understanding of the topic of this research. Three-dimensional nonlinear finite element analysis was carried with the help of the well-known finite element software; ABAQUS. The concrete damaged plasticity model is selected at this study because it is capable of representing the complete inelastic behavior of concrete both in tension and in compression including damage characteristics. The analytical and experimental results were compared and contrasted. Good agreement was obtained.

Flexural behaviour and ductility of high strength concrete (HSC) beams with tension lap splice

An experimental program has been conducted in order to investigate the flexural behaviour of concrete beams with variable length of tension reinforcement lap splice. A test series of eighteen simple beams containing different lap splice length (0, 300, 500 and 700mm) had been conducted in this investigation. The tested beams are of 2200mm total length and 200×150mm cross-section, loaded at the middle third with two equal concentrated loads. The parameters included in the experimental program are the splice length, the bar diameter (12, 16mm), the amount of transverse reinforcement provided within the splice zone, the shape of the anchor at the splices end and the concrete cover (20, 35mm). The test program has been performed on two grades of high strength concretes (0%, 15% silica fume); 55, 65MPa. Two deformed bars of tension reinforcement were spliced in the constant moment zone. The effect of splicing tension reinforcement on the response, cracking load, crack propagation, deflection, ultimate capacity, mid-span reinforcement strain, failure mode and beam ductility, is examined. Results show that by introducing an appropriate amount and distribution of transverse reinforcement, a satisfactory ductility response can be obtained (91% of reference beam).

Verification of cracked section shrinkage curvature models

An attempt is made to theoretically and experimentally verify the shrinkage curvature models presented in Eurocode 2 and BS 8110. These codes claim that the models originally derived and proven for uncracked sections are suitable, with modification, for predicting the behaviour of cracked sections, although this claim has never been proven experimentally. To achieve verification, an alternative theoretical approach is initially proposed in this paper. In this theoretical model, the effect of shrinkage, creep and the variation in the neutral axis position of the section are taken into account. The stresses developed in the steel and concrete at a cracked section according to this theoretical model are then applied to a finite-element (FE) model representing a portion of the beam from the crack to mid-way between the crack and an adjacent crack. Ultimately, the mean curvature is determined. Experimentally, pairs of beams were cast and subjected to a level of flexural loading to produce a stabilised crack pattern in the constant-moment zone. The behaviour of the beams was monitored for up to 180 days. For any pair of beams, one beam was cast using a high-shrinkage concrete and the other with a low-shrinkage concrete. Each concrete type, however, exhibits similar creep. Therefore, shrinkage curvature can be obtained by subtracting the long-term movements of one beam from the other. These experimentally defined curvatures were compared with the mean curvatures obtained from the FE analysis. The comparison showed reasonable agreement. The curvatures were also compared with uncracked and cracked curvatures predicted by the codes. The curvatures derived in this investigation fell within the boundaries of the uncracked and cracked curvatures predicted by the codes and, for the fully cracked case, the curvatures were closer to the uncracked boundary.

Effect of tension lap splice on the behavior of high strength self-compacted concrete beams

Construction using concrete is spreading widely and there is a need for concrete that is capable of flowing under its own weight without mechanical vibration or compaction and fill the places between reinforcement and the complicated form shapes. From here, Self-Compacted Concrete (SCC) appears for the first time. Limited attention has been directed toward the bond between High Strength Self-Compacted Concrete (HSSCC) and spliced bars in beams [1–8].This research studies the bond between HSSCC and spliced tension bars in beams. It is focused on observing the effect of some factors such as; reinforcement bar diameter and ratio, splice length and casting position on the beam flexural behavior. An experimental program consisting of sixteen simply supported beams divided into four groups is considered. All beams are of 1800mm span and 200×400mm cross-section cast with HSSCC. In twelve beams, the tensile steel was spliced in the constant moment zone, and four control beams without splice for comparison purpose. During testing; ultimate capacity, deflection, crack pattern and mode of failure have been recorded. Test results had been compared with proposed values in the Egyptian code of practice, other international design codes and recorded values of other researchers.

Effect of tension lap splice on the behavior of high strength concrete (HSC) beams

In the recent years, many research efforts have been carried out on the bond strength between normal strength concrete (NSC) and reinforcing bars spliced in tension zones in beams. Many codes gave a minimum splice length for tension and compression reinforcement as a factor of the bar diameter depending on many parameters such as concrete strength, steel yield stress, shape of bar end, shape of bar surface and also bar location. Also, codes gave another restriction about the percentage of total reinforcement to be spliced at the same time. Comparatively limited attention has been directed toward the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. HSC has high modulus of elasticity, high density and long-term durability. This research presents an experimental study on the bond between high strength concrete (HSC) and reinforcing bars spliced in tension zones in beams. It reports the influence of several parameters on bond in splices. The parameters covered are casting position, splice length as a factor of bar diameter, bar diameter and reinforcement ratio. The research involved tests on sixteen simply-supported beams of 1800mm span, 200mm width and 400mm thickness made of HSC. In each beam, the total tensile steel bars were spliced in the constant moment zone. Crack pattern, crack propagation, cracking load, failure load and mi span deflection were recorded and analyzed to study the mentioned parameters effect.

Defining the crack pattern of RC beams through the golden section

Both in architecture and arts, the golden section has been exclusively taken into consideration for its geometrical properties. Specifically, among all the proportions, the golden section can inspire beauty. Indeed, it has driven the construction of buildings for centuries. Nevertheless, as discussed for the first time in the present paper, static equilibrium of structures calls the golden section into play. This is the case of reinforced concrete beam in four point bending, whose average crack spacing in the constant moment zone increases of a factor equal to the irrational number 1.61803 when the geometrical dimensions of the beam are doubled. In other words, it can be argued that the centrality of golden section in the art of construction has profound physical meanings, as it can bring together the aesthetic of nature and architecture, and the equilibrium of stress flow in solid bodies.

Influence of loading condition and reinforcement size on the concrete/reinforcement bond strength

The paper reports on a study of bond strength between reduced-water-content concrete and tensile reinforcement in spliced mode. Three different diameters (12, 16 and 22 mm) of tensile steel were spliced in the constant moment zone, where there were two bars of same size in tension. For each diameter of reinforcement, a total of nine beams (<TEX>$1900{\times}270{\times}180mm$</TEX>) were tested, of which three beams were with no axial force (positive bending) and the other six beams were with axial force (combined bending). The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. It was found that there was a considerable size effect in the experimental results, i.e., as the diameter of the reinforcement reduced the bond strength and the deflection recorded at the midspan increased significantly, whilst the stiffness of the beams reduced. It was also found for all reinforcement sizes that higher bond strength and stiffness were obtained for beams tested in combined bending than that of the beams tested in positive bending only.

Localization and Size-Dependent Response of Reinforced Concrete Beams

This paper describes an experimental investigation conducted to determine whether the flexural response of reinforced concrete beams is size (length) dependent. 16 normal- and high-strength concrete beams were tested with 4 different constant-moment zone lengths and 2 different reinforcement ratios. Closed-loop control was used to obtain post-peak behavior, which allowed additional information about the failure mechanisms to be obtained. Results indicate that damage localization occurs in the compression zone, and the size of the damage zone is constant between specimens of different size. Evidence of strain-softening behavior could be seen by comparing local and global strains. While moment-carrying capacity of different size beams was relatively constant, the average strain at yield and maximum moment was observed to be slightly dependent on specimen length. The overall ductility of the beams was dependent on the constant-moment zone length, with larger specimens demonstrating a more brittle response.

Bond between Normal Strength and High-Strength Concrete (HSC) and Reinforcing Bars in Splices in Beams

This paper presents the second part of a study on bond. It reports on the influence of several parameters on bond in splices. The parameters covered are concrete strength, splice length, concrete cover, ratios between side cover, bottom cover, and the spacing between the spliced bars, rib face angle of the reinforcing bar, and admixtures in the concrete mix. The study was conducted in two stages. In the first stage, an analytical equation was developed to calculate the bond strength of splices in normal strength concrete. The mean value of test/calculated bond strength for the proposed equation is 1.007 with a standard deviation of 0.084. These values indicate a significant improvement in the calculation of the bond strength when compared with other proposed equations. The second stage of the research involved tests on 22 beams made of high-strength concrete (HSC). In each beam, the tensile steel was spliced in the constant moment zone. The test strengths are compared with the calculated values. Good correlation between calculated and test strengths of splices in the case of HSC is found.