Strengthening Of Concrete Structures Research Articles

Advanced Composites with Alkali-Activated Matrices for Strengthening of Concrete Structures: Review Study

Old and seismically prone buildings are in need of strengthening in order to comply with the latest building codes and to prolong their service life. For over two decades fiber-reinforced polymers (FRP) have been successfully used for this purpose. However, the poor performance in high temperatures of organic matrices has led researchers to investigate the use of inorganic matrices. Consequently, textile-reinforced mortars (TRM) have been opted for strengthening, since they incorporate textiles impregnated in inorganic cementitious matrices. Lately, in order to promote sustainability and lower the high carbon emissions of cement, alkali-activated mortars, also called geopolymers, have been investigated as an alternative. Their high performance and fireproof properties have made them excellent candidates as matrices in advanced composites for strengthening. This study aims to provide an overview of research in the field of advanced composites with alkali-activated matrices used for strengthening of concrete members. Systems implementing either fiber sheets or meshes have been used so far to strengthen reinforced concrete members, indicating promising results of the new advanced composite.

Effects of temperature and humidity on properties of ethylene/vinyl acetate‐modified ternary complex mortar

AbstractThe ternary complex system of Portland cement, aluminate cement, gypsum is suitable for the repair and strengthening of concrete structures due to its excellent characteristics, such as fast hardening, early strength, micro‐expansion, and so on. However, the application of this ternary complex was limited by the problems of low toughness and strength degradation in the late curing period. Therefore, it is meaningful to study the effects of different curing conditions on repairing materials in concrete engineering. In this paper, the actual repair environment was simulated, and the effects of different curing temperature and humidity on performances of ethylene/vinyl acetate (EVA) modified ternary complex were researched. The initial and final setting time of ternary complex increases with the increase of EVA content. And under different curing temperature and humidity, the addition of EVA improved the toughness of the ternary complex and reduced the water absorption, EVA can effectively prevent the late strength decrease of ternary complex caused by high curing temperature. Under the curing conditions of 40 and 60°C, the addition of EVA greatly improved the flexural strength and toughness of the ternary complex, low temperature environment is not suitable for EVA‐modified ternary complex.

Hygrothermal resistance of pultruded carbon, glass and carbon/glass hybrid fiber reinforced epoxy composites

The hygrothermal resistance of pultruded fiber reinforced polymer (FRP) composites is the key concern when applying in the reinforcement or strengthening of concrete structures in civil engineering. In the present study, the hygrothermal properties of pultruded carbon, glass and carbon/glass hybrid fiber reinforced polymer (CFRP, GFRP, C/GFRP) composite plates were investigated experimentally through the immersion in deionized water at 40 °C, 60 °C and 80 °C as long as 135 days. Water absorption, thermal properties, mechanical properties and microstructure analysis were conducted to evaluate the long-term hygrothermal evolution. It was found that the water absorption response of three kinds of composite plates represented the initial Fick’s diffusion response and a followed long-term deterioration response. Immersed in deionized water led to an obvious degradation of thermal and mechanical properties up to 29.5% for short beam shear strength (SBSS) of C/GFRP, 25.8% for three point bending strength (TPBS) of GFRP and 43.1% for glass transition temperature (Tg) of C/GFRP, respectively. The diffusion of water molecule in the plate brought about the reversible effect of resin plasticization and irreversible effect of resin relaxation. Furthermore, the irreversible interface debonding of fiber/resin aggravated the degradation of SBSS. Based on the Arrhenius accelerating theory, the long-term life of SBSS decreased to stable level of 87.90% (CFRP), 76.64% (GFRP) and 72.43% (C/GFRP), respectively. Meanwhile, the stable retentions of TPBS were 85.99% (CFRP), 77.32% (GFRP) and 81.40% (C/GFRP), respectively.

A Study on Strengthening of Concrete Structures

Concrete is the most commonly used building material due to its high strength, moldability, weather-resistant, and fire resistance, among other benefits. In recent years, there has been a growth in the usage of Micro Silica fume. Through the Ore sand Bridge building, records show that Micro Silica was used in the concrete. These fabrics are not only environmentally sustainable, but they also have technological advantages on both new and hardened mortar products. Both goods are agricultural by-products, but their usage decreases the amount of primary raw materials removed from the earth. Latent hydraulic content is micro silica fume and fly ash. It has intrinsic cementation effects, which must be activated. Combining the powder with Portland cement is a popular way to achieve these results. Pozzolana is the name given to pulverized fly ash. These materials may not have intrinsic cementation properties, but a cementation substance is created when mixed with a high alkaline material.

A Study on Strengthening of Concrete Structures

Concrete is the most commonly used building material due to its high strength, moldability, weather-resistant, and fire resistance, among other benefits. In recent years, there has been a growth in the usage of Micro Silica fume. Through the Ore sand Bridge building, records show that Micro Silica was used in the concrete. These fabrics are not only environmentally sustainable, but they also have technological advantages on both new and hardened mortar products. Both goods are agricultural by-products, but their usage decreases the amount of primary raw materials removed from the earth. Latent hydraulic content is micro silica fume and fly ash. It has intrinsic cementation effects, which must be activated. Combining the powder with Portland cement is a popular way to achieve these results. Pozzolana is the name given to pulverized fly ash. These materials may not have intrinsic cementation properties, but a cementation substance is created when mixed with a high alkaline material.

Review on NSM CFRP Strengthened RC Concrete Beams in Shear

The use of NSM FRP strengthening of concrete structures has become an attractive option to retrofit the existing structures against shear and flexure. This paper reviews only the utilization of NSM for shear in previous review articles. A database of tests of NSM strengthened beams in shear is presented to evaluate the existing design formulations of calculating the NSM contribution in shear. These formulations were in agreement with the experimental results in the database. Further research topics are also identified such as the shape of NSM FRP bars, combined effects of existing steel stirrups, and NSM FRP reinforcement and analytical formulations.

On the shear behavior of natural curauá fabric reinforced cement-based composite systems

The use of fabric reinforced cementitious matrix for shear strengthening of concrete structures has been widely investigated and its field of application is not an uncommon practice. However, the majority of previous works has been limited to synthetic or metallic fabrics. Thus, the present study seeks to investigate the shear mechanical behavior of a cementitious composite reinforced with natural curauá fabrics and its application as a shear structural reinforcement. The experimental program includes Iosipescu tests and the use of Digital Image Correlation technique for material characterization and flexural tests for strengthened structural beams. The developed composite material under shear tests demonstrated a shear strain-hardening behavior with well-defined stages. The beam externally reinforced with the composite presented an improved ultimate strength of 28%. Analytical model predictions were compared to experimental results and a good agreement was achieved.

Strengthening of Concrete Structures Using UHPC - Experimental Verification

UHPC is a material which exhibits excellent mechanical properties and durability. Beside new structures it is also convenient for strengthening of existing structures. The paper investigates the possibilities of strengthening experimentally. A part of the experimental program is presented which is focused on bond of UHPC and existing concrete and on behaviour in bending. Acceptable bond may be achieved when the existing concrete is clean and reasonably rough. Structural elements are exposed to positive and negative bending moments. If UHPC overlay is applied, it works in compression and in tension. The experiments were therefore focused on verification of both alternatives (UHPC in compression and in tension). Finally, the conclusions from the experiments are drawn, which may be applied in practical design.

Torsion behavior of strengthened RC beams by ferrocement

Now a days, construction industry is one of the most over loaded industry and is responsible for carbon emission, the process of enhancing the life of existing structures reduces the impact on the environment. In this regards, external strengthening of concrete structures is necessary under repair and rehabilitation of structures. This will increase the load carrying capacity and life span of concrete structures. In past decade fiber reinforced polymer (FRP) has been widely used as externally strengthening material due to its high strength and light weight. However, ferrocement is one of the suitable alternatives over costly FRP strengthening technique. Ferrocement is thin composite material having better crack resistance ability, ductility, good impact resistance and can be easily wrapped over beam surfaces. Very less research work is available on torsional behavior of concrete beam strengthened by ferrocement due to its complex loading. As practically beam and slab are cast monolithically, strengthening of beam in U- shape is more practical than strengthening all sides. This paper represents an experimental study conducted on reinforced concrete beam strengthened externally by U- shaped ferrocement with different strengths of mortar. This paper also addresses the torsional capacity and angle of twist of reinforced concrete beams. The experimental study was conducted on total 10 number of reinforced concrete (RC) beams of 150 mm × 200 mm × 1500 mm in size. The main purpose of study was to check the effect of different strength of mortar on ferrocement strengthening system under torsion keeping layer of wire mesh and thickness constant. Different strength of mortar mix, fabricated by varying water/cement ratio of 0.400–0.475 at stepped increment of 0.025. Result shows very less improvement in ultimate torsional strength and average 10.33% increment in angle of twist over normal beams.

Experimental study of FRP-strengthened concrete beams with corroded reinforcement

Corrosion of steel reinforcement is the major cause of deterioration in reinforced concrete structures. Strengthening of concrete structures has been widely studied. However, most research was conducted on sound structures without considering the effects of corrosion. This paper presents an experimental study of the feasibility of using externally bonded FRP laminates combined with U-jackets, applied directly without repairing the deteriorated concrete cover, to strengthen beams with corroded reinforcement. Ten beams were tested in four-point bending. Two beams were not deteriorated and non-strengthened; these served as references. The other eight were pre-loaded to induce flexural cracks and then exposed to accelerated corrosion. Two of the deteriorated beams were not strengthened, three were strengthened with glass-FRP (GFRP) laminates and three with carbon-FRP (CFRP) plates on the beam soffits. On the six strengthened beams, CFRP U-jackets were installed along the span. Local corrosion levels were evaluated with a 3D-scanning technique. Pitting corrosion significantly reduced the load-carrying and deformation capacity of the deteriorated beams. Despite average corrosion levels of 20%, local corrosion levels up to 57% and corrosion-induced cracks up to 1.9 mm wide, the FRP-strengthening method (applied directly to the beams without repairing the deteriorated concrete cover) was effective in upgrading the load-carrying capacity and flexural stiffness. The applied U-jackets effectively suppressed the delamination of the concrete cover and led to the rupture of GFRP laminates and a utilisation ratio of CFRP plates up to 64%. However, improvement in the deformation capacity was not noticeable; this requires further research.

Experimental research on hybrid anchorages of prestressed composite strips for structural strengthening

The prestressed unidirectional carbon fibre reinforced polymer (CFRP) strips are currently often used for flexural strengthening of concrete structures. To ensure reliable strengthening, strips have to be anchored at their ends to the surface. However, anchoring of unidirectional CFRP strips is difficult because of their weak transversal mechanical properties. Although several CFRP strip anchorage approaches have been developed to date, only very few were applied on-site. The paper presents the development of hybrid bonded/bolted anchorages of CFRP strips used in a novel prestressing system. The anchorages are made of thin steel plates, clamped with the high-strength friction grip (HSFG) bolts and epoxy adhesive, to fix the CFRP strips. The paper describes the results of full-scale tests carried out on a series of axial tensile specimens. The developed anchorage has an ultimate tensile strength of about 200 kN, i.e. 70% of the ultra-high CFRP strips’ strength, which is sufficient for strengthening purposes. After a series of tests in the laboratory, the anchorages have been implemented on-site and an example is presented in the paper.

Activated Ductile CFRP NSMR Strengthening.

Significant strengthening of concrete structures can be obtained when using adhesively-bonded carbon fiber-reinforced polymer (CFRP) systems. Challenges related to such strengthening methods are; however, the brittle concrete delamination failure, reduced warning, and the consequent inefficient use of the CFRP. A novel ductile near-surface mounted reinforcement (NSMR) CFRP strengthening system with a high CFRP utilization is introduced in this paper. It is hypothesized that the tailored ductile enclosure wedge (EW) end anchors, in combination with low E-modulus and high elongation adhesive, can provide significant strengthening and ductility control. Five concrete T-beams were strengthened using the novel system with a CFRP rod activation stress of approximately 980 MPa. The beam responses were compared to identical epoxy-bonded NSMR strengthened and un-strengthened beams. The linear elastic response was identical to the epoxy-bonded NSMR strengthened beam. In addition, the average deflection and yielding regimes were improved by 220% and 300% (average values), respectively, with an ultimate capacity comparable to the epoxy-bonded NSMR strengthened beam. Reproducible and predictable strengthening effect seems obtainable, where a good correlation between the results and applied theory was reached. The brittle failure modes were prevented, where concrete compression failure and frontal overload anchor failure were experienced when failure was initiated.

Experimental Analysis of Continuous Beams Made of Self-Compacting Concrete (Scc) Strengthened with Fiber Reinforced Polymer (Frp) Materials

Strengthening of concrete structures is applied as a solution for various deterioration problems in civil engineering practice. This also refers to the structures made of self-compacting concrete (SCC), which is increasingly in use, but there is a lack of research in this field. This paper presents an experimental analysis of flexural behavior of reinforced concrete (RC) continuous beams made of SCC, strengthened with fiber reinforced polymer (FRP) materials (glass (GFRP) and carbon (CFRP) bars, CFRP laminates), by the use of near surface mounted (NSM) and externally bonded (EB) methods. Six two-span continuous beams of a total length of 3200 mm, with the span between supports of 1500 mm and 120/200 mm cross section, were subjected to short-term load and tested. The displacements of beams and the strains in concrete, steel reinforcement, FRP bars and tapes were recorded until failure under a monotonically increasing load. The ultimate load capacities of the strengthened beams were enhanced by 22% to 82% compared to the unstrengthened control beam. The ductility of beams strengthened with GFRP bars was satisfactory, while the ductility of beams strengthened with CFRP bars and tapes was very small, so the failure modes of these beams were brittle.

Numerical and experimental investigation into the fatigue life of FRP bonded to concrete and anchored with bidirectional fabric patches

Although extensive research has been conducted into the use of fibre reinforced polymers (FRPs) in strengthening of concrete structures under static loads, their fatigue behaviour when subjected to cyclic loading still requires investigation. The issue of fatigue performance of FRP strengthening systems is mostly relevant to the strengthening of concrete bridges which are subjected to significant traffic loading cycles throughout their lifetime. In order to offset some of the shortcomings of externally bonded FRPs, anchorage systems are commonly used in bridge strengthening applications in conjunction with FRPs externally bonded to concrete in order to mitigate premature debonding failure. This paper presents a summary of numerical and experimental investigations conducted to evaluate the fatigue performance of externally-bonded FRP laminates anchored with bidirectional fibre patch anchors. The anchored laminates were bonded to reinforced concrete (RC) blocks and subjected to various cyclic loading scenarios. Parameters such as the stress range, peak cyclic stress level, and the corresponding number of cycles prior to failure were noted. The results were used to generate an S-N curve relationships. A finite element model was developed and calibrated based on the experimental results. Good correlation was achieved between the experimental and FE results in relation to the maximum number of cycles, strain distribution, and mode of failure. The outcomes of this study showed that no fatigue degradation was observed when the peak cycling stress level was less than 60% of the ultimate static capacity. Further details of the results and recommendations for future research work are also provided.

Fibre optic strain measurements for bond modelling of prestressed near-surface-mounted iron-based shape memory alloy bars

Near-surface-mounted (NSM) iron-based shape memory alloy (memory-steel) bars represent a novel type of reinforcement for prestressed strengthening of concrete structures. To examine the bond behaviour of these embedded reinforcements in detail, fibre optic (FO) methods can be used. In this study, distributed FO strain measurements were performed in small-scale bond experiments and large-scale structural experiments using prestressed and non-prestressed NSM memory-steel bars. The experiments demonstrated the FO measurement technique's feasibility on memory-steel bars, provided insights into the complex strain profiles before and after prestressing, and revealed the active bond lengths. When the memory-steel bar was subjected to an external load, a reduction in the bar's stiffness resulted in an increase in active bond length. The results furthermore highlighted the importance of rigid bar fixations to avoid prestress loss. The obtained strain distributions were also processed to calculate the distributed slip and axial bar stress, and to investigate the parameters of three different bond shear stress–slip laws (BSSLs). A linear increasing BSSL with a nonlinear bar stiffness led to a realistic calculation of the axial stress in the bar, while resulting in acceptable slip and strain. The implementation of a constant Young’s modulus consistently overestimated the axial bar stress.

Nanomechanical properties of cement Paste-Epoxy interphase following hygrothermal conditioning by water immersion

Externally bonded fiber-reinforced polymer (FRP) composites are extensively used for repair and strengthening of concrete structures. The longevity of FRP repairs depends primarily on the durability of adhesive bonds between FRP and the underlying concrete substrate, which is established through epoxy adhesives. In this study, the effect of hygrothermal conditioning by water immersion on epoxy-cement paste interphase was characterized via site-specific nanoindentation. Reduced modulus and hardness were measured across the interphase for a dry and conditioned (water immersion at 30 °C for 4 weeks) cement paste-epoxy sample. The statistical nanoindentation approach, based on Gaussian mixtures decomposition, was used to determine the nanomechanical properties of distinct material phases in the interphase. The results confirm that hygrothermal conditioning by water immersion degrades the nanomechanical properties of the bulk epoxy, interphase, and bulk cement paste. Degradation in epoxy properties is explained by the plasticization of the epoxy due to moisture conditioning. The reduction in nanomechanical properties of interphase and bulk cement paste is primarily accredited to decalcification (also known as ‘calcium leaching’) processes induced by exposure to low-pH water environment.

Analytical study of the section of the rc beams strengthened for flexure with FRP materials

Strengthening of concrete structures is applied as a solution for various deterioration problems in civil engineering practice. This paper presents an analytical study of the behaviour of cross-section of reinforced concrete (RC) beam, strengthened for flexure with fiber reinforced polymer (FRP) materials. Using the balance of internal forces in the cross section through all phases of stress through which the section passes, a program was written in the MATLsoftware, the execution of which produced a curve of dependence between bending moment and curvature, which is one of the most important indicators of cross section behaviour. The parameters varied in this study are the amount and type of FRP reinforcement and the obtained results indicate a significant influence of additional FRP reinforcement both on the yielding and ultimate bending moment, and on the bending stiffness of the strengthened cross section.

Modified Epoxy for Fibre Reinforced Polymer Strengthening of Concrete Structures

Fibre-Reinforced Polymer (FRP) is a preferable material for repairing concrete structure due to excellent material properties and effective installation cost over the long-term maintenance of structures. The successful application of FRP strengthening system very much depends on the bond between the concrete substrate and the FRP material using epoxy adhesive. Epoxy acts as a bridge to transfer stress from the concrete to the FRP material. The use of wet lay-up technique to apply FRP onto concrete structure requires epoxy to undergo a curing process normally referred to as cold curing. This paper intends to give a review of the problems with cold-cured epoxy and its effect on structural performance. Cured epoxy is characterised as brittle; therefore, modifications of epoxy are required to toughen the epoxy to suit the purpose of repairing a concrete structure. The methodological approaches from previous studies on modified epoxy were collected and reviewed in this paper. This review also offers some important insights regarding the use of sustainable materials, as well as recommendations for new epoxy in the future.

Fiber Reinforced Polymer Composite as a Strengthening of Concrete Structures: A Review

For centuries, concrete is the main material most widely used in a building. The strength and durability of this material will decrease over time due to several factors that influence the mechanism of its structural characteristics. Although the compression strength is excellent, it is weak in tensile. Fiber reinforced polymer (FRP) composites are widely used as an alternative material to replace metal-based engineering materials. This material has properties such as lightweight products, low production costs, easy to form, high tensile and flexural strength, and elastic. The purpose of this review paper is to provide information about the application of FRP composites as reinforcement of concrete structures either as repairs or new structures. Initially, FRP composite materials were used as reinforcement structures for reinforced concrete column-beam joints. The resulting structure has increased the flexural ability due to the load given. The addition of natural fiber (jute) to concrete also increases in the hardness of concrete. Other investigations regarding the application of FRP composites in concrete to improve strength performance are discussed in this paper. Finally, the technique of wrapping FRP composites in concrete in an effort to improve the performance of the resulting structure is also discussed in the paper.

Concrete screws as a post‐installed punching shear reinforcement

AbstractThe strengthening of concrete structures is becoming more and more important. To achieve this need for strengthening measures, technically innovative, simple and economical systems for subsequent strengthening are required. At the University of Innsbruck, a system for subsequent strengthening of the punching shear zone through the installation of concrete screws has been developed. These screws are installed from the soffit side of the slab in vertically predrilled holes and are arranged concentrically around the support. This paper discusses the results of four test series, with a total of 21 specimens used to develop the system. The system, which can be very easily installed, demonstrates a significant increase of the shear punching capacity and a reduction in brittle failure modes. In addition, cyclic loading with typical levels of service load has no negative effect on the effectiveness of the system. Finally, the influence of the flexural reinforcement ratio on the effectiveness of the strengthening system was investigated.