•The effective bond length of FRP-concrete was investigated at initiation of debonding.•PIV technique was used to investigate the initiation of debonding of FRP strips.•A simplified analytical method ...was developed to predict the effective bond length.•The proposed procedure showed good performance compared to the other models.
Numerous studies have been so far conducted to better understand the behavior of fiber-reinforced polymer (FRP) sheets adhered to the concrete surface. In most studies, effective bond length has been known as an important concept, which beyond this length, loading capacity cannot be increased. To more scrutinize on effective bond length of FRP-concrete joints, in the present study the bond behavior of FRP-concrete joints was investigated and an attempt was made to determine the effective bond length of the joints. To this end, first, 24 concrete prisms of three grades of different compressive strengths were made and strengthened with various ranges of thickness, elastic modulus, and type of FRP composites; and the specimens were subjected to the single-shear test. The particle image velocimetry (PIV) technique was employed to obtain the load-slip curves and the strain and slip profiles along the joints. An attempt was made to detect the beginning of debonding through investigating the bond behavior of FRP-concrete, and consequently, the effective bond length was measured. A simplified analytical method was then developed to predict the effective bond length such that it be independent of the bond-slip relationship. Finally, comparisons were made between the values predicted by the well-known models and those obtained from the experimental results. Moreover, the new procedure suggested in this study was found capable to predict the effective bond length with reasonable accuracy.
AbstractIn recent years, the grooving method in the form of “externally bonded reinforcement on groove” (EBROG) has been introduced as an alternative method of the conventional externally bonded ...reinforcement (EBR) for strengthening concrete structures using fiber-reinforced polymer (FRP) materials. This paper is a first attempt to develop an FRP-concrete bond strength model based on nonlinear regression on the experimental results. The agreement of the model to the data is verified using different statistical tools and analysis of variance. One hundred and fifty-four single lap-shear tests are conducted on 136 specimens made through the EBROG method and 18 specimens through the EBR method. The design of experiments techniques in the form of response surface methodology and I-optimality criteria are used to design and optimize the test layouts. The effects of groove dimensions, concrete compressive strength, and FRP sheet width and stiffness on the EBROG bond strength and its efficiency are investigated and compared with those on similar EBR specimens. Although debonding failure is observed to occur in all of the specimens, the results obtained confirm the superiority of the EBROG over the EBR specimens, as evidenced by an average enhancement of 31% achieved in bond strength and the postponed debonding. A bond strength model for the EBROG method was proposed by modifying the well-known Chen and Teng model, originally used for EBR.
Application of fiber-reinforced polymer (FRP) composites to enhance the flexural strength of members in the expected plastic hinge regions of ductile reinforced concrete (RC) moment frames for ...resisting seismic loads are stipulated in the current construction codes and guidelines. The main issues of concern in these documents include provisions for appropriate anchorage details for FRP composites at the beam-column interface, debonding of FRP composites off the concrete substrate, and the effect of cyclic load reversal on FRP reinforcement. The present experimental study was conducted to gain insight into the effectiveness of an innovative FRP anchor fan at the beam-column joint interface flexurally strengthened with carbon FRP (CFRP) sheets. To avoid any likely debonding of FRP composites off the concrete substrate, surface preparation of the test specimens was performed according to a recently developed grooving method (GM) in the form of externally bonded reinforcement on grooves (EBROG). For the purposes of this study, seven half-scale RC beam-column subassemblies, including a control specimen and six rehabilitated ones, were tested under constant axial and reversal cyclic lateral loadings. The primary test variables were the FRP sheet length along the beam and fiber volume in the CFRP anchor fan. The results indicated that the adopted rehabilitation strategy enhanced the lateral strength of RC joints up to 80% compared to the control one. It was also found that deformation-controlled classification action could be justified for strengthened specimens under specific circumstances. Keywords: anchor fan; beam-column joints; debonding; EBROG; fiber-reinforced polymer composites; flexure; grooving method; reinforced concrete; seismic rehabilitation; strength.
Bond behavior of adhesively-bonded fiber reinforced polymers (FRPs) to concrete substrate has been investigated by many researchers worldwide. An interesting aspect of FRP-to-concrete bond behavior ...is that there exists an effective bond length beyond which an extension of the bond length cannot increase the ultimate capacity of the joint. Effective bond length of FRP composites, in fact, is an important part of all strengthening calculations, and conservative design guideline predictions can lead to waste of composite materials in strengthening projects. Consequently, the main intention of the current study is to evaluate the accuracy of existing guideline models of effective bond length by means of single-shear bond tests. To do so, carbon FRP (CFRP) sheets with a wide range of bond length from 20 to 250mm, were adhered to 22 concrete prisms using externally bonded reinforcement (EBR) technique. The specimens were then subjected to single-shear test and debonding loads as well as the effective bond length of the CFRP sheets were determined. Moreover, an image based technique, i.e. particle image velocimetry (PIV) was used to verify the estimated effective bond length by analyzing strain distribution along the CFRP strips during loading process. Experimental results of the current study show that fib Bulletin 14 model overestimates debonding loads and effective bond length. The model adopted by ACI 440.2R-08 also overestimates effective bond length while accurately predicts debonding loads. Appropriate calibration factors were introduced to modify the existing models for CFRP sheets.
•The statistical analyses of experimental results indicated that calcium carbonate precipitated by bacteria filled concrete pores to hinder the movement of free ions, which resulted in increased ...concrete electrical resistance around 6%.•The results of RCPT test showed that by adding bacteria and nutrient to water mixture of concrete, the chloride penetration into specimens decreased about 28% due to CaCO3 precipitations which fills pores and enhances concrete performance.•An important finding of this study involved the higher electrical resistance of concrete with bacteria and air voids simultaneously. This was attributed to the fact that air voids in concrete restrain the movement of free ions and protect concrete against aggressive agents such as chlorides; also air voids protect bacteria from harsh environment in cement paste media, so bacteria show better performance to precipitate calcium carbonate.•Another finding of the study concerned concrete surface treatment. It was shown that treatment of specimens cured in media containing minerals enhanced their electrical resistance around 34% and decreased their chloride penetration about 37%. It was speculated that the bacteria precipitated a thin layer of calcium carbonate on the concrete surface which protected the specimen against aggressive agents.•The specimens cured in a solution of calcium lactate and urea were found to exhibit a better performance than the others mainly because the bacteria fed calcium lactate sustained longer inside the concrete.
Use of bacteria in concrete is an eco-friendly technique to enhance concrete durability. Since the concrete is a dry and alkaline medium, the present study implemented air-entrained concrete to protect bacteria against this harsh environment. For this purpose, electrical resistivity experiments were conducted using 90- and 120-days old concrete specimens. Rapid chloride permeability tests were also performed on 90-days specimens. The experiments were designed using fractional factorial split plot (FFSP) to decrease the number of tests while the results remain reliable. Results obtained based on FFSP statistical analyses indicated that inclusion of bacteria enhanced the electrical resistivity and reduced the chloride permeability of concrete. Even better results were achieved with high air content percentages since the bacteria exhibited a better performance near the air voids. Furthermore, it was shown that surface treatment with bacteria increased concrete electrical resistivity and further reduced its chloride penetration for specimens cured in a medium containing minerals. In addition, lower porosity was observed in scanning electron microscopy (SEM) analysis when the specimens contained bacteria.
As yet, no article has comprehensively examined the mechanical properties, durability, and microstructure of ultra-high-performance geopolymer concrete (UHPGC) in comparison with ...ultra-high-performance concrete (UHPC). In addition, no review article has scrutinized the mechanical properties and microstructure of UHPGC exposed to high temperatures. Therefore, this article has undertaken a review of these aspects of UHPGC by examining the fresh properties, hardened properties, microstructure, durability, and specifications of UHPGC exposed to high temperatures. To evaluate the mechanical properties of UHPGC, the tests for flexural strength, compressive strength, modulus of elasticity, and tensile strength that have been performed by researchers thus far have been reviewed. In addition, Fourier-transform infrared spectroscopy (FTIR), energy-dispersive x-ray spectroscopy (EDS), scanning electron microscopy (SEM), and mercury intrusion measurement tests were studied to evaluate the microstructure characteristics of UHPGC. Finally, to evaluate the durability characteristics of UHPGC, the results of rapid chloride penetration, electrical resistivity, and rapid chloride migration tests were compared. The results showed that UHPGC can meet the ideal specifications of UHPC in terms of its mechanical properties. Moreover, the UHPC microstructure is denser than UHPGC. A stronger ion passing was seen in UHPGC because pores larger than 100 nm were found, whereas pores of UHPC were only about 10 nm. It is concluded that the optimal percentage of silica fume as a slag substitute to create the highest compressive strength, tensile strength, and modulus of elasticity in UHPGC was 30, 30, and 20%, respectively. In addition, the best combination of activators to create the best mechanical characteristics and microstructure of UHPGC at temperatures above 600 °C was potassium hydroxide/sodium silicate.
•The current review article has comprehensively reviewed the mechanical properties, durability, and microstructure of UHPGC.•This review has compared the UHPGC microstructure specifications to those of UHPC.
The current study introduces novel mixing ingredients and techniques for high-performance concrete based on alkaline earth metal ions-activated slag (HPC-AAS) and looks into the use of alkaline earth ...activators for the first time. Moreover, the use of limestone powder as the fine ingredient in HPC-AAS has been also evaluated. The specimens of HPC-AAS thus prepared were tested to determine their mechanical characteristics (flexural, compressive, and tensile strength), microstructure, water absorption, and the static and dynamic modulus of elasticity. The results showed that the use of 10% calcium oxide as an activator of HPC-AAS was able to provide the desired characteristics for compressive strength (114 MPa) and flexural strength (9.2 MPa) as well as the dynamic and static moduli of elasticity (43.1 and 34.9 GPa, respectively). Microstructure analyses indicated that the highest peaks for calcium silicate hydrate gel (C–S–H) and Si–O-T (T = Si or Al) and the strongest bond between the geopolymer matrix and aggregates were created for HPC-AAS activated with 10% calcium oxide, which produced the highest ratio of silicon to aluminum.
This paper studied the effects of steel, basalt, and recycled PET fibers on the environmental, social, and mechanical properties of high-performance concrete (HPC) with calcium oxide-activated slag ...(HPC-CAS). The energy consumption, greenhouse gas emissions, and human health impacts of concrete production were determined using resources, climate change, and human health indexes. The compressive strength, tensile and flexural strengths, modulus of elasticity, electrical resistance, and durability of HPC-CAS with different fiber types and contents were also measured and determined. Furthermore, the microstructure of HPC-CAS was examined by scanning electron microscope (SEM). The results revealed that 3 % recycled PET or basalt fibers increased the HPC-CAS tensile strength by 89 % and 55 %, respectively, and the flexural strength by 28 % % and 1.2 times, respectively, compared to specimens without fibers. The specimens with fibers also showed higher ductility and lower electrical resistance than those without fibers. The SEM analysis showed that basalt fibers had a better bond with the geopolymer matrix than recycled PET fibers. Basalt fibers had the lowest environmental impact and recycled PET fibers had the lowest social impact among the three types of fibers. However, recycled PET fibers also had lower mechanical performance and durability than basalt fibers. It was concluded, therefore, that the choice of fiber type for HPC-CAS depends on the balance between social, environmental, and technical criteria.
In this study, a high-performance concrete based on calcium oxide-activated materials (HPC-CAM) was developed and studied. Calcium chloride and calcium nitrate additives were used to improve the ...microstructure of such concrete. In addition, different fibers including steel, recycled PET, basalt, glass, and modified synthetic macro fibers were used to overcome the brittleness of the HPC-CAM geopolymer matrix. Compressive, flexural, water absorption, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) tests were performed to evaluate the overall characteristics of the specimens thus prepared. The results showed that adding calcium chloride and calcium nitrate increased the compressive strength of HPC-CAM by 69% and 29%, respectively. All samples containing calcium chloride had flexural strengths that were 20% higher than those with calcium nitrate. HPC-CAM samples reinforced with recycled PET fibers and modified synthetic macro-fibers recorded flexural toughness values that were 8- and 6-fold greater than those for steel fibers. The highest calcium silicate hydrate gel (C–S–H) and Si–O-T (T = Si or Al) peaks were obtained in HPC-CAM containing calcium chloride.
•A high-performance concrete based on calcium oxide-activated materials (HPC-CAM) was developed and studied.•Calcium chloride and calcium nitrate additives were used to improve the microstructure of HPC-CAM.•Different fibers were used to overcome the brittleness of the HPC-CAM geopolymer matrix.
