Corrosion resistance of self-consolidating concrete in full-scale reinforced beams

The corrosion of steel reinforcement embedded in full-scale self-consolidating concrete (SCC) beams was investigated compared to normal concrete (NC). 400 mm width × 363 mm depth × 2340 mm length beams containing epoxy- and non-epoxy-coated stirrups were monitored under an accelerated corrosion test. The corrosion performance of NC/SCC beams was evaluated based on the results of current measurement, half-cell potential tests, chloride ion content, mass loss and bar diameter degradation. The investigation also included the effect of admixture type and the size of specimen on corrosion performance.In general, SCC beams showed superior performance compared to their NC counterparts in terms of corrosion cracking, corrosion development rate, half-cell potential values, rebar mass loss and rebar diameter reduction. However, SCC beams showed localized corrosion with concrete spalling due to non-uniform concrete properties along the length, which was a result of the casting technique. The results also showed that the difference between SCC and NC mixes in terms of corrosion was more pronounced in large-scale beams, and that types of admixture used in SCC have no influence on corrosion performance.     

Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.     

Bond and interfacial properties of reinforcement in self-compacting concrete

This paper reports a study carried out to assess the impact of the use of self-compacting concrete (SCC) on bond and interfacial properties around steel reinforcement in practical concrete element. The pull-out tests were carried out to determine bond strength between reinforcing steel bar and concrete, and the depth-sensing nano-indentation technique was used to evaluate the elastic modulus and micro-strength of the interfacial transition zone (ITZ) around steel reinforcement. The bond and interfacial properties around deformed steel bars in different SCC mixes with strength grades of 35 MPa and 60 MPa (C35, C60) were examined together with those in conventional vibrated reference concrete with the same strength grades.The results showed that the maximum bond strength decreased when the diameter of the steel bar increased from 12 to 20 mm. The normalised bond strengths of the SCC mixes were found to be about 10–40% higher than those of the reference mixes for both bar diameters (12 and 20 mm). The study of the interfacial properties revealed that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel bar than on the top side, particularly for the vibrated reference concrete. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the corresponding reference mixes.     

Effect of aging on bond of GFRP bars embedded in concrete

This paper presents an experimental investigation of the durability of the bond between GFRP bars and concrete, specifically as it relates to degradation of the GFRP-bar surface and behavior of the bar–concrete interface. The GFRP bars were embedded in concrete and exposed to tap water at 23 °C, 40 °C, and 50 °C to accelerate potential degradation. The bond strengths before and after exposure were considered as a measure of the durability of the bond between the GFRP bars and concrete. In addition, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to characterize how bar aging affected the bond between the GFRP bars and the concrete. The results showed that aging did not significantly affect the durability of the bar–concrete interface under the conditions used in this study.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Evaluation of the bond behavior of steel reinforcing bars in recycled fine aggregate concrete

This paper describes pullout test results on deformed reinforcing bars in natural and recycled fine aggregate (RFA) concrete. The effects of bar location and RFA grade on bond strength between reinforcing bar and recycled aggregate concrete (RAC) were evaluated through the experimental program. A total of 150 pullout specimens were fabricated for the experiment. Two reinforcing bar orientations were considered with respect to the casting direction; vertical bars and horizontal bars, the latter of which was prepared to evaluate top-bar effect. Considered variables included four RFA replacement ratios (RFArs), two water-absorption grades (RFA-A: 5.83%, RFA-B: 7.95%) of RFA and three reinforcing bar locations (75, 225 and 375 mm height from the bottom of the casting mold). In addition, to evaluate the thermal and aging effect on bond behavior between the reinforcing bar and RFA concrete, some parts of pullout specimens had exposed to rapid freeze–thaw environment or been cured at air during 28 or 730 days. Test results demonstrated that bond strength does not seem to be affected by the RFAr for higher RFA grades (RFA-A), at least up to 60% RFAr. In contrast, the RAC including lower RFA grade (RFA-B) showed clear decreases in bond strength with increasing RFAr, similar to the trend observed for compressive strength. For horizontal pullout specimens, RFA concrete specimens showed higher bond strength gap between top and bottom bars than natural aggregate concrete (NAC) specimens. Bond strengths of the horizontally cast pullout specimens were affected by the flowability of concrete rather than the RFAr or RFA grade. No noticeable degradation occurred during freeze–thaw cycling of the RAC specimens, indicating that the RFA used in this study is appropriate for use in freeze–thaw environments.     

Bond analysis of corroded reinforced concrete beams under monotonic and fatigue loads

This study investigated the fatigue bond behaviour of corroded steel reinforced concrete beams. Nine beams (152 × 254 × 2000 mm [6 × 10 × 78.74 in.]) were constructed and tested. Bond failure occurred in all the beams. The variables in this test series were: the type of load applied (monotonic or repeated loading), the repeated load range, whether the reinforcement inside the beam was corroded or not, and whether a carbon fibre reinforced polymer (CFRP) repair method was used or not. The fatigue life of the beams varied linearly with the range of applied load with a very shallow slope. Corroding the beams to a low corrosion level decreased the fatigue bond strength by about 30%. Corrosion caused the concrete in between the lugs of the reinforcing bars to be partially crushed due to the formation of the rust products from the corrosion process. This reduced the strength of the concrete keys and increased the rate of slip in the bar under repeated loading.     

Torsional behaviour of steel fibre-reinforced oil palm shell concrete beams

The increasing demand for curved structural members has prompted an increase in the research on the torsional behaviour of concrete. Recently, oil palm shell (OPS) has received considerable attention as a material that enables the production of sustainable lightweight concrete. This work investigated the effects of steel fibre of 0.25%, 0.50%, 0.75% and 1.00% volume fractions on the mechanical properties and torsional resistance of OPS concrete (OPSC) and OPS fibre-reinforced concrete (OPSFRC) beams. The experimental results showed that the increase in fibre content resulted in better mechanical properties and torsional resistance of OPSFRC. The compressive, splitting tensile and flexural strengths of OPSFRC with 1% steel fibres were found to be 40%, 110% and 150%, respectively, higher than the control mix. The crack bridging effect also improved the pre-cracking and post-cracking torsional behaviour of OPSFRC. The highest cracking torque, ultimate torque, twist at failure and torsional toughness of 8.3 kNm, 8.5 kNm and 1.31 kNm/m were obtained for the mix with 1% steel fibre. Moreover, the crack arrest ability of the steel fibre reduced the primary torsional crack widths and formed multiple fine cracks. Further, a simplified torsional model is proposed to predict the torsional behaviour of OPSC and OPSFRC.     

Engineering properties of lightweight aggregate concrete made from dredged silt

Silt dredged from reservoirs can be hydrated and sintered into lightweight aggregate for producing lightweight aggregate concrete (LWAC). The densified mixture design algorithm (DMDA) was employed to manufacture LWAC using 150 kg/m³ of water at different water-to-binder ratios (w/b = 0.28, 0.32 and 0.4) using lightweight aggregates of different particle densities (800, 1100 and 1500 kg/m³). The engineering properties of the LWAC thus obtained were examined. Results show that the fresh concrete meets the design requirement of having slump of 250 ± 20 mm and slump flow of 600 ± 100 mm. With respect to hardened properties, the compressive strength, ultrasonic pulse velocity and thermal conductivity were found to decrease with increasing w/b ratio but increase with increasing aggregate density. Moreover, higher aggregate density also resulted in less shrinkage. The surface resistivity exceeding 20 kΩ-cm also matched the design objective. The experimental results prove that LWAC made from dredged silt can help enhance durability of concrete.     

Behaviour of hybrid fibre reinforced concrete beam–column joints under reverse cyclic loads

An experimental investigation was carried out to study the effect of hybrid fibres on the strength and behaviour of High performance concrete beam column joints subjected to reverse cyclic loads. A total of 12 reinforced concrete beams column joints were cast and tested in the present investigation. High performance concrete of M60 grade was designed using the modified ACI method suggested by Aïtcin. Crimped steel fibres and polypropylene fibres were used in hybrid form. The main variables considered were the volume fraction of (i) crimped steel fibres viz. 0.5% (39.25 kg/m³) and 1.0% (78.5 kg/m³) and (ii) polypropylene fibres viz. 0.1% (0.9 kg/m³), 0.15% (1.35 kg/m³), and 0.2% (1.8 kg/m³). Addition of fibres in hybrid form improved many of the engineering properties such as the first crack load, ultimate load and ductility factor of the composite. The combination of 1% (78.5 kg/m³) volume fraction of steel fibres and 0.15% (1.35 kg/m³) volume fraction of polypropylene fibres gave better performance with respect to energy dissipation capacity and stiffness degradation than the other combinations.     

Recent trends in steel fibered high-strength concrete

Steel fibered high-strength concrete (SFHSC) became in the recent decades a very popular material in structural engineering. High strength attracts designers and architects as it allows improving the durability as well as the esthetics of a construction. As a result of increased application of SFHSC, many experimental studies are conducted to investigate its properties and to develop new rules for proper design. One of the trends in SFHSC structures is to provide their ductile behavior that is desired for proper structural response to dynamic loadings. An additional goal is to limit development and propagation of macro-cracks in the body of SFHSC elements. SFHSC is tough and demonstrates high residual strengths after appearance of the first crack. Experimental studies were carried out to select effective fiber contents as well as suitable fiber types, to study most efficient combination of fiber and regular steel bar reinforcement. Proper selection of other materials like silica fume, fly ash and super plasticizer has also high importance because of the influence on the fresh and hardened concrete properties. Combination of normal-strength concrete with SFHSC composite two-layer beams leads to effective and low cost solutions that may be used in new structures as well as well as for retrofitting existing ones. Using modern nondestructive testing techniques like acoustic emission and nonlinear ultrasound allows verification of most design parameters and control of SFHSC properties during casting and after hardening. This paper presents recent experimental results, obtained in the field SFHSC and non-destructive testing. It reviews the experimental data and provisions of existing codes and standards. Possible ways for developing modern design techniques for SFHSC structures are emphasized.     

Acoustic emission waveform characterization of crack origin and mode in fractured and ASR damaged concrete

Different constituents of concrete can have cracking behavior that varies in terms of the acoustic waveform that is generated. Understanding the waveform may provide insight into the source and behavior of a crack that occurs in a cementitious composite. In this study, passive acoustic emission (AE) was used to investigate the waveform properties of the individual components of concrete (i.e., aggregate, paste, and interfacial transition zone (ITZ)). First, acoustic events produced by cracks generated using mechanical loading in a wedge splitting test were detected. It was observed that cracks that occurred through the aggregate have an AE frequency range between 300 kHz and 400 kHz, while cracks that propagated through the matrix (paste and ITZ) have a frequency range between 100 kHz and 300 kHz. Second, tests were performed using samples that were susceptible to alkali silica reaction; and AE and X-ray computed tomography were used to detect cracking. AE events with a frequency range between 300 kHz and 400 kHz were detected at early ages, suggesting the initiation of cracks within reactive aggregate. At later ages, AE events were detected with frequency ranges of 100–300 kHz, indicating crack development and propagation in the matrix.     

Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation

The properties of new Interfacial Transition Zone (ITZ) and old ITZ in Recycled Aggregate Concrete (RAC) were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and nanoindentation. From the SEM images, obvious voids and high concentration of calcium hydroxide can be found in both old ITZ and new ITZ in RAC. Based on the nanoindentation study, it is indicated that the thicknesses of old and new ITZs are in the range 40–50 μm and in the range 55–65 μm, respectively. It is also found that the average indentation modulus of old ITZ is 70–80% of that of old paste matrix, while the average indentation modulus of new ITZ is 80–90% of that of new paste matrix. Additionally, the influences of mix proportion, aggregate types and hydration age on the properties of ITZs in RAC are discussed in this study.     

Properties of controlled low-strength materials incorporating cement kiln dust and slag

This research evaluated the potential use of cement kiln dust (CKD) together with slag to replace the use of cement in the production of controlled low-strength material (CLSM). The low strength requirements of CLSM compared to conventional concrete enable the use of industrial by-products for the production of CLSM. In this study, the workability-related fresh properties of CLSM mixtures were observed through slump flow diameter, V-funnel flow time and filling capacity. Setting times, temperature rise, air content and unit weight of CLSM mixtures were also determined as part of fresh properties. The hardened properties that were monitored for 28 days included the unconfined compressive strength. The test results presented herein show that a combination of less than 50 kg/m³ slag and up to 300 kg/m³ CKD provides a good mix that satisfies the requirements of a CLSM with similar or better properties to that of CKD-based CLSM mix containing Portland cement. Suitable CLSM mixtures with reasonable fresh and hardened properties could also be developed by using CKD alone. However, reduced strength in such CLSM mixtures may limit their field application. The slag significantly assisted in increasing compressive strength of CKD-based CLSM mixtures. A CLSM mix containing a combination of slag and CKD was shown to have excellent characteristics for flowable backfill and excavatable base material. Therefore, producing CKD/slag based CLSM through the use of co-generated products from the cement and iron manufacturing processes can provide leadership for the construction industry in the transition for sustainable development.     

Dynamic behavior of concrete at high strain rates and pressures: I. experimental characterization

Understanding the behavior of concrete and mortar at very high strain rates is of critical importance in a range of applications. Under highly dynamic conditions, the strain-rate dependence of material response and high levels of hydrostatic pressure cause the material behavior to be significantly different from what is observed under quasistatic conditions. The behavior of concrete and mortar at strain rates of the order of 10⁴ s⁻¹ and pressures up to 1.5 GPa are studied experimentally. The mortar analyzed has the same composition and processing conditions as the matrix phase in the concrete, allowing the effect of concrete microstructure to be delineated. The focus is on the effects of loading rate, hydrostatic pressure and microstructural heterogeneity on the load-carrying capacities of the materials. This experimental investigation uses split Hopkinson pressure bar (SHPB) and plate impact to achieve a range of loading rate and hydrostatic pressure. The SHPB experiments involve strain rates between 250 and 1700 s⁻¹ without lateral confinement and the plate impact experiments subject the materials to deformation at strain rates of the order of 10⁴ s⁻¹ with confining pressures of 1–1.5 GPa. Experiments indicate that the load-carrying capacities of the concrete and mortar increase significantly with strain rate and hydrostatic pressure. The compressive flow stress of mortar at a strain rate of 1700 s⁻¹ is approximately four times its quasistatic strength. Under the conditions of plate impact involving impact velocities of approximately 330 ms⁻¹, the average flow stress is 1.7 GPa for the concrete and 1.3 GPa for the mortar. In contrast, the corresponding unconfined quasistatic compressive strengths are only 30 and 46 MPa, respectively. Due to the composite microstructure of concrete, deformation and stresses are nonuniform in the specimens. The effects of material inhomogeneity on the measurements during the impact experiments are analyzed using a four-beam VISAR laser interferometer system.     

Quasi-static and dynamic mechanical elastic moduli of alkaline aged pultruded fibre reinforced polymer composite rebar

Due to the increased use of glass fibre reinforced polymer composite (GFRP) rebar in concrete structures, the durability performance of GFRP rebar has been an important research topic in recent years. This paper presents elastic modulus of alkaline environment (pH ≈ 13) aged pultruded GFRP rebar as evaluated by three different methods, namely, quasi-static tensile, quasi-static flexural and dynamic mechanical thermal tests. It was found that elastic modulus of the GFRP rebar samples did not change significantly due to exposure in alkaline environment at 60 °C for 1, 2, 3, 4, 6 and 14 months when compared with that of control sample. Elastic modulus was found to be in the range of 52.5–56.5 GPa irrespective to testing methods and ageing time. In addition, it was estimated from the long time projected results that quasi-static tensile, quasi-static flexural and dynamic mechanical moduli will be retained by about 93%, 95% and 85%, respectively, after 100 years in alkaline environment at 60 °C. Microscopic analysis indicated that quasi-static tensile and flexural failure was mainly due to matrix cracking and shear failure of fibre/matrix interface.     

Experimental study and numerical simulation of the damage mode of a square reinforced concrete slab under close-in explosion

Terrorist attacks using improvised explosive devices on reinforced concrete buildings generate a rapid release of energy in the form of shock waves. Therefore, analyzing the damage mode and damage mechanism of structures for different blast loadings is important. The current study investigates the behavior of one-way square reinforced concrete (RC) slabs subjected to a blast load through experiments and numerical simulations. The experiments are conducted using four 1000 mm × 1000 mm × 40 mm slabs under close-in blast loading. The blast loads are generated by the detonations of 0.2–0.55 kg trinitrotoluene explosive located at a 0.4 m standoff above the slabs. Different damage levels and modes are observed. Numerical simulation studies of the concrete damage under various blast loadings are also conducted. A three-dimensional solid model, including explosive, air, and RC slab with separated concrete and reinforcing bars, is created to simulate the experiments. The sophisticated concrete and reinforcing bar material models, considering the strain rate effects and the appropriate coupling at the air–solid interface, are applied to simulate the dynamic response of RC slab. The erosion technique is adopted to simulate the damage process. Comparison of the numerical results with experimental data shows a favorable agreement. Based on the experimental and numerical results, the damage criteria are established for different levels of damage. With the increase of the explosive charge, the failure mode of RC slab is shown to gradually change from overall flexure to localized punching failure.     

Effects of steel fibers and mineral filler on the water-tightness of concrete pipes

Water-tightness of concrete and reinforced concrete pipes used to convey sewage flow of any kind is extremely important from the aspects of (1) groundwater contamination and (2) durability of the pipes. In this study, water-tightness tests were applied on plain concrete, reinforced concrete, and steel fiber concrete pipes of 500 mm diameter. Standard seepage tests were applied on many pipes of various materials and combinations with the objective of determining the pipes with the best water-tightness. Tests on plain concrete pipes whose concrete included finely ground limestone passing the no. 100 sieve (d < 0.15 mm) at an amount of 7% by dry weight of the total aggregates revealed that the water-tightness of former was 57% better than that of pipes manufactured without the filler. The water-tightness values measured on steel fiber concrete pipes with a fiber dosage of 25 kg/m³ turned out to be 47% and 15% better than those of plain concrete pipes and reinforced concrete pipes, respectively. Those findings tangibly reveal that the addition of steel fibers and mineral filler in concrete pipes improve their seepage property.     

Prediction of ductility factor of corroded reinforced concrete beams exposed to long term aging in chloride environment

An experimental study using a three-point bending test on RC beams with dimensions of 150 × 280 × 3000 mm, naturally corroded over many years was conducted to evaluate the influence of steel corrosion on structural performance and, in particular, to better understand the change in ultimate deflection in bending and then in ductility. Some previous works by different authors are also discussed. The results show that the conventional ductility factor hardly applies to the assessment of ductile behaviour of corroded beams. A new ductility factor, based on the ratio between ultimate deflection of corroded and non-corroded beams, is proposed. In addition, the relation between ductility factor of corroded beams and cross-section loss in the corroded reinforcing steels was studied on the RC beams tested. The service life of corroded structures appears to be limited by the reduction of ductility in bending behaviour, which is more pronounced on the reduction of load-bearing capacity. This was linked to the change in mechanical properties of corroded steel bars in comparison with non-corroded steel bars.