Correlations among mechanical properties of steel fiber reinforced concrete

In this paper, applicability of previously published empirical relations among compressive strength, splitting tensile strength and flexural strength of normal concrete, polypropylene fiber reinforced concrete (PFRC) and glass fiber reinforced concrete (GFRC) to steel fiber reinforced concrete (SFRC) was evaluated; moreover, correlations among these mechanical properties of SFRC were analyzed. For the investigation, a large number of experimental data were collected from published literature, where water/binder ratio (w/b), steel fiber aspect ratio and volume fraction were reported in the general range of 0.25–0.5, 55–80 and 0.5–2.0%, respectively, and specimens were cylinders with size of Φ 150 × 300 mm and prisms with size of 150 × 150 × 500 mm. Results of evaluation on these published empirical relations indicate the inapplicability to SFRC, also confirm the necessity of determination on correlations among mechanical properties of SFRC. Through the regression analysis on the experimental data collected, power relations with coefficients of determination of 0.94 and 0.90 are obtained for SFRC between compressive strength and splitting tensile strength, and between splitting tensile strength and flexural strength, respectively.     

Influence of ultra-high strength infill in slender concrete-filled steel tubular columns

This paper describes 24 tests conducted on slender circular tubular columns filled with normal, high, and ultra-high strength concrete for plain, bar reinforced and steel fiber reinforced columns. These were reinforced and subjected to both concentric and eccentric axial load. It is a continuation of a previous research paper (Portoles et al., 2011 [1]), which presented test results on eccentrically loaded plain concrete columns. The test parameters are nominal strength of concrete (30, 90 and 130 MPa), eccentricity e (0, 20 and 50 mm) and type of reinforcement. A comparison with the corresponding empty tubular columns is performed, as the aim of the paper is to analyze the influence of each type of infill and establish the best option for practical application. For the limited cases analyzed the results show that the addition of high or ultra-high strength infill is more useful for concentric loaded cases than for eccentric loaded ones, where it seems that the best design option is the utilization of bar reinforced concrete filling rather than steel fiber to reinforce CFST columns. The experimental ultimate load of each test was compared with the design loads from Eurocode 4, accurate for the eccentrically loaded tests.     

Performance of an alkali-activated slag concrete reinforced with steel fibers

This study investigated mechanical and permeability properties at early ages of an alkali-activated slag concrete (AASC) reinforced with steel fibers. The compressive, splitting tensile and flexural strengths, flexural notch sensitivity, pull-out and water absorption properties were evaluated. Test results reveal a reduction of AASC compressive strengths with fiber incorporations. However, splitting tensile and flexural strengths were largely improved with increasing fiber volume, varying from 3.75 to 4.64 MPa and from 6.40 to 8.86 MPa at 28 days of curing, respectively. The properties related to durability performance as water absorption, capillarity and water resistance penetration were enhanced with the steel fibers addition. The results show the enormous potential of AASC as building material with and without steel fiber reinforcement.     

Influence of fly ash on strength and sorption characteristics of cold-bonded fly ash aggregate concrete

Cold-bonded fly ash aggregate concrete with fly ash as part of binder or fine aggregate facilitates high volume utilization of fly ash in concrete with minimum energy consumption. This paper investigates the influence of fly ash on strength and sorption behaviour of cold-bonded fly ash aggregate concrete due to partial replacement of cement and also as replacement material for sand. While cement replacement must be restricted based on the compressive strength requirement at desired age, replacement of sand with fly ash appears to be advantageous from early days onwards with higher enhancement in strength and higher utilization of fly ash in mixes of lower cement content. Microstructure of concrete was examined under BSEI mode. Replacement of sand with fly ash is effective in reducing water absorption and sorptivity attributable to the densification of both matrix and matrix–aggregate interfacial bond. Cold-bonded fly ash aggregate concrete with a cement content of 250 kg/m³, results in compressive strength of about 45 MPa, with a total inclusion of around 0.6 m³ of fly ash in unit volume of concrete.     

Corrosion of steel reinforcement in concrete of different compressive strengths

Corrosion of steel bars embedded in concrete having compressive strengths of 20, 30 and 46 MPa was investigated. Reinforced concrete specimens were immersed in a 3% NaCl solution by weight for 1, 7 and 15 days. In order to accelerate the chemical reactions, an external current of 0.4 A was applied using portable power supply. Corrosion rate was measured by retrieving electrochemical information of polarization technique. Pull-out tests of reinforced concrete specimens were then conducted to assess the corroded steel/concrete bond characteristics.Experimental results showed that corrosion rate of steel bars and bond strength between corroded steel/concrete were dependent on concrete strength and accelerated corrosion period. As concrete strength increased from 20 to 46 MPa, corrosion rate of embedded steel decreased. First day of corrosion acceleration caused a slight increase in steel/concrete bond strength, whereas sever corrosion after 7 and 15 days of corrosion acceleration significantly reduced steel/concrete bond strength. Visual and metallographic observation of steel bars removed from concrete samples after testing revealed that the severity of corrosion reactions and reduction of steel bar diameter increased as the corrosion acceleration period increased. Presence of localized corrosion pits as well as severe corrosion grooves of steel bars was confirmed after 7 and 15 days of corrosion acceleration, respectively.     

Improvement of residual compressive strength and spalling resistance of high-strength RC columns subjected to fire

This paper investigates the spalling properties of high-strength concrete in order to improve the residual compressive strength and spalling resistance in specimens subjected to 3 h of unloading fire conditions. This study consists of three series of experiments with eighteen different specimens varying in fiber type and content, finishing material and simultaneous fiber content and lateral confinement. They were fabricated to a 300 × 300 × 600 mm mock-up size. Results of the fire test showed that the control concrete was explosive, while the specimens that contained more than 0.1 vol% of polypropylene (PP) and polyvinylalcohol (PVA) fibers were prevented from spalling. One specimen, finished by a fire endurance spray, exhibited even more severe spalling than the control concrete. The specimen containing 0.1 vol% of PP fiber and using a confining metal fabric at the same time, showed the most effective spalling resistance; in particular, the residual compressive strength ratio was even higher than that of the control concrete before the fire test. It was demonstrated that adding fibers in concrete prevented the spalling occurrence and confining metal fabric around the main bars of concrete specimens can secure the strength of structures during the conditions of elevated temperature.     

Mechanical properties of natural fibers reinforced sustainable masonry

This research evaluates the physical and mechanical properties of Portland cement masonry blocks reinforced with lechuguilla natural fibers, that were lightened with 2-l bottles of polyethylene terephthalate.A concrete mix was designed for a target compressive strength of 16 MPa at 28 days, and slump of 70 mm. Masonry concrete blocks with dimensions of 730 × 340 × 130 mm were produced for two different fiber lengths (25 and 50 mm) and with fiber contents of 0.25%, 0.50%, 0.75% and 1.0%.Based on the obtained results, it was found that as the aspect ratio decreases the compressive strength increases and that the use of natural fiber (Vf = 0.5–0.75%) improves masonry post-cracking features, showing a ductile behavior and generating a uniform cracking pattern in the longitudinal sides of the blocks.     

High strength characteristics of cement mortar reinforced with hybrid fibres

An experimental study was conducted on high strength mortar reinforced with steel fibres and hybrid fibres consisting of steel fibre, palm fibre and synthetic fibre (Barchip). The inclusion of fibres was maintained at a volumetric fraction of 2%. The compressive strength, splitting tensile strength, static modulus of elasticity, shrinkage, flexural strength, and flexural toughness were determined to study the effect of the hybrid fibres on the properties of high strength cement mortar (HSCM). The results showed that hybridization of fibres in the quantities 1.5% steel fibres + 0.25% palm fibres + 0.25% Barchip fibres, improved the compressive strength and flexural toughness significantly, and also enhanced the splitting tensile strength and flexural strength of the mortar by about 44% and 140%, respectively.     

Mix proportion of high-strength,roller-compacted,latex-modified rapid-set concrete for rapid road repair

In this study, we optimized a blend of high-strength, roller-compacted, latex-modified rapid-set concrete (RCLMC) that can be re-opened to traffic after 4 h. To this end, we tested several variables in laboratory experiments, including hardening acceleration agents, cement type, latex addition, and CSA admixture ratios. The target compressive strength was 21 MPa after 4 h. A mixture of Type III cement to CSA admixture at 235:165 kg/m³ (400 kg/m³ total binder) and 23.5 kg/m³ latex (10% of the cement weight) achieved the target compressive strength and was the most economically efficient.     

The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete

This study describes a laboratory investigation of the influence of thiosemicarbazide (TSC) on the corrosion of reinforcing steel and the compressive strength of concrete. The effect of TSC on the corrosion resistance of steel reinforced concrete was evaluated by carrying out electrochemical tests in NaCl and NaCl + TSC solutions for 60 days. Polarisation resistance (R_p) values of TSC added reinforced concrete were much higher than those without TSC. Similarly, AC impedance spectra revealed that the resistance of TSC mixed electrodes were also quite higher than those without. The compressive strength of concrete specimens containing TSC was measured and an increase of 20–25% was observed.     

Post-crack (or post-peak) flexural response and toughness of fiber reinforced concrete after exposure to high temperature

Several studies have already reported on the various effects of high temperature on the mechanical properties of fiber reinforced concrete (FRC). Some of these effects include changes in; compressive strength, compression toughness and splitting tensile strength. None of the previous studies have investigated the changes that might occur on the post-crack flexural response and flexural toughness. Post-crack (or peak) response and toughness is considered one of FRC’s key beneficial characteristics – as the purpose of adding fibers is to increase the energy absorption and load carrying capacity after an initial crack. In this study, the flexural toughness test according to ASTM C1018 was carried out on two types of concrete: plain concrete and fiber reinforced concrete with three different types of fiber (steel, polypropylene, and polyethylene) at 0.5% and 1.0% by volume fractions. Prior to the flexural test, the specimens were put in an oven chamber and subjected to high temperatures using the ISO/TR834 standards of: 400 °C, 600 °C and 800 °C. The results showed the typical load–deflection response of FRC was a double-peak response. The first peak represented the properties of concrete matrix and the second peak represented the properties of the fibers used. Under flexural load, instead of dropping (or remaining unchanged), the post-peak load and the toughness were found to increase at lower temperatures (400 °C) and later, decreased as the temperature increased (600 °C and 800 °C). Fiber type and content also played an important role. At a temperature of 400 °C, all FRCs exhibited higher flexural strength and increased post-peak response and toughness. A significant decrease in strength, toughness and load–deflection response was observed with synthetic or plastic FRC (PFRC) when the temperature approached 800 °C. When steel FRC (SFRC) was used, those effects were relatively small. It appears, SFRC has better heat resistance than the PFRC. The density (measured by ultrasonic pulse velocity) was found to decrease more in the PFRC than in the SFRC.     

Effects of tensile reinforcement ratio and compressive strength on the behaviour of over-reinforced helically confined HSC beams

The technology of high strength concrete has improved over the last decade. High strength concrete (HSC) is more brittle than normal strength concrete. The brittleness increases with the use of over-reinforced section, which fails suddenly without warning. Use of over reinforced sections is restricted in codes of practice of concrete design. This paper presents an experimental study of the behaviour of five HSC beams confined with helical reinforcement. Concrete compressive strength in the range 72–95 MPa and tensile reinforcement ratio in the range 5.24–7.86% were used. The main results indicate that as the concrete compressive strength increases the displacement ductility index decreases and the load at spalling-off the concrete cover increases. Also, the displacement ductility index increases as the longitudinal reinforcement ratio increases and the load at spalling-off the concrete cover decreases.     

Effects of steam curing on strength and porous structure of concrete with low water/binder ratio

A research program was carried out to investigate the effects of duration of initial steam curing at atmosphere pressure on compressive strength of concrete with low water/binder ratio. The results showed that the compressive strength of samples steam cured for 5, 10, 14 h increased, while it decreased distinctively for sample steam cured for 24 h. Mercury intrusion porosimetry (MIP) method and scanning electron microscope-backscattered electron (SEM-BSE) image analysis technique were adopted to measure the corresponding variation of porous characteristics caused by the increasing duration of steam curing. The changes in coarse porosity and total porosity calculated by SEM-BSE image analysis and MIP method respectively could indicate the relationship between porosity and mechanical properties of the concrete subjected to different duration of steam curing. Compared with total porosity obtained by MIP method, the coarse porosity by SEM-BSE image analysis was in better accord with the compressive strength because the coarse pores measured by SEM-BSE image analysis were larger than 0.5 μm and included not only the interconnected pores but also the closed ones. An empirical model was developed to evaluate the influence of duration of initial steam curing on the compressive strength of concrete. By comparison, the measured compressive strength was in great accordance with the compressive strength calculated by the proposed model.     

Compressive strength loss and reinforcement degradations of reinforced concrete structure due to long-term exposure

Degradations due to long-term weathering actions on a reinforced concrete structure were investigated. Compressive strength and reinforcement corrosion developments of a prototype RC structure were monitored for 6 years using destructive and non-destructive tests which include periodic coring, compressive strength, rebound hammer, ultrasonic pulse velocity, carbonation, half-cell and tensile strength tests. Eventually, results have shown that more than a quarter of peak compressive strength can be lost within 5 years of continuous exposure. Corrosion of the exposed bars within the range of the testing period was also observed to be quite alarming. Thus, defects caused by prolonged actions of environmental factors may pose serious threats on the integrity of partially completed structures especially abandoned projects.     

Effect of GGBFS on time dependent compressive strength of concrete

This paper addresses the compressive strength properties when GGBFS is used to make concrete and discuss in detail the compressive strength development of concrete cubes and cylinders. The uniaxial compression tests have been conducted on these concrete specimens with and without GGBFS at the ages of 3, 7, 28, 56, 90, 150 and 180 days. Experimental values show increasing concrete strength with GGBFS up to 40% but at a higher period of maturity (56 days and more). Sixty percent GGBFS blending, however, shows reduced strength when compared to 40% blending. A comparative study has also been carried out between the experimentally obtained compressive strength and strength predicted by the models given in SP-24 (Indian Standard), ACI-209, CEB-FIP and GL-2000. Based on the experimentally obtained results a strength predicting models has also been proposed for GGBFS based concrete.     

Critical embedment length and bond strength of fully encapsulated rebar rockbolts

A series of rock bolt pull tests were carried out in the laboratory to determine the critical embedment length of a specific type of fully cement-grouted rebar bolt. The rebar bolt is 20 mm in diameter, and it is widely used in underground excavations in Norway. Three water-cement (w/c) ratios were used in the tests. It was discovered that the critical embedment length of the rock bolts was approximately 25 cm for the water-cement ratio 0.40 (the corresponding uniaxial compressive strength (UCS) of the grout is 37 MPa), 32 cm for the ratio 0.46 (UCS 32 MPa), and 36 cm for the ratio 0.50 (UCS 28 MPa), for the specific type of cement, Rescon zinc rock bolt cement. It was found that the bond strength of the rock bolt is not a constant but is related to the embedment length. The bond strength was linearly proportional to the UCS of the grout.     

Strength and deformability of waste tyre rubber-filled reinforced concrete columns

This study aims to investigate the efficiency of waste tyre rubber-filled concrete to improve the deformability and energy absorption capacity of RC columns by considering different concrete compressive strength, size of waste tyre rubber particles and rubber content. Twelve column specimens were tested using concrete of compressive strength 24 and 28 MPa mixed with 0.6 and 1 mm tyre rubber particles. For each concrete batch, 27 control specimens were prepared to examine the concrete properties. Using waste tyre rubber-filled concrete leads to a slightly lower compressive strength and modulus of elasticity, but the curvature ductility can increase up to 90%. It is concluded that this type of concrete can offer good energy dissipation capacity and ductility, which makes it suitable for seismic applications.     

Effect of rice husk ash on the strength and durability characteristics of concrete

This work investigates the effects of adding residual rice husk ash (RHA) from South Vietnam, generated when burning rice husk pellets in the boiler, to cement. To improve pozzolanic reactivity, RHA was ground for 1 h. The non-ground RHA and ground RHA were used to test strength activity index according to ASTM C311. The properties of the concrete were investigated, including compressive strength, concrete electrical resistivity, and ultrasonic pulse velocity. Results show that the non-ground RHA can be applied as a pozzolanic material. Decreasing the non-ground RHA average particle size provides a positive effect on the compressive strength of mortar. Compressive strength of cylindrical concrete in the 47–66 MPa range was obtained in this study. The results also indicate that up to 20% of ground RHA could be advantageously blended with cement without adversely affecting the strength and durability properties of concrete.     

Oil palm shell as a lightweight aggregate for production high strength lightweight concrete

In Malaysia, oil palm shell (OPS) is an agricultural solid waste originating from the palm oil industry. In this investigation old OPS was used for production of high strength lightweight concrete (HSLC). The density, air content, workability, cube compressive strength and water absorption were measured. The effect of five types of curing conditions on 28-day compressive strength was studied. The test results showed that by incorporating limestone powder and without it, it is possible to produce the OPS concretes with 28-day compressive strength of about 43–48 MPa and dry density of about 1870–1990 kg/m³. The compressive strength of OPS HSLC is sensitive to the lack of curing. The water absorption of these concretes is in the range of good concretes.     

Experimental study of hybrid composite beams

In this study, a new type of hybrid confining device, a perforated steel tube that is externally protected by a thin fiber reinforced polymer skin is proposed and experimentally investigated. Hybrid composite beams were fabricated by filling fresh concrete into the hybrid composite tube. Fifteen scaled-down square beams, which had varying numbers of perforated steel faces or ‘steel grids’ and a dimension of length 55.9 cm, height 10.1 cm, and width 10.1 cm, were prepared. Four-point bending tests were conducted on all the specimens. In addition to the load–displacement curves obtained from the tests, strain gages were installed to monitor the local strain distributions. Test results show that the grid tube encased specimens lead to higher specific strength and ductility than the solid steel tube encased counterparts. Compared to other configurations, the specific strength and ductility are the highest when all the four faces are made of steel grids.