Full-scale test of a pile supported steel fibre concrete slab

The aim of the short-term studies is to investigate the structural behaviour of pile supported slabs made of steel fibre concrete (SFC) only and combined reinforced steel fibre concrete. The studies include tests on an elevated slab where a combination of reinforcement bars and steel fibres have been used in one half of the slab and SFC only in the other half. The tests were performed on a column-supported elevated slab that simulates a half scale model of an industrial pile-supported floor slab. The short-term tests showed considerable structural and crack arresting performance that also increased with a higher dosage of fibres. A small addition of conventional reinforcement bars further increased the ultimate load capacity P _Max. P _Max was in the range of 125–298 kN for the two types of slab. The results indicate that SFC can be used with verifiable results in structural applications for elevated slabs and pile-supported floor slabs despite that the material testing from the ordered SFC showed a larger scatter in properties and that the calculated load capacities were only 40–220 kN. Main causes of deviance are arch and membrane effects.     

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.     

Effectiveness of the use of steel fibres on the torsional behaviour of flanged concrete beams

The behaviour under torsion of reinforced concrete beams with steel fibres as mass reinforcement is experimentally investigated. Short hooked-ended steel fibres with aspect ratio l_f/d_f = 37.5 are used. Test results of 35 beams with rectangular, L-shaped and T-shaped cross-sections tested in pure torsion are presented and discussed. Various configurations of conventional and fibre steel reinforcement are examined. The experimental program includes (i) plain concrete beams (control specimens), (ii) specimens with longitudinal reinforcing bars and (iii) specimens with bars and stirrups. All cases are examined with 0%, 1% and 3% steel fibre volume fractions. The use of steel fibres as the only shear torsional reinforcement is also reported herein, in an attempt to examine the effectiveness of fibres as a potential replacement of stirrups. Test results indicated that fibrous concrete beams exhibited improved overall torsional performance with respect to the corresponding non-fibrous control beams. The addition of steel fibres was essential to the tested beams without or with inadequate conventional steel reinforcement. Fibres prevented the sudden brittle failure of both rectangular and non-rectangular beams and proved to be under some circumstances adequate to provide for enhanced torsional moment capacities, even in the case of full replacement of stirrups with steel fibres.     

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.     

Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars

Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

Fatigue analysis of concrete bridge deck slabs reinforced with E-glass/vinyl ester FRP reinforcing bars

FRP composites have been widely used as internal reinforcement for concrete bridge deck slabs. However, experimental researches on the behavior of such FRP-reinforced elements in general have been limited, especially those on fatigue performance. This research is designed to investigate the fatigue behavior of concrete bridge deck slabs reinforced with GFRP bars. A total of six full-size deck slabs were constructed and tested under concentrated cyclic loading conditions. Different reinforcement types, ratios, and configurations were used. Also, different schemes of cyclic loading were applied till failure. Finite element modeling was used to investigate the effect of different parameters on the ultimate static capacity. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with GFRP composite bars compared to the steel reinforced ones.     

Assessment of the effectiveness of steel fibre reinforcement for the punching resistance of flat slabs by experimental research and design approach

The present paper deals with the experimental assessment of the effectiveness of steel fibre reinforcement in terms of punching resistance of centrically loaded flat slabs, and to the development of an analytical model capable of predicting the punching behaviour of this type of structures. For this purpose, eight slabs of 2550 × 2550 × 150 mm³ dimensions were tested up to failure, by investigating the influence of the content of steel fibres (0, 60, 75 and 90 kg/m³) and concrete strength class (50 and 70 MPa). Two reference slabs without fibre reinforcement, one for each concrete strength class, and one slab for each fibre content and each strength class compose the experimental program. All slabs were flexurally reinforced with a grid of ribbed steel bars in a percentage to assure punching failure mode for the reference slabs. Hooked ends steel fibres provided the unique shear reinforcement. The results have revealed that steel fibres are very effective in converting brittle punching failure into ductile flexural failure, by increasing both the ultimate load and deflection, as long as adequate fibre reinforcement is assured. An analytical model was developed based on the most recent concepts proposed by the fib Mode Code 2010 for predicting the punching resistance of flat slabs and for the characterization of the behaviour of fibre reinforced concrete. The most refined version of this model was capable of predicting the punching resistance of the tested slabs with excellent accuracy and coefficient of variation of about 5%.     

Damage evolution and energy absorption of E-glass/polypropylene laminates subjected to ballistic impact

High-velocity transverse impact of laminated fiber reinforced composites is of interest in military, marine and structural applications. The overall objective of this work was to investigate the behavior of laminated thermoplastic composites of varying thicknesses under high-velocity impact from an experimental and modeling viewpoint. In order to analyze this problem, a series of ballistic impact tests have been performed on plain weave E-glass/polypropylene (E-glass/PP) composites of different thicknesses using 0.30 and 0.50 caliber right-cylinder shaped projectiles. A gas gun with a sabot stripper mechanism was employed to impact the panels. In order to analyze the perforation mechanisms, ballistic limit and damage evaluation, an explicit three-dimensional finite element code LS-DYNA was used. Material model 162, a progressive failure model based on modified Hashin’s criteria, has been assigned to analyze failure of the laminate. The projectile was modeled using Material model 3 (MAT_PLASTIC_KINEMATIC). The laminates and the projectile were meshed using brick elements with single integration points. The impact velocity ranged from 187 to 332 m s⁻¹. Good agreement between the numerical and experimental results was attained in terms of predicting ballistic limit, delamination and energy absorption of E-glass/PP laminate.     

Effect of reinforcement type on high velocity impact response of GRP plates using a sharp tip projectile

High velocity impact performance of glass reinforced polyester (GRP) resin composite plates with different type of reinforcement was investigated. The projectile used was a sharp tipped (30°) conical head with total length of 30 mm and shank length of 15 mm with weight of 9.74 g. Five different types of E-glass fiber reinforcement were used, including chopped strand mat (CSM), plain weave, satin weave, unidirectional and cross-ply unidirectional fiber reinforcements. A smooth barrel gas gun was used to conduct high velocity impact tests in the velocity range of 80–160 m/s. Composite plates with size of 15 cm × 15 cm were prepared in 3 and 6 mm thickness. Results showed higher ballistic limit velocity (velocity at which samples fully penetrated the target plates with zero residual velocity) for 3 mm GRP plates with cross-ply unidirectional reinforcement followed by unidirectional reinforcement and plain weave, the plates with satin weave and CSM reinforcements were almost in same level. The thicker specimens (6 mm), plates with plain weave reinforcement showed better ballistic performance towards sharp tipped conical projectile impact, followed by cross-ply unidirectional, satin weave, unidirectional and CSM reinforced plates. Experimentally determined ballistic limit velocity for all specimens correlate well with estimated ballistic limit values obtained in full perforation tests. Damage assessment conducted on all specimens indicated fiber tension and shear failure for thin-walled and sever delamination for the thick-walled specimens as the dominant failure modes.     

Evaluating the fire endurance of concrete slabs reinforced with FRP bars: Considerations for a holistic approach

In recent years, there has been increased interest in the use of fibre reinforced polymers (FRP) bars as flexural and shear reinforcement in concrete slabs and beams. When used in building applications, provision of appropriate fire endurance in structural members is a major safety requirement that must be considered. This paper presents an overview of experimental and numerical studies carried out on FRP-reinforced concrete slabs under standard fire conditions. The factors that are likely to influence the fire performance of FRP-reinforced concrete slabs are presented and discussed, and the development of initial design guidance for evaluating of the fire resistance of concrete slabs reinforced with FRP bars is outlined. Examples are presented for illustrating the application of existing design charts for evaluating the fire endurance of concrete slabs reinforced with FRP bars. Suggestions for further research are presented.     

Early-age restrained shrinkage cracking of GFRP-RC bridge deck slabs: Effect of environmental conditions

Most of codes and guidelines for glass fiber reinforced polymers (GFRP) - Reinforced Concrete (RC) are based on modifying corresponding formulas, originally developed for steel bars, taking into account the differences in properties and behavior between FRP and steel. The main objective of this research is to investigate the effect of cyclic environments on early-age cracking of GFRP-RC bridge deck slabs experimentally. Two full-scale (measuring 2500-mm long × 765-mm wide × 180-mm thick) cast-in-place slabs reinforced with similar amounts of reinforcement ratio of 0.7% with GFRP and steel bars, respectively, were tested in adiabatic laboratory conditions as control specimens. In comparison, two other GFRP-RC deck slabs were tested under freezing–thawing and wetting–drying conditions. The test results are presented in terms of materials degradation, cracking pattern, crack width, and spacing, and strains in reinforcement and concrete. Test results indicate that the minimum reinforcement ratio (0.7%) recommended by the Canadian Highway Bridge Design Code 2006 (CHBDC 2006) for bridge deck slabs reinforced with GFRP bars satisfied the serviceability requirements after being subjected to the simulated cyclic exposures.     

Oblique and normal perforation of concrete targets by a rigid projectile

The oblique and normal perforation of a concrete target subjected to a rigid projectile impact is studied in this paper. A general three-stages model, i.e., initial cratering, tunnelling and shear plugging, is developed based on dynamic cavity expansion theory and plug formation. Analytical solutions for the ballistic performance and the perforation limit of concrete target are obtained. The proposed formulae are consistent with other empirical formulae and correlate well with the experimental data.     

钢纤维混凝土遮弹层抗弹丸侵彻效应试验研究与分析

钢纤维混凝土遮弹层抗常规武器侵彻效应问题,是防护工程界亟待解决的一个崭新课题。为研究这种新型防护材料的抗侵彻性能,利用Φ12.7mm弹道炮-测速靶系统对混凝土及钢纤维混凝土进行了弹道冲击对比试验,获得了弹丸着靶速度及对应的最大侵彻深度、弹坑直径、靶体破坏形态等试验参数,并利用高速摄影系统记录了靶体的动态破坏过程。针对现有经验公式均不能反映钢纤维混凝土材料高韧性影响的不足,引入钢纤维混凝土材料韧度R,对试验数据进行了回归分析,导出了侵彻深度工程计算公式。计算结果与试验数据对比表明,预估公式计算精度较高,公式中相关参数简单易于确定,且能反映钢纤维混凝土的高强高韧性特点,在实际工程应用中具有重要的参考价值。     

弹丸冲击贯穿有限厚混凝土材料靶板的背面成坑效应

弹丸冲击贯穿有限厚混凝土靶板后,靶板背面有大块的混凝土剥落,形成近似的锥形坑。为了研究背面弹坑半锥角θ这一作为混凝土靶板贯穿后破坏范围问题研究中的重要参量,将贯穿问题等效为轴对称条件下的冲切破坏问题。采用双剪应力三参数强度准则及刚塑性模型,得到了极限应力圆的包络线方程,给出了轴对称破坏机构,进而求得θ值的表达式。理论计算结果与数值模拟结果、实验数据三者之间吻合度较好。研究表明,θ值由混凝土材料抗压强度与抗拉强度比值决定。基于上述研究,提出在混凝土中掺加钢纤维以提高有限厚靶板抗贯穿能力的实际方法,并进行了弹道实验。实验结果表明,贯穿破坏后靶板碎片的数量及θ大幅降低,显示了高含量异型钢纤维混凝土在抗贯穿方面的适用性。     

Comparative analysis of deformations and tension-stiffening in concrete beams reinforced with GFRP or steel bars and fibers

Present research experimentally and theoretically investigates deformations and tension-stiffening in concrete beams with different types of reinforcement. The paper reports test results of eight beams reinforced with glass fiber reinforced polymer (GFRP) or steel bars, combined with steel fibers. For given uniform reinforcement ratio, different number and distribution of bars was assumed in the section. Experimental curvatures were checked against the predictions by design codes (Eurocode 2, ACI 318 and the new Russian code SP 52-101) and recommendations (Italian CNR-DT 203 and American ACI 440). The study examined capability of different code techniques to predict deformations of beams with varying reinforcement characteristics. It has been shown that distribution of reinforcement had a significant influence on the prediction accuracy. In a more elaborate analysis, the tension-stiffening effect was investigated using an inverse technique earlier developed by the authors. Stress–strain tension-stiffening relationships were obtained for each of the beams using the test moment–curvature diagrams. Unlike the common practice, the analysis took into account the shrinkage effect which was different for steel and GFRP reinforced elements. To verify adequacy of the obtained results of constitutive modeling, the derived tension-stiffening relationships were implemented into finite element simulation as material laws for tensile concrete. It was shown that the above inverse approach offers an alternative and versatile tool for constitutive modeling.     

Resistance of fibre concrete slabs to low velocity projectile impact

An investigation on fibre concrete slabs subjected to low velocity projectile impact was carried out to assess impact resistance. The main variables of the study were type of fibre and volume fraction of fibres. The types of fibres chosen were polyolefin, polyvinyl alcohol and steel. The volume fraction of fibres examined were 0%, 1% and 2%. A total of 10 square slabs of size 1 m and 50 mm thickness were cast and tested. Impact was achieved by dropping projectile of mass 43 kg from a height of 4 m, by means of an instrumented impact test facility. Test results indicate that hooked-end steel fibre concrete slabs have better cracking and energy absorption characteristics than slabs reinforced with other fibre types. Slabs reinforced with polyvinyl alcohol fibres exhibited higher fracture energy values compared to slabs reinforced with polyolefin fibres.     

CFRP筋增强ECC梁弯曲性能试验研究

为研究碳纤维增强树脂复合材料(Carbon fiber reinforced polymer,CFRP)筋/超高韧性纤维增强水泥基复合材料(Engineered cementitious composite,ECC)梁的抗弯性能,对3根CFRP筋/ECC梁、1根玻璃纤维增强树脂复合材料(Glass fiber reinforced polymer,GFRP)筋/梁和1根CFRP筋混凝土梁进行了四点弯曲试验,分析了配筋率、纤维增强树脂复合材料(Fiber reinforced polymer,FRP)筋类型和基体类型对梁抗弯性能的影响。试验结果表明:CFRP筋/ECC梁与GFRP筋/ECC梁和CFRP筋混凝土梁类似,均经历了弹性阶段、带裂缝工作阶段和破坏阶段;配筋率对CFRP筋/ECC梁的受弯性能影响较大。随着配筋率的增加,CFRP筋/ECC梁的承载能力不断提高,延性性能逐渐减弱;ECC材料优异的应变硬化能力和受压延性,使得CFRP筋/ECC梁的极限承载能力和变形能力均优于CFRP筋混凝土梁;由于ECC材料多裂缝开裂能力,CFRP筋/ECC梁开裂后,纵筋表面应变分布比CFRP筋混凝土梁更均匀; 由于聚乙烯醇(Polyvinyl alcohol,PVA)纤维的桥联作用,CFRP筋/ECC梁破坏时,其表面出现了大量的细密裂缝,且能保持较好的完整性和自复位能力;正常使用阶段,CFRP筋/ECC梁的最大弯曲裂缝宽度均小于CFRP筋混凝土梁。最后,根据试验结果,建立了基于等效应力图的CFRP筋/ECC梁弯曲承载力简化计算模型,确定模型中的相关系数。由简化模型计算的极限承载力与试验结果具有较好的相关性。