Response of SIFCON two-way slabs under impact loading

An experimental program was carried out to investigate the behaviour of slurry-infiltrated fibrous concrete (SIFCON) slabs under impact loading. Fibre-reinforced concrete (FRC), reinforced cement concrete (RCC) and plain cement concrete (PCC) slabs were also cast and tested for comparison purposes. The impact force was delivered with a steel ball drop weight. The test results revealed that SIFCON slabs with 12% fibre volume fraction exhibit excellent performance in strength and energy-absorption characteristics when compared with other slab specimens. Regression models have been developed to estimate the energy absorption for SIFCON slab specimens.     

Flexural behaviour of small steel fibre reinforced concrete slabs

Influence of length and volumetric percentage of steel fibres on energy absorption of concrete slabs with various concrete strengths is investigated by testing 28 small steel fibre reinforced concrete (SFRC) slabs under flexure. Variables included; fibre length, volumetric percentage of fibres and concrete strength. Test results indicate that generally longer fibres and higher fibre content provide higher energy absorption. The results are compared with a theoretical prediction based on random distribution of fibres. The theoretical method resulted in higher energy absorption than that obtained in experiment. A design method according to allowable deflection is proposed for SFRC slabs within the range of fibre volumetric percentages used in the study. The method predicts resisting moment–deflection curve satisfactorily.     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

Critical parameters of nylon and other fibres for spalling protection of high strength concrete in fire

This study investigated various types of fibre length, fibre diameter, fibre type and fibre content on the degree of spalling of concrete in fire. Four types of fibres, namely, polypropylene, polyvinyl alcohol, cellulose and nylon with various lengths and diameters were studied. Fibre contents ranged from 0.05 to 0.15% by volume of concrete. Fire tests were conducted according to the ISO 834 standard heating curve. Results showed that when comparing all the fibres under the same fibre content levels (% volume of concrete), the nylon fibre was the most effective in protecting concrete from spalling. This is because the diameter of the nylon fibres were significantly less than the other fibres, hence there were significantly more number of nylon fibres present for the same fibre content (% volume) in concrete. Analysis revealed, regardless of the amount of fibre, the type of fibre, diameter of and length of fibre, there is a strong relationship between the total number of fibres present per unit volume, length of fibres and the degree of spalling observed. Based on this relationship, the authors established a critical minimum for total number of fibres per unit volume for spalling protection in fire.     

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%.     

Repairing reinforced concrete slabs using composite layers

There are several strengthening methods for rehabilitation of RC structural elements. The efficiency of these methods has been demonstrated by many researchers. Due to their mechanical properties, using fibrous materials in rehabilitation applications is growing fast. Therefore, this study presents rehabilitation of slabs in such a way that plain concrete layers on top, on bottom, on the entire cross section are replaced by reinforced concrete layers. In order to reinforce the concrete, Polypropylene (PP) and steel fibers were used by 0.5%, 1% and 2% fiber volume fractions. Nineteen slabs were studied under flexural loadings and fibrous material effects on the initial crack force, the maximum loading carrying capacity, absorbed energy and ductility were investigated. The obtained results demonstrated that increasing the fiber volume fraction or using reinforced concrete layer on top, bottom, or at the entire cross section of the slabs not only always leads to improvement in the slab performance, but also sometimes debilitates the slab performance. Hence, this study will propose the best positioning of reinforced concrete layer, fiber volume fraction and fiber type to achieve the best flexural performance of slabs.     

Mechanical characterisation and impact behaviour of concrete reinforced with natural fibres

It is well known that high strength concrete displays a brittle behaviour and less tough characteristics than normal strength concrete. This type of behaviour can be enhanced by incorporating various types of fibres which lead to better mechanical properties and impact resistance. In this paper, an experimental study conducted on high strength concrete reinforced with glass fibres and natural fibres (palm tree leaves), both used at a relatively low volume fraction, is presented. Compressive, splitting, three-point bending and impact test methods have been used to characterise reinforced concrete materials, and the results are analysed statistically. It is observed that natural fibres enhanced the mechanical properties and impact resistance of concrete and exhibit comparable response to the glass fibres. A finite element model using ANSYS was employed to study the flexural behaviour of fibre reinforced concrete. It is concluded that both experimental and numerical results are in good agreement.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.     

Permissible crack widths in steel fibre reinforced marine concrete

The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. The overall aim of the investigation is to develop the material for marine applications. The results reported here pertain to pre-cracked specimens of steel fibre reinforced concrete which were exposed to wet-dry cycles of marine spray in the laboratory simulating tidal zone conditions of exposure. Two types of concrete mixes were used in the investigation—one with standard concrete constituents and OPC and the second replacing about 26% of cement with pfa. The cement content of the mixes was 590 and 435 kg m⁻³, respectively. Fibre reinforcement was provided by means of low carbon steel fibres and melt extract steel fibres at a v _f ℓ/d ratio of 100 and 147. Prism specimens were manufactured and these were precracked to induce cracks of width ranging between 0.03 and 1.73 mm. After cracking, both sealed and unsealed specimens were exposed to laboratory marine spray cycles using sea water. Some control specimens were cured in the laboratory air throughout. Tests were carried out after 650 marine cycles (450 days) and 1450 marine cycles (900 days). Based on data on flexural strength, energy absorption capacity, stiffness and state of corrosion of the fibres, recommendations are made regarding suitable permissible crack widths for the design of steel fibre reinforced concrete for marine applications. The results indicate that a permissible crack width of 0.2 mm is satisfactory for concrete reinforced with melt extract fibres. A smaller value is recommended for concrete reinforced with low carbon steel fibres. Complete healing of open cracks of small widths is observed under exposure to marine cycles.     

Observations on the electrical resistivity of steel fibre reinforced concrete

Steel fibre reinforced concrete (SFRC) is in many ways a well-known construction material, and its use has gradually increased over the last decades. The mechanical properties of SFRC are well described based on the theories of fracture mechanics. However, knowledge on other material properties, including the electrical resistivity, is sparse. Among others, the electrical resistivity of concrete has an effect on the corrosion process of possible embedded bar reinforcement and transfer of stray current. The present paper provides experimental results concerning the influence of the fibre volume fraction and the moisture content of the SFRC on its electrical resistivity. The electrical resistivity was measured by alternating current (AC) at 126 Hz. Moreover, an analytical model for the prediction of the electrical resistivity of SFRC is presented. The analytical model is capable of predicting the observed correlation between the fibre volume fraction and the electrical resistivity of the composite (the SFRC) for conductive fibres and moisture saturated concrete. This indicates that the steel fibres were conducting when measuring the electrical resistivity by AC at 126 Hz. For partly saturated concrete the model underestimated the influence of the addition of fibres. The results indicate that the addition of steel fibres reduce the electrical resistivity of concrete if the fibres are conductive. This represents a hypothetical case where all fibres are depassivated (corroding) which was created to obtain a conservative estimate on the influence of fibres on the electrical resistivity of concrete. It was observed that within typical ranges of variation the influence of the moisture content on the electrical resistivity was larger than the effect of addition of conductive steel fibres, but also that the relative impact on the electrical resistivity due to conductive steel fibres increased when the moisture content of the concrete was reduced.     

On the prediction of the orientation factor and fibre distribution of steel and macro-synthetic fibres for fibre-reinforced concrete

The orientation and distribution of the fibres is decisive in the mechanical behaviour of fibre-reinforced concrete. Several classical models have extensively been used for the case of rigid steel fibres. The increasing interest in structural synthetic fibres that can bend demanded new considerations in this matter. A probabilistic model considering the previous research with stereographical assumptions has been performed allowing the use of fibres that can bend. This paper also provides significant tools for design engineering in order to predict and confirm the number of fibres crossing a vertical surface using fibre reinforced concrete with steel and polyolefin fibres. Additionally, the proposed model coincides with the most accepted values and represents with accuracy the existence of boundaries.     

Shrinkage of fibre and reinforced fibre concrete beams in hot-dry climate

The influence of steel fibre inclusion on the shrinkage of 16 full-size plain and reinforced concrete beams was assessed. Shrinkage measurements, at three levels over the depth of the beams, were carried out for 200 days. Half of the beams were cured in a controlled laboratory environment and the other half cured under hot, dry and windy climatic conditions.

Test results show that under laboratory curing conditions adding 1% by volume of steel fibres reduced the ultimate shrinkage at the top, mid-height, and bottom of the plain concrete by 16, 23, and 28%, respectively. However, in the reinforced concrete beam the presence of longitudinal reinforcement rendered it less significant.

Under the uncontrolled severe curing environment, the addition of 1% by volume of fibres produced a reduction of 30% in shrinkage at the bottom level of both the plain and the reinforced concrete beams. At the top level, however, the geometry constraints and the compaction techniques influenced the fibre contribution to shrinkage.     

TOUGHNESS CHARACTERISTICS OF SYNTHETIC FIBRE-REINFORCED CEMENTITIOUS COMPOSITES

The brittle nature of concrete is a reflection of its low toughness and the presence of defects. One effective way to enhance the toughness of concrete is by fibre reinforcement. Synthetic fibres, mostly polypropylene, have been widely used as a concrete additive at 1% or less volume fraction for toughness enhancement and shrinkage control. Pull-out of fibres bridging a concrete matrix crack absorbs a significant amount of energy. This paper reviews some of the fracture mechanics approaches to the prediction of failure of fibre-reinforced concrete structures, methods used to rank the toughness of fibre-reinforced concrete, toughness optimization, and the properties of concrete reinforced with selected synthetic and recycled fibres.     

Energy absorption capacity of a sustainable Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) in quasi-static mode and under high velocity projectile impact

This paper investigates the energy absorption capacity of a sustainable Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) in quasi-static mode and under high velocity projectile impact. The design of the sustainable concrete mixtures aims on achieving a densely compacted cementitious matrix with a relatively low binder amount, employing the modified Andreasen & Andersen particle packing model. The experiments on UHPFRC are performed using a 4-point bending test and high velocity projectile impact tests. The obtained results show that although the utilization of hybrid steel fibre enhances the mechanical properties of the developed UHPFRC, the application of fibres with hooked ends is crucial in improving the energy absorption capacity of the sustainable UHPFRC in quasi-static mode. However, under high velocity projectile impact, the UHPFRC mixture with hybrid fibres shows a much better energy absorption capacity than the one with hooked steel fibres only, particularly in resisting the scabbing at the rear surface. The intrinsic mechanisms for the energy absorption capacity of the sustainable UHPFRC in quasi-static mode and under high velocity projectile impact are studied and analysed.     

Mixed-mode fracture performance of fibre reinforced concrete under impact loading

A test procedure developed to determine the mixed-mode impact resistance of fibre reinforced concrete materials is described. Results are presented from a series of experiments using a repeated drop-weight impact apparatus for the impact resistance of both polypropylene and steel fibre reinforced concrete. The behaviour of the mixed-mode specimen geometry was investigated under impact loading condition. The effect of the fibre types and contents on the impact fracture energy of the specimens was investigated. A close investigation was made of the positions and formations of the crack patterns. The fracture performance of the plain and fibre reinforced concrete was investigated with the proposed geometry using 1.0, 2.0 and 3.0% by weight in the case of steel fibre, and 0.1, 0.2 and 0.3% by weight in the case of polypropylene fibre.

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Resume Aux charges statiques auxquelles les structures en béton sont soumises s'ajoutent souvent des charges dynamiques, parfois significatives, qu'il convient de prendre en compte dans le calcul. Cet article décrit un mode opératoire visant à déterminer la résistance au choc en mode mixte de béton renforcé de fibres (FRC). On y présente les résultats d'une série d'essais utilisant une machine d'essai au choc à répétition (mouton) pour évaluer la résistance de béton renforcé de fibres d'acier et de fibres de polypropylène. On a mis au point une géométrie d'éprouvettes en mode mixte qu'on a soumises à des essais au choc, et on a étudié l'effet des types et des pourcentages de fibres sur la résistance des éprouvettes à la rupture par choc. On s'est aussi livré à une étude précise des emplacements et de la formation des fissures. On a examiné la résistance à la rupture de béton ordinaire et de béton de fibres dans la géométrie proposée avec des pourcentages de 1, 2 et 3% en poids de fibres d'acier, et de 0,1, 0,2 et 0,3% pour les fibres de polypropylène.

     

Fracture properties of concrete reinforced with steel–polypropylene hybrid fibres

This research discusses polypropylene fibres and three sizes of steel fibres reinforced concrete. The total fibre content ranges from 0% to 0.95% by volume of concrete. A four-point bending test is adopted on the notched prisms with the size of 100×100×500 mm³ to investigate the effect of hybrid fibres on crack arresting. The research results show that there is a positive synergy effect between large steel fibres and polypropylene fibres on the load-bearing capacity and fracture toughness in the small displacement range. But this synergy effect disappears in the large displacement range. The large and strong steel fibre is better than soft polypropylene fibre and small steel fibre in the aspect of energy absorption capacity in the large displacement range. The static service limitation for the hybrid fibres concrete, with “a wide peak” or “multi-peaks” load–CMOD patterns, should be carefully selected. The ultimate load bearing capacity and the crack width or CMOD at this load level should be jointly considered in this case. The K_IC and fracture toughness of proper hybrid fibre system can be higher than that of mono-fibre system.     

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.     

Crack growth resistance in fibre reinforced alkali-activated fly ash concrete exposed to extreme temperatures

This paper examines the crack growth resistance of alkali-activated fly ash concrete under extreme temperatures. Plain and hybrid fibre reinforced alkali activated concrete prepared with fly ash were subjected to a range of temperatures from ? 30 to 300 °C, sustained for 2 h. The alkali activation was effected with a blend of sodium hydroxide and sodium silicate. A fibre blend of steel to polypropylene in the volume ratio of 4:1 and a total as high as 1% by volume fraction reinforced the mixtures. The resulting systems were examined for residual strength under compression and splitting tension. Further, notched prisms were loaded under 4-point flexure to evaluate the residual fracture toughness. Based on the results, four different stages for fracture behaviour were identified with superior fibre efficiency seen at sub-zero temperatures. Across the breadth of temperatures examined, adding fibres led to higher residual fracture toughness for those systems that displayed a narrow range of thermal conductivity.     

Structural behaviour of basalt fibre reinforced glass concrete slabs

Small-scale slab tests at ambient and elevated temperatures, conducted on horizontally unrestrained simply supported slabs, are presented in this paper. The aim of this research is to investigate the structural behaviour of concrete produced from different percentages of glass sand (20, 40, and 60 % by weight) and reinforced with different volume fractions of basalt fibre (0, 0.1, 0.3, and 0.5 % by total mix volume), when subjected to large vertical displacement. The results were also compared against similar structural members with concrete that did not contain glass or fibres. The results showed that the fracture of the reinforcement was the mode of failure for all the slabs and the load carrying capacity was enhanced above the theoretical yield-line load. For the slabs tested at elevated temperatures, the enhancement due to membrane action was at least twice as high as that recorded in the ambient temperature tests. The slabs with higher glass sand and basalt fibre content also exhibited greater enhancement and failed at higher displacement. The results also showed that the enhancement in the concrete with glass aggregate and basalt fibre was greater than that in concrete that contained no glass or fibre by up to 26 and 31 % at ambient temperature and in fire respectively.     

Shrinkage of steel fibre reinforced cement composites

The paper presents results of an experimental investigation on the influence of steel fibres on the free shrinkage of cement-based matrices. Shrinkage tests were carried out on cement paste, mortar and two types of concrete mixes for a period of up to 520 days. Melt extract, crimped and hooked steel fibres were used for reinforcement at volume fractions ranging between 1 and 3%. The results indicate that fibres restrain the shrinkage of the various cement matrices to a significant extent, resulting in reductions of up to 40%. Crimped fibres are the most efficient in providing shrinkage restraint. The paper also presents a theoretical expression and an empirical expression which can be used to predict shrinkage strains of steel fibre reinforced cement matrices. The analysis requires a knowledge of the values of coefficient of friction, μ, at the fibre-matrix interface, which are also derived in this paper. The μ values for steel fibres in normal concrete, mortar and cement paste range between 0.07 and 0.12.