Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.     

Properties of strain hardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading

Enhanced matrix packing density and tailored fiber-to-matrix interface bond properties have led to the recent development of ultra-high performance fiber reinforced concrete (UHP-FRC) with improved material tensile performance in terms of strength, ductility and energy absorption capacity. The objective of this research is to experimentally investigate and analyze the uniaxial tensile behavior of the new material. The paper reviews and categorizes a variety of tensile test setups used by other researchers and presents a revised tensile set up tailored to obtain reliable results with minimal preparation effort. The experimental investigation considers three types of steel fibers, each in three different volume fractions. Elastic, strain hardening and softening tensile parameters, such as first cracking stress and strain, elastic and strain hardening modulus, composite strength and energy dissipation capacity, of the UHP-FRCs are characterized, analyzed and linked to the crack pattern observed by microscopic analysis. Models are proposed for representing the tensile stress–strain response of the material.     

Permeability of ultra high performance fiber reinforced concretes (UHPFRC) under high stresses

Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability which make them very attractive for the rehabilitation of existing structures and for new conceptions. UHPFRC are characterized by a significant tensile strain hardening (multiple cracking stage) that can be used to optimize the mechanical performance of composite structural elements. In order to validate this assumption, permeability tests were carried out on UHPFRC specimens previously submitted to various levels of tensile deformation with progressive damage. Based on permeability results, it was possible to define maximal tensile deformations whereby the water permeability of a specific UHPFRC remains low for various exposure conditions.     

Effects of silica powder and cement type on durability of ultra high performance concrete (UHPC)

Ultra-high performance concrete (UHPC) achieves extraordinary strength characteristics through optimization of the particle packing density of the cementitious matrix. The dense matrix also promotes exceptional durability properties and is arguably the biggest benefit of the material. A durable concrete enables structures to last longer, reduces the cost of maintenance and helps achieve a significantly more sustainable infrastructure. To assess the durability of UHPC, the performance of several non-proprietary blends are investigated by assessing the materials' resistance to freeze-thaw cycles, ingress of chlorides as well as the presence and distribution of air voids. The main experimental variables are cement type and the quantity of silica powder, which varies from 0% to 25% of the cement weight. All mixes displayed negligible chloride ion penetration and high resistance to freeze-thaw with mass loss well below the limit in over 60 cycles of freeze-thaw. Analysis of the test data indicates that the silica powder content has little influence on performance.     

Material and bond properties of ultra high performance fiber reinforced concrete with micro steel fibers

For investigating the effect of fiber content on the material and interfacial bond properties of ultra high performance fiber reinforced concrete (UHPFRC), four different volume ratios of micro steel fibers (V_f = 1%, 2%, 3%, and 4%) were used within an identical mortar matrix. Test results showed that 3% steel fiber by volume yielded the best performance in terms of compressive strength, elastic modulus, shrinkage behavior, and interfacial bond strength. These parameters improved as the fiber content was increased up to 3 vol.%. Flexural behaviors such as flexural strength, deflection, and crack mouth opening displacement at peak load had pseudo-linear relationships with the fiber content. Through inverse analysis, it was shown that fracture parameters including cohesive stress and fracture energy are significantly influenced by the fiber content: higher cohesive stress and fracture energy were achieved with higher fiber content. The analytical models for the ascending branch of bond stress-slip response suggested in the literature were considered for UHPFRC, and appropriate parameters were derived from the present test data.     

Toughness testing of ultra high performance fibre reinforced concrete

In this paper an investigation is made of the applicability of the ASTM C 1609 procedure for testing toughness of ultra high performance fibre reinforced concretes containing a large amount of fibre (≥2% by volume) and exhibiting deflection hardening behaviour. All mixtures exhibited deflection hardening behaviour, and the parameters varied included (1) the amount of steel fibres, (2) the type of steel fibres, (3) the size of the longest fibre, (4) the addition of polypropylene fibres, and (5) the size of the maximum aggregate grain in the concrete matrix. Based on comparison of the curves obtained from flexural toughness tests with the evaluation of the test results obtained according to ASTM C 1609 and with the statistical analysis, the authors recommended additional toughness parameters (P_100,3.00, P_100,4.00, P_100,6.00, T_100,3.00, T_100,4.00, and T_100,6.00) for the evaluation of toughness results. Such additional toughness parameters are calculated using a similar procedure as that specified in ASTM C 1609.     

Influence of ring size on the restrained shrinkage behavior of ultra high performance fiber reinforced concrete

In order to evaluate the restrained shrinkage behavior of ultra high performance fiber reinforced concrete (UHPFRC), ring-tests with three different wall thicknesses and two different diameters of inner steel ring were performed. Partially exposed free shrinkage and tensile tests were carried out simultaneously to assess the theoretical elastic stress, stress relaxation, degree of restraint and potential for cracking in the concrete. Test results indicated that the UHPFRC ring specimen with a thicker steel ring demonstrated a faster theoretical cracking time, higher stress relaxation and degree of restraint than that of a thinner steel ring, whereas those factors were rarely affected by the diameter of the inner steel ring. About 39–65 % of the theoretical elastic stress was relaxed by the sustained interface pressure. Since the actual residual tensile stress of all specimens was less than the tensile strength, the computed cracking potential varied from 0.43 to 0.7, and thus no shrinkage crack was observed. Finally, the degree of restraint provided a linear relationship with the ratio of steel and concrete wall thickness.     

Tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC)

The tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) under constant amplitude fatigue cycles is presented. Three series of uniaxial tensile fatigue tests up to a maximum of 10 million cycles were conducted with the objective to determine the endurance limit of UHPFRC that was supposed to exist for this material. The fatigue tests reveal that an endurance limit exists in all three domains of UHPFRC tensile behaviour at S-ratios ranging from 0.70 to 0.45 with S being the ratio of the maximum fatigue stress to the elastic limit strength of UHPFRC. Rather large variation in local specimen deformations indicates significant stress and deformation redistribution capacity of the UHPFRC bulk material enhancing the fatigue behaviour. The fatigue fracture surface of UHPFRC shows features of the fatigue fracture surfaces of steel, i.e. fatigue crack propagation is identified by a smooth surface while final fracture leads to rather rough surface. Various fatigue damaging mechanisms due to fretting and grinding as well as tribocorrosion are identified.     

Discrete fracture in high performance fibre reinforced concrete materials

In this paper a simple, but effective methodology to simulate opening mode fracture in high performance fibre reinforced concrete is presented. The main contribution of the paper is a technique to extrapolate the load displacement curves of three point bending experiments on fibre reinforced concrete. The extrapolation allows the full work of fracture to be determined, from which the fracture energy may be obtained. The fracture energy is used in the definition of a cohesive softening function with crack tip singularity. The softening relation is implemented in an embedded discontinuity method, which is employed for the numerical simulation of three point bending experiments. The experimental work includes a size effect study on three point bending specimens. The numerical simulation provides a satisfactory prediction of the flexural behaviour and the size effect observed in the experiments.     

The fracture and fatigue performance in flexure of carbon fiber reinforced concrete

The fracture parameters and fatigue performances of carbon fiber reinforced concrete is investigated by three point bending tests. In comparison with the results of quasi-static tests where no pre-cyclic loading is applied, the influence of pre-cyclic loading history on fracture parameters was researched by using compliance calibration. The test results show that the fracture parameters of carbon fiber reinforced concrete and plain concrete will be reduced if the pre-cyclic loading stress levels are higher than a certain threshold, and this threshold value for carbon fiber reinforced concrete is higher than that of plain concrete. The critical effective crack length for carbon fiber reinforced concrete is significantly larger than that of plain concrete and independent of the pre-cyclic loading history and fatigue life. Carbon fiber reinforced concrete has a considerable beneficial effect on the behaviour of concrete subjected to flexure fatigue loading.     

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.     

Some characteristics of high strength fiber reinforced lightweight aggregate concrete

The effect of polypropylene and steel fibers on high strength lightweight aggregate concrete is investigated. Sintered fly ash aggregates were used in the lightweight concrete; the fines were partially replaced by fly ash. The effects on compressive strength, indirect tensile strength, modulus of rupture, modulus of elasticity, stress–strain relationship and compression toughness are reported. Compared to plain sintered fly ash lightweight aggregate concrete, polypropylene fiber addition at 0.56% by volume of the concrete, caused a 90% increase in the indirect tensile strength and a 20% increase in the modulus of rupture. Polypropylene fiber addition did not significantly affect the other mechanical properties that were investigated. Steel fibers at 1.7% by volume of the concrete caused an increase in the indirect tensile strength by about 118% and an increase in the modulus of rupture by about 80%. Steel fiber reinforcement also caused a small decrease in the modulus of elasticity and changed the shape of the stress–strain relationship to become more curvilinear. A large increase in the compression toughness was recorded. This indicated a significant gain in ductility when steel fiber reinforcement is used.     

Hybrid fiber reinforced concrete (HyFRC): fiber synergy in high strength matrices

In most cases, fiber reinforced concrete (FRC) contains only one type of fiber. The use of two or more types of fibers in a suitable combination may potentially not only improve the overall properties of concrete, but may also result in performance synergy. The combining of fibers, often called hybridization, is investigated in this paper for a very high strength matrix of an average compressive strength of 85 MPa. Control, single, two-fiber and three-fiber hybrid composites were cast using different fiber types such as macro and micro-fibers of steel, polypropylene and carbon. Flexural toughness tests were performed and results were extensively analyzed to identify synergy, if any, associated with various fiber combinations. Based on various analysis schemes, the paper identifies fiber combinations that demonstrate maximum synergy in terms of flexural toughness.     

Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements

Ultra-high performance concrete (UHPC) is promising in construction of concrete structures that suffer impact and explosive loads. In order to make UHPC structures more ductile and cost-effective, hybrid fiber reinforcements are often incorporated. In this study, a reference UHPC mixture with no fiber reinforcement and five mixtures with a single type of fiber reinforcement or hybrid fiber reinforcements of 6 and 13 mm in length at a total dosage of 2%, by the volume of concrete, were prepared. Quasi-static compressive and flexural properties of those mixtures were investigated. Split Hopkinson press bar (SHPB) testing was adopted to evaluate their dynamic compressive properties under three impact velocities. Test results indicated that UHPC with 1.5% long fiber reinforcements and 0.5% short fiber reinforcements demonstrated the best static and dynamic mechanical properties. The static compressive and flexural strengths of UHPC with 2% long fiber reinforcements were greater than those with 2% short fiber reinforcements, whereas comparable dynamic compressive properties were observed. Strain rate effect was observed for the dynamic compressive properties, including peak stress, dynamic increase factor, and absorbed energy. The reinforcing mechanisms of hybrid fiber reinforcements in UHPC were eventually discussed.     

Experimental evaluation of fiber reinforced concrete fracture properties

Concrete is now universally recognized a construction material vital and essential for the regeneration and rehabilitation of the infrastructure of a country. The last few decades have now shown that high strength concrete, with a compressive strength of 100–120 MPa can be readily designed and manufactured. There have also been several advances made in the development of fiber reinforced concrete to control cracking and crack propagation in plain concrete, and to increase the overall ductility of the material. However, there are now many types of fibers with different material and geometric properties, and the exact fracture behavior of fiber reinforced concrete materials is not clearly understood. The overall aim of this paper is to establish the fracture properties and fracture behavior of concrete containing two widely used types of fibers, namely, steel (high modulus) and polypropylene (low modulus). The experimental investigation consisted of tests on cubes and notched prismatic specimens made from plain concrete and fiber concrete with 1% and 2% of steel or polypropylene fibers. The cube tests and the three point bending tests on notched specimens were carried out according to RILEM specifications, and extensive data on their compressive and flexural tensile behavior and fracture energy were recorded and analyzed. The results obtained from the tests are critically assessed, and it is shown that fibers contribute immensely to the structural integrity and structural stability of concrete elements and thereby improve their durable service life.     

Characterization of the interfacial bond between old concrete substrate and ultra high performance fiber concrete repair composite

The interfacial bond characteristics between normal concrete substrate as old concrete and ultra high performance fiber concrete as repair material have been investigated. Normal concrete substrates were first subjected to different surface preparation methods prior to bonding the ultra high performance fiber concrete to form repair composites. The interfacial mechanical bond of the composites was assessed using slant shear and tensile splitting strength tests. In addition, rapid chloride permeability test was performed to ascertain the potential chloride resistance of the composites. The microstructure of the transition zone between the normal concrete and ultra high performance fiber concrete was also studied using scanning electron microscope. The results generally indicate that surface preparation of the substrate is very much required to obtain superior mechanical bond of the composites; whereby the composites with the sand-blasted substrate providing the most superior mechanical bond. The excellent bond of the composite is also evident through the rapid chloride permeability test, as well as confirms by the scanning electron microscope image of the interface. Hence, the ultra high performance fiber concrete exhibits significant potential as an excellent material for repair and rehabilitation of concrete structures.     

Time dependent behavior of elements combining ultra-high performance fiber reinforced concretes (UHPFRC) and reinforced concrete

Structural elements combining Ultra-High Performance Fiber Reinforced Concretes (UHPFRC) and concrete offer a high potential in view of rehabilitation and modification of existing structures. The investigation of the time-dependent behavior of composite “UHPFRC-concrete” elements is a fundamental step in the determination of durability and serviceability. For this, an experimental program was conducted on large composite “UHPFRC-concrete” beams and a numerical model was validated with the test results. The experimental results and a parametric study performed with the numerical model showed that UHPFRC and normal strength reinforced concrete are compatible in the long-term and that the critical period of composite “UHPFRC-concrete” elements are the first 90 days after the casting of the UHPFRC layer. Thus, the high potential of such composite elements can be exploited also in the long term.     

方形中空夹层钢管超高性能钢纤维混凝土柱抗爆性能数值模拟与实验验证

建立了爆炸荷载作用下方形中空夹层钢管超高性能钢纤维混凝土(Ultra-High Performance Steel Fiber Reinforced Concrete Filled Double Skin Steel Tube,UHPSFRCFDST)柱动态响应及其损伤破坏三维有限元数值模型。首先通过模拟结果与爆炸破坏试验结果的对比分析,验证了数值模型和计算方法的有效性。进而运用参数化分析方法,研究了空心率、含钢率、内、外层钢管厚度及其强度等关键参数对UHPSFRCFDST柱抗爆性能的影响。研究结果表明,UHPSFRCFDST柱具有优越的抗爆性能,所建立的三维有限元模型能够有效地分析UHPSFRCFDST柱在爆炸荷载作用下的动态响应及其损伤破坏;在一定范围内减小空心率及提高外层钢管强度可有效提升UHPSFRCFDST柱抗爆性能;提高含钢率、减小内、外层钢管高厚比均能够显著提升UHPSFRCDST柱抗爆性能;内层钢管强度对UHPSFRCFDST柱的抗爆性能影响并不明显。