1,720,982 research outputs found
repetability of self healing in fiber reinforced concretes with and without crystalline admixtures: preliminary results
No full textThis paper analyzes the repeatability of autogenous and engineered self-healing in fiber reinforced concrete (FRC) with and without crystalline admixtures. To this purpose, the tensile behavior of two different mixes, differing by the presence of the crystalline admixture, has been investigated after a series of pre-cracking and conditioning cycles. An indirect testing methodology has been employed to the aforementioned purpose, i.e. Double Edge Wedge Splitting (DEWS) test. Three different exposure conditions were considered: open air exposure, water immersion and wet/dry cycles. Specimens were pre-cracked up to a crack width of 0.25mm (0.01 in.) (precrack cycle). Then, specimens were healed for one month and tested again up to a crack width of 0.25mm (0.01 in.) (cycle after 1st healing). After that, specimens were healed for two months further (2nd healing) and finally, they were cracked once again up to 0.25mm (0.01 in.). The highest healing rate was reached for specimens immersed in water; moreover, as expectable, the larger the initial crack width, the lower is the percentage of crack closure. Regarding the repeatability, a general better trend was found for the mix with crystalline admixtures, in which, in addition, the maximum load regain was measured after the 2nd healing cycle rather than after the 1st healing
Self-healing capacity of fiber reinforced cementitious composites. State of the art and perspectives
Full text linkDesign of building structures and infrastructures is mainly based on four concepts: safety, serviceability, durability and sustainability. The latter is becoming increasingly relevant in the field of civil engineering. Reinforced concrete structures are subjected to damages that produce cracks which, if not repaired, can lead to a rapid deterioration and would result into increasing maintenance costs to guarantee the anticipated level of performance. Therefore, self-healing concrete can be very useful in any type of structures, as it allows to control and repairing cracks as soon as they are likely to occur. The effectiveness of self-healing can be improved with the use of fibers due to their capacity to control crack width and enhance multiple crack formation. In that way, researchers should use advanced cement based materials (FRCC, HPFRCC, etc.) and techniques (autogenous and engineering healing) to satisfy all demands in which sustainability and durability are key factors. Compared to the large number of investigations on selfhealing of plain concrete, self-healing studies on Fiber Reinforced Cementitious Composites (FRCC) are still limited. Therefore, the main objective of this paper is to provide a deep literature review on this subject in order to clarify what is known (What now?) and finally to identify those gaps which still require further studies (What next?) such as: healing capacity under sustained stress, repeatability healing/cracking cycles as well as healing capacity for cracks and damages occurring at later concrete ages
Autogenous self-healing capacity of early-age ultra-high-performance fiber-reinforced concrete
Full text linkThis paper analyzes the autogenous self-healing capacity of early-age Ultra-High-Performance Fiber-Reinforced concretes (UHPFRCs) by measuring the crack closure and the possible mechanical recovery on healed specimens. The main parameters considered in this research were the healing exposure conditions (humidity chamber, immersion in tap water, immersion in seawater and heat curing) and the precracking levels (microcracks and macrocracks). For the microcrack level, four-point bending tests were performed on prismatic specimens (100 × 100 × 500 mm3 ) obtaining a multiple cracking pattern characterized by crack widths ranged from 10 to 20 μm. Whereas for the macrocrack level (behavior after crack localization), splitting tests were carried out on notched cubic specimens (100 × 100 × 100 mm3 ) obtaining crack widths of up to 0.4 mm. For both precracking levels, specimens were precracked at two days and were cured for one month in the mentioned exposure conditions. Healing products were analyzed on the specimen surface and also inside the cracks; to this purpose, their microstructure was analyzed by means of SEM and EDS analyses. The results have shown that the highest crack closure values were obtained for the heat-cured specimens and for the specimens immersed in water (tap water and seawater) whereas the less efficient condition was the humidity chamber
Shear Behavior of Self-Compacting Concrete and Fiber-Reinforced Concrete Push-Off Specimens
No full textThe shear behavior of reinforced "Z"-shaped push-off specimens made with self-compacting concrete (SCC) and self-compacting fiber-reinforced concrete (SCFRC) was analyzed by means of experimental tests. Testing consisted of two phases. Firstly, specimens were precracked subjected to linear load along the shear plane. During this first phase three precracked levels were distinguished (without precrack, thin and thick precrack). Then, precracked specimens were tested under direct shear load. The shear behavior along the shear plane was analyzed by means of the crack opening and shear displacement versus shear load process. Variables were: the type of concrete (SCC, or SCFRC with different fibers contents: 40 kg/m3 or 60 kg/m3), the transversal reinforcement (TR) and the precrack width. The analysis was specially focused on the study of aggregate interlocking. The failure occurrence is better controlled thanks to the presence of fibers, the shear behavior is more ductile. © RILEM 2010
Influence on Shear Strength of Fibre Reinforced Concrete: Experimental and Numerical Study on Deep Beams
No full textThe use of Fibre Reinforced Concrete (FRC) has gained considerable attention in the last few years, particu-larly when crack propagation control is of primary importance, i.e. in beams when shear reinforcement is partly or totally absent. Many experiments available in the literature showed that fibres limit the growth of shear inclined crack, give visible warning prior the structure collapse and also provide a stable and diffused crack pattern within the shear critical area. However, the issue of size effect in members containing steel fibres, has not been deeply investigated and evaluated yet. In this context, nine experimental tests on full-scale beams (six with steel fibres and three with plain concrete only), with a depth ranging from 0.5 m up to 1.5 m, are herein presented. Experimental results will be extensively evaluated in terms of strength, ductility, shear cracking, collapse mechanism and effect of fibres. Moreover an extensive parametrical numerical study, performed by means of a FE program based on the Modified Compression Field Theory (MCFT), suitably adapted to FRC materials, will be presented
Influence of the fiber reinforcement in concrete under direct shear
No full textShear behaviour in cracked concrete is still a matter of discussion into the scientific community. This paper reports on an experimental campaign which focused on the shear behaviour of pre-cracked concrete under direct shear. The push-off test with confined specimen is used to this aim. The influence of the granular skeleton (one mix with aggregate maximum size of20 mm, and the other with12 mm), concrete compressive strength (30 and 50 MPa), shear reinforcement (close stirrups, steel fibres or plain concrete) and the initial crack width (fixedin two levels: low (0 mm) and high (1mm)) are studied. Results show that the reinforcement transmits shear stress up to a slip of 0.4 mm (for uncon-fined specimens), and1 mm (for confined ones). After that, macro roughness and confinement force are the mechanisms which transmit shear stress
An application oriented state-of-art and research-need perspective on self-healing fibre-reinforced cementitious composites
No full textThe design of building structures and infrastructures is mainly based on four concepts:
safety, serviceability, durability and sustainability. The latter is becoming increasingly
relevant in the field of civil engineering. Reinforced concrete structures are subjected to
conditions that produce cracks which, if not repaired, can lead to a rapid deterioration and
would result in increasing maintenance costs to guarantee the anticipated level of
performance. Therefore, self-healing concrete can be very useful in any type of structure, as it
allows to control and repair cracks as soon as they to occur.
As a matter of fact, the synergy between fibre-reinforced cementitious composites and selfhealing
techniques may result in promising solutions. Fibres improve the self-healing process
due to their capacity to restrict crack widths and enable multiple crack formation. In
particular, cracks smaller than 30-50 μm are able to heal completely. Moreover, in the case of
High Performance Fibre Reinforced Cementitious Composites (HPFRCC), high content of
cementitious/pozzolanic materials and low water-binder ratios are likely to make the
composites naturally conducive to self-healing.
In this framework the main goal of this paper is twofold.
On the one hand, a state-of-the-art survey on self-healing of fibre-reinforced cementitious
composites will be provided. This will be analysed with the goal of providing a “healable
crack opening based” design concept which could pave the way for the incorporation of
healing concepts into design approaches for FRC and also conventional R/C structures.
On the other hand, the same state-of-the-art will be instrumental in identifying research
needs, which still have to be addressed for the proper use of self-healing fibre-reinforced
cementitious composites in the construction field
Shear database for reinforced and prestressed beams made with fiber reinforced concrete
No full textA total number of 215 structural elements were used to prepare a complete database to analyze the shear behavior and the influence of each parameter on shear out of 363 elements of the experimental database. 148 elements were eliminated for various reasons. Thus, the following items were removed: those with different failure modes to shear, those beams which are not known in some detail, also the beams containing a mixture of more than one fiber type, those for which values of strength are not available and all those elements with ratios a/d smaller than 2.5, where the arching action is dominant [1]. The database is made up of elements from databases of the University of Brescia and of RILEM, in addition to all the shear tests carried out within the Brite/Euram project [2], beams tested by Dupont & Vandewalle [3], other beams [4] and the tests presented in the Ph.D. thesis of Cuenca [5]. The input parameters used were: the shear span-to-depth ratio (a/d); the effective depth (d); the concrete cylinder compressive stress (fc); the residual flexural tensile strength (fR3) corresponding to a crack mouth opening displacement CMOD=2.5 mm, according to EN 14651 [6]; the longitudinal reinforcement ratio (ρl); the average stress acting on the concrete cross-section for an axial force due to prestressing actions (σc); the amount of steel fibers (kg/m3) and transverse reinforcement area per unit length (Asa/s). The output value was the safety margin (SM) obtained as Vtest/Vtheo (the shear test value divided by the shear theoretical value). The theoretical shear (Vtheo) was calculated for each of the beams according to three calculation codes: the Spanish Standard EHE-08 [7], the RILEM approach [8] and the first complete draft of Model Code 2010 [9]
Upgrading the push-off test to study the mechanisms of shear transfer in FRC elements
No full textIn this paper an upgrading of the push-off test in pre-cracked specimens is presented. The test is performed under conditions of crack control both in the pre-cracking and in the push-off stages. To this end, transversal forces to control the crack opening are introduced. Additionally, this confinement system avoids unnecessary movements that may introduce strains in the specimen. It is also included the methodology to perform the pre-cracking of the specimens prior to the push-off test. Specimens of 40 MPa of concrete compressive strength are used with two types of steel fibers type as well as with transverse reinforcement. It can be concluded that it is possible to perform the push-off test under crack control as it is shown with the results of crack width obtained with different measurement techniques. Different types of behavior after cracking can be detected such as hardening or the evolution of post-cracking residual stresses
Are steel fibres able to mitigate or eliminate size effect in shear?
No full textThis paper reports some recent results of an experimental campaign on fibre reinforced concrete (FRC) beams under shear loading tested at the University of Brescia: nine full scale beams, having a height varying from 500 to 1,500 mm, were tested for investigating the effect of steel fibres on key-parameters influencing the shear response of concrete members, with special emphasis on size effect. All tested members contained no conventional shear reinforcement and different amounts of steel fibres: 0, 0.64 or 1 % by volume. Results show that a relatively low volume fraction of fibres can significantly increase bearing capacity and ductility. The latter determines visible deflection and prior warning of impending collapse, which is not possible in plain concrete beams (without transverse reinforcement). The size effect issue is substantially limited. Results were compared against the shear formulation incorporated in the final draft of fib Model Code, which can be considered a valuable analytical model for FRC beams under shear loading, even in the case of the three deepest elements herein tested
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