49 research outputs found
Bond of Reinforcing Bars to Steel Fiber Reinforced Concrete (SFRC)
The use of steel fiber reinforced concrete (SFRC hereafter) is becoming more and more
common. Building codes and recommendations are gradually including the positive effect of
fibers on mechanical properties of concrete. How to take advantage of the higher ductility
and energy absorption capacity of SFRC to reduce anchorage lengths when using fibers is
not a straightforward issue.
Fibers improve bond performance because they confine reinforcement (playing a similar
role to that of transverse reinforcement). Their impact on bond performance of concrete is
really important in terms of toughness/ductility.
The study of previous literature has revealed important points of ongoing discussion
regarding different issues, especially the following: a) whether the effect of fibers on bond
strength is negligible or not, b) whether the effect of fibers on bond strength is dependent
on any other factors such as concrete compressive strength or concrete cover, c)
quantifying the effect of fibers on the ductility of bond failure (bond toughness). These
issues have defined the objectives of this thesis.
A modified version of the Pull Out Test (POT hereafter) has been selected as the most
appropriate test for the purposes of this research. The effect of a number of factors on bond
stress¿slip curves has been analyzed. The factors considered are: concrete compressive
strength (between 30 MPa and 50 MPa), rebar diameter (between 8 mm and 20 mm),
concrete cover (between 30 mm and 5 times rebar diameter), fiber content (up to 70
kg/m3), and fiber slenderness and length.
The experimental program has been designed relying on the principles of statistical Design
Of Experiments. This has allowed to select a reduced number of combinations to be tested
without any bias or loss of accuracy. A total of 81 POT specimens have been produced and
tested.
An accurate model for predicting the mode of bond failure has been developed. It relates
splitting probability to the factors considered. It has been proved that increasing fiber
content restrains the risk of splitting failure. The favorable effect of fibers when preventing
splitting failures has been revealed to be more important for higher concrete compressive
strength values. Higher compressive strength values require higher concrete
cover/diameter ratios for splitting failure to be prevented. Fiber slenderness and fiber
length modify the effect of fiber content on splitting probability and therefore on minimum
cover/diameter ratios required to prevent splitting failures. Two charts have been
developed for estimating the minimum cover/ diameter ratio required to prevent splitting.
Predictive equations have been obtained for estimating bond strength and areas under the
bond stress¿slip curve as a function of the factors considered. Increasing fiber content has a
slightly positive impact on bond strength, which is mainly determined by concrete
compressive strength. On the contrary, fibers have a very important effect on the ductility of
bond failure, just as well as concrete cover, as long as no splitting occurs.
Multivariate analysis has proved that bond stress corresponding to the onset of slippage
behaves independently from the rest of the bond stress¿slip curve. The effect of fibers and
concrete compressive strength on bond stress values corresponding to the onset of slips is
mainly attributable to their influence on the material mechanical properties. On the
contrary, the effect of fibers and concrete cover on the rest of the bond stress¿slip curve is
due to their structural role.García Taengua, EJ. (2013). Bond of Reinforcing Bars to Steel Fiber Reinforced Concrete (SFRC) [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/32952TESI
Bond of reinforcing bars to steel fiber reinforced concrete
Steel fiber reinforced concrete (SFRC) has been increasingly used during recent years. Regarding bond of rebars to concrete, fibers provide passive confinement and improve bond capacity in terms of bond strength and, more importantly, toughness. An extensive experimental programme has been carried out, and SFRC specimens with embedded rebars have been subjected to the Pull Out Test to obtain the bond stress slip curves, retaining the bond strength and the area under the curve as measures of the bond capacity of concrete. The following parameters were considered: concrete compressive strength (30-50 MPa), rebar diameter (8-20 mm), concrete cover (between 30 mm and 5 times rebar diameter), fiber content (up to 70 kg/m(3)), and the slenderness and length of the steel fibers used. Predictive equations have been obtained to relate the experimental results to the factors considered, and the trends observed have been analyzed and discussed. (C) 2015 Elsevier Ltd. All rights reserved.The authors of this work wish to thank both the Research Bureau of the Spanish Ministry of Science and Innovation for the funding of the projects 'BIA 2009-12722' and 'BIA 2012-35776-AR', this research being a part thereof.García Taengua, EJ.; Martí Vargas, JR.; Serna Ros, P. (2016). Bond of reinforcing bars to steel fiber reinforced concrete. Construction and Building Materials. 105:275-284. https://doi.org/10.1016/j.conbuildmat.2015.12.044S27528410
Influence of Fibre Reinforcement on the Long-Term Behaviour of Cracked Concrete
The influence of fibre reinforcement on the long-term behaviour of cracked concrete is analysed in this work by means of a creep test. Nine concrete mixes were prepared (7 SFRCs and 2 conventional RCs) based on two basic mix designs. Concretes type I were conceived for structural precast applications and concretes type II reproduce a general purpose. Fibre dosages and conventional reinforcements were varied to represent a wide spectrum of post-peak flexural responses. In all cases with fibre reinforcement steel fibres were used. Conventional RC specimens were reinforced with two steel rebars. In addition to the variables of mix design of concrete, there are two significant variables related to the creep test: the pre-crack opening level (CMODpn) and the stress level (Ic) sustained during the test. Creep tests were performed by applying a constant flexural load on notched pre-cracked specimens and controlling crack opening evolution. Some of the specimens developed a sudden increase of crack opening deformations during the creep test. Creep coefficients and Crack Opening Rates were calculated and analysed. Creep coefficients show significant dependence on the analysed variables. The results of this experimental campaign show that creep of SFRC specimens may be similar to a traditional RC
Effect of nanosilica addition on the fresh properties and shrinkage of mortars with fly ash and superplasticizer
The ongoing use of various mineral additions along with chemical admixtures such as superplasticizers justifies the need for further research. Understanding and quantifying their effects and possible synergies on the fresh and hardened properties of cement-based materials is necessary, especially if some of these components are known to have a pozzolanic effect. This paper describes and models the fresh and hardened properties of cement mortars including nanosilica and fly ash, and relates their properties to the proportioning of these materials and the superplasticizer dosage. Mini-slump, Marsh cone and Lombardi cone tests were used to examine the properties of the fresh mortars, and to assess density, plastic shrinkage, and drying shrinkage up to 20 days. The equations presented in this paper make it possible to optimize mortar proportionings to the required levels of performance in both fresh and hardened states
Influence of paste thickness on the coated aggregates on properties of high-density sulphoaluminate cement concrete
An improved method for the densified mixture design algorithm and Fuller curve were used to design high-density sulphoaluminate cement concrete (HDSC). The performance of HDSC is significantly influenced by the paste thickness on the coated aggregates. Sulphoaluminate cement concrete mixtures containing aggregates coated with 3 different paste thickness of t=10μm, 20μm, and 30μm and water-binder ratios (W/B) of 0.25, 0.30 and 0.35 were prepared. The results of experiments show that paste thickness on the coated aggregates significantly influences the mechanical properties and durability of HDSC. With the increase of paste thickness, the compressive strength is increased, but the electrical resistivity is decreased, particularly at the early ages of 1 and 3 days. The sulfate corrosion resistance coefficients of HDSC are larger than 1.0, the total porosity can be less than 7%, and the micropore (i.e. with pore size less than 20nm) can be larger than 70%
Consecuencias del uso de distintos áridos reciclados sobre la resistencia a compresión simple del hormigón y su variabilidad
Se presentan los resultados de resistencia a compresión de hormigones producidos con cinco áridos reciclados distintos. Se analizan e interpretan los efectos que la composición y propiedades del árido tienen sobre la resistencia media del hormigón y su variabilidad. Se llega a un modelo claramente interpretable mecánicamente.García Taengua, EJ. (2011). Consecuencias del uso de distintos áridos reciclados sobre la resistencia a compresión simple del hormigón y su variabilidad. http://hdl.handle.net/10251/15379Archivo delegad
Bond of rebars to steel fibre reinforced concrete: Minimum concrete cover requirements to prevent splitting
Statistical Approach to Effect of Factors Involved in Bond Performance of Steel Fiber-Reinforced Concrete
[EN] The positive effect of fibers on the bond of reinforcing bars in
concrete is widely recognized. Different authors, however, come to
different conclusions regarding particular points.
This research analyzes the results of a series of pullout tests to
obtain statistically supported conclusions regarding the bond
performance of normal-strength steel fiber-reinforced concrete
(SFRC). To do so, the experimental program was conceived by
observing statistical criteria (design of experiments [DOE]
technique), and the results were studied using the analysis of
variance (ANOVA).
It has been shown that the role fibers play in the bond of
reinforcing bars in concrete is of the same importance as that of
concrete cover or reinforcing bar diameter. It is especially
remarkable that the mere fact of adding fibers¿regardless of the
amount¿considerably increases the ductility of the bond failure,
thus underlining the role of fibers in bond performance as
passive confinement.García Taengua, EJ.; Martí Vargas, JR.; Serna Ros, P. (2011). Statistical Approach to Effect of Factors Involved in Bond Performance of Steel Fiber-Reinforced Concrete. Structural Journal. 108(4):461-468. http://hdl.handle.net/10251/30811S461468108