The addition of steel fibers improved the hardened properties of the concrete mixes with RCAs to an extent that exceeded the original value of some properties of their NA-based counterparts. Experimental tests revealed that the addition of steel fibers in solid RC deep beams made with RCAs at vf of 1 and 2% restored 80 and 90%, respectively, of the shear capacity of a similar beam with minimum steel stirrups. The shear strength enhancement of the RCA-based deep beams with openings due to the inclusion of steel fibers was in the range of 39 to 84%, whereas the use of conventional steel stirrups resulted in a strength increase of 18%.
Results of the numerical parametric study indicated that the shear strength increase caused by the addition of steel fibers at ff of 1 and 2% was higher in the deep beam models with the lower shear span-to-depth ratio (a/h) of 0.8, relative to those of their peers with a/h of 1.6. The effect of a/h on the shear strength increase of the solid deep beam models decreased at the higher ff. She is the founder of the ACI Student Chapter at the United Arab Emirates University, UAE.
Introduction
Overview
This CDW is mostly disposed of in stockpiles or landfills, causing serious environmental hazards (Radonjanin et al., 2013). The use of RCAs in concrete mixes is accompanied by a decrease in its mechanical performance compared to its conventional counterpart (Debieb et al., 2010; Guo et al., 2020; Wagih et al., 2013). Results of steel fiber reinforced RCA concrete showed that the compressive strength, elastic modulus, tensile strength, toughness and impact resistance increased (Bencardino et al., 2008; Meddah & Bencheikh, 2009).
This is mainly due to the bridging effect of steel fibers that can reduce the initiation and propagation of cracks, leading to an increase in the energy absorption capacity of the material (Bencardino et al., 2010). The use of steel fiber reinforced recycled aggregate concrete in structural application is limited, even though significant potential exists for full or partial replacement of expensive, hand-placed, steel bar reinforcement. One of the reasons for the limited use of steel fiber reinforced recycled aggregate concrete in structural applications is the lack of standardized design procedures and information on properties of such a composite material.
Statement of the Problem
Insight into the Benefits of Recycled Aggregates
While the addition of steel fibers would result in a direct cost increase, there is a potential to reduce or eliminate the use of conventional shear reinforcement in large-scale RC beams, resulting in direct and indirect cost savings. Although the financial and environmental aspects are beyond the scope of the current study, Table 1.1 shows typical quantifiable measures that could assist practitioners and researchers in evaluating the environmental benefits and associated costs of RC structures made with RCAs and steel fibers. Table 1.1 clearly shows that the use of RCAs together with steel fibers to replace NAs and conventional steel stirrups results in a significant reduction in GWP.
However, the economic analysis should include other factors that would affect the cost benefit analysis, such as landfill space saving, construction time reduction, raising of reinforcing bars on upper floors, service life of the structure and other and indirect jobs. associated costs. The total cost savings in construction due to the use of steel fibers in replacing conventional steel reinforcing bars can be up to 30% (ACI Committee 544, 2015). Previous studies have also verified that the additional cost caused by the inclusion of steel fibers can be significantly countered by determining the quantity.
Research Objectives
Relevant Literature
- Characteristics of Concrete with Recycled Aggregates
- Shear Behavior of RC Beams
- Published Analytical Models
The tensile strength of concrete reinforced with steel fibers increased by 40 to 75% compared to the control mix. The drying shrinkage of concrete reinforced with steel fibers decreased when compared to the control mix without steel fibers. The compressive strength of RCA-based concrete containing steel fibers increased by 3 to 14% compared to their NA-based counterparts.
This section contains a review of the available literature on the shear behavior of RC beams made with RCAs and steel fibers. Beams with steel fibers showed a smaller crack width than the control beam without fibers. For the beams without steel fibers with an a/d of 0.7, the creation of an opening in the shear span reduced the shear strength by 33% compared to that of the solid beam.
For the beams without steel fibers with an a/d of 0.6, creating an opening in the shear span reduced the shear strength by 50% compared to that of the solid beam. The initial stiffness of steel fiber reinforced specimens was almost equal to that of the RC beams without steel fibers.
Research Significance
Numerical simulation models were created for large-scale deep beams made of RCA and steel fibers. A comparative analysis between numerical and experimental results was performed to verify the accuracy and validity of the deep beam simulation models.
General Discussion
Research Design
- Material Characterization
- Details of Deep Beams
- Numerical Simulation
Predictions from the numerical deep-beam models were validated through a comparative analysis with results obtained from laboratory tests. In the present study, test mixes were performed prior to pouring the beams to ensure adequate workability of the concrete and homogeneous distribution of the steel fibers in the concrete mix. Segregation, bleeding and agglomeration of steel fibers were not observed in any of the blends.
In the deep beam specimens of group N, two circular openings, one in each shear span, were placed symmetrically around the center of the beam to completely interrupt the natural load path as shown in Figure 2.6. The center of each opening was located at the mid-height of the beam (250 mm below the top surface) and at the center of the shear span (200 mm away from the center of the support). Based on testing of three replicate steel specimens, the average measured yield strength of the No.
The concrete cover at the center of the steel reinforcement was 50 mm, giving an effective depth (d) of 450 mm. The models used realistic tensile and compressive constitutive laws that account for the nonlinear behavior of the concrete material. Monitoring points were added to the FE models to obtain values of applied load, midspan deflection and steel strain.
This concrete material model allows the users to edit and modify key values of the constitutive law of concrete materials, such as the tensile softening behavior that describes the post-peak tensile strength of concrete, which is crucial in the case of steel fiber reinforced concrete. A linear relationship characterizes the compressive stress-strain relationship of the concrete material in the elastic phase with a slope equal to Ec. ATENA software recommends having a minimum of 4–6 elements in the shortest dimension of the member to ensure solution convergence while minimizing computation time (ATENA).
A displacement-controlled applied load was induced at the center of the upper steel plate at a rate of 0.1 mm per step.
Data Collection
- Sample Preparation
- Testing Procedure
- Tension Function of Concrete with Steel Fibers
- Deep Beam Tests
The post-peak behavior of steel fiber reinforced recycled aggregate concrete mixes was identified by performing inverse analysis of the load-deflection curves obtained from the bending tests on prisms using finite element software ATENA. The inverse analysis technique started by developing FE models for the tested prisms and setting specific tensile parameters in a user-defined tensile softening law of the concrete. The stress function of the concrete with ff of 2 and 3% showed a slightly reduced rate of degradation in the tensile stress after cracking relative to that of the concrete with ff of 1%.
The agreement between the predicted and experimental loading responses of the prisms confirmed the validity of the stress function developed from the inverse analysis. Two actuators 1300 mm apart were used to load the upper surface of the beams. For beams in Group S, four strain gauges (SGs) each 5 mm long were glued to the bottom layer of longitudinal steel reinforcing bars within the shear span at 200 mm spacing to measure the tensile strain of the steel.
In each shear span of the beams with steel stirrups, a 5-mm SG is installed on the vertical stirrup next to the opening. The locations of the SGs bonded to the tension bars and stirrups are illustrated in Figure 2.22(a). The SGs of the shear reinforcement were used to check stirrup yielding and investigate the effect of test parameters on the rate of increase of the stirrup deformation.
Concrete SGs are also attached to the concrete surface of the beams with stirrups, but not shown in Figure 2.21(a) for clarity. The SGs attached to the concrete surface were used to check the level of deformation in the concrete and to better understand the effect of test parameters on the behavior of the tested beams. The locations of the concrete SGs used in all deep beams with openings are shown in Figure 2.22(b).
Before performing the tests, the front surfaces of the girders were painted white to facilitate crack tracking.
Results
- Characterization of Concrete made with RCAs
- Shear Behavior of Solid RC Deep Beams made
- Shear Behavior of RC Deep Beams with
- Parametric Study and Discussion
The addition of steel fibers did not always increase the uniaxial tensile strength of the RCA-based specimens. It is clear that the influence of steel fiber incorporation is more dominant than the RCA replacement. The RCA specimens with steel fibers exhibited higher maximum longitudinal concrete stresses at the shear capacity than that of the control RCA-based specimen SR100-SF0.
The shear load-deflection curves of the solid AB deep beam models are shown in Figure 2.51. The addition of steel fibers greatly improved the shear capacity and stiffness of the RCA-based designs [Figure 2.51(b)]. The responses of deep beams with openings to shear loading are shown in Figure 2.57.
Such an increase is due to the improved tensile properties of steel fiber composites. The inclusion of steel fibers significantly increased the shear capacity of spandrel beams made with 100% RCA. Swaddiwudhipong (1988) tends to overestimate the shear capacity value of steel fiber beams by up to 17%.
The addition of steel fibers improved the stiffness and significantly increased the shear capacity of the deep beam models, as shown in Figure 61 (b). The shear load-deflection responses of the models without steel fibers predicted numerically are compared with those obtained experimentally in Figure 2.62. The numerical shear-deflection load responses of the steel fiber models are compared with those obtained from the tests in Figure 2.62.
The crack patterns of the deep beam models with openings predicted numerically are shown in Figure 2.63 (a). The shear capacity of the deep beam models with steel fibers increased almost linearly with an increase in the steel fiber volume fraction. It is clear that the shear capacity of the deep-beam models with a/h of 0.8 was higher than that of their counterparts with a/h of 1.6.
Conclusion and Future Perspectives
Overview
Conclusions Related to Material Characterization
Conclusions Related to Shear Behavior of
Conclusions Related to Numerical Modeling
Limitations
Future Research
