All reinforced concrete elements must comply with the conditions established in the last limit states and service limit states. In SLS, particular attention must be paid to limiting the width of cracks. In the case of structures in highly aggressive environments or in liquid deposits, the width of the cracks is important in determining the amount of reinforcing steel required.
In concrete mixtures, reinforced with steel fibers in low or moderate volumes (0.4% – 1%), a notable improvement is observed in the ductility and in the cracking energy of the piece, but no influence on the structure resistance capacity is showed. Surely, conventional passive reinforcing steel cannot be replaced with fibers, but reinforcing the concrete matrix with moderate amounts of steel fibers produces more efficient control of cracking. The combination of reinforcing bars and fibers leads to a low crack distance and a greater resistance capacity of the concrete against cracking, which translates into smaller crack widths.
The present research work develops the experimental campaign carried out by the Structural Concrete Group of the UPM in the use of reinforced concrete reinforced with fibers in beams and columns of a conventional concrete (fck≈30 MPa), passive reinforcement of conventional reinforcement (B -500 S) and recycled fibers from used tires. The results of previous research works confirm that it is feasible to reinforce concrete with fibers, however, the nature of the fibers (heterogeneous, twisted, and difficult to control due to current recycling technologies) makes their use recommended only in small volumetric quantities in order to guarantee the workability of the concrete.
The experimental campaign includes tests on reinforced concrete elements using different contents of recycled and commercial fibers. The results are compared to analytical predictions to develop a model that determines crack widths taking into account the type of fiber used. The main objective of this research is the evaluation of the efficiency of a new type of fiber technology with advantages from the point of view of sustainability due to the use of recycled material.
The experimental program has the aim to evaluate, on one hand, the employment of recycled fibers as reinforcing elements in concrete (considering their influence on the workability of the mixture) and on the other hand, the effectiveness of steel fibers in controlling the crack thickness. For this reason, beams with zones of constant moment and reinforced with both steel fibers and passive steel bars were tested.
The structural campaign controls the following data:
• Applied load
• Deformations at the load application points and in the central span
• The deformation in the compression zone on both faces, corresponding to the location of the longitudinal reinforcement reinforcement
• The deformation in the tensile zone on both faces, corresponding to the location of the longitudinal reinforcement reinforcement
The following conclusions can be drawn from the present study:
• Recycled fibers from used tires provide results and properties similar to commercial fibers in flexural tensile tests
• The addition of fibers in reinforced concrete improves crack width control by increasing stress stiffening and reducing transfer length, both leading to smaller cracks
• The average width of cracks is reduced in each case that fibers were considered
• The models available in the technical literature and in the MC2010 describe the average crack spacing with sufficient precision
The tests were carried out in the Structural Laboratory at the ETSICCP of the UPM within the research program: Use of Concrete with Recycled Fibers of NFU in Integral Structures, sponsored by COMSA-EMTE, S.A. and partially financed by the Center for Industrial Technological Development (CDTI) part of the Spanish Ministry of Science and Technology, under project number IDI_2011480