Development of Flexible Link Slabs using Ductile Fiber Reinforced Concrete

Civil engineering structures with large dimensions, such as multi-span bridges, overpasses and viaducts, are typically equipped with mechanical expansion joints. These joints allow the individual spans of the structure to undergo unrestrained deformations due to thermal expansions and load-induced deflections of adjacent spans.

Mechanical expansion joints, commonly used between simply supported spans in bridge structures, tend to deteriorate and require significant maintenance activities. Deterioration is often caused by ingress of water and chlorides into the joints, which leads to corrosion of the joint and spalling of the surrounding concrete, and more importantly to corrosion of the bridge substructure including girders and bearings.

Deterioration and the resulting rehabilitation and maintenance needs of such structures constitute a significant infrastructure deficiency. In this study, it is suggested to replace the mechanical expansion joint and implement a flexible, precast ductile concrete link slab element between simply supported bridge spans.

To design and analyze the suggested link slab element, each constituent of the element, i.e. the structural reinforcement and the cementitious composite material, as well as their interaction is investigated and characterized in detail. These characteristics are especially important as all aspects of the composite behavior of reinforcement and surrounding cementitious matrix are governed by the material properties of the constituents and their interfacial bond characteristics.

Research presented in this thesis focuses on four main aspects of the composite tensile behavior, including: i) the material characterization of the cementitious composites and reinforcement, ii) the interface between the reinforcement and surrounding matrix, iii) the load-deformation response and crack development of representative sections of the reinforced composites, and iv) detailing, designing and testing of large scale prefabricated link slab elements.

In addition, an application of ductile Engineered Cementitious Composite (ECC) in prefabricated floor panels is presented in this thesis as an example of the versatility of the material and the applicability of the mechanisms investigated in the main part of this study. The conventional combination of ordinary brittle concrete and elastic plastic steel reinforcement in tensile loading has in past research shown the typical localized cracking and debonding behavior at the steel rebar-concrete interface.

In this thesis, focus is on combining relatively soft, elastic FRP reinforcement with ductile concrete, which is contrary to the conventional combination of matrix and reinforcement materials in structural engineering. The mechanical response and detailed deformation characteristics of FRP reinforced ECC are the focus of research described in this thesis.

In chapter 2 on material characterization, the material properties of the cementitious composites are characterizedin terms of their elastic modulus and compression strength, as well as the first tensile crackstress and strain. For ductile cementitious composites with multiple cracking such as ECC, the tensile stress–strain behavior is of particular interest as it illustrates the pseudo strain-hardening ability of ECC.

By utilizing Digital Image Correlation (DIC), accurate crack widths and crack spacing measurements are obtained, which can characterize the tensile behavior of ECC. In chapter 3 on interfacial bond, the bond slip behavior and crack development, between the reinforcement and surrounding cementitious matrix is investigated in a unique test setup with special emphasis oncrack formation and development at the rebar-matrix interface during direct tensile loading.

Utilizing a high definition DIC technique in a novel approach, detailed measurements of the crack formation and debonding process are obtained. It is found that ductile ECC, in contrast to conventional brittle concrete, reduces interfacial debonding significantly, resulting in a more uniform load transfer between the reinforcement and surrounding matrix.

In chapter 4 on the tension stiffening process, the load-deformation response and crack development of reinforced prisms, the tension stiffening and tension strengthening effects are addressed in particular. The tensile load-deformation response of reinforced concrete members is typically characterized by the tension stiffening effect, which relates to the degree of rebar-matrix interface degradation.

The comparison of strain hardening ECC with conventional brittle concrete showed a tension strengthening effect in addition to the tension stiffening effect. This conceptually new tension strengthening effect is a direct result of the ability of ECC to maintain or increase its load carrying contribution in the post-cracking deformation regime.

As a result of the multiple cracking ability, limited crack widths and additional load carrying ability of ECC, deformation compatibility between reinforcement and cementitious matrix is established as an important interfacial bond mechanism to maintain structural integrity at relatively large deformations and under cyclic loading.

The findings from the investigations on bond-slip, tension stiffening and tension strengthening are used in Chapter 5 as inputfor the design and analysis of the loaddeformation response and the crack development of a prefabricated flexible link slab elements potentially connecting two adjacent bridge deck segments.

The link slab element, composed of GFRP reinforced ECC,exhibited the same tension stiffening and tension strengthening behavior with limited crack widths as was observed in the reinforced prisms under monotonic and cyclic loading. The combination of ductile ECC and low stiffness GFRP resulted in the highly flexible link slab, capable of facilitating relatively large deformations, contrary to the heavily reinforced and stiff continuous link slabs implemented in the field.

In addition, the structural details of the suggested link slab concept, including a de-bonded active middle section and passive load transfer zones at each end, concentrated the induceddeformations in the active part of the link slab as intended. The uniform deformations with limited crack widths during both monotonic and cyclic loading indicate promising results for the concept, which will be implemented in a future full-scale field demonstration.