# Working State of ECC Link Slabs Used in Continuous Bridge Decks
## 5 Characterization of the ECC Link Slabs' Working State
### The GSED-Based Investigation into the Working States of ECC Link Slabs
The CFRP bar reinforcement condition raises the yielding load of ECC link slab by 166.67%. Although Type I and Type II ECC link slabs yield to the same magnitude of load, the \(E_{j,\text{norm}}\) of the latter is relatively slow to reach M2. Compared with Type I ECC link slab, ECC link slab strengthened by CFT is observed to produce an extended elastic-plastic period. The structural toughness index \(T\) is proposed herein to quantify the energy dissipation capacity of ECC link slab working in the elastic-plastic stage, which is formulated as following: \[T=\frac{S}{\Delta E/E_{\text{M1,norm}}}=\frac{S}{(E_{\text{M2,norm}}-E_{\text{ M1,norm}})/E_{\text{M1,norm}}},\] (13) where \(E_{\text{M1,norm}}\) and \(E_{\text{M2,norm}}\) are the GSED sum at the loading steps corresponding to M1 and M2, respectively; \(S\) is the span of elastic-plastic stage (the difference in loading steps between M1 and M2). The results are illustrated in Figure 10 and it is evident that Type II ECC link slab exhibits superior structural toughness to Type I ECC link slab. The structural toughness index is raised by 78.79% owing to the CFT grid reinforcement, which could be attributed to the enhanced deformation capacity and cracking resistance. Figure 9: \(E_{j,\text{norm}}\)-\(L_{j}\) curves for the three types of ECC link slabs. 3. The degrading stage. Despite the presence of plastic behavior in some parts of link slabs, the whole structures are able to bear increasing loads until M2. The \(E_{j,\text{norm}}\) of Type I and Type II ECC link slabs begin to increase rapidly and sharply at M2 as shown in Figure 9, implying that the whole structures qualitatively leap to a new stage different from the previous ones. The number of cracks remained unchanged while the crack width and extent of cracking kept increasing [26]. It demonstrates that the ECC materials did not exhibit strain hardening characteristics and multiple cracking behavior any more. Moreover, forces cannot effectively be transferred between ECC and CFT grid after M2. The loss in normal properties of materials leads to the degrading structural performance within this stage. Therefore, the load corresponding to the critical point M2 can be defined as the structural failure load of the link slab. Actually, this updated failure is the initiation of structural performance deterioration (i.e., the initial structural failure), which is different from the existing structural ultimate failure. From this critical point on, the interfacial bonding between components and structural integrity start to be gradually impaired. This updated concept of failure is reliable as it is revealed by delving into the structural working state characteristics complying with the natural course from quantitative change to qualitative change. It is noted that the critical point M2 is not observed for Type III ECC link slab based on the existing test data. In other words, the degrading stage discussed herein is not applicable to Type III ECC link slab. Both the high flexural stiffness of CFRP bars and their intact bonding with ECC materials help to maintain the desired load capacity and ductility after M1 [26]. The development of local plastic deformation in Type III ECC link slab was far from enough to affect the working behavior of the whole structure, hence the initial failure point has not been reached in this investigation. Provided that the time-varying test data is complete, the initial structural failure load can be accurately predicted for Type III ECC links slab.