# Effects of Freeze-Thaw and Wet-Dry Cycles on Tension Stiffening Behavior of Reinforced RAC Elements
## 3 Results and Discussion  
### Effects of Freeze-Thaw and Wet-Dry Aging on Main Properties of Reinforced RAC Elements  
Figure 4a proposes a schematic representation of the typical load-strain curve for reinforced concrete element and the isolated matrix contribution during the tension stiffening test. The figure also highlights the main parameters (E\({}_{\text{pre}}\), E\({}_{\text{post}}\), F\({}_{\text{1 st}}\), F\({}_{\text{yield}}\), \(\epsilon_{\text{1 st}}\), \(\epsilon_{\text{final}}\), C\({}_{1}\), C\({}_{n}\), f\({}_{\text{m,1 st}}\) and f\({}_{\text{m,final}}\)) that will be considered for the analysis of the experimental results. The reinforced concrete elements subjected to tension stiffening tests behave in accordance with the well-known ACK model [44], characterized by the following four phases: (i) Pre-cracking elastic phase; (ii) Multiple crack formation; (iii) Stabilized crack phase (post-cracking); and (iv) Yielding of reinforcement. On the other hand, Figure 4b,c present the curves obtained for the reference natural mixtures, confirming the above-described trend. The curves plotted in Figure 4b,c show an improvement (higher loads for the same strain) in the behavior of non-degraded REF(FT300) in comparison with the companion REF(FT150), which is explained by the higher cement hydration degree of older REF(FT300) samples. As a general trend, all mixtures presented a reduction in the load-strain behavior after degradation processes as for the same strain level, but the non-degraded samples reached higher strength values than the degraded ones (see Figure 4b). The main representative parameters (and the corresponding coefficients of variation expressed in percentage) for the tested reinforced concrete elements subjected to freeze-thaw and wet-dry cycles are presented in Tables 3 and 4, respectively. Until the appearance of the first crack (phase I in Figure 4a), since the composite is in the elastic phase, the load-strain curve slope is governed by the pre-cracking elastic modulus of the composite (E\({}_{\text{pre}}\)): the results show that for unconditioned elements, the presence of RCAs did not significantly influence this property. For instance, the values obtained for REF(FT150) of mixtures C35-NAT, C35-L-C1 and C35-D-C1 are equal to 25.8, 26.1 and 25.5 GPa, respectively. On the other hand, all tested mixtures presented elastic modulus reduction when undergoing degradation cycles. The highest degradation impact in both cases (300 freeze-thaw and 50 wet-dry cycles) was observed in the mixtures with the highest amount of attached mortar (i.e., RCA_L_C1). This behavior is related with the higher values of concrete open porosity (see Table 2) that allows an easier flow of water into the concrete element. For example, the mixtures subjected to 150 freeze-thaw cycles (Table 3) suffered decreases in the elastic modulus in the range of 6% (from 5.4% to 6.2%) in relation to the non-degraded references. The damage was more pronounced when the samples were submitted to 300 cycles. The modulus reduction reached 16% for the C35-NAT mixture and 22% for the C35-L-C1 concrete. After wet-dry cycles, the elastic modulus of the tension stiffening elements submitted to 25 cycles was only slightly decreased (3.0% to 5.5% (see Table 4). After 50 cycles, the elastic modulus of C35-NAT and C35-L-C1 mixtures was reduced by about 10% and 16%, respectively. The multiple crack formation (phase II in Figure 4a) begins at the moment of the appearance of the first crack, and at that moment the first crack strength (\(\rm f_{1 st}\)) and strain (\(\rm\varepsilon_{1 st}\)) were determined. For the lower level of degradation (i.e., 150 freeze-thaw and 25 wet-dry cycles), the higher porosity of the RCAs does not seem to have affected the composite response. Conversely, for higher number of cycles, the presence of a more porous aggregate caused a slightly higher reduction in the first crack strength of the degraded elements.