## 4 Discussion
In this case, the maximum textile axial stress decreases slightly (by 14%) for specimens subjected to 120 \({}^{\circ}\)C (with single-layer TRMs) and by 50% and 36% for specimens subjected to 200 \({}^{\circ}\)C with single-layer and double-layer overlays, respectively, in comparison to the maximum textile axial stress of the reference specimens. An increase of mortar thickness (to accommodate higher fiber volume fractions, i.e., more textile layers) seems to be beneficial in terms of residual bond capacity. In TRLM overlays, the maximum textile axial stress decrease for increasing exposure temperature is mainly attributed to the cracking that this exposure caused. Despite the fact that these cracks did not increase in number or in width during shear bond testing, it is believed that they comprised textile-to-matrix bond breaker points. Bond damage could also occur as the combined result of: (i) early-age swelling and (ii) stress buildup during moisture evaporation by heating. The latter should theoretically be minimal in lightweight mortars with highly porous aggregates. However, according to [PERSON] and [PERSON] [20], lightweight cementitious matrices with a dense cement paste (like the one used herein) exhibit low vapor permeability and, hence, moisture transport results in the development of high internal stresses. The latter is expected to mainly affect the bond between the textile and the top (atmosphere-exposed) mortar layer. In double layer systems, the stress-relieving effect of higher fibers' content is combined with the preservation of the bond quality of the bottom textile layer with the surrounding (unexposed) matrix. The plot of the textile relative displacement corresponding to the maximum textile axial stress, \(d_{r,max}\), versus the target furnace temperature is presented in Figure 7b for each TRM configuration. These displacement values are less reliable in comparison to the stress ones due to their (inherently) higher CoV. Therefore, the corresponding data trends are only qualitatively commented. Average \(d_{r,max}\) values: (i) depend more on the number of textile layers than on the type of matrix used and (ii) increase with increasing exposure temperature for single-layer configurations (for which \(d_{r}\) measurement is more straightforward compared to double-layer ones). The relative displacement is the result of two synergistic phenomena: the elongation and the slippage of the longitudinal yarns' fibers in the matrix (see [PERSON] and [PERSON] [5]). In the case of single-layer TRNM overlays, the increase of \(d_{r}\) is attributed mainly to the increase of the textile slippage from within the normal weight mortar with increasing exposure temperature (see slip values recorded by DDG at the TRNM overlays' free ends in Figure 5c). In the case of TRLM overlays, the textile-to-matrix bond is more severely damaged by volume change phenomena (especially those heat-induced), which lead also to sleeve fibers' rupture within the matrix (hence, the lower slip values compared to single-layer TRNMs in Figure 7c). Average \(d_{r,max}\) values of double-layer TRM overlays are higher than the respective values of single layer ones. This is in agreement with observations done by [PERSON] and [PERSON] [6] who also provide the relevant reasoning. The response (textile axial stress versus relative displacement curves) of representative specimens of both TRM systems (TRNM and TRLM) is given in Figure 8. Most curves can be approximated as bilinear up to failure. In the first branch, both components (textile and mortar) behave in an elastic composite manner. The second branch extends between the change of the curve's inclination and the maximum load where the bond between fiber yarns and matrix is gradually deteriorated along a part of the bond length. It is highlighted that failure manifests the commencement of the load-aligned yarns' debonding from the matrix in combination with their sleeve fibers' progressive rupture.