# Durability of Fabric-Reinforced Cementitious Matrix (FRCM) Composites: A Review
## 4 Durability of Composite
### Durability of the Matrix-Fiber Interface Capacity
[34] observed a slight reduction (less than 20%) of the retained ultimate strength for FRCM including glass fibers and lime-based matrix. Figure 8: Effect of freeze–thaw cycles on the retained ultimate strength ratio of composite coupons. It should be noted that a significant scatter was observed among the results, which indicate the need to perform a large number of tests to obtain reliable results. [PERSON] et al. [21] investigated the effects of wetting in a saline solution (5 wt. % of NaCl) and drying at room temperature on the bond behavior between a cementitious matrix and a hybrid glass-carbon textile, using a pull-out set-up. Since also the effect of drying was considered, these test results are not reported in Figure 9. However, no significant difference in the maximum pull-out load was observed after 90 and 120 cycles. Nevertheless, a certain degradation was observed after 150 cycles due to the crystallization of the salt that affected the matrix-fiber interface. The effect of moisture and wet-dry cycles on the bond capacity can be controversial. For not entirely hydrated matrix, they can be beneficial due to the continuation of the hydration process [35; 36]. However, it should be noted that the continuation of the hydration process can cause a densification of the matrix next to the multi-filament yarn with the penetration of hydration products within the voids among the filaments, which may lead to a reduction of the matrix-fiber bond properties [26]. More studies are needed to identify the dominant effect among these two competing factors. For completely hydrated matrix, moisture and wet-dry cycles may not directly affect the bond behavior. However, by damaging the fibers (e.g., corrosion of steel cords or swelling of glass fibers [33; 43]) and the matrix due to the expansion of water within the pores, they can lead to a decrease of the matrix-fiber bond properties [35; 43]. The stress-transfer between the fiber and matrix can be also influenced by the alkalinity of the matrix, as discussed in Section 4.2. [PERSON] et al. [27; 47] utilized pull-out tests and uniaxial tensile tests to investigate the effect of the matrix alkalinity on the bond between cement-based matrices and AR-glass fibers. Specimens were aged in a humid chamber at 40 \({}^{\circ}\)C and 99% RH. One year of this condition was assumed to be equal to 50 years of exposure to the middle European climate. Three cementitious matrices with different level of alkalinity and hydration kinetics were employed. The low alkalinity matrix (M1) had a pH of 12.4 at the mixing time, which gradually reduced to 11.8 after 1 year. The pH of the high alkalinity matrix (M3) was 12.7. For the medium alkalinity matrix (M2), the pH value was between M1 and M3. The results of unconditioned specimens showed that the maximum pull-out force of specimens made with high alkalinity matrix (M3) was higher than the others due to the fast hydration process and homogeneous structure of calcium silicate hydrate (CSH) phase close to the fiber. However, an opposite trend was observed after ageing. Figure 10a shows the behavior of the maximum pull-out force ratio, which progressively decreased with proceeding the ageing period. The reduction was higher for specimens with matrix M2 than with matrix M1. Results of specimens with matrix M3 are not shown in Figure 10a because they failed at the formation of the first crack, without slippage of the fibers within the matrix. Figure 9: Effect of immersion in saline solutions on the retained ultimate strength ratio of fabric-reinforced cementitious matrix (FRCM) coupons. The retained ultimate strength ratio of corresponding FRCM coupons subjected to uniaxial tensile test is shown in Figure 10b. The behavior observed is consistent with that of pull-out tests, with the exception that the ultimate strength of the coupons made with matrix M1 increased after 30 days of ageing due to the continuation of the matrix hydration process.