# Durability of Fabric-Reinforced Cementitious Matrix (FRCM) Composites: A Review
## 4 Durability of Composite
### Durability of the Matrix-Fiber Interface Capacity  
The effectiveness of externally bonded reinforcements depends on the stress-transfer between its components and between the reinforcements and the substrate. When a single layer of fiber is employed, externally bonded FRCM composites have two main interfaces, i.e., the matrix-fiber interface and the matrix-substrate or composite-substrate interface [57]. Although the long-term bond behavior of the various interfaces of an inorganic-matrix composite should be studied with bond tests, some studies  
Figure 7: Retained tensile capacity ratio of AR-glass exposed to alkaline environments (blue bin = room temperature and orange bin = temperature equal to 60 \({}^{\circ}\)C). available in the literature derived information on the effect of various environmental factors on the matrix-fiber interface from uniaxial tensile tests of composite coupons [22; 28; 34]. In particular, the degradation of the matrix-fiber interface was assessed by direct observation using scanning electron microscopy (SEM) [26] or evaluated by studying the differences in the load response of control and conditioned specimens that can be explained by the matrix-fiber bond behavior. In this section, the effect of various environments on the matrix-fiber bond behavior, either directly studied with bond tests or derived by uniaxial tensile test results, is discussed, whereas the durability of the composite-substrate interface is discussed in Section 4.4. Details of the tests included in the figures below are provided in Table 1.  
Figure 8 presents the effect of freeze-thaw cycles on the ultimate strength of FRCM coupons comprising different types of textile. The range of cycles adopted by each research group was discussed in Section 3.1. [PERSON] et al. [22] investigated the effect of freeze-thaw cycles on uncracked specimens and on specimens cracked by applying a uniaxial tensile load 20% higher than the cracking load. It can be noticed that the retained ultimate strength of the uncracked specimens progressively decreases with increasing the number of cycles until it reaches 0.8 at the end of 500 cycles. However, cracked specimens showed fluctuating results and a clear trend could not be identified. [PERSON] [28] reported a slight increment (approximately 10%) in the ultimate strength of PBO-FRCM coupons, whereas the ultimate strength of carbon-FRCM coupons did not change significantly. [PERSON] et al. [14] reported a slight reduction (approximately 16%) in the case of a biaxial fabric. [PERSON] et al. [26] reported a small reduction (approximately 16%) in the ultimate strength of TRC coupons with AR-glass fibers after 100 cycles. Anyhow, freeze-thaw cycles had a slight effect on the coupon ultimate strength, being the variation always lower than 20%. [PERSON] et al. [21] utilized a pull-out test to investigate the combined effect of freeze-thaw cycles (\(-18\)\({}^{\circ}\)C to \(5\)\({}^{\circ}\)C with 3 h cycle duration) and saline environments (5 wt. % of NaCl) on the matrix-fiber bond capacity. The results obtained in [21] are not listed in Figure 8 since the authors considered the effect of saline solution and used a different test set-up. Similarly, [PERSON] et al. [21] reported not significant effect of the ageing condition. The effect of saline environments on the matrix-fiber bond behavior was investigated by immersing FRCM coupons in saline solutions (seawater or 3.5 wt. % of NaCl) and observing the variation of the ultimate strength obtained, which was attributed to the degradation of the matrix-fiber interface [28; 34]. Figure 9 shows the retained ultimate strength ratio obtained by FRCM coupons immersed in saline solutions for different periods. [PERSON] [28] reported an increase of the retained ultimate strength for FRCM including PBO and carbon fibers with cement-based matrix. This increment (larger than 40%) was attributed to the continuous hydration of the mortar, which may improve the matrix-fiber bond behavior leading to high ultimate strengths. [PERSON] et al.