# Experimental Study on Shear Strengthening of RC Beams with an FRP Grid-PCM Reinforcement Layer
## 1 Introduction  
Over the last two decades, there has been a great demand for the strengthening or retrofitting of existing reinforced concrete (RC) structures due to ageing, the increment of the service load, the fact that they were designed using former guidelines, or even due to earthquakes. For example, a large number of concrete structures designed and constructed according to former standards had collapsed or were damaged in the Great East Japan Earthquake in 2011. At that time, a shuice with an I-shaped profile (Figure 1), placed across a river embankment to drain wastewater and pump water from the river, was damaged due to the occurrence of uneven settlement. Moreover, it has been found that the steel reinforcement ratio of existing concrete structures designed using the former standard was inadequate. For this reason, a certain strengthening method is required to make sure the capacity of shuice is sufficient in shear. If using reinforced concrete to enlarge the cross section, the inhibition rate of the cross section becomes too large and a thicker concrete covering should be applied to prevent corrosion of steel bars. As a result, this method may have an adverse impact on the normal use of shuice, so another method should be used to instead of enlargement of the cross section. Some strengthening and retrofitting materials have been developed for existing concrete structures in the last decade, including textile reinforced mortar (TRM) [1; 2; 3; 4; 5; 6], fiber reinforced cementitious matrix (FRCM) [7; 8; 9; 10; 11; 12], textile reinforced concrete (TRC) [13; 14; 15; 16], steel-reinforced growth (SRG) [17; 18], and fiber reinforced polymer (FRP) composites [19; 20; 21; 22; 23; 24; 25]. Among them, fiber reinforced polymer (FRP) composites, including FRP sheets, FRP plates, FPR grids, and so forth, have been demonstrated to be an effective solution due to their favorable and prominent properties (e.g., light weight, high tensile strength, excellent corrosion resistance, and durability in harsh environments) [26; 27]. FRP grids (Figure 2), a kind of FRP composite that has been proposed in the past decade, are made of high-strength fibers such as carbon fibers impregnated with a suitable resin system to form a grid pattern, and the main difference between FRP grids and FRP sheets/plates include the production process, cross section, and stiffness. Currently, numerous experimental studies have been performed on RC beams strengthened with externally bonded FRP sheets/plates, but those were always conducted together with epoxy-based materials used as bonding agent. Thus, the strengthening effect in flexural or in shear largely depends on these organic systems (usually an epoxy system) at the bonding interface between FRP sheets/plates and the concrete substrate. However, the epoxy-based materials have their own weaknesses, which primarily include the poor fire resistance, easy degradation under strong UV radiation, and low efficiency in both over-high/low temperature and moisture environments [28; 29; 30]. These disadvantages have partially compromised the attractive properties of strengthening techniques mentioned before in some engineering applications. For these reasons, the bonding behavior at the interface may deteriorate rapidly if existing concrete structures lie in harsh environments, such as over-high/low temperature, in the presence of moisture, or even near fire and underwater. Therefore, some scholars have suggested replacing the epoxy systems with some inorganic cementitious materials to develop two major strengthening systems which combine FRP composites and cement-based materials [31], namely FRP sheets/plates bonded with a cementitious material [32] and FRP grids bonded with cement mortar [33; 34]. Compared to the former strengthening method,  
Figure 1: Sluice. Figure 2: Carbon fiber reinforced polymer (CFRP) grid and schematic illustration of the strengthening method.