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The need for retrofitting and strengthening of structures has increased dramatically in recent years. This is mainly to overcome deterioration in material performance resulting from aging of structures, material degradation, disaster related structural damage, and construction defects, or to enhance capacity of structures to withstand higher levels of loading. A range of new strengthening materials, such as ECC[1] (engineering cementitious composites), TRM[2] (textile reinforced mortar) or TRC[3] (textile reinforced concrete), FRP[4] (fiber reinforced polymer), SRG[5] (steel reinforced grout), have emerged over the years for retrofitting and strengthening of civil engineering structures. Among them, TRM or TRC is one of the most popular and innovative strengthening materials[6]. TRM or TRC comprises of a binder such as mortar or concrete and a strengthening textile like carbon fiber, basalt fiber or glass fiber. Due to the use of inorganic binder, TRM exhibits better performance under high temperature exposure than FRP, in which epoxy resin is commonly used as the binder. Thus TRM is an ideal strengthening binder, specifically in applications where fire safety is a critical concern[7]. In addition, low cost and better compatibility with concrete substrate also popularized the use of TRM strengthening[6] technique.

There have been considerable amounts of researches concerning the strengthening effect of TRM. Studies on flexural strengthening of RC beams have shown that TRM, as strengthening material, significantly enhances flexural capacity, as well as lowers deflection and crack widths in beams[8], [9], [10]. It is widely accepted that beams strengthened with more TRM layers have larger bearing capacity increment. Ombres[10] reported that the bearing capacity of RC beams strengthened with one to three layers of TRM increased 16%, 33% and 40% respectively, compared to an un-strengthened RC beam. However, increasing the number of strengthening layers may change the failure mode of TRM flexural strengthened beams. Ebead et al.[11] demonstrated that FRCM (fibric-reinforced cementitious matrix) strengthened beams failed by fabric slippage when using one or two layers of strengthening, but failed through delamination of layers when three layers of strengthening was adopted. The same phenomenon was also observed in Ref. [9], [10], [12], [13]. Generally, only strengthening layer debonding was found in FRP strengthening. D’Ambrisi et al.[12], who investigated the failure modes of TRM and FRP strengthened beams, concluded that fiber debonding of TRM strengthening involves the fiber/ matrix interface but does not involve the concrete as happened in the case of FRP strengthened beams. Compared with FRP strengthening, TRM strengthening has inferior strengthening effect in terms of load bearing capacity increment. Raoof et al.[13] investigated the strengthening effect of TRM and FRP, indicating that the strengthening effect of TRM was only 46% of that of FRP when one-layer of strengthening was used, but the effectiveness ratio increased as the number of strengthening layers increased from one to three. Elsanadedy et al.[14] also reported that FRP provides better strengthening effect than TRM, but further concluded that the ductility of TRM flexural strengthened beams were superior to that of FRP strengthening.

In comparison with RC beams, RC slabs have larger area of bottom surfaces and are usually stressed biaxially, and therefore the use of bi-directional textiles is more suitable for strengthening of slabs. Schladitz et al.[15] strengthened slabs with TRC and found significant enhancement in load bearing capacity, along with decrease of deflection in slabs. Koutas and Bournas[16] also found that TRM strengthening can increase the pre-cracking and post-cracking stiffness, as well as the cracking load, in two-way slabs.

There are also studies concerning the finite element analysis and flexural capacity calculation of TRM flexural strengthened members. Elsanadedy et al.[14] simulated TRM strengthened RC beams with LS-DYNA software, and defined the interaction between TRM strengthening layer and concrete substrate using surface to surface contacts. Portal et al.[17] modelled TRC strengthened slabs with DIANA software and defined the interaction between textile fiber and mortar by bond-slip relation. Similarly, Schladitz et al.[15] inserted interfaces between TRM strengthening layer and strengthened members to get more accurate results. A review of previous numerical studies clearly shows that, it is crucial to accurately define the bond-slip relation between strengthening fibers and binding mortar to correctly simulate the behavior of TRM strengthening.

As for the flexural capacity calculation of TRM/TRC strengthened members, Schladitz et al.[15] presented a simplified calculation model for TRC strengthened slabs with epoxy resin coated textiles and with failure of fiber rupture. Raoof et al.[13] modified the calculation model of FRP strengthening in FIB Model Code 2010 to obtain calculation model for TRM strengthened members, which was only suitable for strengthened members with failure of strengthening layer debonding. There are still limited models for bearing capacity calculation of TRM strengthened members, and the existing models are usually restricted to be used under specific conditions, thus, a universal model for TRM strengthening is in great need.

Currently, the most used binder in the existing TRM strengthening is high performance cement mortar or polymer-modified cement mortar. Elsanadedy[14] investigated the strengthening effect of ordinary cement mortar and polymer-modified cement mortar, revealing that polymer-modified cement mortar had higher bond strength between textile and concrete substrate than ordinary cement mortar. However, even though polymer-modified cement mortar can improve cohesion between strengthening layers and concrete substrate, further studies also revealed that bond of the mortar degraded severely when exposed to fire. Hu[18], [19] investigated the performance of TRM strengthened slabs under fire exposure, using polymer-modified cement mortar as the binder, and reported that the strengthening layer fell off in 30 min in fire and cannot meet the required fire resistance needed in most applications. Therefore, it is necessary to propose substitute with high bond strength and good fire resistance for the cement-based repair mortar.

In the past few decades, geopolymer has gained significant attention for its potential to replace cement because of its environmentally friendliness and similar or even better properties as compared to cement[20]. Geopolymer is synthesized by mixing alkali solution and aluminosilicate source material, including industrial waste like fly ash and slag, or agricultural byproduct like rice hush ash, and even metakaolin calcined at lower temperature[21], [22], [23], [24]^. A great number of studies have been conducted to investigate the properties of geopolymer. It is found that geopolymer has higher early strength[25], [26] and bond strength[27], [28], better durability[29], [30] as well as high temperature performance[31], [32] than ordinary Poland cement. Laskar et al.[33] studied the bond strength of newly-developed ultra-fine blast furnace slag based geopolymer, Zanotti et al.[34] investigated the effect of curing pattern and PVA fiber reinforcement on the bond strength between concrete substrate and metakaolin-based geopolymer, both indicating that geopolymer had strong potential to be used as concrete repairing agent because of high early strength and bond strength. Pacheco-Torgal[35] also investigated the bond strength between OPC concrete and tungsten mine waste (TMWM) geopolymeric binder, demonstrating that the TMWM geopolymeric binders possessed higher bond strength than the existing cement based commercial repair product. Geopolymer have high bond strength not only at ambient temperature, but also at elevated temperatures. Zhang et al.[36] investigated the bond strength of metakaolin-fly ash based geopolymer, and compared it with ordinary Portland cement mortar and polymer-modified repair mortar, reporting that the bond strength of geopolymer mortar is close to or even higher than that of polymer-modified repair mortar at ambient temperature and after exposure to elevated temperature. Due to good mechanical properties, especially high bond strength at ambient and elevated temperature, geopolymer mortar is a promising binder material to replace cement mortar for structural strengthening.

For the application of geopolymer on structure strengthening, there are researches investigating the feasibility and effectiveness of geopolymer binder on carbon fiber sheets strengthening[37], [38]. These studies indicate that the flexural capacity of beams strengthened with carbon fiber sheets using geopolymer is comparable to that using organic binder. However, limited studies are undertaken concerning the strengthening effect of geopolymer mortar in TRM strengthening. The authors previously investigated the shear strengthening effect of TRGM in RC beams, reporting that TRGM strengthening material can effectively enhance the shear capacity of RC beams[39], but the flexural strengthening effect and mechanism of TRGM were left unknown. Besides, very limited numerical studies were undertaken to evaluate the effect of TRGM strengthening on RC slabs.

Considering that the large surface area of slabs is more suitable for textile reinforcing, geopolymer mortar was adopted as the binder in TRM flexural strengthening of slabs in this research. The flexural behavior of five slabs strengthened with textile reinforced geopolymer mortar (TRGM) was investigated under static loading, and the critical parameters such as the number of textile layer, lateral textile spacing, presence of anchorage were varied. Additionally, two slabs strengthened with textile reinforced polymer-modified cement mortar (TRPM) and one un-strengthened slab were also tested for comparison. Since the goal of the experiment in this paper is to investigate the influence of the above-mentioned parameters on the strengthening effect of TRGM, one-way slabs are used to simplify the test apparatus. Following experimental study, numerical simulations were carried out on TRGM strengthened slabs, with different slab thickness, reinforcement ratio and number of TRGM strengthening layer. Finally, calculation formulae for predicting the flexural capacity of TRGM strengthened slabs is proposed, which can be applied for the design of TRGM strengthened slabs.