## 3 Results  
### Relevant Results, Failure Modes and Classes  
Table 6 presents the relevant results obtained on the seven tested manhole specimens, namely, maximum load (\(P_{max}\)), centre deflection at this maximum load (\(u_{P_{max}}\)), and initial tangent stiffness (\(K_{Ti}\)). According to BS EN 124: 1994, the class of the manhole cover depends not only on its maximum load-carrying capacity (\(P_{max}\)) but also on the experimentally registered permanent deflection (\(u_{pd}^{exp}\)) over the admissible permanent deflection (\(u_{pd}^{adm}\)). The \(u_{pd}^{adm}\) is equal to \(1/L\cdot CO\), where \(CO\) is the clear opening of the specimen's support (600 mm, Figure 5a), while \(L\) is 100 for the manholes of class A15 and B125, and \(L=300\) for the other classes. To determine \(u_{pd}^{exp}\), to the displacement at \(F_{2/3 Cl}=2/3\cdot F_{Cl}\) in the force-deflection relationship registered experimentally (\(u_{2/3 Cl}^{exp}\)) is subtracted the recuperated elastic deformation (\(u_{el}^{exp}=F_{2/3 Cl}/K_{Ti}\)) due to the unloaded phase: \(u_{pd}^{exp}=u_{2/3 Cl}^{exp}-u_{el}^{exp}\)  
Figure 8: Typical damages observed on the GFRP gratings after their testing. Figure 9: Modelling the GFRP gratings to obtain the modulus of elasticity of their GFRP by inverse analysis: (**a**) 4 was simulated by taking advantage of the double symmetry of the structure, (**b**) support and boundary conditions, and (**c**) vertical displacement field. The values of \(u_{pd}^{exp}\) and \(u_{pd}^{adm}\) are indicated in Table 6, from which is determined the manhole class, which is included in the last column of the table. It is verified that all manhole specimens of 100 mm depth made by HFRC attained class C250 or higher class. Despite the HFRC100_c30_g35 has included G_type1 grating (smaller stiffness and load-carrying capacity), it attained Class C250, to which it has contributed the relatively large top HFRC cover thickness (35 mm). This characteristic may justify the fact that HFRC100_c00_g50 has attained class D400, since it has the highest top HFRC cover thickness (50 mm). The HFRC100_c00_g50 has null bottom HFRC cover thickness, therefore it disposes of the highest internal arm for the reinforcement composed by the G_type2 grating, which may have contributed to the highest structural performance of this specimen. Since this reinforcement is immune to corrosion, its disposition without the bottom cover is possible, which, when combined with the highest top HFRC cover thickness, had a significant impact on the manhole load-carrying capacity (the highest of the experimental program). The influence of the top HFRC cover thickness was also visible when comparing the results of HFRC100_c30_g35 and HFRC100_c25_g50. Despite the G_type1 grating has been adopted in the HFRC100_c30_g35, the top HFRC cover thickness with more than 10 mm than the one in the HFRC100_c25_g50 has contributed to the larger stiffness and load-carrying capacity registered in HFRC100_c30_g35. The class D400 was also attained by HFRC110_c40_g50, which, despite having the highest bottom concrete cover thickness (40 mm), an increase of 10 mm on the manhole's thickness (110 mm) allowed a peak load of 423.87 kN. All tested manhole specimens failed by punching, with the typical failure shown in Figure 10. The delamination of the elements forming the GFRP grating is visible, revealing the lack of fibres in the orthogonal direction to the longitudinal axis of these elements. The typical pattern of diffuse circumferential cracks on the top surface of the manhole specimens is also visible, indicating the high fibre reinforcement effectiveness in arresting the propagation of cracks that they cross, delaying the damage localization due to punching.