## 4 Results and Discussion
### Stress of the Anchorage Zone at the Bottom Slab
#### 4.2.2 Stress Distribution of Concrete  
Concrete stresses at Sections A-A through E-E were measured. Cracking was observed at the top surfaces of anchorage blocks at Sections B-B and C-C during the loading process. No cracking was formed at other sections. The linear relationship between the stress and load were observed. Therefore, the concrete was in good mechanical performance and elastic condition under each loading case.  
When the prestressing tendons were tensioned, the top surface of concrete at Section A-A was under tension due to the tie-back actions of ahead anchors [33]. With the increase of prestressing force, the tensile stress was gradually increased. The maximum tensile stress of 1.02 MPa was measured at the top surface of Section A-A when the prestressing load was equal to the prestressing force of 100%. No cracking was formed due to no tensile stress exceeding its limit (2.35 MPa).  
Higher tensile stress was formed at the top and lateral surfaces of tooth plate (Test Point 65) at Section B-B. The other test points at this section were under compression at various loading cases. Section C-C was under compression at these loading cases. The stress level at this section was relatively  
Figure 12: The relationship between load and stress of the steel bars on the bottom slab specimen: (**a**) the upper layer steel of Section B-B; and (**b**) the upper layer steel of Section D-D. MP, measuring point.  
high due to the vertical eccentricity of prestressed ducts. The concrete at the top surface of anchorage block of Section C-C was under higher stress condition than other locations. It can be concluded that the stress concentration near the anchorage zone was very obvious.  
The linear relationship between the stress and load were observed at Sections D-D and E-E during various loading cases. Therefore, the concrete showed good mechanical performance and elastic condition under each loading case. The reason for linearity is that the stress increasingly extended to the entire section, because the effect of local bearing pressure progressively decreased when the distance from anchorage section gradually increased. The relationship between concrete stress and load at Section D-D in the bottom slab is illustrated in Figure 13.  
The tensile stress that was formed at the area near ahead anchor was relatively small when the triangular anchorage block was applied to the bottom slab of the box beam. No crack was observed at the concrete at this area. However, the tensile stress level that was formed at the top surface of the anchorage block was higher during the tensioning progress, which might make concrete crack. When the distance from anchorage section gradually increased, the tensile stress gradually decreased due to the effect of local bearing pressure.  
Based on the analysis above, we can see that both the concrete and reinforcement of the specimens under anchorage was mainly subjected to compression, and the stress was larger. Meanwhile, there was local tensile stress near the anchorage zone, especially on the anchorage block. However, no tensile stress exceeded the limit. When the tensile load gradually increased, excessive local deformation and increasing tensile stress may lead to concrete cracking in anchorage zone. Therefore, excessive deformation caused by concentrated compressive stress under anchorage had a negative effect on concrete in anchorage zone. Moreover, cracking of concrete may be caused, and even the normal service of the component might be affected.