4.1 Relationship Curves of Drawing Force and Elongations
Relationship curves between the drawing force and elongation with three types of plates and two kinds of shotcretes (plain and steel fiber reinforced concrete) are shown in Fig. 5 and Fig. 6, respectively. The mechanical behavior of the anchor and shotcrete support system with the steel fiber reinforced concrete spray layers, as shown in Table 1.
According to Fig. 5, Fig. 6, Table 1 and related test data, we can find that:
(1) Under different plate/shotcrete combination conditions, the relation curves between the drawing force and elongation present similar variation trends. By taking the combined influence of three types of plates and two kinds of shotcretes (plain and steel fiber reinforced concrete), the pull-out process can be divided into three stages with obvious features, which are elastic (OA stage), yield (AB stage) and hardening (BC stage).
(2) The different plate types has significant influence on the tensile stiffness at the elastic stage. In the condition of plain concrete, when flat plates, butterfly plates and new-type plates are used, the tensile stiffness is 8.83 MN/m, 5.59 MN/m and 4.71 MN/m, respectively. This is mainly attributed to that the lack of pressure-yielding function of the flat plate. As for the steel fiber reinforced concrete, with the same steel fiber content, the pull-out curves slope of the new-type plate is lesser than those of the butterfly plate and flat plate at the elastic stage. This means that the new-type plate can extend the elastic stage of pull-out curves. Thus, it is indicated that the flat plate is not beneficial to the full development of the elastic deformation stage of the anchor and shotcrete support system, thereby affects the controllable release of surrounding rock during the early excavation of tunnels or roadways.
(3) In the condition of plain concrete, when flat plates, new-type plates and butterfly plates are used, the yield platform widths is 41 mm, 47 mm and 50 mm, and the yield strength of bolt is 175 kN, 181 kN and 181 kN, respectively. This shows that the pressure-yielding and deformation resistance of butterfly plate and new-type plate are higher than that of flat plate in the anchor and shotcrete support system. As for the steel fiber reinforced concrete, when flat plates are used, the yield platform widths in the anchor and shotcrete support system is not obviously affected by the steel fiber content, and the bolt yield strength are all 181 kN. Compared with plain concrete, the yield strength of bolt is increased by 4%. When butterfly plates are used, the pull-out curves slope of steel fiber reinforced concrete in curves elastic stage is greater than plain concrete, and the yield platform widths is almost similar. When new-type plates are utilized, the bolt yield strength gradually increases with the accession of steel fiber content. This shows that the steel fiber can improve the yield strength of bolt and slope of curves elastic stage.
4.2 Shotcrete Strain in the Pull-out Process under Different Plate and Shotcrete
(1) Analysis of shotcrete strain under different plate
According to test data, draw out 20 mm outside the rockbolt hole at the interface between initial shotcrete layer and resprayed shotcrete layer, 70mm outside the rockbolt hole at the external surface of resprayed shotcrete layer, 20 mm outside the rockbolt hole at the interface between initial shotcrete layer and surrounding rock mass, and 70 mm outside the rockbolt hole at the interface between initial shotcrete layer and surrounding rock mass of relation curves between the shotcrete strain and drawing force (The steel fiber content is 1.0%.), as shown in Fig.7. Since the variation tendencies of shotcrete strain corresponding to steel fiber reinforced concrete spray layer with 0.5% and 1.5% steel fiber content, are similar to that of 1.0% steel fiber content, only the 1.0% steel fiber content is discussed here. [Note: The above 4 different positions are denoted as: M1, M2, M3 and M4.]
According to Fig. 7 and related test data, we can find that:
1) When butterfly plates are used, the initial strain around the rockbolt holes at the interface between the initial shotcrete layer and the resprayed shotcrete layer, and the interface between the initial shotcrete layer and the surrounding rock mass is lower than when flat plates or new-type plates are used. This is attributed to that at the early stage of loading, the center of the butterfly plate cannot contact with the initial shotcrete layer, and with the loading increasing, the butterfly plate deforms and contacts the initial shotcrete layer until the test ends. Thus, it is demonstrated that the butterfly plate has the function of pressure-yielding. 2) When new-type plates are adopted, the slope of the strain-drawing force curve around the rockbolt hole at the external surface of the resprayed shotcrete layer experiences a sharp increase. This is caused by that with the load increasing, the new plate deforms and introduces pressure on the shotcrete, which hinders the further increase of strain. The new plate presents the property of limiting the exorbitant deformation of shotcrete. 3) The interface between the initial shotcrete layer and surrounding rock mass has strain slightly greater than that of the interface between the initial shotcrete layer and resprayed shotcrete layer. The strain at the interface between the initial shotcrete layer and surrounding rock mass is greater than that of the external surface of the resprayed shotcrete layer, though they are equal far away from the rockbolt hole. This phenomenon also explains why the shotcrete often starts to break around the rockbolt hole during pull-out tests under the coupled action of plates and shotcrete in anchor and shotcrete support system.
(2) Analysis of shotcrete strain under different shotcrete
On the basis of test data, the relation curves between the shotcrete strain and drawing force (M1-M4) with butterfly plate and two kinds of shotcretes (plain and steel fiber reinforced concrete) are displayed in Fig.8. Since the variation tendencies of shotcrete strain corresponding to different combination conditions between new-type plate and flat plate with plain shotcrete and steel fiber reinforced concrete, are similar to that of butterfly plate, only the butterfly plate is discussed here.
Some results drawn from Fig.8 are as fellows: 1) The shotcrete strain increases with the accession of the drawing force. Around the rockbolt hole, the shotcrete is obviously yielded at the late loading stage. For strain values on the same position, the higher they are the steel fiber content, the lower their strain will be. 2) When the drawing force is small, the slope of relationship curves between the drawing force and shotcrete strain increases with the improvement of steel fiber content. This is mainly attributed to that the steel fiber increases the toughness of concrete. 3) The strain of steel fiber reinforced concrete is lower than that of plain concrete, and the strain values decreases with the improvement of steel fiber content. This shows that steel fiber reinforced concrete significantly improves the anti-cracking performance of concrete. 4) The strain values of steel fiber reinforced concrete spray layer with 1.0% and 1.5% steel fiber content is similar. This indicates that the strain values will not decrease infinitely with the increase of steel fiber content.
4.3 Failure properties comparison of different shotcrete
The failure properties of different shotcrete, as shown in Table 2. Since the failure properties of steel fiber reinforced concrete spray layer with 0.5% and 1.5% steel fiber content, are similar to that of 1.0% steel fiber content, only the 1.0% steel fiber content is discussed here.
(1) In the condition of plain concrete, the failure properties of the flat plate and butterfly plate are similar. The cracks extend from the vicinity of rockbolt hole in the initial shotcrete layer to the border as well as the resprayed shotcrete layer, until the shotcrete gets failure. The crack pattern is outward radial with the rockbolt hole as the center point. Under the coupled action of new plates and plain shotcrete, the failure properties are cracks radiating outward in the initial shotcrete layer with the rockbolt hole as the center point. The cracks in the resprayed shotcrete layer run through along the same direction, which is attributed to that the plate types with the drawing force increasing and introduces pressure on the shotcrete to hinder the generation of cracks. Thus, it is indicated that the new plate has the function to prevent shotcrete fracturing.
(2) As for the steel fiber reinforced concrete, the failure properties of the flat plate, butterfly plate and new-type plate are similar. The cracks extend from the vicinity of rockbolt hole in the initial shotcrete layer, and the cracks in the resprayed shotcrete layer are very small and almost invisible to the naked eye. At the end of pull-out tests, the shotcrete was not obviously damaged, and the integrity is better. Failure properties indicates that the steel fiber makes the shotcrete better tough and obviously improves the anti-cracking of shotcrete.