An investigation of early age strength in tunnel shotcrete applications

This thesis is structured to better understand early age compressive strength of the shotcrete and aims at determining the optimal time for re-entry to excavation in tunnels, based on numerical data gathered from field and laboratory testing of shotcrete. Shotcrete has an important role in New Austrian Tunneling Method (NATM) tunneling. Introduced by Rabcewicz in 1962, NATM tunneling method or the principal relies on the controlled deformation of the excavated area and allowing rock mass to carry itself. This control is achieved by applying support elements such as shotcrete, wire mesh and, rock bolts to the excavated area. Among the aforementioned support systems development of shotcrete's early age compressive strength is one of the most important factors that determines the time of return to next phase of excavation in both mining and civil tunnels. For meeting the aims of this thesis, proctor penetrometer, Schmidt hammer and, uniaxial compressive strength tests were conducted. Samples obtained from two different tunneling locations and consisting of different compositions were cast into block samples for proctor penetrometer and Schmidt hammer testing and cylindrical samples for uniaxial compressive strength (UCS) testing. Furthermore, core samples were obtained from block samples to be used in UCS testing as well. Proctor penetrometer test were done with six different needles on block samples from both locations. Testing was done immediately after obtaining samples from the nozzle with accelerative additive. Schmidt hammer testing, obtaining the samples, started after two hours following a predetermined pattern at hourly and daily intervals. The tests were done on the troweled surface of the block samples on 5 different locations covering the surface of the sample at a time. Schmidt hammer was applied 12 times on the same testing location, considering consolidation and plasticity of freshly obtained shotcrete. UCS tests were done at second, third and fourth weeks after obtaining the samples on cylindrical sample as well as on the 45th day with core samples. These samples were obtained from the truck-mixer without accelerative additive. All testing was done according to respective ASTM standards. Test results showed the clear increase of the compressive strength of the shotcrete. Testing with proctor penetrometer both samples has satisfied the requirements of General Directorate of Highways of Turkey. Results of sample A showed an increase from 1.04 MPa to 4.24 MPa at between 45th and 120th minutes with 65 mm2 needle. For sample B the increase was from 1.23 MPa to 3.87 MPa between 15th and at 60th minutes with 65 mm2 needle. Furthermore, proctor penetrometer proved a valuable testing method for the first two hours of shotcrete sampling regarding its early age compressive strength. Schmidt hammer testing were not able to yield a rebound value within the first day of testing. Instead spring energy turned into excessive crushing of the shotcrete. Resulting plunger depths from the surface of the samples were recorded. It was concluded that there was a direct connection with excessive crushing of the shotcrete under plunger and the energy released from the spring of the Schmidt hammer. Rebound values obtained from sample A on the first day had a minimum value of 21 and maximum value of 47 with a mean of 34.41 and at the 28th day, minimum rebound value of 24 and maximum rebound value of 48 with a mean of 40.48. Sample B for the first day had had a minimum rebound value of 10 and maximum rebound value of 34 with a mean of 22.95 and at the 28th day minimum rebound value of 16 and maximum rebound value of 45 with a mean of 36.2, respectively. Uniaxial compressive strength tests showed the expected development of compressive strength of the shotcrete. On the 14th day, sample A sample had failed at 29.3 MPa, while sample from sample B failed at 34.8 MPa. On 28th day A sample failed at 42.04 MPa and B sample failed at 44.1 MPa. Results obtained from testing clearly shows the development of the shotcrete and it was determined that, shotcrete's early age strength, the rock mass and, the forces coming to excavated area must be known to correctly calculate optimal time for re-entry to tunnel face. It was also determined with the aforementioned connection and further testing Schmidt hammer could be used to determine the early age strength of shotcrete after first day.