FIBRE REINFORCED SHOTCRETE FOR LONG TUNNEL PROJECTS IN SWITZERLAND

Transcription

1 FIBRE REINFORCED SHOTCRETE FOR LONG TUNNEL PROJECTS IN SWITZERLAND Volker Wetzig, Roland Weiss VSH Hagerbach Test Gallery, Switzerland Abstract Fibre reinforced shotcrete is nowadays a common support mean in tunnels. Fibres, and especially steel fibres, are added into shotcrete to improve the ductility and energy absorption. But also polypropylene fibres are in use as structural fibres to prevent spalling of the concrete. The determination of the properties of the material is normally done with rectangular or circular plate tests. The plates are supported on the whole circumference. Bending tests with beams are not very common to determine the properties of fibre reinforced shotcrete. The result from different test methods are compared to prove the existing relation between the different methods 1. Introduction In recent years fibre reinforced shotcrete has increasingly been used for tunnel linings. A common use of steel fibres ranges from 30 to 50 kg/m³ of concrete. Sometimes values out of this range could be found. Fibres, and especially steel fibres, are added into shotcrete to improve the ductility and energy absorption. Next to technical and security aspects there is also an economical reason to use fibre reinforcement in the heading of a tunnel. Using fibre reinforced shotcrete; both shotcrete and reinforcement are put into place within one step. This makes the heading of the tunnel much faster, could save in the end a lot of money and during spraying the workers are not in the dangerous area as they are if they have to place a net. Therefore to guarantee a certain safety standard in the tunnels a quality control system is necessary to confirm the properties of the shotcrete. This quality control system has to take into account the special influences on properties of fibre reinforced shotcrete. Just after spraying there must be easy, quick and strong evidence tests which have a well-known relation to the expected properties of ductility and energy absorption. 545

2 2. Actual Tunnel projects in Switzerland Actually the most famous tunnel projects in Switzerland are the railway base tunnels at the Gotthard (57 km) and at the Lötschberg (35 km) which are now under construction. Conditions concerning rock pressure, rock temperature and logistics are unique for tunnel constructions. Next to this two large tunnel projects several other railway and road tunnels with a length of more than 4 km are under construction. Independently to the concept of the project and contractor fibre, reinforced shotcrete is used more or less on all jobsites. 3. Systems of quality control at tunnel jobsites The system of quality aim has two aspects. First the system should guarantee that the quality requirements were full filled and second it is a documentation of the object for the future. In the control plan the projecting engineers fix the test methods, frequency, requirements and rules if quality is not reached. An example for shotcrete with steel fibres is shown in Table 1. Table 1: control plan Aspect Test method Freq. Requirement If not reached Comprehensive SIA 198, test on 150 m³ B 35/25 1) strength drilled cores D=50mm,L=50mm Density SN EN m³ Water penetration 1) Workability and content of steel fibres < 25 mm SIA 162/ m³ specific fracture energy > 5000 N/m 1) It is important that the rules have been fixed before starting production in case the quality can not be reached, for example: - Verifying the test results (if there was something wrong, correcting) - If non conformity is proved true (for example with a second test or cores from the object), take correcting steps for the future and settle the costs. Normally the contractor has to consider all works and costs to improve the quality. If it is not possible to improve the quality to the required level, price reductions for the part of construction are possible or the part of construction has to be rebuilt. 1) 546

3 In addition the design engineer also requires a quality system which shows how the contractor wants to reach the quality requirements. The contractor takes the control plan and adds all the needs to reach the requirements with high probability. Let s have a look on shotcrete with steel fibres, the quality system could be based on the following: Once concrete is sent to the site in the tunnel, it is nearly impossible to send it back. This is a psychological aspect, because if concrete is sent back the costs and waste of time are sure. If concrete is casted or sprayed even it is out of the good consistency range as long, as its pumpable, there is the hope the loss in quality is not as big as thought. The main aspects for a contractor s quality control are the following. Only concrete with the right consistency is sent to the tunnel. That means all the produced concrete must be controlled before delivering. Normally it is enough if the correlation between a consistency test (e.g. flow table value) and parameters of the mixing plant (like energy of the mixer) is found, controlled and properly documented. On the site, worker have to check if the delivered concrete is the required concrete and if the concrete is still in the right consistency range. Since the rebound consists also of steel fibres. So the content of steel fibres in the shotcrete has to be checked regularly. With a wash out test on fresh concrete, or from drilled cores, the real content of steel fibres in the shotcrete can be controlled. The content has to be high enough. The specific fracture energy will be reached with high probability but the fibre must be higher than 20 kg/m³ (referring to SIA 162/6 guidelines). 547

4 Table 2 Aspect Test method Frequency Requirement If not reached Properties of aggregates, cement, admixtures, steel fibres a.s.o. check delivery note every delivery specific norms sent back the delivery consistency at production concrete at the site content of steel fibres Comprehensive strength correlation between consistency and energy of the mixer check delivery note demanded concrete? in the range of consistency? SIA 162/6, washing out or breaking cores SIA 198, test on drilled cores Ø 50 mm, high 50 mm Density SN EN , penetration of water under pressure Workability and content of steel fibres SIA 162/6, plate test and fibre content every charge every delivery correlation to consistency and water cement ratio visible and consistency test 150 m³ enough to reach specific fracture energy but > 20 kg/³ range within can react with the next charge; out of that range throw away the concrete sent concrete back check on the site whether sufficient (cracks?) perhaps a second layer necessary 150 m³ B 35/25 1) see above 150 m³ Water penetration < 25 mm 1000 m³ specific fracture energy > 5000 N/m 1) see above 1) see above 548

5 4. Fibres for Shotcrete Next to steel fibres, nowadays structural polypropylene fibres are also available. Steel fibres for shotcrete have a length ranging from 25 to 50 mm and a diameter ranging from 0.4 to 0.8 mm. The cross-section of the fibres depends to the production process of the fibres; it could be: - circular (made out of wire) - rectangular (cut sheet metal) - irregular (planed steel) The fibres itself are hooked in different manners to improve the load bearing capacity of the fibre-concrete-system. 5. Testing of fibre reinforced shotcrete 5.1 Preparing samples The preparation of the samples has the most important influence on the results compared to all other influences of the testing process. Therefore this step of a test has to be done with big accuracy and is simpler for cast concrete than for shotcrete. The samples should represent the work done on the jobsite concerning strength, water tightness and also energy absorption. To achieve best results for strength and tightness a good compaction of the concrete is necessary. The energy of the spraying process which is needed in the arch of the tunnel is too high to spray samples in a mould. When samples sprayed in a mould with the same energy as the arch, the rebound, especially of the fibres, will be much higher than on the jobsite.table 3 gives the results of fibre contents which have been measured during one spraying test. Table 3: Fibre content Content [kg/m³] Content [%] Dosage of fibres Fibres in shotcrete from the arch Fibres in samples sprayed in mould After reducing the energy when spraying the samples in the mould the fibre content increases up to the level which has been measured in the concrete from the tunnelling. Further attention has to be paid to the edges and corners of the samples. Rebound settles very often in these areas which may result in lower values of the measured energy absorption. 549

6 5.2 General tests Fibre reinforced shotcrete will be tested according to all common standards which are used for sprayed concrete. In the past we did not find any significant influences of fibres on parameters like strength or water tightness. 5.3 Tests related to fibres Fibre content The determination of fibre content in the concrete is mainly interesting for shotcrete, where, due to the rebound, a fibre loss of about 20 % may occur. The fibre content is measured at the fresh concrete by washing out the fibres. Another method is to smash hardened cores and to collect the fibres Bending strength The bending test at beams is normally not used to investigate fibre reinforced shotcrete EFNARC-Plate test The EFNARC plate test, which is described also in other standards, is the most common test to determine the energy absorption of sprayed concrete. The energy absorption is quite easily calculated by integration of the load-deflection diagram. The value of energy-absorption does not relate directly to any calculations for the dimension of the tunnel construction. The energy absorption will be used to compare different reinforcement systems. For each type of fibre and concrete a certain relation between the fibre content and the energy-absorption could be obtained. Fig 1 shows such a relation for one type of fibres. It does not seem to be a linear but an increasing relationship. This could be explained in by considering that a minimum dosage of fibres is necessary to achieve a good ductility. It is expected that the energy absorption will not increase beyond a certain fibre content (in Fig. 1 this level is not reached). This effect will occur due to the bad compaction of dense fibre network. Recent tests with different types of polypropylene fibres have shown, that also this type of fibres can achieve highest requirements. The load deflection curve differs a little bit from those of steel fibres. With the relationship between the energy adsorption and the fibre content the contractor is able to determine the minimum fibre content to reach the asked energy adsorption. Referring to SIA Guidelines the fibre content has to be higher than 20 kg/m³. 550

7 Energy-Absorption [J] Content of fibers [kg/m³] Fig. 1: Energy absorption as a function of the fibre content SIA Plate test The SIA test procedure concerns square plates and circular slabs. The tests procedure is more or less the same, except the size of the sample. According to the SIA-guidelines also provide a method to determine specific fracture energy G f. This value allows also a classification of the fibre reinforced shotcrete Comparison of test methods For a long time the energy absorption value, measured in J, was the criteria to classify a fibre reinforced shotcrete. With the new SIA-Guidelines [2], the specific fracture energy G f was introduced. The values of Energy absorption and the specific fracture energy was compared for more than 100 tests. The results are shown in Fig. 2 which shows a linear relation between the two parameters. 551

8 Comparison Assessment of Plate Test SIA 162/6 vs EFNARC correlation y = 7.21 x tests Energy Absorbtion in accordance EFNARC at 25 mm Deformation [J] Fig. 2: Relation between energy absorption and specific fracture energy References 1. EFNARC, Specifications for sprayed concrete 2. SIA, Empfehlung SIA 162/6, Stahlfaserbeton (Recommendation on Steel Fibre- Reinforced concrete), Marti, P.; Pfyl, T.; Sigrist, V.;Ulaga, T., Harmonized Test Procedures for Steel Fibre-Reinforced concrete. 552