A number of test methods are used around the world to evaluate the behaviour of fibre-reinforced shotcrete (FRS) for permanent sprayed concrete linings (PSCL). However, as Martin Knights, independent consultant and former President of the International Tunnelling Association (ITA) says in his review of the ITA Working Group No. 12 on Permanent Sprayed Concrete Linings: “Confusion exists between strategies to characterise material by testing and then how to use information for design.”1
A good understanding of some of the key concepts around the testing of FRS is therefore essential to avoid this kind of confusion and any other potentially controversial discussions.
INTRODUCTION TO FIBRE-REINFORCED SHOTCRETE
Fibre-reinforced shotcrete (FRS) is a composite material consisting of a concrete or mortar matrix and discontinuous, discrete fibres of steel, polymers, carbon, glass or natural materials. The properties of the composite depend on the characteristics of the constituting materials as well as on their dosage. Other factors such as the geometry, the volume fraction and the mechanical properties of the fibres, the bond between the fibres and the matrix, as well as the mechanical properties of the matrix, significantly affect the properties of FRS.
The behaviour of FRS also depends heavily on the transfer of load from the concrete matrix to the fibre system. For efficient load transfer, the following three conditions must be satisfied:
? A sufficiently large exchange surface, which depends on the number, length and diameter of fibres.
? The nature of the fibre-matrix interface which should be optimised for efficient load transfer.
? The intrinsic mechanical properties (Young’s Modulus, anchorage and tensile strength) of the fibre should allow the forces to be absorbed without breaking or excessively elongating the fibres.
In a hyperstatic mechanical system, the better the cracking is ‘controlled’ as soon as it arises (limiting cracking to small openings), the better will be the multi-cracking process and thus the more the structure will tend to show ductile behaviour.
According to ISO 132702: “Steel fibres are suitable reinforcement material for concrete because they possess a thermal expansion coefficient equal to that of concrete, their Young’s Modulus is at least five times higher than that of concrete, and the creep of regular carbon steel fibres can only occur above 370°C.” For these reasons FRS is an ideal material for permanent sprayed concrete linings.
HOW TO DETERMINE DUCTILITY OF FRS
European standard EN 14487-13 mentions two different ways to specify the ductility of FRS, namely in terms of the energy absorption capacity and the residual strength. It also states that both ways are not exactly comparable.
? The energy absorption capacity that is measured on a panel can be prescribed when – in the case of rock-bolting – emphasis is put on the energy that has to be absorbed during the deformation of the rock. This is especially useful for permanent sprayed concrete linings4
? The residual strength method can be prescribed when the concrete characteristics are used in a structural design model.
For each approach, two different testing methods are possible, as described below.
ENERGY ABSORPTION CAPACITY TEST ACCORDING TO EN 14885-5
The EN 14488-5 plate test (Figure 1) is designed to determine the absorbed energy from the load/ deformation curve as a measure of toughness. The test is designed to realistically model the biaxial bending that can occur in some applications, particularly in rock support. The central point load can also be considered to replicate a rock-bolt anchorage. This test has proved to be considerably useful to design engineers.
The test simulates at laboratory scale the structural behaviour of the system anchor bolt – sprayed concrete under flexural and shear load.
No numerical material properties, such as post-crack strength values, can be determined from the test due to an irregular crack pattern. However, this has never been the intention of this test method; the method serves to quantify and illustrate the ductile behaviour of an FRS concrete tunnel lining.
The test method is probably the closest to the actual loading conditions in a ground support scheme with bolts. It is a statically indeterminate setup, which allows for load redistribution, and creates both flexural and punching shear stresses, leading to more realistic failure modes. The well-known Barton Chart initially included a reference to the energy absorption obtained with this test, considering 700J or 1,000J, according to the ground condition.
Table 1 shows the three energy-absorption performance classes for a deformation of 25mm according to this test.
ENERGY ABSORPTION CAPACITY TEST ACCORDING TO ASTM C1550
The ASTM C1550 test method is an ASTM standard that is used to evaluate the flexural toughness (or energy absorption) of FRS. However, this test method does not allow for load redistribution between different cracks as it is statically determinate. It is not a substitute for an indeterminate test setup (such as the EN 14488-5 test described above) that allows for mobilising the FRS in hyperstatic conditions and generating a multi-cracking response. (Rock Tech Centre – MIGS III – WP 24, 5 July, 2019)
The ASTM C1550 test typically reports values of energy absorption at deflections of 5mm for civil works (which is relevant to most cases), although higher deformations of 10, 20 or 40mm are possible. The distinction between the values of energy absorption at different central deflections is essential, as the interpretation of these results depends on the intended use of the FRS. In fact, different deflections lead to different crack rotations and thus different crack openings. Recent testing on a broader range of fibre types and concrete strengths unfortunately shows that this correlation does not exist, and that using a direct relationship between the results of both tests can lead to unsafe ground support assessments (‘Doing More and Doing Better with Fibre-Reinforced Shotcrete,’ Design and testing comparison by Antoine Gagnon and Marc Jolin).
It should be noted that for ASTM C1550, the energy absorbed up to 5mm central deflection is applicable to situations in which the material is required to hold cracks tightly closed at low levels of deformation. Examples include final linings in underground civil structures such as railway tunnels that may be required to remain watertight. The energy absorbed up to 40mm is more applicable to situations in which the material is expected to suffer severe deformation in situ, such as shotcrete linings in mine tunnels and temporary linings in swelling ground. Energy absorption up to intermediate values of central deflection can be specified in situations requiring 20mm performance at intermediate levels of deformation.
RESIDUAL STRENGTH TEST ACCORDING TO EN 14651
For structural use, mechanical performance of FRS must be verified according to a performance-based approach. Model Code 2010 allows a comprehensive design approach for ordinary (not sprayed) fibre-reinforced concrete (FRC). It can be extended to FRS if a correct characterisation of the mechanical properties of the material is made, with particular regard to the residual tensile strength.
Concerning the behaviour of the FRC in tension, which is the most important aspect of FRC, various test methods are possible. Model Code 2010 refers to bending tests that can be carried out to determine the load-deflection relationship of a beam under a threepoint loading configuration, in accordance with EN 14651 (Figure 2).
Nominal values of the material properties are determined, and a graph of the applied force (F) versus the deformation is produced, such as shown in Figure 3. The deformation is generally expressed in terms of Crack Mouth Opening Displacement (CMOD).
The residual flexural tensile strength fRj can be evaluated from the Force-CMOD relationship, as follows:
where: fRj [MPa] is the residual flexural tensile strength corresponding to CMOD = CMODj
Fj [N] is the load corresponding to CMOD = CMODj
l [mm] is the span length
b [mm] is the specimen width
hsp [mm] is the distance between the notch tip and the top of the specimen (125mm).
The FRC can be classified according to the compressive strength (with the same approach adopted for normal reinforced concrete) and the tensile post-cracking performance measured at 28 days.
Two parameters in particular are used to describe the behaviour, namely fR1k (representing the strength interval) and a letter: a, b, c, d or e (representing the fR3k/fR1k ratio).
The strength interval is defined by two subsequent numbers in the series:
1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, … [MPa]
while the letters a, b, c, d, e correspond to the residual strength ratios:
a if 0.5 ≤ fR3k/fR1k ≤ 0.7
b if 0.7 ≤ fR3k/fR1k ≤ 0.9
c if 0.9 ≤ fR3k/fR1k ≤ 1.1
d if 1.1 ≤ fR3k/fR1k ≤ 1.3
e if 1.3 ≤ fR3k/fR1k
The designer should specify the residual strength class and the fR3k/fR1k ratio as well as the type of the fibre and its material properties, such as fibre strength.
It is difficult to make a characterisation of a sprayed FRC following the EN 14651 procedure. In fact, tests are made on beams having a square section of size 150 x 150mm and a length between 550 and 700mm. This beam geometry is suitable for cast (poured) concrete. This procedure is however difficult to perform due to the difficulty in preparing specimens with sprayed concrete.
RESIDUAL STRENGTH TEST ACCORDING TO PREN 14487-3
To solve the problem of the characterisation of a sprayed concrete, a procedure was proposed by EFNARC5 in accordance with prEN 14487-3 Method B. The procedure proposes a bending test on a square panel of size 600 x 600mm and thickness 100mm (Figure 4). This geometry is derived from the square panels typically adopted for temporary sprayed concrete in traditional excavated tunnels suggested by EN 14488-56.
The bending test is performed with a span of 500mm (the same as adopted in EN 14651) and a notch having a depth of 10mm is sawn at mid-span. The geometry and dimensions of the specimens, as well as the spray method adopted, will ensure distribution of the fibres in the matrix, which is as close as possible to that encountered in the real structure. The dimensions of the test specimen will be acceptable for handling within a laboratory, as it does not involve excessive weights or dimensions.
The test can be carried out, as far as the experimental means permit, in a normally equipped laboratory, as no exceptionally sophisticated equipment is necessary. The geometry will be the same as in the plate test for energy absorption (EN 14488-5). The plate could be sprayed on the job site, and there is no need to saw a prism from a panel, which could influence the result.
The scatter will be lower than the current standardised beam test, and the notch will provide a slower cracking process, thereby reducing the risk of a sudden fail.
As with EN 14651, this test defines residual flexural strength (fr1, fr2, fr3, fr4) according to the updated international standard, fib Model Code 20107. The obtained mechanical property will serve as input for the dimensioning method.
By adopting the prEN 14487-3 method B testing procedure for the post-peak characterisation of the sprayed FRC, it is also possible to derive the parameters fct,L, fR1, fR3, that are necessary for the design according to Model Code 2010, and propose some minimum requirements (Table 2):
CONCLUSION
Complete information concerning the material properties of fibre-reinforced shotcrete should be collected by executing on-site spray panel testing to obtain the relevant residual strength to meet the minimum criteria required by Model Code 2010 to substitute partially or totally the mesh reinforcement.
The mechanical characteristics of FRS present an early and important challenge before gaining acceptance of this solution. Consequently, the right testing for the right information during deformation should be specified according to the project requirements.
FRS is a relevant material to be used both for preliminary lining and for final lining. All the relevant standards concerning the product, the testing, the design and quality control are currently available to allow designers to specify the right performance. A good understanding of the material however requires complete information to be gathered on the properties of the FRS material.
For this reason it is recommended in all cases to specify the energy absorption and the residual strength. For structural use, mechanical performance of FRS must be verified according to the fib Model Code 2010 requirements and material properties that are determined based on the three-point bending test to limit the structural effect. The three-point bending test on a square panel with a notch appears to be a suitable method for characterising FRS, as demonstrated by the results of the laboratory tests. This test for all sprayed concrete applications is needed to check the minimum performance required by Model Code 2010.
A permanent sprayed concrete lining should be considered in the same way as any other permanent concrete structure. Hence, codes such as Eurocode 2 and ACI 318 should be applied for the design and acceptance of the requirements for normal loading conditions in the long term.
Quality and safety can be achieved using the relevant product for the right use. The use of the finished material should be considered along with the test and performance criteria, namely: post-crack behaviour, and matching crack widths and deformation in the test so as to meet expectations in the project as well as the requirements for durability.
