Critical Assessment of Testing Methods

Having presented a brief overview of some common beam and plate test methods, this section critically reviews these test methods. Emphasis is placed on the development of design guidelines for SFRC pile-supported slabs.

Beam Tests

The prime benefit of beam tests is that they give material properties (Sukontasukkul, 2003). The test however suffers from the disadvantage that the results can exhibit considerable scatter. However there is a widespread belief, amongst many steel fibre suppliers that beam tests do not model accurately the response of pile-supported slabs (Concrete society, 2007). The main reasoning behind this argument is the fact that simply supported beams do not exhibit the load re-distribution that occurs in pile-supported slabs on cracking. As a result the post-peak response does not correspond with that of a statically indeterminate pile-supported slab (Destree, 2004). After the initiation of the crack in the beam test, a drop in flexural load is typical of a tension softening response. On the other hand, pile-supported slabs do not necessarily behave in such fashion as they are statically indeterminate. Consequently, a re-distribution of stresses occurs within the slab after initial cracking (Lambrechts A. N., 2007) which can result in an initially hardening response. The redistribution of stresses can occur from adjacent bays or piles.

The counter argument is that the beam tests give material properties whereas indeterminate plate tests give the structural response. Hypothetically, the structural response of the statically indeterminate test, or the pile supported slab, should be predictable once the relevant material properties are known.

The previous sub-sections described various standard beam tests in which variations included both the loading arrangement (three point bending – RILEM, BS EN 14651, four point bending – ASTM C1399, C1609) and loading method (continuous – RILEM, BS EN 14651, non-continuous – ASTM C1399). Each test exhibits its own distinct benefits and drawbacks as described below.

50

Before the introduction of BS EN 14651, the JCI-SF4 test was commonplace. Although easy to execute, due to its arrangement it provided no information regarding the crack mouth opening displacement. The fact that the crack could, in theory, initiate anywhere along the middle third of the beam would make the measurement of the crack difficult. Even if such a measurement was possible, the location of the crack would have an effect on the crack width. The inability to record CMODs is a significant drawback, particularly as a major reason for the introduction of steel fibres in the concrete is the reduction of crack widths.

The RILEM beam test however, enables the accurate measurement of the CMOD. The sawing of the notch predetermines the crack location making it is possible to measure the CMOD during the test (RILEM Technical Committee, 2000) (Kooiman, 2000) (Destree, 2004). Both the RILEM (RILEM Technical Committee, 2000) and the BS EN 14651 (British Standards Institution, 2005) provide equations for the calculation of the CMOD based on the beam’s central displacement.

On the other hand, the incorporation of the notch does not allow the beam to fail at its weakest section. As a result, the notched beam test exhibits additional scatter (coefficient of variation) (Kooiman, 2000).

Previous research has also highlighted that the scatter in results (the coefficient of variation) is also directly related to the ‘cracked area’ (Lambrechts A. N., 2007). The fracture plane in an ASTM C1550 statically determinate plate is around five times more than in a RILEM beam test. The orientation of even a few fibres in a beam test can have a greater effect which can partly explain the difference in variation (Lambrechts A. N., 2007).

The high scatter in the results is a drawback which nevertheless can be addressed by executing a larger amount of tests. Variation coefficients for beam tests are in the region of 30% (Lambrechts A. N., 2007).

Statically Determinate Plate Tests

A number of round plate and square panel test are available, each with its distinct benefits and drawbacks.

Round Panel Tests (RDP) have a number of distinct benefits, as well as shortfalls, in comparison to beam tests. The response of the RDP is arguably more representative of the in situ structural response of a pile-supported slab due to the fact that multiple cracks develop (The Concrete Society, 2007). The repeatability of the crack pattern is a distinct benefit which translates to the low coefficient of variation exhibited by such tests like the ASTM C1550 (Bernard & Pircher, 2000)

51

(Lambrechts A. N., 2003) (Marti, Pfyl, Sigrist, & Ulaga, 1999). According to Lambrecht (2007) ‘the average variation coefficient is around 10%’. This is approximately a third of the variation of coefficient for standard beam tests.

Previous research has demonstrated that the influence of the location of the three radial cracks has a very small effect on the load resistance of the plate (Bernard & Xu, 2008).

As plate tests generate a larger number of cracks, they are able to absorb more fracture energy than beams (Sukontasukkul, 2003). That can enable the fibres to demonstrate their ability in bridging a crack. A few researchers categorise the RDP test as a more logical choice for the determination of the fracture toughness of SFRC (Banthia, Gupta, & Yan, Impact Resistance of Fibre Reinforced Wet- Mix Shotcrete-Part 2: Plate Tests, 1999) (Sukontasukkul, 2003).

The ASTM C1550 is a RDP which has been designed for easy fabrication as well as execution (Bernard & Pircher, 2000). The three pivotal points ensure that the test is always in conctact with the support regardless of the flatness of the specimen itself.

The limitation of this test is that it is not easy to record the crack width. The fact that the crack width tends to vary along the yield lines makes such a calculation even more difficult.

Statically Indeterminate Plate Tests

Whereas the statically determinate tests (beam tests and round determinate round panels) help to extract material properties, tests of statically indeterminate nature (Arcelor plate test, EN 14488, EFNARC panel test) are predominately used to understand the structural behaviour of SFRC with regard to specific applications (Lambrechts A. N., 2007).

Their statically indeterminate nature makes it difficult to extract the intrinsic material properties of the SFRC. Primarily, this is due to the fact that the stress distribution is not known and cannot be derived due to the indeterminate boundary conditions. Such tests do not exhibit a consistent mode of failure as in the case of the RDP (Bernard, 2000). The cracking pattern observed can also be unpredictable, particularly in the case of the square panel specimens, postulated in EN 14488-5. The addition of the fibres into the mix can make the crack pattern even less predictable given the ability of the fibres to transfer stresses across the concrete matrix after fracture has initiated (Lambrechts A. N., 2007).

A distinct advantage of the RDP over the ENFRAC test is the even load distribution. The three symmetric pivotal supports ensure an even load distribution ‘regardless of tolerances’ and surface flatness (Bernard & Pircher, 2000). Furthermore, the flexural resistance of such tests is not directly

52

related to the crack-width (Concrete Society, 2007). This is due to the known fact that the flexural resistance tends to vary along the yield lines as the CMOD is not known.

For this reason, it can be argued that indeterminate plate and slab tests should not be used to measure the material properties but rather to monitor the overall structural response.