Holdfast Training Services Trial Blog

The last blog saw me trying to understand the issues surrounding the high failure rate in the concrete in the primary lining, with respect to flexural strength. In a pincer movement reminiscent of the Zulus at Rorke’s Drift, Richard and John exposed my mild bluffing surrounding the testing procedure….

So here is me attempting to be John Chard…

Why test for flexural strength?
Concrete is a brittle material with a low tensile strength. Adding steel fibres to the mix will enhance toughness and ductility defined by the BS as follows:

Toughness – the ability of fibre reinforced concrete to sustain loads after cracking of the concrete ie its energy absorption capacity

Ductility – general ability of the material to sustain load beyond a yield point that defines the limit of elastic behaviour (onset of cracking). (Concrete without reinforcement would be brittle and demonstrate an abrupt loss of strength beyond elastic range.

The sprayed concrete that delivers the structural propeties of the tunnels is steel fibre reinforced (SFR).This ensures that the structure is able to sustain significant ground movements without sudden brittle failure. Therefore the trials procedures will seek to test the ‘Residual Flexural Strength’ ie the flexural tensile strength retained after cracking.

Testing
The CrossRail specification for Sprayed Concrete Linings specifies BS EN 14651 as the test method for approving:
1. Flexural Strength
2. Residual Flexural Strength

The test beams are centrally loaded as shown below, by the 3 point method. The specimens are notched at mid span to induce a crack via stress raising

Performance of the test sample is specified by the relationship between the applied load and the Crack Mouth Opening Displacement (CMOD). This measurement can either be measured directly, or calculated in terms of central deflection

Measurements are taken as follows:

F(L) is the load corresponding to the limit of proportionality (LOP)

F1 @ CMOD1 = 0.5mm

F2 @ CMOD2 = 1.5mm

F3 @ CMOD3 = 2.5mm

F4 @ CMOD4 = 3.5mm

The slide shows a typical graph of load, F, against CMOD (BSEN 14651:2005)

Flexural Beam Test Correlation

Flexural Strength

f(L) is calculated in terms of the centre span load, F(L) as follows:

f(L) = 3FL/2bh

h = depth of beam above the notch

b = section width

L = span

Residual Flexural Strength

Crossrails specification tests values of residual flexural strength @ deflections of 1mm and 4mm respectively. However, this is not measured directly, and is measured as a CMOD, which adds an approximation to the calculation (I’ve asked the test centre why they do this…awaiting response). At these values, the beam must retain values of 1.8 and 1.4 respectively.

FAR 0053-10.Beam & Cores

REFLECTIONS

1. CMOD v Deflection. Other than reliability of measurement, I dont know why CMOD would be used over direct deflection measurement (yet). Whatever the reason, the approximation adds a level of uncertainty that we can ill afford. The example test result above shows a beam which has failed, by a tiny amount, and which is typical of many of the beam failures. Perhaps a wider understanding of this testing method may mean that a tolerance is imposed such that this group of slim failures may edge over the line and achieve a pass?

2. Testing Method. Prompted by the Great Orator, I considered the 3 point testing method as discussed above. This method implies a failure method of coexistent shear and bending. 4 point testing methods for pure bending are specified in the EFNARC beam test (European Federation of Producers and Applicators of Specialist Products for Structures). The BS EN 14488 beam test largely adopts this method, and is mentioned in the CrossRail spec under structural requirements. I’m wondering whether the performance requirements were designed to one test procedure (4 point), and the 3 point test, and its differing pass criteria, were defined for use?

Am I John Chard yet????????