Holdfast Training Services Trial Blog

Throughout my time here on the materials trials team, a constant headache has been the high failure rate of the flexural strength beam tests. The flexural beam tests for fibre reinforced concrete in the primary lining have been failing at a rate of ~40% at 28 and 90 days. For comparison, BFK understand that across all other Crossrail SCL works the rate is down at 5%. It turns out this issue has been going on for months prior to my arrival. The issue has become toxic, with a number of different and conflicting opinions. Unhelpfully, the issue has now become so prevelent that it is being formally discussed and debated in too many forums. Finally, CrossRail have issued an ultimatum…BFK are to provide a robust explanation as to the cause/causes of the high failure rate including action plan by the end of the month, or all SCL work stops.

So here’s what Ive learnt….

Testing.

BS EN 1465 dictates the test critieria. In outline, a set test panels of concrete are sprayed at the face of the advance, at a rate of 1 set/month. Each panel is cut into 3 beams, which are wrapped and sent to TestConsult…our subcontracted test laboratory. The beams are notched at mid section to induce a crack in the centre of the beam. The beam is then tested to failure using the 3 point testing procedure on a Universal Testing Machine….note the band layers in the top right of the beam

As the beam fails, note the faint band layer below which crosses it from approximately 10 – 4 on the clock face. The crack appears to follow the band layer as a failure plane up and to the left. The beam is now loaded off centre, perhaps contributing to a more rapid failure.

The test is dictated by the CMOD or Crack Mouth Opening Dimension. Measured in mm by electrical contacts, an average test result looks like this.FAR 0053-10.Beam & Cores. Note here the Residual Flexural Tensile Strength section…two snapshots are taken…at a deflection up to 1mm, the specification dictates the CMOD must be above 1.8, whilst up to 4mm the CMOD must maintain above 1.4. CMOD is a function of width of crack to length of crack

Action Plan

BFK are beginning to pull together an action plan to resolve this issue. Due to the complexity of the potential contributory factors, they have cast their net widely. Pulling in advice from their parent companies BAM Nuttall and Wayss & Freitag, aswell as specialist suppliers Normet/Tam alongside our own design team. The collective thoughts have distilled down to 6 areas of consideration:

1. Materials/Mix Design.

• A ‘sandy’ texture has been observed on the sprayed material when cured, which may contribute to the laminations. It is possible that this is due to a lack of cement and microsilica (packs the mix and reduces voids)
• Laminations in the beams indicate that the accelerent is reacting too quickly. A less reactive product is being trialled.
• Beam tests have shown fibres ‘pulling out’. This could be for two reasons: firstly insufficient fibre anchorage due to the fibre profile (length, dimensions, shape etc). Secondly, poor concrete ‘grip’. The density of the beam could have been affected by poor compaction, or indeed that the concrete was too old at point of spray.
• Fibre tensile strength. Fibres could be snapping instead of pulling out. Tensile strength currently 1200N/mm2

2. Spraying

• Nozzlemen competancy and technique varies significantly. In addition, the pump speed can be adjusted over a flow rate range between 8 – 22 m3/hr. High flow rates can lead to poor compaction, high rebound and affect fibre orientation.
• Accelerator dosing. Dosing can be varied on the machine over a range 6%-8% and it has been seen that higher dosing making the concrete cure quicker could be leading to higher lamionations
• Test panel. Perhaps the most signinficant factor in spraying is that the test panels are not representative of what is going on in the lining. The panel area is clearly small, meaning that the nozzle movement is smaller leading to increased rebound which is caught in the layers.

3. Plant

• Mechanical factors. Each of the suite of spraying machines work slightly diffently, but critically the concrete is dosed at the nozzle by pump. There is a period of pulsation…flow of concrete…flow of accelerant…before a constant flow is established. When the rate is slowed to spray into the panel, this phenomenon is exacerbated, contributing to laminations.

4. Testing

• Test house procedures. Are the testing houses complying with standards. Members of the quality team have been despatched to conduct a ‘surveillance’!

5. Environment

• How well are the panels and beams protected from damage? The panels are demoulded at 18hrs, before being cut and transported. This demoulding is conducted by using plant to lift and bang out the panel from the mould. There is definitely a likelihood of damage.

6. Batching

• Conducted by concrete subcontractor, Hansons and therefore outwith of our quality control procedures. Fibre storage is important..if not properly protected, they will rust detrimentally impacting their strength..
• Fibre dosing. Is trhe required amount being added to the mix? This is checked by a process of fibre wash out, and weighing samples which arrive at site. These tests are mandated to be conducted monthly, so perhaps the frequency of testing needs to be increased?
• Mixing. Fibres are poured into the back of the wagon at the end of the process, relying on the motion of the concrete lorry to spread them throughout the mix. There has been evidence that fibres in some instances, ball up together and are not spread consistently throughout the mix.

REFLECTIONS

Technical. The factors affecting the many and varied, but in most cases they are measureable. Whilst I have detailed a number fo factors above, the root causes seem to be related to the presence of band layers in the test panels, which cause a failure plane. There is a definite correlation that says the higher the number of band layers in the specimen, the more likely and quicker it will fail. The reduction of the band layers is withiin our gift, as many of the contributing factors occur on site. Secondly, the issues surrounding the steel fibres, are easily resolved, but are entirely under control of the concrete supplier.

Managerial. This management of this issue has been dominated by infighting and opinionating. It has become very devisive…criticism of the mix design means criticism of the materials team…criticism of the workmanship means criticism of the site operatives…an one thing everyone agrees on is criticism of the concrete supplier. Therefore people have become very emotive and unwilling to sit down with a blank sheet of paper and revisit every aspect of this process. Until now. It has taken an ultimatum from CrossRail to drive BFK to go outside of their team and bring in the big guns. This has meant an objective study carried out without the emotional baggage of the last few months. Consequently, a detailed and far reaching root cause analysis was conducted with a very thorough and logical action plan which will test and discount each theory individually. This process has taken 4 days….versus 9 months of bickering.

Contractual. To my mind the concrete conctractor need to be held to account. I suggested having BFK quality assusrance reps at the batching plant…turns out they have reps down there. 2 materials techs who no one can ever reach for comment. It feels to me that this needs to be upgunned, but I darent suggest it for fear of getting the job.

After all that though…I’m not sure why we just don’t go and ask what the other contractors are doing/not doing and make sure we do the same! Maybe thats just not the done thing around here….