Holdfast Training Services Trial Blog

No doubt you have all had trouble sleeping recently. Understandable…waiting in trepidation for the outcome of ‘McGuirk’s Magical Mix’ trial for the cast in situ secondary lining! In the seminal works, ‘Red Card Moments – Parts 1 and 2’, I detailed the genesis of this mix, following the realisation that there was a national shortage of Pulverised Fuel Ash (PFA). So like a slightly confused contestant on Great British Bake off, I was set the challenge of developing a new recipe for self compacting mix by the site sub agent (who is no Mary Berry!). This mix was to include Ground Granulated Blast Furnace Slag (GGBS) as a cementious filler, in place of PFA. The mix design is as follows:

kg/cubic metre

Ordinary Portland Cement                                  – 270

GGBS                                                                        – 180

Agg (Limestone, 20mm, 10mm, 4mm)             – 1300

Marine Sand                                                            – 325

Filler                                                                          – 100

Admixtures including superplaticiser (water reduction for the low water/cement ratio), stabiliser (modifies viscosity, and eliminates segregation) and retarder(controls the rate of cement hydration). An importanly, 2kg of polypropylene fibres for passive fire protection.

Results

This table shows the performance requirements specified by the client…and how my mix got on!

Performance Requirement:	KF10 rev 10.0:	BFK024A SCC
Cementitious Content	> 320 kg/m3 & < 450 kg/m3	450 kg/m3
Water / Cement Ratio BS EN 206-1: 2000	< 0.50	0.50
Density	>2300kg/m3	2207kg/m3 average @ 28days
Target Flow BS EN 12350-5: 2009	To be determined during site trials	500 mm
Concrete Temperature	15°C – 35°C	26°C
Early Age Strength Development	C8/10 @12hours or	6MPa (achieves 9.2MPa at 24hrs)(please see general comments)
Long Term Compressive Strength BS EN 12504-1: 2009	28 days > C32/40	43.8 MPa(Must exceed specification by current margin, 7.5MPa minus 3.5)
	90 days > C32/40
Concrete Shrinkage ASTM C341/C341M-06	<0.03%	0.003%
Exposure class	XC3	DS2

Many of you will not be surprised to hear that despite a valiant effort…it failed. Two criteria. Density and compressive strength.

Analysis

The strength of the concrete is influenced by the water/cement ratio and the relative volume of air in the mix. All Crossrail mixes that I have encountered so far have a specified w/c ratio of <0.5 and in the main, have easily achieved the required density. This mix achieves a figure of 0.5. Whilst I would have preferred a slightly lower figure to achieve a greater strength, this is still within specification, and therefore is not affecting the density in this instance.

The cementitious content adheres to the specification of no more than 450kg/m3, and so cannot be increased. Further, at no point can the mix achieve a temperature of 70degrees C during curing. Using thermocouple data loggers I recorded a peak temperature at the core of 66degrees, so I dont feel I can increase the proportion of cement for fear of too much heat gain.

Therefore, I am focussing my efforts on the volume of air in the mix. The low density may indicate a higher than normal volume of air in the mix. Crossrail have an air entrained mix which is used as a sacrificial medium for the Tunnel Boring Machines to ‘pull’ their way through the station boxes on their drives. The air entrainment has left them with a similiar density to my mix at around 2100kg/m3. This mix is designed to have a relatively low compressive strengh to allow the TBM to easily plough through it. I therefore suspect something in the mix causing it to retain air voids which reduces the density, and further the compressive strength.

Polyfibres

By comparison to other mixes in the project, the only real variable in this mix, is the addtion on polyurethane fibres. Following a series of high profile tunnel fires and the increased threat of terrorism, the safety of underground structures have gained public atention. Polypropylene fibres have been developed as a means of passive fire protection to prevent explosive spalling, and maintain the structural integrity of the concrete.

How do they work?

This is quite interesting…so much so that I found myself spinning the dit to my girlfriend at the weekend…before I had a moment of clarity, whereupon I fell silent for fear of getting chucked!

Imagine the concrete is exposed to high temperatures, such as those in a tunnel fire, or indeed my fire trials (Refer to the tour de force that is “I am the god of hell fire!! (testing) for more details). In high quality concrete,t he density prevents the moisture contained within the concrete lining escaping quickly enough. Any voids that are present will become saturated. The heat will gradually increase and overtake the moisture front, whereupon the moisture will vapourise and increase the pressure in the body of the concrete. This increased pressure can ultimately lead to explosive spalling.

Polyfibres are introduced to increased permeability during heating and ultimatley reducing pore pressure. At approx 160degrees they will begin to melt, before disintegating at about 360degrees. This has the effect of providing millions of capilliaries in the concrete which allow moisture to escape. Brilliant…

However….

Having been tipped off by a guarded response by the manufacturer, it seems the particular brand of fibres…IGNIS…have a tendency to trap air in the mix during curing. As a fag packet calculation, a percentage air change of 1% in your mix can affect the compressive strength by as much as 5%. As the mix had cured there wa no means of conducting an air entrainment test on it, but this confirmed in my mind that this was a prime suspect.

Next Steps.

The pressure is definitely on. We are programmed to pour this mix in the permanent works in 5 weeks time. 28day results, plus a 2 week contracted response time from the client for material approvals, puts us behind by a week.

I have initiated two trials, whih were poured this morning. The same mixes, but with two alternate fibres: one from the same manufacturer, one from a competitor. Initial results as follows:

Fibremesh 150.

Slump/Flow.                                                  –            660mm

Ambient temperature.                                 –             15.7°C

Concrete temperature.                                –             17.6°C

Fresh Density.                                               –             2275

Air Content.                                                   –              2.1%

M320P 32F.

Slump/Flow.                                                  –            640mm

Ambient temperature.                                 –             14.5°C

Concrete Temperature                                –             19.7°

Fresh Density                                                –             2270

Air Content                                                    –             3.7%

 

NB. The air entrainment test is not a requirement byt specification, but I requested it given my theory that air content is at the heart of this. Using previous mix performances, and the fag packet from above, I reckon the percentage of air content in the first, failed mix could have been upto 10%. The result above are a good start

Testing

I have taken a total of 30 cubes over the two tests, which will allow the following (increased) testing regime. Those required by the spec are highlighed. In addtion, cores will be taken at 28days the test dry shrinkage.

12hrs                     –             3 Cubes               –             Crushed at 27 Aug 14, 1930

24hrs                     –              3 Cubes               –             28 Aug 14, 0730

7 Days                   –             3 Cubes               –             03 Sep 14

14 Days                 –             3 Cubes               –             10 Sep 14

28 Days              –             3 Cubes               –             24 Sep 14

56 Days                 –             2 Cubes               –             22 Oct 14

(2 spare)
In order to expedite this, I have gained provisional agreement from the client to submit a Materials Compliance Report at 14 days in the hope that I can show a marked improvement in material strength against the first mix. If Im really lucky it may have gained its 28day strength by then, in which case they will approve subject to dry shrinkage results at 28days, meaning McGuirks Magic Mix will be holding up the ‘cross’ in Crossrail!