Holdfast Training Services Trial Blog

The Plan

The casting of the column bases into the pile heads is one of the key early stages in my project. To (try to) ensure the quality of this operation a carefully staged process was developed with hold points and hand over inspections involving the concrete and steel sub-contractors, the principle contractor and the designer.

Design build up beneath the base plate within the pile casing

To leave space for the steel shear keys beneath the base plates the pile concrete cut-off level was left short of the top of the casing. The process for installing the base plates became a 6 stage operation as follows:

1. Prepare the pile head as handed over by scabbling the surface and extending the rebar cage where required. Pour C70/85 concrete to achieve desired pile level, ~50mm below the bottom of the shear key.
2. Install a 10mm steel levelling plate.
3. Grout beneath the levelling plate using a high strength (100MPa) ‘non-shrink’ grout.
4. Install the main baseplate with shear key and temporary works.
5. Pour C70/85 concrete to within 100mm of the bottom of base plate.
6. Pour high strength grout (100MPa) beneath the bearing surface of the base plate (max 100mm thickness) and up the sides to top of steel. Grout was specified to ensure full dispersal of the mix beneath the plate to ensure full contact with the bearing surface to satisfy the design bearing pressures from the column into the pile.

All stages had gone as planned until the final (and most important) stage.

The Execution

Unfortunately, our ‘specialist’ sub-sub-contractors clearly didn’t get the memo (or any training in mixing grout it seems). While mixing the bagged grout on site they ignored quality procedures and, following a mechanical failure with the volumetric doser, ended up adding water to the mix using the ever accurate bucket method.

The result is unlikely to surprise anyone. The grout was considerably over watered and separated in the pile casing. The cementitious content has fallen to the bottom topped by a layer of polymer additives, all submerged under 90 mm of bleed water.

Bleed water from the separated grout pour

There were immediate concerns over the strength of this grout layer and the 7-day cube result shows 39.1 MPa against a forecast compressive strength at this age of 75 MPa and a design target strength (28 day) of 85 MPa. This raises all the usual issues with failed quality tests but owing to the criticality of this bearing surface a remediation plan must be put in place within the month. Any longer than that and the designers cannot guarantee the structure for further steel erection – all of which is on the critical path.

With no float left in the programme following delays elsewhere (see my earlier blog on off-site programme) any critical delay to the programme will incur damages and costs totalling close to £1M per week.

So what are the options?

The Hope

Owing to the cause of this non-conformance (over-watering) there is no confidence that the cube result represents the in situ condition so additional testing is required. We will also take smaller core samples through the base plate’s vent holes to ascertain the profile of the grout across the bearing surface (see below). 

I have uploaded an In Situ Testing Techniques guide from one of our sub-contractors for interest but for this problem we are taking simple core samples for compressive strength testing (BS EN 12504). 

Core locations in failed grout layer

The hope is that a core can return a result approaching 50MPa and that the design engineer can justify this reduction in strength from the design. I suspect this is highly unlikely on both counts so what is the fall back?

The Contingency

The only fall back is to remove the grout but the access is extremely difficult. The current proposal is for hydro demolition with a curved lance to access the gap between the casing and the base plate. This is currently under review but adjusting the power of the lance should allow for the removal of the weak grout without damaging the concrete of steel beneath.

This requires considerable planning effort and time to execute. This will be followed by installing the grout again properly. And the clock is ticking before damages begin.

Lessons Identified

1. A specialist is not necessarily any more competent than any one else in any given task.
2. Adding water to concrete/grout, beyond mix design, is a bad idea.
3. Paying to do the job right the first time is cheaper than rectifying mistakes if they happen.
4. It doesn’t take long for a simple task to slip onto the critical path.