Holdfast Training Services Trial Blog

Saturday 3^rd May has become a dark day on the progress of the Liverpool street station. Following on from the success of the level 106 slab the level 101 slab should have been more straight forward. The amount of concrete to be poured for the 101 level was calculated at 220m3 which was 100m3 less than the 106 level. In addition the steel fixers had work extra shifts to ensure that all the steel was in place prior to the pour, the 106 level had seen steel fixers completing the steel as the concrete was being poured.

With all this in place the pour should have taken only six hours however 8hrs after starting, the pour was not complete. The principle delay to the pour was due to the poor standard of concrete that arrived at site and a miscalculation in the quantity required, with a further 7 loads having to ordered.

At the pre pour meeting on the Friday I had raised my concern over the fact that the previous pour had seen concrete being pumped into form work before the slump tests were complete. If any of the slump test had failed dramatically we would have had severe difficulties in removing the failed concrete or even identifying were it had been poured. In my mind this would have meant the entire slab would have not met the required strength. With my concerns aired I found myself armed with the clip board and in charge of the slump test. Unfortunately this would result in me being struck of many a Christmas card list. The concrete supplier, Cemex, had won the contract based on price. Although I don’t have the exact contract comparison details, I have been informed that Cemex use their own brand of additives to keep cost down however this would seem to have disastrous consequences on their mix.

The concrete specified was a C50/60 mix and the slump was specified between 580 – 620 average diameters. The first delivery of the day managed a less than impressive 520mm. A discussion with the site engineer saw the concrete accepted. However the second delivery spilled off the slump testing board managing a estimated diameter of 680-700mm. Further discussion with the site engineer and the Cemex technician saw the concrete accepted with promises of improvements in the mix. However this inconsistency was set to continue and of 34 loads (full load 7.6m3) of concrete on order I ended up rejecting 6 loads with the majority of the remaining loads being highly questionable. Many of the questionable loads that were accepted were overly loose rather than stiff. Despite failing the slump test they were accepted on the basis of perceived risk of the concrete reaching the required strength against the risk of creating a cold joint.

A cold joint is formed when the time between concrete loads is sufficient to allow the curing of the poured concrete to be different to that of the next load, ultimately creating a concrete joint. As the position of this joint can’t be controlled this can result in a weakness as this may be at a steel reinforcement lap joint or an area of steel without the required anchorage to achieve the required tensile stress.

The risk was transferred to Cemex with verbal and written confirmation that the concrete will reach the required strength. Despite this reassurance from Cemex this does not sit easy in my mind as the question that was not addressed was whether the concrete strength would be reached in time to allow us to strike the formwork and then to excavate.

In addition to the structural issues the loose concrete also prevented the power floating of the surface to achieve the required smooth finish. Power floating requires the concrete to be firm enough to be stood on with enough flex to allow the power float to smooth the surface. Three hours after completing the pour the concrete was to loose to power float and a paddle finish had to be used to complete.

This has thrown up a number of contractual issues concerning the strength and finish of the concrete. The concrete was required to meet 30Mpa cube strength in 24hrs, however this took over 48hrs to achieve. The delay has had the knock on result of delaying the excavation through the 101 level to the 95 level. With over 3000m3 of soil to remove the two week excavation to working level is now going to be very tight with very little scope for further delays due to unforeseen ground conditions. The finish surface of the concrete is to be exposed the paddle finish applied as power floating could not be achieved has had to be raised a s a ‘Non Compliance Report’ and negotiations with the client and designers have continued throughout the week to seek a resolution and to avoid arbitration.