Holdfast Training Services Trial Blog

I have mention this in an earlier blog however, Richard asked me to highlight this again as he’s currently delivering the concrete package.

The practice of adding water to a concrete mix in order to brings it’s slumps back inside tolerance is poor at best. It alters the water/cement ratios and affects the whole mix, however, we are allowed to add water on site!

The concrete specified for our bridge is a C40/50, 20mm limestone mix. The limestone aids with the strength but also assists with ensuring that no chlorides are in the concrete which would cause long term PT tendon corrosion issues.

The problem is that limestone draws moisture out of the mix at a hell of a rate and during the trials we had real issues in maintaining a workable mix. Plasticisers and retarders have been added but altering a mix is not as simple as adding more….

As a result, the supplier monitors the mix, travel time and ambient temp and provides a certificate that details how much water we can add to each load. Every load that arrives is different and ranged from 0 to 72 litres (the most I’ve seen to date). Very rarely do we have to add this, normally the slumps are within tolerance but when loads get held up or arrive marginally out of spec, it has allowed us to keep batches alive.

Today’s pour should have had a 180mm +/-20mm slump but one load was measured at 130mm. The ticket specified up to 33l could be added. I added 10l to a 6m wagon which brought the slump up to 190mm, a little water makes a big difference.

Concrete ticket – max additional water is marked on every ticket

Water gauge – For QA we have to monitor how much water is added.