The mixes were originally designed, so that 50% of the fly ash in the mixes P08 and P12 were substituted with bio ash; Together the fly ash and the bio ash made half of the powder content in these mixes (in kg’s).
Later the bio ash content was increased to 100% of cement content in kilos (“half and half”), i.e. no fly ash in the mixes P08 and P12.
This lead to major problems on the consistency of the concrete, and the slump was much too low although the water content was very high compared to a similar concrete without bio ash, and superplastizer was added.
Further, a decrease in the compressive strength level (7 days) was observed.
Subsequently the mix design was changed to 70% bio ash, 30% fly ash. So far the results on this combination have shown promising results, all though the water content is still app. 10 litres higher than for “normal” concrete, and adding superplastizer is necessary.
This led to the decision to change the mix design to the original, so that 50% of the fly ash in the mixes are substituted with bio ash. Concrete in Passive and Extra Aggressive Environmental class is pretested with a good result (50% bio ash). Pretest of concrete in Moderate and Aggressive Environmental class is to be repeated during the summer due to a problem with some of the laboratory equipment.
Concrete for placing cobblestones (low slump concrete) is no longer produced with bio ash. The concrete does not reach the same strength level on the construction site, and is difficult to get out of the trucks.
For the past period, Unicon has had a claim from a costumer, where we had to pay a larger amount for re-placing the cobblestones, originally placed in concrete with bio-ash. A part of the project was carried out with “normal” concrete, not showing the same defect.
Status per 2007.05.11