Designing concrete mixes for specific purposes often raises questions that you don’t know the answers to. Familiar products have fallen into disuse and new products are just new. A recent project in Shetland required a mix able to get into areas of heavily congested reinforcement that had previously failed the first time around.
Highly plasticised mixes look and behave differently from “ normal “ concrete and the current mix was a 100% Portland cement mix that appeared to have followed me over from Abu Dhabi with the same original contractor having developed a similar mix for two different projects. Shetland and Abu Dhabi having in common a lack of cement replacements particularly in the oil & gas industry. A situation further complicated as the current mix had a very high cement content was plasticised but not retarded making it very difficult to use in all but the simplest pours. It has very little time to be placed.
The rework had a number of challenges that followed from the earlier pour.
- Hanging shutters that obstructed the passage of concrete.
- A relatively small volume; less than 10m3
- Over detailing of reinforcement; surplus reinforcement that had not been removed during original fixing.
- Complicated shuttering and finishing that required considerable time to complete after the bulk of the concrete had been placed.
For the revised rework mix a new mix with the following requirements was developed.
- A 10mm aggregate for congestion and cover issues.
- Place using the chute from the back of the truck.
- Delay the setting time to around 3 hours.
- Retain the waterproof qualities of the existing mix.
- Characteristic strength of 35N
- Satisfy durability requirements for a Marine Environment.
TESTING & APPROVAL
Sika provided an S4 mix design based on the above requirement; to obtain client approval and to understand what we had and how it worked a simple test was done twice.
Batches of the revised concrete mix were mixed and held in a truck mixer replicating a long discharge time on site.
The truck mixer was progressively emptied over 3 hours.
Slump test were taken every 20 minutes and cubes at hourly intervals (1,2 and 3 hours) for testing at 7 and 28 days.
they were very good
- the mix stayed at or round the design slump only marginally dropping off in the last two results but still within limits.
- cube results were consistent and all passed.
One important thing that was noted was that the consistency (slump) potentially drops for a short period below the specification ,about 1 hour after batching was completed, and then regains the required consistency. It occurred on both batches.
The lesson for Contractors, Consultants and QA /QC processes is that
- The time from batching to the initial test is important and that there is the potential to reject concrete that is actually ok.
- When doing trial mixes develop tests that explore what could happen when a concrete pour goes wrong and placing of concrete is delayed identifying any characteristics that might explain a rogue test result during placing operations.