Lightweight Concrete Submittal
Lightweight Concrete Submittal
(OP)
I recently rejected a LW concrete submittal becuse it came in a 120 pcf, and my spec required 115 pcf max. The contractor just called me to say that the supplier assures me that if they mix it at 120 at the plant - by the time it gets/delivered/placed it loses about 6 pcf. Anybody ever hear of that one?
Thanks,
Thanks,






RE: Lightweight Concrete Submittal
RE: Lightweight Concrete Submittal
RE: Lightweight Concrete Submittal
What may have happened was the lightweight aggregate was wetter than normal. This means that less agregate and more moisture was include in the weight of lightweight aggregate. The result is heavier concrete and higher compressive strengths if the aggregate batch weights are not adjusted for moisture.
Normal coarse heavyweight aggregates usually are assumed to have about 1% moisture contistantly and cannot get too much higher than that even when they have surface water. Lightweight aggregates can range between 5% and 40% depending on what conditions they were subjected. Because of the high range, it is necessary that the batch weights of the lightweight aggregate be adjusted for contained moisture if the moisture content of the batched aggregate is higher than normal. Rains on a stockpile or an open bin will cause the bottom portion of the aggregate to be much wetter.
I would suggest looking at the aggregate stockpiling and sampling methods for future lightweight aggregate. The dry lightweight aggregate density should also be verified.
RE: Lightweight Concrete Submittal
If they want to push the issue you could always require onsite unit weight testing, but if the tests came out bad...well you would have a real mess on your hands. Better to head it off at the pass then deal with it later.
RE: Lightweight Concrete Submittal
But you should know that the entrapped water lb-loss number all depends on how porous the lightweight agg is which is regional. it isn't loss during delivery or placement. it's lost after a couple of weeks.
All that being said, reject it. you're owed a good submittal. What if that 120 pcf number IS based on dry aggregate? Then it could be showing up at 125 wet.
RE: Lightweight Concrete Submittal
RE: Lightweight Concrete Submittal
I did not want to get into the pressure effects caused by pumping.
Some types of uncrushed rotary kiln aggregate may require substantial times (days) to saturate the aggregate completely. Other types are much easier to saturate. In Russia, I have seen even where pressure was used to increase the saturation.
The condition of the aggregate used in the production mix must be the same as the aggregate in the samples used for the mix design.
theclipper -
Ask your concrete supplier to work closely with his lightweight aggregate supplier. The lightweigh suppliers have a vast knowledge based on many different applications and specifications. Many local concrete suppliers just do not produce enough concrete to be as well versed as the aggregate suppliers. In my experience, many technical support people in the lightweight industry approach being fanatical about quality, uniformity and performance since the product is used because of its properties.
RE: Lightweight Concrete Submittal
RE: Lightweight Concrete Submittal