Lightweight Concrete Submittal

[theclipper]

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,

 

[jike]

Wow! That is a lot of evaporation!

 

[PMR06]

Is there an air entraining admix? Maybe the 120pcf is before the admix, and the "loss" of 6pcf is actually a reduction in density from the air entraining as the concrete mixes in the truck? If so, they should clarify their submittal.

 

[concretemasonry]

The supplier is in error because you cannot know what the concrete density is at the concrete plant. You can only follow the mix designs. Deviations from dry batch weights (more likely) or lightweight agrregate desity (not as likely) are 2 possibilities.

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.

 

[WBUW]

Seems like you took the right approach to me.

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.

 

[DarthSoilsGuy]

to expand on or 2nd concretemasonry's post. They should be soaking the aggregate well and this water will be evaporate out much later similar to how moisture vapors pass through your basement slabs. the lightweight aggregate gets hardcore soaked because most LW mixes are pumped. If the agg isn't saturated, slump loss can be huge b/n the truck and the end of the hose. The pressure from the pump mechanism fills the porous air voids in the aggregate with your convenience water. i've seen this just kill a concrete pour over and over again as they have to break down the whole clogged pipe system while trucks buildup. On one pour, we did testing and another company was doing Special Inspections. The SI was absolutely flipping out about all the extra water (40+ gals./truck) being dumped in the mix to get the stuff to come out of the hose (<4 slump). The cylinders broke well (taken from end of the hose, of course). We had a few jobs where air-dry cylinder testing to confirm the unit weight was specified (don't have the ASTM handy). The air-dry test should cost you an extra 2 cylinders/test and maybe some lab time if you spec it. On these jobs, we would always do field unit weight testing. A 4 pound drop between the wet and dry unit weight was pretty typical.

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.

 

[msucog]

is the 120pcf the wet unit weight or dry unit weight? i recently had a job that the concrete was running 123pcf...wet. after running the additional testing per astm, it came in at 116 pcf for the dry unit weight.

 

[concretemasonry]

Darth - Thanks for adding to my opinions -

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.

 

[msucog]

oh, and pertaining to my previous post, the 116 pcf was at 28 days...

 

[concretedoc]

The concrete producer may have been referring to equilibrium density as opposed to fresh density. If the fresh density was 120 pcf, the equilbrium density could be expected to fall between 116 and 112 pcf as a result of natural drying (not oven drying). The 4 to 8 pcf difference depends primarily on the amount of water lost from the aggregate (which depends on the amount of presoaking) and the cement content. See Fig. 4.1 in ACI 213R-03, Guide for Structural Lightweight-Aggregate Concrete.