Lightweight Concrete Strength Testing

[GTSLABS]

We are testing concrete for a project that requires 3500 psi lightweight concrete. We are not casting the samples, just picking up, lab curing and breaking. Our Coefficient of Variation for all other concrete on this job ranging from 2500 to 8000 psi is from 1.4 to 2.6 for which <3.0 is the best ACI rating. However the LW concrete has a variation of 6.3 for a total of 15 sets of 2 breaks @28 days. We are curing and breaking the same as all the other concrete. This "with-in" test variation is is a function of the lab and field casting and curing.
The suppiler Overall Std. Dev is 474 corrected for 15 sets which is Very Good even though the average strength is 3290

This material is being used as a parking garage topping and is being pumped multiple stories. The slump ranges from 7.5"-10.5" and air between 7.75% and 10% via Rolometer. But there is no clear trend between these and the strength.

ASTM C330 talks about ater curing for 7 days then floor curing for the remaing 21 days. But this is a standard specification and not meant for job site control.

The Oven Dry Density and Equilibrium Density has not been requested.

Any ideas on what to look for here to improve the testing?

 

[civilperson]

Testing is not the problem. Consistency of the product is the problem. Dual sets of cylinders made by two different technicians out of the same sample will confirm testing proceedures correctness.

 

[concretemasonry]

I agree about using multiple technicians to make cylinders. All to often, people believe that all technicians are the same. A technician can generally follow all procedures can skew results easily by sloppiness or intentionally. - We had contests in college seeing who could skew results the most by using different cylinder making techniques. Angled rods instead of vertical, different penetrations, speed of rodding, etc., but not allowing sugar or rebar.

Lightweight concrete offers many chances for variation if there is poor control over materials and procedures.

One thing thta is not obvious, is that lightweight cylinders are lighter and earier to handle, but have the same dimensions. They can also be tossed too easily since they can be 30% lightwer. Mishandling is a big part of all testing variations, especially when they are large.

From a production standpoint, a critical first thing to look at is the absorbed moisture condition at batching time. Prewetting is far more important than people realize. A aggregate with no internal moisture will increase the amount of aggregate (volume) used and will suck up excessive moisture and in the critical early curing period. There is also a situation where surface moisture can give a false idea of saturation. This is usually very evident if the concrete is pumped, but can be a large factor in unpumped concrete.

The major key to uniform lightweight concrete is UNIFORM moisture condition of the aggregate. - surface and absorbed separately.

This may not directly adress the testing variability, but does provide some insight into a solution if the concrete actually varies.

Dick

 

[Rjeffery]

Also at issue is the initial field cure conditions. are the cylinders being cured as per the ASTM standard or just left where they were cast. A variation of the curing conditions could be the cause of the great variation. Also transportation cushioning, time of transport and temperature that the specimens are subject to will cause variation as well.

The best way to test something is to squeeze it, slowly, until it breaks!

 

[concretemasonry]

GISLABS -

I just had a late thought about the variations you are seeing.

Your testing may not be a problem, but the sampling. Even when going from one load to another on the same day, it is possible to see some of the variations you are seeing.

Lightweight (depending on the type) can vary from 5% moisture to over 40% moisture based on the dry weight. That can range represent a lot of water in the aggregate and less aggregate. If there was a variation in the moisture and it was not compensated for, you are seeing a very large difference in the actual amount of lightweight aggregate in the mix.

Is there any correlation between the weights of the cylinders and the compressive strength? Higher weights and higher strengths could be an indication of less actual lightweight aggregate. => out of spec density and higher strengths.

Dick

 

[GTSLABS]

The correlation is not too great. We were not asked to do an ovendry density. But we weight and get 2 diameters of every specimen we break. So I know a nominal Wet Density assuming a 12" heigh specimen.

I started to break some 56 day samples and there was virtually no increase from 28 days.

I have attached a plot of Wet Density vs 28 day strength.