Cylinders from mixes with silica fume

[didoT]

I have read in the silica fume's mfg. data sheet that Silica fumes react with free lime to form calcium silicate hydrates, which is responsible for the strength of concrete. Could this be an explanation to why i'm getting higher compressive strength reults on cylinders cured in tanks (which contain lime) than cylinders cured in my moist room???

 

[Ron]

It's possible, but there's plenty of CaO in the concrete to react with the silica fume, and the curing water that is lime saturated does not reach the interior of the cylinders, so no contribution except on the exterior face of the cylinder.

It is more likely that your tank conditions are better controlled than your moisture room. Just a couple of degrees cool in the curing process will cause a strength reduction...not much, but enough to vary the strength between the two methods of curing. Further, the tanks provide true 100% relative humidity, whereas the moisture room might vary a bit.

 

[concretemasonry]

The moist cure only provides atmospheric moisture that is relatively pure and no minerals. The tank could and probably will contain lime that was leached out of previous samples. Also, the pressure exerted on the samples by the water could force more lime (CAO) into the samples since the detention time of previous samples could increase the concentration.

I feel the immersed samples would be more realistic of the real world since the concrete in place does have pressure or enhancement from the silica fume or the natural long term process of carbonation that can increase the strength.

Testing and curing processes are tightly controlled but can only go so far to predict the properties of the materials in the real world. This is a constant problem to balance a strict, repeatable process and yet be realistic. Concrete is relatively easy to test on a long term basis (except for durability) because the real results are better than lab tests. - This is always a concern when being involved in writing ASTM procedures and practices. Some procedures and practices are good for research and others for real world conditions.

 

[Ron]

Also, remember that the cylinders are not intended to reflect the properties of the concrete in place, but to check the mix design. In-place properties of the concrete can vary significantly from the standard specimens and lab curing. As CM noted, there are many ways to assess the strength and other properties of the in-place concrete.

 

[didoT]

Ron,
the comparisson was cylinders to cylinders. I was suprised to see 1000 psi difference between 2 sets of cylinders taken from the same wheelbarrow.End treatment the same, size of sample the same and age @ test the same.The only difference was tank vs moist room.

 

[concretemasonry]

Since you are dealing with a microscopic sample (wheel barrow) you can expect variations because the actual material and segregation can affect the results. Add in the technician variable (at one time, I could easily add or reduce a cylinder strength by 500 psi just by still barely keeping within the ASTM sample preparation requirement. - Only two sets of cylinders?

If you had different curing, you must have had different handling and preparation of samples. - What was the age of the cylinders? The chemistry of cement hydration over time can get very complex, especially when less common materials are introduced.

Dick

 

[didoT]

2 sets, each set had 8 cylinder. the difference was in the 7-day results. & now the 28-day results today were basicly the same.

 

[conceng]

DidoT,
Depending on your w/c ratio, it may be that the moist room is preventing loss of moisture from within the concrete where the tank is providing a constant source of external water to help hydrate particles that may otherwise run out of water for hydration.

 

[Ron]

A 1000 psi difference at 7 days is significant, but the fact that the two converged at 28 days leads me to believe you have a moisture room issue. Check temperatures and relative humidities more accurately than your strip chart recorder. You likely have a moisture-humidity control issue.