Steam Curing 2

[dik]

It's my understanding that steam curing is a means of accelerating the strength gain of concrete but has negligible effect on the historic overall strength. The concrete in question has added flyash material.

Can anyone provide a reference that illustrates/refutes this?

Dik

 

[Ron]

Dik...that's correct. Ultimate strength not greatly affected but early strength is enhanced. Strength gain with flyash is slower than without, so you might get a reduced early strength gain as well, but still more than without steam curing.

There have been several studies in ACI on autoclaved concrete (not AAC, but conventional concrete)

 

[dik]

Thank-you, sir... Do you by chance have any citations for the test results.

Dik

 

[Ron]

Dik...I'll take a look at my references. I had a client at one time that made steam cured concrete railroad ties and we had quite a bit of data. I'll see what I can find.

 

[concretemasonry]

If you are talking about accelerated curing of concrete, much depends on the curing temperature, since once you get beyond a certain temperature, the chemical reactions and end products are very different since accelerated high temperature does convert the excess lime from the normal chemical reaction to an additional cementing and strength material and reduces the possible efflorescence since free lime is not available for future expansion, contraction or dissolving in water.

If you are talking about high pressure accelerated curing (over 300 degrees and about 140 psi in an autoclave) it is a different world than atmospheric pressure with a little bit of extra heat (below 212 F). Typical autoclaved products would be AAC and autoclaved concrete block (CMUs) (not AAC since there is little aggregate strength contribution). In the curing of normal concrete, you get about 90%+ of the ultimate strength in a short cycle (4.5 to 8 hours of curing.

For atmospheric pressures, the amount of accelerated curing depends on the temperatures the concrete (not surrounding temperatures) reaches and the duration.

In general, the ultimate strength of low pressure curing are about the same as for unassisted curing (which is greatly affected by the time and conditions.

In an earlier life, we would cure cores (from suspected frozen concrete) in an autoclave for a local testing laboratory to assist them in the determination of the supposed ultimate strength. Because they had a normal mix for ready mix, the chemistry was not ideal or what would be used in an autoclaved product, the resulting strength was lower than it could have been if it was designed for an autoclaved product. Because of the poor mix (for autoclaving), it was thought that the results were reasonable and approximate enough to decide if the on site concrete was acceptable and have to be torn out and replaced. This could be done in one day after coring. - One important difference relating to high (over 300F) and low temperature (less than about 250F) is the amount, chemistry of the fines in the aggregate and cement replacement materials (fly ash, etc.) and the fineness (whether they were really reactive). An autoclaved block (not AAC) can easily and economically made to over 8000 psi because it optomizes the chemistry and the strength of the compacted aggregate.

After all that verbage, atmospheric, accelerated curing with steam or other sources, the ultimate compressive strength will ultimately depend on the time/temperature relationships. This is also limited by the conditions for concrete batching, aggregate gradation and site curing that is not nearly as accurate, controlled or reliable as plant conditions.

Dick

 

[dik]

Thanks CM... a brief recap.

Using high temperature curing can more effectively utilise the lime, then. Does this have an effect on the long term strength? Does it marginally increase it? I've not encountered high temperature/pressure curing.

I'm a little confused about your last paragraph. Does the time:temperature relationship have an effect on the historic strength, or does the strength gain just happen more quickly.

Dik

 

[concretemasonry]

Are you concerned with a job site situation or a factory situation? What is the size of the product if it is a plant/factory setting? there are limitations on the size of the units that can be cured using high temperature moist curing, but atmospheric curing can be done if the moisture is contained and the temperature is controlled. In this case, steam, circulating oil or electric resistance heat can be used, such as is done in different pre-cast plants for larger concrete elements.

Regarding the temperatures, high temperatures can only be achieved in an autoclave (pressure vessel) in order to provide a saturated atmosphere at the higher temperatures. (water evaporates at 212F under atmospheric conditions). This required a factory setting and not a site setting. Steam is a lower tech option, but it is commonly used is the production of concrete elements in a plant, must the moisture must be maintained because moisture is required for concrete to hydrate.

In general, with the same type of cement the ultimate strength of similar mixes would be about the same. Some exotic admixtures and conditions may be the exception (possibly costly). Many tests have shown that high early strength cement (Type III) will give a SLIGHTLY lower ultimate strength than Type I cement, but the desired high early strength is much better. In some cases the chemistry of the aggregates can have an effect on the ultimate strength. Despite the conditions, concrete can be designed to give whatever results desired for the local materials and the method of curing desired.

 

[dik]

Thanks CM:

The concrete work is critical in nature and the specified 50 MPa (7200 psi) concrete has a 3 day strength of about 15 MPa (2200 psi). The contractor has provided (non-standard) test results for a similar mix that is steam cured that indicates this will achieve the 28 day spec'd strength.

I'm a little sceptical <G>.

The other issue is that the work is critical and if the 28 day strength is not achieved, the area will be inaccessible for several months with additional costs (to the contractor, I assume). Because the concrete has not failed the 28 day strength criteria, yet, it cannot be outright condemned.

I've indicated to others, with sufficient 'wiggle room' that my understanding of steam curing was as noted above and what I hear, is that this is generally correct. I'm looking for reference or anecdotal information.

Another interesting thing. At the ACI (Manitoba Chapter) dinner tonight, one of the engineers at the table was of a similar opinion. He works with a precaster but when I mentioned that a couple of days earlier I had heard about high temperature curing he had never heard of it (fot that matter, I hadn't heard of it either).

Dik

 

[concretemasonry]

High temperature (over 212F) curing is limited to plant production and is limited to units of specific sizes and shapes because of the cost and capacity of a pressure vessel that can be used at 150 psi pressure in a 8' diameter by 140' long vessel. If you are dealing with smaller sizes and shapes it is very economical and unbelievably predictable and reproduceable. I have made 8700+ psi block with a 6 hour curing cycle that agreed with units made 2 years earlier from similar materials and the same mix design within 2% unit compressive strength. - From a practical standpoint, it is not economical for large irregular concrete units.

Any good producer of concrete products usually has a good handle in the curing required using whatever energy source the plant is designed around. - It is far more predictable, reliable and provides much quicker verification the in-situ concrete, especially for higher strengths. Precasters have a good background for production of precast units using lower, but still accelerated curing, but little experience in the world of high temperature curing because concrete chemistry is different and more efficient.

If you are dealing with 7000 psi and very early strengths, I would suggest contacting one of the major cements companies (usually European - Holcim/Holderbank, Lafarge, etc.) since they have done a great deal of research and background. Most concrete producers rely on them, depending who they buy cement from. - There is a vast knowledge of research in the European concrete journals.

Dick

 

[hokie66]

dik,

Just my 2 cents worth. A 50 MPa mix which tests at 15 MPa at three days has no chance of reaching the design strength, no matter how cured.

You have not identified the size of the concrete element, but Dick is correct that steam curing is usually only applicable to plant precast work.

 

[dik]

Thanks holkie,

Well, maybe I was a little more than skeptical. The dilemma is that I can't do anything other than suggest that the mix is inadequate and be removed. The Contractor will not remove it until the 28 day strength is in, and then the area will be inaccessible for several months and will have to be re-worked at a later date.

Dik