Need to accelerate Conc Hardening

[Plainsboro]

I need to obtain the full strength of concrete in 14 days rather than 28 days. I believe use of admixture will be cheaper than rapid hardening cement. Please give me your tips

 

[SlideRuleEra]

Calcium Choride is a traditional, inexpensive accelerator. However it can cause problems with embedded steel (rebar, anchor bolts, etc.)
Here is a link for general information

http://www.gogenchem.com/calcium/Ind05.pdf

 

[whymrg]

Call your local concrete supplier. They will give you info for early strength concrete chemistry, test history and price. We used it on some projects, but price was concern of general contractor.
Good luck.

 

[jheidt2543]

When you talk to your concrete supplier ask about past concrete cylinder breaks. It has been my experiance, that for some Type 1 cement mix designs in the 3,000 to 4,000 psi range, the DESIGN 28 day strength is reached in 7 days. There are a lot of reasons for this, but I'm sure your ready-mix supplier can verify it with past test reports.

Another way to get an earlier high strength is to add a 1/2 to 1 bag of cement/cy to the mix design. It is less expensive than high early cement or admixtures. The thing to be careful of is that above 6-1/2 bags of cement/cy shrinkage becomes more of a concern.

During your pouring, take a couple of sets of test cylinders (3 cylinders/set) so you can break them a 3 days, 7 days, and 28 days to verify your final mix design.

 

[Ron]

Plainsboro....there are several ways to accomplish your goal. I would not use calcium chloride as it serves only to accelerate the set, not the strength gain.

Be careful with adding cement to the mix as this will increase your overall shrinkage and water demand, thus increasing your crack potential. This could be detrimental if you are placing slabs or exposed structural components with minimal cover.

I would suggest you check with a reputable ready mix supplier as whymrg suggested. There are admixtures that achieve the same result as using a Type III cement (high early strength). As jheidt2543 stated, most good quality concrete mixes will achieve design strength long before 28 days.

I would suggest a mix with a low water-cement ratio (0.50 or less), and a high-range water reducing admixture to achieve workability. Then wet cure the concrete and watch the temperatures (colder weather will cause slower strength gain).

Good luck.

 

[dicksewerrat]

If the concrete can be tented, use live steam to hasten the cure. This how rcp pipe companies can ship pipe made only three days earlier

 

[Zambo]

All points made so far are good suggestions. I would ask the concrete supplier to supply concrete that has the required strength at 14 days, he should have all the test records to be able to achieve this. The other approach would be to determine that the loadings will be less during the first 14 days of use of the structure. For example if a storage tank initially you need only consider the self weight of the structure and then the contents later.

Zambo

 

[jheidt2543]

In the interest of adding some some more information regarding the addition of chloride in concrete, I submit the following quotations. Like any material or admixture, under the right circumstances, the addition of calcium chloride has a place in the construction industry.

Reference: Materials of Construction, 6th Edition by Lloyd F. Rader, John Wiley & Sons, Inc., 1967:

Page 413:
“The effect of adding SODIUM chloride to concrete cured at temperatures BELOW 32 degrees F is to REDUCE the freezing temperature and to retard the freezing of the concrete, thus permitting its setting and hardening. For curing BELOW 32 degrees F, the addition of UP TO 12 % by weight of mixing water INCREASES the concrete strength, but higher percentages are detrimental to strength. Approximately 1% of salt in the mixing water lowers the freezing point 1 degree F (0.55 degrees C). For curing at NORMAL temperatures, the addition of salt causes a DECREASE in the strength of the concrete. The integral use of CALCIUM chloride causes a more rapid rate of liberation of heat of hydration of the cement, thus promoting early strength development and thereby furnishing materially greater resistance of the concrete to the detrimental effect of low temperatures.” (Emphasizes are mine.)

Page 420:
“T.E. Stanton in a comprehensive series of long-time tests reported a definite increase in compressive strength at 1 year for Portland-cement concrete containing additions of calcium chloride up to 5 % by weight.” (T.E. Stanton: A Study of Calcium Chloride as a Strength Accelerator in Portland Cement Concrete, Proceedings of the Highway Research Board 1950, p. 232)


These statements pertain to the strength characteristics of the concrete and were made without reference to the effects that sodium and calcium chloride may have on the reinforcing steel or other embedded items. However, the addition of .5% to 2% calcium chloride to reinforced concrete that is not exposed to the weather has not been found detrimental. The finishing time of floor slabs is reduced considerably with the addition of calcium chloride and is used more than most engineers realize.

 

[Ron]

jheidt2543...while I agree that accelerators such as calcium chloride can be useful under the right conditions, the original query was regarding strength gain, not set acceleration. Calcium chloride does not significantly change the strength gain curve as is need by Plainsboro.

 

[SlideRuleEra]

While I'll be the first to agree that calcium chloride has its limitations, the original question gives equal emphasis to both strength gain AND cost. Since we know nothing about the reason for the question, calcium choride may, or may not be an option - the link given provides an assessment of pros and cons. Plainsboro can make their own decision; many other, and likely better, choices have been presented.

 

[samdamon]

Howdy Plainsboro-

I live in Plainsboro NJ on Maple Ave. Where are you located?
Regards

 

[BigH]

A kind of interesting discussion. I'd like to put out another thought. Ready mix plants "play" their design pretty close to the characteristic strength that is needed - i.e., if 35MPa is required at 28-days, the ready mix plant knows that a certain design will achieve this where the standard deviation is quite small, say 1.5MPa - so the "target" strength is, say 35+(1.5*1.65)= 37.5MPa. This is because they know their mixes quite well and have good control. If the same characteristic strength is specified, say, for an area where the mix design must be done from scratch, the "new guy's" target strength might be characteristic strength plus as much as 11MPa more (standard deviation of 6.7MPa), hence the target strength of the 35MPa would be 46MPa. So, you might find yourself setting a 28-d strength of 35MPa that the ready-mix guy can't achieve until about 23 or so days but the "new guy" can achieve the 35MPa in 14 days or less - why, because he had to design his mix in such a fashion as to achieve a real strength that is much much higher. Point on this - if you want 35MPa at 14-days, then you might, with the ready mix guy, order a different mix - say a 40MPa mix (but still have the "new guy's" 35MPa mix). Similar comparison of one ready mix plant with another would yield a similar relationship although much closer.
One other point that is very important is that the cylinder or cube strengths that are cast to "meet" the characteristic strength have no reflection on the real strength in the structure - not only does curing matter, but poor vibration (i.e., compaction) in placement may reduce the structural value by as much as 15%.

 

[DRC1]

We often specify a higher strength concrete to get an earlier set. Published curves exsist for % of f'c vs time.
I think 14 days is about 70% or so (but check to be sure - I don't have them now), Thus if you want 3500 psi order 3500/.7 = 5000 psi. This eliminates a lot of chemistry (never my best subject)

It has been my experience that concrete will break about 12 to 1500 psi high on the average. I think it is because plants would rather buy a little more cement than have problems with a pour.

If I am trying to do something like this, I always get imput from the quality engineer for the redi-mix supplier

One final note is that Big H has an excellent point that is often forgotten. Cylinders are cured in a warm humid condition which is ideal for strength gain. Curing of the concrete in the field can be radically different (especially this time of year). Willow Island was a tragic example of that.

 

[dicksewerrat]

You might want to add some micro-silica to get your rapid strength.

 

[pba]

To get 28 day strength at 14 days means that you need to do SOMETHING to the concrete. In reality as discussed above, the difference between 14 day actual strength and target 28 day strength may be very small. Can't you relax the the partial safety factors for two weeks?

 

[BigH]

One point I forgot - concrete is judged on 28-day strength test result. As you want the 14-d to be equivalent, make sure you have extra cylinders (cubes) cast to break at 14 days - or you may not have the 28-day ones to break. (May also want to cast more for strengths at 10 days, 21 days, etc. to better guage the strength gain with time curve.