Concrete Curing 2

[Zambo]

I planning the construction of a 30m high reinforced concrete tower. Section in plan 5m x 2m, concrete grade 90N/mm2.

The Resident Engineeer does not like curing agents (worried about them breaking down in the hot climate) and we have to paint the surface later which rules out some membranes due to compatibility problems.

The Resident Engineer would like to see water curing, sheeted in with mist sprays, high humidity etc. etc. I am considering whether shrink wrapping polythene sheet would be an alternative. The cost doesn't worry me as this is already an expensive structure, but anyone ever seen or tried this, will it work?

Zambo

 

[kenvlach]

I suggest the following: After stripping the forms off each stage, wrap the exposed concrete in several layers of burlap, placing a perforated polyethylene hose (irrigation drip tubing) around the top. Keep the burlap wet. Do not overwrap with polyethylene sheeting; it may save water, but due to the hot climate, I suggest simply using a little more water for evaporative cooling. Continue for at least 2 weeks (preferably 4) after completion of the pour.

This procedure is easier said than done, but it will keep the Resident Engineer happy, and give better concrete with minimal surface cracking. My limited experience with stiff, medium strength concrete mixes about half as strong as your 90 N/mm² (13053 psi) concrete suggests to me that it will absorb a lot of water while curing. I also coated several of these concrete pours with penetrating sealants for chemical resistance. The concrete kept wet for 28 days was ~fully cured and coated well; less critical areas that hadn’t been as well moistened were still highly alkaline and ‘ate’ the epoxy sealant (although we did a 3 wt% HCl neutralization and rinse).

Hope this helps. There may be higher tech methods or concrete admixtures with which I am not familiar, but old fashion methods are often still good.
Ken

 

[neil10]

Curing compounds are manufactured to breakdown over a period of time under UV light.They only lock in the water present in the mix from evaporation. Water curing is the best method to achive higher strength. There is an admixture available which is added with the mix water and claims to achieve internal curing.
contact MBT or Sika admixture manufacturers.

 

[flamby]

90MPa is a lot of strength. Can you give me an idea of mix composition?

Just curious.

 

[Zambo]

Been a bit of a delay in reply as we've been casting a prototype.

The charactersitic strength is 80MPa with a 12MPa margin giving a design strength of 92MPa. Concrete of this strength is achieved by using plasticisers to keep the water cement ratio low, silica fume and a PBFA or PFA OPC mix.

Curing has now become a bigger problem as early age curing with water was increasing the temperature gradient in the section and causing thermal cracking. We had temperatures of 70 deg. C in the core and 30 Deg C at the water cooled surface. Now we intend to keep the forms on for longer then wrap with polythene until the tmperature gradient reduces followed by water curing.

Any thoughts,

Zambo

 

[dirtsqueezer]

I have seen misting of a wall as you describe under visqueen under these conditions. I think plasticisers are always a good idea to use, as the contractor will have much less reason to add water on site. It takes out the element of low strenght results and all the problems that go along with a high w/c ratio- contractors don't add water to a mix that's already wet (using superplasticisers).

One more thing. If it's a massive pour, I have heard of suppliers producing using chipped ice in the mix. I believe it was for dam construction. That sounds a little different than what you're doing, but I just hadn't seen it mentioned elsewhere.

 

[flamby]

Mixing ice flakes in concrete mix and cooling aggregates by spraying water before mixing are all common practices, but you need some liberty in w/c ratio to have room to play. M90? Sorry, never tried. Ask your designer, why he specified M90 in a tall structure which will most probably depend upon steel for its strength. And also ask him how he thought the curing will be carried out. Design is not only calculation but construction feasibility too.

Based on your description, I believe the design is not sound. Or the structure is very special. What is it?

flame

 

[22jpTX]

This is actually part of my dissertation. Hope the following helps. I have instrumented a number of bridge columns with strengths a third of what you propose whose adiabatic temperature rise is even greater than what you report. I suggest you look at several of the RILEM pubilcations on thermal stresses in mass concrete. RILEM 15 - Prevention of Thermal Cracking in Concrete at Early Ages - ed. R. Springenschmid (1998) provides excellent guidance for this problem.

A 13000 psi mixture, with both the low w/c and silica fume, will probably have awful problems with autogenous shrinkage, as well as thermal cracking. Unfortunately, the two issues are interrelated, and their occurance depends heavily on a number of factors. Thermal cracking doesn't depend solely on the maximum temperature gradient in the structure. More critical is the temperature gradient at which mechanical properties begin to develop, and subsequently the final temperature gradient from that point.

The following suggestions are pulled from RILEM 15 (1998)
1. Reduce fresh concrete temperature - as low as possible
2. Lower cementitous content - how much strength do you really need?
3. Use aggregate with low CTE - e.g. limestone
4. Use larger max aggregate size, and reduce paste content.
5. If possible, reduce the restraint
6. Use air entraining admixture

You must water cure such as low W/CM mixture, or autogenous shrinkage will crack the structure. Do NOT use curing compound in addition to water curing. Unfortunately, it seems contractors never water cure properly anyway (I used to work for one!)

Hope this helps.

Take care

Jon Poole