Concrete Accelerator 1

[SperlingPE]

What do I need to be aware of if accelerator is used for a concrete wall? Accelerator is being considered to be able to backfill sooner. It is an 18" thick, 14' tall foundation wall. It will be supporting first floor framing and have some columns bearing on it.

 

[SlideRuleEra]

If (of course it does) the wall includes any steel (rebar, anchor bolts, embedded pipe, inserts, etc.) do not use a chloride based accelerator... a lot of them are, including some products from reputable additive manufactures. There are alternative products that do not use the chlorides which promote steel corrosion inside the concrete.

For unreinforced concrete, straight calcium chloride works like magic. I've used it for underground plastic pipe thrust blocks and public highway crossings that had to be backfilled quickly. Within a couple of hours of completing concrete placement, do the backfill.

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[SperlingPE]

I am aware of the chloride based accelerators and the effect on reinforcing.
I can't recall ever using an accelerator in a cast wall situation.
good idea? bad idea? what should I look out for?

 

[TehMightyEngineer]

I'd reach out to Euclid and BASF and see what they recommend for non-chloride accelerating admixture. Ideally you'd find one that has water-reducing properties and a delayed set property as well. This allows the contractor to pour and finish their concrete like normal without trying to race against the clock and the reduced water helps with shrinkage cracking and concrete density.

Accelerated mix design should really be under the means and methods of the contractor mixing the concrete. I'd get some spec language from BASF and/or Euclid and have the contractor submit a mix design for your approval. As with all new mixtures the first batch will be a bit of a learning experience and they'll fine tune the properties on the subsequent batches. Make sure proper testing is done on the batches unless they already have the required historical data to prove it meets strength and other design properties.

The only thing I'd really worry about is it setting up in the forms between pours. I reviewed a job a while back where the concrete was shipped 2+ hours and even with set retarders the foundation wall ended up full of cold-joints and honeycombing. Thus, be very clear that if they delay too long between pours they may junk the entire wall or require extensive repair.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[SperlingPE]

I appreciate the information. This is a CM project. The wall is 150' long, 14' high, and 18" thick.
I'd like to hear about any issues to be concerned with using an accelerator.
The goal here is to be able to back fill sooner than 28 days.
I know strength will likely be achieved in 5-10 days, but that concrete is still 'green'.
The process for the design mix and testing and any other items are surely important.
At this time, I want to hear about your experience with using an accelerator for a cast in place formed wall.
I may just have to go to a supplier.
My hesitation to go there first is that they are in the business to sell a product.
I would like to hear from engineers with first hand experience for this particular situation.

 

[TehMightyEngineer]

My hesitation to go there first is that they are in the business to sell a product.

You're 100% right, they will tell you only good news and no bad news. Their product will solve all your ills and has no flaws. That said, their job is also to help you use their product correctly. They'll help select the correct product, get you info on it's use, help with the mix design requirements, etc.

In reality most accelerators will work fine, but often times you may have unintended side effects (workability loss, poor air entertainment, too sticky, etc.) most of these can be solved by tweaking the mix design and careful testing and monitoring during the first pours. If you're really concerned maybe you can setup a batch mixed in a portable mixer and placed in a small test form. This would help you evaluate the mix, the set time, and the finished surface. Costly but would help guarantee success.

For your need an accelerator is definitely the way to go. Other than it setting up too quickly or the initial tweaking of the mixture, I'd say this isn't an unusually difficult task.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[MotorCity]

My concern would be that you start to get an initial set DURING the pour, given the size of wall we are talking about. You may want to consider a super plasticizer to facilitate quick placement. I am not sure if the accelerator would be compatible with a superplasticizer or if they would tend to counteract each other.

 

[SlideRuleEra]

This is a CM project.
The wall is 150' long, 14' high, and 18" thick.
The goal here is to be able to back fill sooner than 28 days.
I would like to hear from engineers with first hand experience for this particular situation.

I would not use an accelerator. The problem is wall proportions - long (150') and high (14'). Assuming there is one concrete placement crew, concrete will have to be deposited along the entire wall length in layers each less than 4' thick. The crew will have to make at least 4 passes along the entire wall length... this will not be quick. Even without an accelerator a real challenge for the Contractor.

Making things even slower, if the initial concrete is allowed to free-fall 14' into the forms, all the rebar (think top to bottom) inside the forms gets coated with concrete. By the time the upper layers of concrete are placed, concrete coating the upper rebar will destroy the concrete-rebar bond. Solution is to use a tremie or pump the concrete directly to the lower layers. Of course the crew has to continue to move the length of the wall at the same time... slow.

If fact, as a former bridge contractor, I would consider using a retarder, not an accelerator... and just build the forms for increased hydrostatic pressure.

IMHO, the best compromise is use a high-early-strength additive. Any Contractor that is qualified to build this wall will certainly be capable of responsibly backfilling well before 28 days.

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[hokie66]

Agree with SRE. But if this wall is not designed as a cantilever, you still need to brace it before backfilling, either by the floor which it supports, or by temporary means.

 

[SteelPE]

I like the discussion about a non chloride additive to the concrete. In all my years I have never seen a chloride accelerator additive be proposed for a mix (at least none to my knowledge). I didn't even know they still existed. Then I made a site visit about a month ago to a small project. The site super and the concrete foreman were onsite and were firing tons of questions at me (an unnerving amount for such a small project). At then end, they mentioned something about the approved mix design to which I asked if they still made/used chloride accelerators. The foreman said "Yes, they are used all the time". I was surprised because I have never seen one come through in a mix.

It was a good conversation and I'm glad I know that they are still out there and that I need to still stand on the wall.

 

[hokie66]

SteelPE,
Ignorant and/or unscrupulous operators don't put it in the mix design, they just put it in the concrete.

 

[Ron]

Agree with SRE...you're more likely to need a retarder than an accelerator.

I think you're in an area that gets pretty cold, so you might want to check out ACI's Cold Weather Concrete procedures, assuming you have a placement coming up soon.

 

[TehMightyEngineer]

I (obviously erroneously) assumed you would be pouring the wall in sections with expansion joints at the boundary between pours. Agree completely with SRE about pouring this whole wall in one go.

Interesting thought about the concrete setting up on the rebar. Generally never considered that but I could definitely see that happening.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Ron]

TME....I think you recently got licensed in Florida. Hot rebar and slow sequencing are facts of construction life in the southeast! Be ready!

Yes...concrete setting up on the rebar before encapsulation is a problem as SRE noted.

 

[TehMightyEngineer]

True that; toasty Florida rebar probably cures any concrete stuck to it very quickly.

If you were pouring this wall in one go Ron in a hotter clime, would you use construction joints or set delay admixtures? Both?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[SlideRuleEra]

TME - To a Contractor, the wall contains 117 yd³ of concrete, for pay purposes. For a moment, disregard the schedule, which the OP implies is important. Financially, 117 yd³ is a reasonable size pour considering the work involved. If the pour is divided into sections... how many?

Half (each 75' long) does not help.

Half, say 6 sections at 25' each, with odd number (1,3 & 5) being placed one day and even numbers (2, 4 & 6) placed another day makes appears to make some sense... however, formwork becomes a pathwork quilt and there are complications with rebar extending through the ends of each form (14' of hydrostatic concrete pressure, since each 25' long section will be placed fairly quickly). All this takes extra time (labor cost) with no increase in pay quantity.

Contractor would have to evaluate weather, other job conditions, schedule, workforce, available equipment, concrete supply dependability, etc. to make an informed decision. If one placement crew cannot handle the work, probably best to add a second placement crew for the single pour. A qualified Contractor would have picked this up and incorporated conservative means and methods pricing into the bid price - the Engineer and Owner would never know it.

Here is a photo of an overpass railroad crash wall under construction on one of our project that gives an idea of (wooden) formwork needed for 14' hydrostatic pressure. Note the worker on the right, he is about to lower a concrete vibrator into the forms on a rope... just another detail means and methods that is not obvious, there are others.

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