precast concrete well cellars - advice on production issues

[ARS97]

The company I work for has decided to fill a niche in our area by building precast concrete well cellars. These well cellars are essentially a four-sided box with no top or bottom that are buried in the ground to provide access to gas well heads.

First of all, we're not a certified precast manufacturer......we're a LONG way from that type of quality control. Nonetheless, they're wanting to build these things.

Anyway, I'm trying to develop a written procedure that (a) meets acceptable ACI standards, and (b) the production guys, who aren't well versed on any concrete codes/specs, can use and understand. This procedure must cover the complete production process, including time frames for form removal, curing times, possible cold weather & hot weather protection, etc.

We're going to be using a high early strength concrete mix (5,000 psi) with a superplasticizer.

1. ACI 318-11 Section 5.11 - with high early strength concrete, the concrete must be maintained above 50 degrees and kept moist for a minimum of 3 days, unless "alternative" curing methods are used. I'm being told that some of the production guys see similar precast units having the forms stripped and just left exposed on the shop floor with no wet burlap applied. Does anyone (maybe some precast people) know how this is accomplished? I've downloaded ACI 308.1-11 and briefly read through it - I noticed that there is something called "liquid membrane-forming curing compound" that apparently "locks-in" moisture. (There are other methods discussed as well.) Perhaps this is what precast manufacturers are using? otherwise, I'm not aware how it would be possible to NOT take the proper precautions to prevent excessive moisture loss during the curing process.

2. They want to know how soon they can strip the forms on these units. From what I'm being told, the forms will be stripped and the units will just sit there until there is enough strength for lifting purposes (whole other discussion). Due to the low levels of anticipated stress (selfweight of the walls themselves), as long as the walls remain undamaged from the removal, the forms can be removed. ACI 347R-14 Table 5.7.2.3 provides some general guidance to this time frame - it lists 12 hours for walls. I suspect that it could be earlier since it's a high early strength mix though. I think the best way to handle this is to start with the 12 hour time frame, and reduce it as needed for our specific purposes. Make sense?

3. Some guys are trying to steer me towards NPCA standards instead of ACI. I have no knowledge of NPCA, but I would assume that it circles back to ACI in some fashion?

Any precast people out there that can lend a hand? Thanks!

 

[precast78]

I think ACI is written mostly for cast in place. If your precast plant is outdoor with high temperature in the summer or cold temperature in the winter, you really have to follow it. If you are doing it indoor, I would really follow it for the first 12 hour.

1. As long as you do not see surface cracking from curing too fast, you are ok. At our company we use concrete mix that is 1000 psi above the design. Yes perhaps putting it in the yard after 1 day will reduce strength, but we are probably talking about a few hundred psi. Some DOT jobs do require curing compound to "lock" the moisture.
2. You need to analyze the precast piece at whatever strength you strip it. I would add 1.3 factor on the dead load to account for suction when you stip it from the form. As long as your moment does not exceed cracking moment, you are good to go. You also need to make sure your lifter will handle the load at lower concrete strength.
3. NPCA is just a group of precasters. They have a certification program. If your plant is NPCA certified, it gives your customer extra confidence on your products.

This is just my limited knowledge from working in the industry for a couple of years. I could very well be wrong.