Tilt-Up Construction References?

[Aton]

I am researching tilt-up construction methods for several designs currently on my desk. I am planning on using a high-volume fly ash (ASTM Class F) reinf. concrete (56% replacement of portland), with which I have had some good experience already, and whose properties I like. I have successfully designed ICF walls, but have never actually done a horizontal tilt-up placement. I have searched the threads in this forum, and have checked out the FAQs and done a search, but this would seem to be a new topic. This is not a common type of technique yet in my area. After several hours unsuccessfully searching the WWW, I am coming up blank. Does anyone know of any references that deal with Tilt-Up design in particular? For example, anchor placement, connections for double wythe construction, proven methods of exterior wall treatment for architectural effect, etc.? I do not mind inventing a wheel - but it's silly to try to re-invent one. Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

 

[ishvaaag]

I have some pdfs downloaded but it is easier to search the web

http://www.tilt-up.com/LearningCenter/Default.asp

and browse links resulting from search

http://www.google.com/search?hl=es&q=Tilt-Up+Wall+Design+Manual+pdf&btnG=B%FAsqueda+en+Google&lr=

 

[JAE]

PCA has some publications on tilt-up as well as Dayton-Superior.

With a high fly ash content, I would expect (but don't know for sure) that your modulus of elasticity (E) would be affected. With tilt-up construction, many times, you have a slender element where the design is affected greatly by the second-order effects (PDelta) which are in turn affected by the E. Be sure to have a good handle on the Ec of your mix design.

 

[Aton]

Thanks, Ishvaag! The websites are helpful - some PDFs to download, and some books to buy, so much reading lays ahead! I guess I'll set aside Plato for a while...

And thanks for your comments, JAE. I think E is OK. From research at CANMET, the Ec of test cylinders (of a similar design mix to mine) at 28d averaged 36.8 GPa - as compared with the control (no fly ash) mix which hovered around 32 GPa. I know that one drawback with high-volume fly ash (HVFA) concrete is that it has a fairly low early strength, so I cannot lift the slabs too soon. However, once in place, there are several advantages:

HVFA concrete tends to be far more dense than regular mixes, and thus shows a far greater impermeability to capillary suction (nice if you live in a climate of terrific freeze/thaw cycles as we have here!).

Continued hydration of the fly ash pozzolan (provided hydration is allowed to continue!) means that the strength curve keeps increasing for an extremely long time, and 2 year strengths far exceed those of regular portland cement cured under similar conditions. In one standard foundation placement I did in '94 (with no special continued hydration), I made some test cylinders and found their compressive strength, f'c to be 32.2 MPa, and the Modulus of Rupture, fr, worked out to be 3.4 MPa [from the equation fr = 0.6*lambda*SQRT(f'c), where lambda = 1.0 for normal density]. In other preliminary compressive tests I myself have performed, f'c worked out to be extremely close in both designs - one with 56% fly ash (f'c = 30 MPa), and the other with a standard 20 MPa design mix (f'c = 28 MPa from tests). At any rate, for my tilt-up, I may be specifying two layers of reinforcing, to allow for bending moments during lifting. But that is what this reading will tell me, I hope.. .. Regular P-delta effects (once the slab is upright) are already worked into a spreadsheet I made for other non-tilt-up projects. With no experience in the actual tilting design, I need to do some study, and I am happy to listen to anyone's words of caution!

This HVFA is an interesting mix, with extremely low w/c (made possible by superplasticizers, of course, but also by the fact that the fly ash particles are spherical in nature, and thus act as dry lubricants between the shards of portland and sand particles). Fly ash spheres average less than 45 microns. Its on-site slump should be almost zero, until the SP is added, and then it is as if the flu bug hit - which means that formwork must be extremely tight. This, coupled with a lower heat of hydration, makes for a condition where shrinkage cracks are far less than regular concrete – so larger slabs may possibly be practical.

In addition, there is something about the chemistry of the Class F fly ash that makes even a 20% replacement mix virtually inoculated against alkali-aggregate reaction - not to mention the reduction of CO2, a greenhouse gas, due to less portland.

The above comments are about ASTM Class F ash, and NOT Class C, which is not to be recommended in these designs (in fact, some Class C mixes will actually increase the susceptibility to AAR).
Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

 

can i know the advantages of lift slab construction

 

for lift slab construction, one thing for sure is that you will no longer need underpinnings and bottom form works of your slab. For a better finish and rigid monolithic construction,pre cast half slab(pre-stress)construction technology is employed.

 

[Jim6758]

I am surprised that you are using a HVFA mix for tilt slab construction since the strength gain is so slow. Around here, the contractors would like to lift as soon as possible and often use rich mixes to speed the time when they can lift the tilt up slab. Make sure the contractor's schedule will allow enough time for the panel to cure before lifting.

 

[Aton]

Jim6758: Thanks for your thoughts. Yes, I am aware that HVFA concrete can be slow to set. In my case, I will be in control of the placement and schedule, since I will be the contractor for this experimental building, as well as doing the engineering. No client, small building, my schedule.... The experience I have had with HVFA to date has shown that the set times were not as long as the literature suggests. Since the properties of this mix are superior to regular concrete, I think the experiment is worth it. At present, I am still working out the design, and hope to begin work later this year - but it may be next spring before I can do it.

Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.