concrete without coarse aggregate 1

[YKalaf]

Is it possible to justify that a proper mixture of sand, water and cement (concrete witout coarse aggregate) with strenght of +3000psi, placed pnewmatically (shotcrete) is still concrete and can thus be designed by ACI requirements.

 

[henri2]

This is covered in ACI 506 and IBC Sec 1914. The are two types of shotcrete as explained below.

Dry mix shotcrete (gunite) does not include coarse aggregate. It's mixed at the jobsite and water is added immediately just before discharge from nozzle. It can easily meet f'c of 3000 psi. 1 part cement to between 4 to 4-1/2 parts sand is very common.

When it's ready mixed, its referred to as wet mix shotcrete; essentially a low slump(2 inches) pea gravel (3/8 inch MSA) concete mix is typically used on commercial projects. Some public works projects specify 3/4 MSA.

Regarding maximum aggregate size, here is what IBC Sec 1914.3 states. "Coarse aggregate IF used, shall not exceed 3/4 inch. So I guess this permits a fine aggregate wet or dry shotcrete mix.

Shotcrete can be designed as concrete using ACI 318 but check relevant sections in ACI 506 Guide to Shotcrete (explains what design code to use) like Sec 5.4.2 and IBC Sec 1914.4 for splice requirements...contact lap splices are not permitted. Check the FAQ page at

http://www.shotcrete.org/ASAfaqs.htm

question 38 is a good one.

 

[JAE]

Nice post henri2

 

[YKalaf]

Thanks a lot henry for your post. Its been very helpful. One last question before I go and order the ACI 506 and related documents if you would be so kind: Does it specifically says in the aci 506 that, be it dry mix or wet mixed, shotcrete structures shall be designed as per ACI318?

 

[henri2]

YKalaf, I cannot find my copies of ACI 506 specs/practice..but I do recall an older version I gave away stating that ACI 318 is the design method and it did not make a distinction between the dry or wet mix.
FYI, there are 3 companion documents for ACI 506...namely: 506R, 506.2 and 506.4R

An internet search indicates ACI 506R-90 (5.4.1) states reinforcement for shotcrete shall be designed per ACI 318.

Further Evidence:

In the last version of the UBC(97) according to Sec 1900.4.2 Shotcrete...it states" In addition to the requirements of Division II, design and construction of shotcrete structures shall meet the requirements of Division IV". Division II of the 97 UBC was essentially ACI 318-85 and Div IV is similar to IBC Sec 1914 and both are based on ACI 506.

Fast forward to the present. In the IBC-03, Sections 1901.2 and 1914.1 are the references stipulating ACI 318 be used for design...but as mentioned in previous post, amongst other things, IBC Sec 1914, modifies reinforcement requirements of ACI 318.

 

[YKalaf]

Thanks a lot henri2, again.

 

[BigH]

One more point - in wet mix. henri2 indicated a slump of about 50mm. This seems very low to me. Of course, it depends on the type of pump you are using - whether a piston pump or a pneumatic one. In the later, we are using slumps for wet mix over 200mm.

 

[henri2]

BigH, up in Canada I guess the shotcrete is air-entrained so I imagine the slump will be higher as it is difficult to achieve adequate air-entrainment with stiff low slump mixes. However, 200 mm slump which is about 8 inches slump seems way too high even for regular concrete except when a super-P (high range water reducer) is used.

Check paragraph 5 of the response to question 44

http://www.shotcrete.org/ASAfaqs.htm

it indicates a typical slump for air-entrained concrete in Canada. Now check Corp of Engrs Standard Practice for Shotcrete Sec 3.7 (e) Wet Mix proportioning

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2005/basdoc.pdf

and also read what 3.7 (e)1 says when the slump is too high.

Perhaps what you have in mind is a pea gravel (3/8 in/ 10mm MSA) mix that has a high range water reducer...a mix which is commonly used for situations where the reinforcement is congested..no?

 

[BigH]

henri2 - I'm not in Canada presently. We found that our pneumatic pump (not a piston pump) would not pump the wet mix (5-10mm coarse agg, and fine agg with ribtec fibres) with a slump of 150mm. We are using rheobilt plasticizer and intraplast Z (sort of like intraplast N).

 

[henri2]

BigH, a few points:

1. The adjectives 'Wet-mix' and 'dry-mix' as used in this thread are construed to be part of shotcrete terminology consistnet with ACI 506.

2. Wet-mix shotcrete has a much stiffer consistency than the 5- 10mm mix you describe. The mix you describe has a much wetter consistency and is typical of small aggregate concrete with a super-plasticizing admixture, in fact very similar to masonry grout; it will not be used for shotcrete.

3. Positive dispacement piston type sytems and pneumatic pumping systems are used for both wet-mix shotcrete and regular concrete(including the super-plasticized small aggregate concrete mix you describe).

4. The pneumatic type of pumping system used for regular concrete is similar in certain operational aspects to the one used for shotcrete. However, in addition to the differences in operating air pressures there are other major differences.

5. Shotcrete is delivered at a much higher velocity (for effective impact so it adheres to surface it impinges on)..just look at the wet-mix shotcrete nozzle in Fig 4-9 and how it fits into the system in Figures 4-6 and 4-7

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2005/basdoc.pdf

check where the air compressor fits in..does your system use a set-up like that? Discharge in the pneumatic pump you are using would be at a much lower velocity.

 

[YKalaf]

I thank you all for your answers to this thread. My inquire was posted because I am currently working for a company that sells shotcrete-polyurethane-shotcrete sandwitch panels in Dominican Republic. The bilt dimensions of this panel is 35 mm shotcrete - 80 mm plyurethane - 35 mm shotcrete. Shotcrete is reinforced with 2.5 to 3.5 mm galvanized steel mesh spaced at 65 mm with yiel strenght of 87,000 psi. Both shotcrete layers are interconected trough the polyurethane by 82, 3.0 mm conectors per M2, that make them work as a single 70 mm wall panel.
While submiting design calculations as per ACI 318, city official told me that this was not possible and that ACI 318 does not cover this type of construction.
Henri2 your answer was right on the money. Found .pdfs of both ACI 506 and IBC and I have corroborated your post.

Thanks Again