Any thoughts on Sand-Cement mix for structural components? (no coarse aggregate)

[structuresguy]

Hi all. We are working on a project which is located in an area where there is no coarse aggregate at all, only sand. We had desired to create a 4000 psi sand-cement mix to use for structural foundations, slab on grade, 8" thick 2-story high walls, CIP stairs, and infill on composite metal deck.

We are concerned about being able to achieve the 4000 psi, and about the suitability of such a mix for the intended use. Our local ready-mix supplier had no experience with such a mix, though his gut feel would be that it would be too brittle for our use.

Just wondering if anyone has any experience using a sand-cement mix for structural components. Some thoughts we had to make it more durable were adding synthetic fibers to the mix, and changing the reinforcing from a typical single layer mat (#5 @ 12" o.c.) in the walls to a double layer mat of smaller, closer spaced bars, or even heavy WWF.

Any thoughts you have are much appreciated.

Thanks.

 

[concretemasonry]

I would be concerned if the local supplier has no experience with a similar mix. It puts the onus on you to determine if the 4000 psi is actually needed and/or accept some test reports on proposed mix designs.

Much has to do with the actual gradation of the sand AND the particle shape. I assume it may be a glacial deposit with rounded particles. A single size gradation could be very difficult.

The ASTM aggregate specs have the tradition option of saying essentially non-spec aggregate can be used if a HISTORY of performance can be shown.

I boils down to getting test reports on the aggregate and mix designs.

In a plant/manufacturing situation, that strength can be easily achieved, but that is using almost zero slump concrete and they have the advantage of having tightly controlled vibration and accelerated curing that is not available in the field.

Monitoring placement and testing will be required if you vary from the ASTM aggregate specs.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[structuresguy]

Sorry, should have mentioned that project is in the amazon jungle, so unlikely to get any good historic test data or ASTM style testing of the sand. When i said local supplier, i meant local here in the US. I have no contact with any supplier at the job site. cement will be bagged product shipped in by barge. Sand will be locally sourced, whatever clean sand (hopefully) they pull out of the ground. I saw some concrete work in the town when i was there, but mostly limited to tie beams/columns used with hollow clay tile masonry, and also slab on grade. Elevated slab construction that i know of is a CIP joist/slab combo made from laying the same hollow tile sideways on forms, leaving a couple inches between successive rows of the blocks. They then drop a single bars into each "joist", and lay a mat of bars in the 2-3 slab poured over all the tile. The tile is grooved and gets bonded to the slab, which they then stucco the bottom for the ceiling. However, none of the native construction really meets modern building code standards and seismic design criteria imposed by our client.

We probably don't really need 4000 psi. I am sure anything 2500 psi or better would work just fine, compressive strength-wise. I'm just really more concerned with the behavior of the finished product. Will it behave like normal concrete structures, or will it be really brittle, and have higher shrinkage problems?

 

[concretemasonry]

Trying to reach 4000 psi will certainly require more cement because of the surface area of the aggregates and the shrinkage will also be higher. That is typical with going away from the ideal gradation for concrete.

Despite its size, Brazil/Brasil has some unique aggregate situations.

The floor system they use in some cities is usually referred to as a filler block system (concrete block or clay tile) that is very common for high rise (7 to 20 stories) construction. Many buildings are designed by engineers with the same floor plan and wall thicknesses for either block or poured concrete (usually 6" walls).

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.