AAC Block for Residences 1

[bkgd]

I have a client that would like to use a Autoclaved Aerated Concrete Block (AAC Block)for residential construction either in a high seismic design regions or otherwise. One manufacturer recommended following ACI 530 for strength design of masonry. The block has a compressive strength varying from 290 psi to 760 psi.

If you type in AAC block you will hit numerous websites.

let me know, it seems like a good product for residential, comes in 6", 8", 10" and 12" can be reinforced as needed

opionions would be intersting



thx

 

[dik]

Seem awfully low strength to use for flexure in a seismic area. I've not been involved in high seismic areas with masonry. There may be local codes or bylaws that come into play. Also use as foundation will be determined by soils type and seismic effect of these soils. Maybe someone with greater exposure to masonry foundations in seismic areas can fill in some details.

Dik

 

[bkgd]

ya , i hear you on the strength side of things but when you compare this to a 2x6 stud wall versus a 8" or 12" aac wall with 435 psi it is much stronger, and i would prefer a masonry wall built out of masonry compared to a plywood shearwall, particularly when you get the strength from a substance that is barely heavier than wood based on unit weights 37 pcf for AAC

thanks for the input

 

[concretemasonry]

AAC is an older European building material. Most of the experience is in one and two story reisdences in eastern/central Europe. These are not critical seismic area. Because of the low strength, hollow masonry units (clay block/tile or concrete block) are used in critcal locations and building over 2 stories.

The first AAC was produced in the U.S. in the 1960's, but all these plants are now closed. The recent large U.S. plants were originally built by European interests (Ytong and Hebel), but have changed ownership and names.

The production is quite energy intensive because of the autclave process. In eastern Europe (Poland through romania), most plants were government owned and built in conjuction with power plants. Recent privatization has changed the industry significantly.

The compressive strength varies with the density. Many plants in Europe offer only one density, but advertise the strength of the dense material and the insulation values of the lighter materials. In reality, the properties of AAC vary with the orientation of the material during the maufacturing process. During manufacture, the material rises like bread contained in a large bread pan, so the expansion is all in one direction. I have not checked out how this was handled in the current ASTM specifications.

I believe there is also a tenative specification for a specialized mortar in preparation. I will check to see if it is published or approved.

Some panels/slabs are avaiable. Most of the production is solid units that are laid in a thinset mortar. Because of the sold units, reinforcement and continuity are difficult and the original details included routing out a vertical channel in multi-story building for the placement of cables. Usually, channels and holes for mechanical and electrical must be chipped/sawed/drilled after the wall is built. The large, solid units can be difficult to handle in comparison to conventional hollow concrete block that are easier to grasp and lay.

The material must be covered in exterior applications. Traditionally, the interior walls in Europe were plastered.

 

[concretemasonry]

There are several current ASTM standards appropriate to AAC design and construction. They are:

1. ASTM C1386-98 - Standard Specification for Precast Autoclaved Aerated Concrete (PAAC) Wall Construction Units.

2. ASTM C1555-03a - Standard Practice of Autoclaved Aerated concrete Masonry.

There is current ASTM standard for the set mortar for AAC. There is a draft document for thin set mortar for AAC currently ungoing review. ASTM C1555 states that the mortar shall meet the performance standards prescribed by the AAC manufacturer.

ASTM C1555 gives the practices for the use of the units. It refers to the material standards contained in ASTM C1386, workmanship and the "Exterior Surface Treatment". Recommended required surface treatments are an exterior wyhe of masonry, a cladding system or a coating meeting liquid and vapor permeability requirements.

ASTM C1386 is a material standard similar to ASTM C90 for block with different requirements and also some unique testing requirements to replace the ASTM C140 procedures for concrete masonry. The standard lists 3 strength categories that may be available and the compressive strength testing procedure to reflect the orientation and three different locations samples should be cut from. The direction of the rise should be noted on all specimens.

Testing is only for units inspected and sampled at the place of manufacture from lots ready for delivery. There is no provision for site sampling or insurance that the orientation of the solid units in the actual construction is consistant with the orientation during testing (similar to parallel to grain or perpendicular to the grain).

The use of the product and the specifications are a "work in process" to become compatible with traditional U.S. methods.

 

[bkgd]

Concretemasonry

thanks very informative and useful, i appreciate the fast and thorough response.


bp

 

[dik]

CMU
thanks for the added info

 

[rgerk]

We used it on a condo project and decided we wouldn't use the product again for several reasons.

It is brittle and easily damaged if not handled properly.
Had several crack issues the manufacturer couldn't explain.
The contractor wasn't familiar with the product which made construction a bear when issues arose.

Bottom line, interesting product, but I won't ever try it again.

BTW, it was a low seismic area, so no help from me there.