weight classes of CMU

[woody1235]

What are the relative advantages and disadvantages of normal, medium, and light weight structural CMU? As a typical matter of course, we specify medium weight (115 pcf, but really don't have a knowledge base of why this should be so. Seems like, in most cases, the primary building structural properties can be obtained in all three,(but would expect durability to be somewhat different if used in exterior).

Have Googled around, and didn't find a related thread here.

Any anticipated pluses and minuses?

Thanks!

 

[JedClampett]

Some sharper contractors will give you a discount (not insignificant) for the lighter block. But the surface is a little rougher and doesn't take coating as nice.

 

[concretemasonry]

There is no such thing in ASTM C90 as weight classification, which was eliminated many years ago. The units are determined by density, although some old codes and local union agreements may refer to the weight of the individual units. ASTM C90 defines normal weight as greater than 125 pcf, medium weight as 105 to 125 pcf and lightweight as less than 105 pcf.

The economics of density depend on the the location and materials commonly used, although other aggregates can be specified for certain applications. I have seen high density units made using iron or even steel shavings for nuclear or X-Ray protection since the handing and molding equipment can handle most material because concrete block are commonly used is almost every country.

Usually, the local aggregates available are used and dictate the cost of normal units, although a producer can not afford to use crude dirt, sand and rock with cement added. In many areas, lightweight CMUs may be the most economical to buy and lay, but in other area normal weight CMU units may be cheaper to buy and it is up to the contractor to decide how much higher the labor may be. Labor is the major cost factor in the cost of a wall.

Normal weight CMUs offer more mass and less sound transmission than others. Lightweight CMUs off greater fire protection and better acoustics (depending on the surface texture). Medium weight CMUs are usually a blend of lightweight aggregate (natural or manufactured) or in some areas, waste materials.

The minimum strength requirements (a performance requirement)for all types (used by most engineers) is 1900 psi (net area) and it is really not economical to produce the minimal strength units because of the clash of technology, manufacturing methods with the olde tradition. It is possibile to get much higher strength if the project is large enough, I have seen 8500 psi units (net area), which is 4 times the ASTM minimum. In South America, 3 to 5 different strengths are used on the common high rise load bearing structures.

The National Concrete Masonry Association (ncma.org) has over 100 TEK notes available on their web site (upper right corners as I recall). Just enter any location and any producer to get the access. - It is a well hidden resource.

Dick

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

 

[concretemasonry]

I neglected to mention that for structural purposes, the wall is meant to be treated as structural element. The nit-picking analytical people have a difficult time accepting this and look on each of the individual materials (units, mortar, and grout) without looking or understanding the concept in ACI 530, the code requirements or the years of research. Some engineers even advocate full grouting of core and high strength grout, which is counter-productive. Better engineers place an upper limit of grout strength to preserve the wall properties to conform with the basic assumptions.

The ultimate measure is the prism test (f'm), although the relationship between unit strength and mortar is much easier to administrate and shows the low influence of the thin layer of mortar.

Dick - opinionated as usual.

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

 

[woody1235]

Thank You for your input.

It follows fairly well with what I had thought. In this case, we had spec'd normal weight and the contractor is asking for the change.
Part of the project is exposed vehicle work bays, other part is offices.
Some shear walls heavily loaded, but no real high f'm required.
Owner expects a very high quality building, so will consider all these aspects to decide.

Thanks!