lightweight vs. normal weight CMU 1

[structuralnerd]

Are there any structural property differences I should be aware of for lightweight CMU vs. normalweight CMU? Obviously lightweight weighs less, but is that the only difference? Will it affect fire rating of a building?

 

[mjl23]

This one doesn't look like it's available yet:
TEK 2-6 Density-Related Properties of Concrete Masonry
[see Section 7.4 here

http://www.ncma.org/members/minutes/2007MY/MasonryDesign07.pdf

]

In the meantime, you could try: Tek 14-13A Concrete Masonry Wall Weights (2002)

Concrete masonry walls provide a range of benefits,
including structural integrity, fire and sound resistance, energy efficiency, insect resistance, durability and architectural interest. Many of these attributes are measured quantitatively, and often [COLOR=red yellow]vary directly with the weight[/color] of the concrete masonry wall.
Wall weights are used directly to calculate:
- sound transmission class (STC) ratings, with heavier walls providing higher STC ratings and hence better sound insulation (see TEK 13-1A, ref. 1, for further information),
- seismic base shear force, with heavier walls resulting in higher base shear values (see TEK 14-12A, ref. 2, for further information),
- heat capacity, an indication of thermal storage capacity,
with heavier walls providing higher heat capacities (see
TEK 6-16, ref. 3, for further information), and
- gravity loads (wall dead loads) on structural members such as lintels and foundations.
Additional wall properties are also impacted by wall
weight, although estimates of [COLOR=red yellow]these properties are based on concrete density [/color] or aggregate type, rather than directly on wall weight. Examples of these properties include:
- fire resistance ratings, with lighter weight walls providing more fire resistance in general (see TEK 7-1A, ref. 4, for further information), and
- thermal resistance, with lighter weight walls providing higher R-values (see TEK 6-1A, ref. 5, for further information).

Also maybe check these:

http://www.ncma.org/online/Fire/TEK.pdf

http://www.nwcma.org/TekNotes/CMFireRes0205.pdf

http://www.ncma.org/fire_acoustic_calc/

[purple](calculator)[/purple]


If you don't know already, go here and pick a state to get into an access point to view the teks:

http://www.ncma.org/map_pages/state_finder.cfm

 

[JKW05]

As mjl notes, thermal resistance, fire resistance, and sound transmission properties are affected. And obviously the dead load from the masonry would be reduced. But the compressive strength shouldn't be affected. If you specify an ASTM C 90 block, it has to meet the minimum compressive strength and face shell/web thicknesses of that standard.

You may want to check with local suppliers on the availability of the different weight units.

 

[JLNJ]

You specify the PRISM strength you need. The masonry contractor will consult with his supplier to choose the block which meets the specs. ASTM C90 requires a minimum of 1900psi but if your specified prism strength is above 1500psi then the block will be higher strength.

Most suppliers have test data for things like sound transmission and fire rating specific for their lightweight units.

 

[CJJS]

Fire rating is affected. Compare UL ratings of lightweight vs. normal weight.

 

[JKW05]

ACI defines f'm as the specified "compressive" strength. And I agree, if the required compressive strength is above 1500 psi, then a stronger block would be required.

The PRISM test is one way to determine the compressive strength. But Section 1.4 of ACI 530.1-02 also permits the Unit Strength method to be used which requires:
1) ASTM C 90 units
2) Maximum bed joints of 5/8 inch
3) ASTM C476 grout.
4) Minimum grout strength = 2000 psi.

So if you specify a C-90 block, 2000 psi grout, and the type of mortar to be used, the PRISM strength does not have to be specifed, and the compressive strength can be obtained from Table 2 of the masonry code.

For the low- to mid-rise structures I have dealt with, I have not found that I've needed more than the 1900 psi block, (f'm=1500), which manufacturer's can easily meet.

 

[ARLORD]

Lightweight concrete has a lower shear capacity than normal weight concrete. You would think the same applies for lightweight cmu, however, I don't see any indication of that in the masonry code.

 

[concretemasonry]

ASTM C90 specifies that Normal weight block should have a density of over 125 pcf. and lightweight should be less than 105 pcf. Depending on you location lightweight will usually be between 90 and 100 pcf. The specification requires that both must have the same compressive strength to meet the loadbearing requirements.

Normal weight CMUs are made using normal weight aggregates. Lightweight units are made using normal weight aggregate with some lightweight aggregate added.

Codes permit taking advantage of the much higher strengths possible and do not differentiate based on weight because they are performance oriented. Both Lightweight and Normal weight are basic masonry building materials.

In most areas, both lightweight and normal weight are available. In some areas, only lightweight are used, while in other areas, most will be normal weight. Some areas also have Medium weight (105-125 pcf) available.

ASTM C90 requires 1900 psi unit strength based on the net area. Normal weight can go as high as 8000 psi in certain plants, awhile lightweight usually tops out at about 5000 psi. These high strengths are rarely needed or justified.

The unit strength is used in the tables correlating with the mortar properties to provide a quick, easy determination of the compressive strength of the masonry f'm). The compressive strength of the masonry can also be determined by testing a prism constructed of the materials to be used in the wall. This usually gives higher strengths, but is more time-consuming and costly. Neither of these methods differentiate between the type of aggregates used to make the CMUs.

For the same block shape and configuration -

1. Generally, lightweight block have better acoustic properties because of the typical texture. Normal weight block will have greater sound reduction properties

2. Lightweight block have superior fire ratings, with the actual rating depending on the type of aggregate used.

3. Lightweight block has superior insulating properties.

4. Both lightweight and Normal weight block can be used for much higher strength engineered walls (f'm up to 4500 psi)based on the strength of the units and mortar.

All block can be obtained in different configurations - core number, core arrangement, face shell thicknesses and web thicknesses to obtain additional benefits such as strength, fire resistance, insulating properties and reduced sound transmission. The NCMA TEK notes are valuable source for ranges, minimum and average properties. Individual producers can provide more detailed infprmation on units readily available AND for othe units that could be provided, if needed.

 

[Unionmason]

Lightweights cost more usually per unit some which costs can be recouped by lower unit freight costs. But masons will lay more of the lightweights per hour especially in a vertically reinforced wall panel where most units are hoisted over a re bar. The larger/longer the bar size/lap the more the lighter units increases production until it pays buy/make open ended units. Some Lwts in this area are actually stronger and more brittle than the standard wt. Locally the new style lwt. (smart block) are almost as smooth as standards and don't reflect sound differently as the old school 'Haydites' made with larger flakes of autoclaved cinders did. Work rules regarding the maximum weight of units that may be laid by one mason also may make lwt.system 'cheaper' than the two mason regular unit system at certain wall wythes (thickness).