lightweight vs. normal weight CMU
lightweight vs. normal weight CMU
(OP)
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?






RE: lightweight vs. normal weight CMU
TEK 2-6 Density-Related Properties of Concrete Masonry
[see Section 7.4 here ht
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 vary directly with the weight 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 these properties are based on concrete density 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/ (calculator)
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
RE: lightweight vs. normal weight CMU
You may want to check with local suppliers on the availability of the different weight units.
RE: lightweight vs. normal weight CMU
Most suppliers have test data for things like sound transmission and fire rating specific for their lightweight units.
RE: lightweight vs. normal weight CMU
RE: lightweight vs. normal weight CMU
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.
RE: lightweight vs. normal weight CMU
RE: lightweight vs. normal weight CMU
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.
RE: lightweight vs. normal weight CMU