FIRE RESISTANCE
Concrete block wall systems are unsurpassed in functioning
as a barrier to contain the spread of fire. These systems
effectively resist transmission of intense heat through
the wall while also preventing the passage of flames and
hot gases. The fire resistance rating period of concrete
masonry elements is determined by testing, by calculation
methods or through a listing service.
Testing of representative elements of the wall construction
in accordance with standard fire test methods is usually measured
by ASTM E 119, Standard Test Methods for Fire Tests of Building
Construction and Materials.
Calculation methods determine fire resistance ratings based
on physical and material properties of the concrete masonry
unit, such as the equivalent thickness and aggregate types
used in the manufacture of the concrete masonry units.
These calculation methods are based on extensive research which
has established a relationship between physical properties
of materials and the fire resistance rating. The rating is
a function of the aggregate used in the manufacturing of the
units and the equivalent thickness of the unit.
Private listing services allow a designer to specify a fire
rated assembly which has been previously classified. The listing
service monitors materials and production to verify that the
concrete masonry units are in compliance with appropriate standards.
EQUIVALENT THICKNESS
The equivalent thickness of a unit is the solid thickness that would be obtained
if the same amount of masonry contained in a hollow unit were recast without
core holes. The equivalent thickness of a hollow unit is equal to the percentage
solid times the actual thickness of the unit. The equivalent thickness of a
100% solid unit or a solid grouted unit is equal to the actual thickness.
LOOSE FILL MATERIAL
If the cells of hollow unit masonry are filled with approved material, the equivalent
thickness of the assembly can be considered the same as actual thickness. The
list of approved materials includes: sand, pea gravel, crushed stone, or slag
that meets ASTM C33 requirements; pumice, scoria, expanded shale, expanded
clay, expanded slate, expanded slag, expanded flyash, or cinders that comply
with ASTM C331 or C332, or perlite or vermiculate meeting the requirements
of ASTM C549 and C516 respectively.
BLENDED AGGREGATES
The fire resistance rating of units manufactured with a combination of aggregate
types is determined by linear interpolation based on the percent, by volume,
of each aggregate used in the manufacture. This may be expressed by the following
equation:
Tr = (T1 x V1) + (T2 X V2)
Where: Tr = required equivalent thickness
for a specific fire resistance rating of an assembly
constructed of units with combined aggregates. (Value
may be inches
or mm.)
T1, T2 = required equivalent thickness
for a specific fire resistance rating of a wall constructed
of units with aggregate types 1 and 2, respectively.
(Value may be inches or mm.)
V1, V2 = fractional volume of aggregate
types 1 and 2, respectively, used in the manufacture
of the unit. (Value is expressed in % volume of aggregate
type.)
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SOUND REDUCTION
Sound absorption involves reducing the sound emanating from
a source within a room by diminishing the sound level and changing
its characteristics. Sound is absorbed through dissipation
of the sound wave energy. The effectiveness of the absorption
method is dependent on the ability of the room surfaces to
absorb the noise rather than reflect it. Sound Absorption Coefficient
(SAC) is an indication of the sound absorbing efficiency of
a surface. The Noise Reduction Coefficient (NRC) is the average
SAC taken at different frequencies.
NRC values depend on the porosity of the material and the surface.
An open rough textured surface will have a higher NRC value.
This means that a more porous block such as a Splitface or
Fluted block will have a higher NRC rating. Also a medium weight
block, because of its porosity will perform better from an
absorption point of view, than a normal weight block. If the
NRC = 1, then no sound is reflected back. The percentage that
is reflected back is fractionalized, subtracted from 1 and
this figure is the NRC value.
Sound transmission is concerned with sound traveling through
barriers from one space into another. To prevent transmission
the walls just have enough density to stop the energy waves.
Sound Transmission Loss is the total amount of airborne sound
lost at a given frequency, as it travels through a partition.
The STL, which is measured in decibels, is measured at 16 frequencies
and the loss at these frequencies is used to plot a curve,
which is used to determine the Sound Transmission Class (STC).
The STC of a wall is determined by comparing its sound transmission
loss curve with a set of standard curves or contours. There
is a definite correlation between Sound Transmission and the
weight of the wall. If a wall is heavier and more dense then
the Sound Transmission Coefficient will increase. For concrete
masonry units this means that a Normal Weight Block would have
a higher STC rating because of the mass of that block. Porosity
of the units is also an important aspect, as the tighter a
texture on the surface, the greater the resistance to sound
penetration. Therefore a painted surface will increase the
STC, but will decrease the NRC. If a sound of 100 decibels
is generated on one side of a wall and 40 decibels is measured
on the other side, then the reduction in sound intensity is
60 decibels. The wall then has a 60 decibel rating.
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CONTROL JOINTS
Control joints are vertical separations built into a concrete
masonry wall at locations where stress concentrations may occur.
They are one method used to relieve horizontal tensile stresses
due to shrinkage of the concrete masonry units, mortar, and
when used, grout. Control joints reduce restraint and permit
longitudinal movement.
Control joints should be located where volume changes in the
masonry due to drying shrinkage, carbonation, or temperature
changes are likely to create tension in the masonry that will
exceed its capacity. Care should be taken to provide joints
at locations of stress concentrations such as:
1. at changes in wall height,
2. at changes in wall thickness,
3. at movement joints in foundations and floors,
4. at movement joints in roofs and floors that bear on a wall,
5. near one or both sides of door or window openings,
6. adjacent to corners of walls or intersections within a distance equal to half
the control joint spacing.
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COMPRESSIVE STRENGTH
Structural performance of masonry is based on the physical
characteristics of its components, and on the construction
methods used in assembling these components. The strength
of masonry is influenced by the structural properties of
units,
grout, mortar and reinforcement. In engineered masonry
structures, the required strength of structural elements
is determined
by distributing design loads to the various resisting elements
in accordance with a structural analysis. After the required
strength is determined, the designer uses it as a basis
for specifying f'm, which is defined as the "specified compressive
strength of masonry". This property is noted in the
project documents and is used in accordance with masonry design codes
to establish allowable stresses for masonry elements.
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WEIGHT CLASSIFICATIONS
Concrete block has three classifications according to weight.
Lightweight units weigh less than 105 pounds per cubic foot.
Medium Weight units weigh between 105 and less than 125 pounds
per cubic foot, while Normal Weight units weigh 125 pounds
per cubic foot or more. Oldcastle adams manufactures all three
weight classifications. Lightweight block is the lightest,
has the highest fire rating, the highest R value and will be
more effective for sound absorption (NRC rating). Normal Weight
block will have a tighter texture and will be more effective
for sound transmission (STC rating). All weight classifications
meet or exceed ASTM C90 Standard Specification for Loadbearing
Concrete Masonry Units.
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COLORED MASONRY UNITS
Economy, superior design and function, and appealing aesthetics
are readily achieved in projects constructed with integrally
colored concrete masonry units.
It is important to note that minor variations in tone and texture
are inherent in all masonry products. Factors influencing these
variations in C.M.U. include: color variation in pigments,
aggregates, cement, water content, degree of compaction achieved
during manufacture, kiln conditions, and atmospheric conditions
including temperature and humidity.
If properly dispersed throughout the wall these variations
can enhance rather than detract from the appearance of a project.
There are several steps that can be taken to ensure that the
product performs as intended:
1. The architect should request samples
for verification purposes showing the full range of exposed
color and texture to be expected in completed construction.
2. A sample panel should be built on the jobsite. This sample panel should be
used as a reference for the color of the CMU, the texture of the CMU and the
workmanship. In case of disputes over these issues on the project, the sample
panel should be referred to.
3. The mason should pull product to be placed in the wall from opposite corners
of three randomly selected pallets to ensure dispersion of variability.
4. The initial order should be as accurate as possible, as Betco Supreme cannot
guarantee an exact match from subsequent production runs.
5. Following construction the concrete masonry units should be cleaned using
well-established and specified techniques.
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