NCMA Teknotes
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 E119, Standard Test Methods for Fire Tests of Building Construction and Materials.
Calculation methods determine fire resistance ratings based on physical and materials properties of the concrete masonry unit such as the equivalent thickness and aggregate types used in the manufacture of concrete masonry units.
These calculation methods are based on extensive research with an established 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.
Equivalent Thickness
The equivalent thickness of a unit is the solid thickness that would be obtained f the same amount of concrete contained in a hollow unit was recast as a solid unit.
Loose fill Insulation
The fire resistive time period for concrete masonry units meeting the equivalent thickness required for a two-hour fire-resistive rating in item 6 below and having a thickness of not less than 7 5/8 inches is four hour when cores which are not grouted are filled with silicone-treated perlite loose-fill insulation, vermiculite loose-fill insulation, or expanded clay shale or slate lightweight aggregate, sand or slag having a maximum particle size of 3/8 inches. (Table 7-B, footnote 16).
Blended Aggregates
The fire-resistance rating of concrete masonry units composed of a combination of aggregate types shall be based on equivalent thickness values determined as follows:
Tr = T1 x V1/ T2 x V2 / Tn x Vn
Where:
TR =“The minimum required equivalent thickness corresponding to the desired fire-resistance rating as listed in Table 7-B for concrete masonry units (cmu) manufactured with a particular combination of aggregate types.
T1, T2, Tn=The equivalent thickness for each aggregate Type1
Respectively, used as indicated in Table 7-B for the desired fire resistance rating.
V1, V2,Vn = The percentage by volume of each aggregate Type 1,2, respectively, which is used in the manufacture of cmu.
Sound Reduction with Concrete Masonry Walls
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 our 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 splitface will have a higher NRC rating. Also as 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 to another. To prevent transmission the walls have just enough density to stop the energy waves. Sound Transmission Loss (STL) is the total amount of airborne sound lost at a given frequency as it travels through a partition. THE STL which is measured I 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 the wall is heavier and more dense than 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 the block Porosity of the units is also an important aspect as the lighter a texture on the surface, the greater the resistance to sound penetration. Therefore, a painted surface will increase te 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.
These types of walls are known to provide greater thermal comfort, largely because of the significant heat capacity available in their thickness and density. Masonry walls slow down the passage of heat from the inside out during the winter and from the outside in during the summer. Significantly better than lighter weight walls.
Thermal performance cannot be measured by insulation along. Still, many people have been led to believe that simply comparing R-values will allow them to make an informed decision when it comes to energy efficiency issues. R stands for resistance. In the case of a building, it refers to the resistance of a wall to heat passing through it. This measurement is assigned a numerical value such as R-8 rated wall. A certain type and thickness of insulation materials may have a specific R-value. Unfortunately R-values are not measured by testing walls or ceiling under actual conditions, Instead, R-values are determined in a laboratory, where small samples of building materials are measured under constant, controlled temperatures and humidity.
Control Joints
Control Joints are employed in crack control to reduce restraint by accommodating movement of the masonry wall, or movement of structural elements adjacent to the wall. Their spacing along the wall will depend upon, the movements of the wall or other causes of stress concentration. Control joints should be used in conjunction with bond beams and joint reinforcement for crack control purposes.
Control Joint Slot
All the units drawn in the Shapes and Sizes Directory are drawn without any control joint slots. Northfield manufactures a certain ratio of regular block to control joint block within a pallet of material. The general rules for the amount of control joint block manufactured with regular units are as follows:
6″ wide block 1 control joint block for every 4 regular block
8″ wide block 1 control joint block for every 3 regular block
10″ wide block 1 control joint block for every 2 regular block
12″ wide block 1 control joint block for every 2 regular block
All half block are manufactured with a control joint slot. The only exception to this is when we manufacture a splitface on side on one end and a splitface two sides one end. Northfield manufacturers its units in the this manner unless specified otherwise.
Compressive Strength of Masonry
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, from 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.
Weight Classifications and Aggregates
Concrete Block has three classifications according to weight. Lightweight units weigh between 85 and 105 pounds per cubic foot. medium weight units weigh between 105 and 125 pounds per cubic foot and Normal Weight units weigh over 125 pounds per cubic foot . Northfield currently only manufactures only medium weight and normal weight units. Medium weight block will be lighter, have a higher fire rating, a higher R-Value and will be more effective for sound absorption (NRC rating).Normal weight block will have a higher compressive strength, a lighter texture and will be more effective for sound transmission (STC rating).
Integrally Colored Concrete Masonry Units
Economy, superior design and function, and appealing aesthetics are readily achieved in projects constructed with integrally colored concrete masonry units (cmu).
It is important to note that minor variations in tone and texture are inherent in all masonry products. Factors influencing these variations in cmu 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 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 a reference for the color of the cmu.
