INTRODUCTION

 Concrete masonry is a versatile, durable material for the construction of building walls that provide load-bearing strength, fire resistance, a high degree of resistance to sound penetration and other desirable features. The density of the concrete mix design used to create a concrete masonry unit (CMU or commonly called block) can vary greatly. This Viewpoint addresses the effects that density has on the physical, aesthetic, engineering and economic characteristics of concrete masonry walls.

Throughout North America, a wide variety and blends of aggregate materials are used to manufacture concrete masonry units.  Concrete block density can range from less than 85 pcf to more than 140 pcf. Table 1 shows a range of concrete densities and approximate resulting weights for typical concrete masonry units. ASTM C 90 (Ref. 1) concrete masonry units must be made with aggregates that meet either ASTM C 33 "specification for concrete aggregates" or ASTM C 331 "specification for lightweight aggregates for concrete masonry".

PHYSICAL PROPERTIES

Fire Resistance

The fire resistance of concrete masonry walls depends on the geometry of the concrete masonry units, and the density of the concrete block. The unit geometry affects the effective or "equivalent" unit thickness. For a given unit configuration, the fire resistance of a concrete masonry wall increases as the concrete density decreases. (See NCMA TEK 7-1A Ref. 4).

Extensive testing has established a relationship between the fire resistance and the equivalent solid thickness for concrete masonry walls as shown in Figure 1. Equivalent thickness is essentially the solid thickness that would be obtained if the same amount of masonry contained in a hollow unit were recast 100% solid without core holes. The equivalent thickness of a hollow unit is equal to the percentage solid times the actual thickness of the unit.  The percentage solid is determined in accordance with ASTM C 140 (Ref 9). Table 2 shows the relationship between fire resistance and equivalent thickness for concrete masonry units of varying densities.

Thermal Resistance (R-Values)

R-Values of concrete masonry are correlated to concrete density, since thermal conductivity of concrete increases with increasing density. Table 3 shows the R -Values for a typical 8" CMU of various densities with and without insulated cores. The R value of 12" insulated and reinforced cmu for various densities is listed in Fig 5.

Thermal Mass

The effect of thermal mass (also known as thermal inertia) on walls is well documented. High thermal inertia walls, such as concrete masonry, have the ability to delay and reduce the impact of outdoor temperature changes on conditioned indoor environments, improving energy efficiency. The International Energy Conservation Code (1994) recognizes most masonry walls that weigh more than 25 lb per square foot as mass walls. For example, this wall weight is attained with a 90 pcf 8" un-reinforced cmu.

Load Bearing Properties

All concrete masonry units, regardless of density, must conform to ASTM C 90. ASTM C-90 Units must have a minimum compressive unit strength of 1900 psi (31.1Mpa). If required, compressive strengths considerably higher than ASTM C 90 minimum are readily achieved with low and high density aggregates.

Structural and Seismic Loading

Lower density units reduce the dead load on supporting beams, columns and foundations of a structure. This is especially important in mid-rise or high-rise structures. Lower density units also reduce the inertia of a masonry wall, which improves the building's seismic performance and makes masonry a more structurally efficient material. 

Economic Characteristics

In estimating masonry construction, a common method employed by mason contractors is the production rate method. The production rate of a given unit is expressed in the total units a mason can lay in one work day (8 hours).  Figure 2 shows the relationship of CMU weight to daily production of concrete masonry units.

The figure shows that productivity increases as the weight of the CMU goes down. Figure 3 shows the amount of wall area that a typical crew of 7 people can build in 5 days using 12" CMU of varying densities.

Initial Wall Costs and Density

The initial cost of a concrete masonry wall is a function of material cost, labor cost and contractor's overhead and profit. The largest single factor is the cost of labor required to construct the wall. Many experienced estimators and project managers recognize the relationship between the density of the concrete masonry units and the labor cost. This relationship is reflected in the cost data obtained through market research in Illinois (Table. 4).

LIFE CYCLE COST

A life cycle cost analysis performed by Henderson Engineering,, Table 5 compares climate data and energy costs for heating and cooling and shows the life cycle energy cost examples. 

AESTHETIC CONSIDERATIONS

One of the most significant architectural benefits of designing with concrete masonry is its versatility in the finished appearance of a concrete masonry wall can be varied with the unit size and shape, color of units and mortar, bond pattern, and surface finish of the units. The term "architectural concrete masonry units" typically is used to describe units displaying any one of several surface finishes that affects the texture of the unit, allowing the structural wall and finished surface to be installed in a single step.

Architectural concrete masonry units are used for interior and exterior walls, partitions, terrace walls, and other enclosures. Some units are available with the same treatment or pattern on both faces, to serve as both exterior and interior finish wall material, increasing both the economic and aesthetic advantages. Architectural units ranging in density from 85 to 140 pcf comply with the same quality standards as conventional concrete masonry, ASTM C 90.