By Ethan Davis
Continuing our discussion of Foundations, we'll discuss concrete masonry units.
Concrete masonry units are classified in accordance with ASTM C90 as Type I or II (ASTM, 1999). Type I is a moisture-controlled unit that is typically specified where drying shrinkage of the block due to moisture loss may result in excessive cracking in the walls. Type II is a non-moisture-controlled unit that is suitable for all other uses. Residential foundation walls are typically constructed with Type II units.
Concrete masonry units are available with different densities by altering the type(s) of aggregate used in their manufacture. Concrete masonry units are typically referred to as lightweight, medium-weight, or normal-weight, with respective unit weights or densities less than 105 pcf, between 105 and 125 pcf, and more than 125 pcf.
Residential foundation walls are typically constructed with low- to medium-weight units because of the low compressive strength required. However, lower-density units are generally more porous and must be properly protected to resist moisture intrusion. A common practice in residential basement foundation wall construction is to provide a cement-based parge coating and a brush- or spray-applied bituminous coating on the below-ground portions of the wall. This treatment is usually required by code for basement walls of masonry or concrete construction; however, in concrete construction, the parge coating is not necessary.
Hollow or Solid
Concrete masonry units are classified as hollow or solid in accordance with ASTM C90 (ASTM, 1999). The net concrete cross-sectional area of most concrete masonry units ranges from 50 to 70%, depending on unit width, face-shell and web thicknesses, and core configuration. Hollow units are defined as those in which the net concrete cross-sectional area is less than 75% of the gross cross-sectional area. Solid units are not necessarily solid but are defined as those in which the net concrete cross-sectional area is 75% of the gross cross-sectional area or greater.
Masonry mortar is used to join concrete masonry units into a structural wall; it also retards air and moisture infiltration. The most common way to lay block is in a running bond pattern where the vertical head joints between blocks are offset by half the block's length from one course to the next. Mortar is composed of cement, lime, clean, well-graded sand, and water, and is typically classified into Types M, S, N, O, and K in accordance with ASTM C270 (ASTM, 1999). Residential foundation walls are typically constructed with Type M or Type S mortar, both of which are generally recommended for load-bearing interior and exterior walls, including above- and below-grade applications.
Grout is a slurry consisting of cementitious material, aggregate and water. When needed, grout is commonly placed in the hollow cores of concrete masonry units to provide a wall with added strength. In reinforced load-bearing masonry wall construction, grout is usually placed only in those hollow cores containing steel reinforcement. The grout bonds the masonry units and steel so that they act as a composite unit to resist imposed loads. Grout may also be used in unreinforced concrete masonry walls for added strength.
In our next blog, we'll cover some information on soil bearing issues.
(This information is taken from an article by Nick Gromicko and Ben Gromiko on the International Association of Certified Home Inspections website)