Q: Most architectural specifications do not allow bed joints to be furrowed, but most masons are doing it.
Is this a problem? And if it is not, why do architectural specifications still include statements prohibiting furrowing?
A: There can be a problem if the furrow is so deep that the joint is not filled when the unit is placed. With shallow furrows, the mortar flows laterally and fills the furrow when the unit is placed, as shown in Fig. 1.
However, if the furrow is very deep (down to the surface of the brick), there is not sufficient mortar to fill the furrow, resulting in a channel within the bed joint, as shown in Fig. 2. This channel can allow water to flow laterally, or create a pocket that can hold water.
Most masons are trained to lightly furrow. However, furrowing must be shallow so that it can be shown to fill up when the units are placed. A rule of thumb is if you can hear the trowel scraping, or even intermittently contacting the top of the brick units below, the furrow is likely too deep.
When in doubt, remove random units after they have been placed to confirm complete filling of bed joints.
Cells and cores
Q I have seen the terms “cells” and “cores” used to describe the same things. I usually think of the term cell as large openings in concrete masonry, and core as smaller holes in brick units.
What is correct?
A There is no consistent use of these terms in the industry. In several references for concrete masonry, the terms “cell” and “core” are used interchangeably to describe the void spaces within a unit.
The International Building Code (IBC) defines cell as a void space having a gross cross-sectional area greater than 1½ sq in. IBC, however, does not define the term core.
ACI 530-05/ASCE 5-05/TMS 402-05 (MSJC 2005) does not define either term in Section 1.6 - Definitions. However, in Table 1.16.1 -Grout Space Requirements in Section 1.16.1 Grouting, Minimum Spaces, the term cell is used to describe the void space in hollow masonry units. The smallest cell in this table is for use with fine grout. This cell is 1½ in. x 2 in., which would be equivalent to 3 sq in., or roughly twice the definition of cell in the IBC.
The best definition that I could find for these two terms is in ASTM C652 Standard Specification for Hollow Brick (Hollow Masonry Units Made from Clay or Shale). This specification covers clay or shale units where more than 25% of the gross cross sectional area can be voids. In Section 10 - Hollow Spaces, a core is defined as a “void space having Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill. a cross-sectional area equal to or less than 1½ sq in. (9.68 sq cm),” and a cell as a “void space having a cross-sectional area greater than 1½ sq in.”