Norbert V. Krogstad
Norbert V. Krogstad

Q: What is the difference between a bond beam block and a lintel block in concrete masonry?

Is it possible to use lintel block over windows and as bond beams within the field of the wall?

A: Bond beam blocks are concrete masonry units that are similar to standard CMUs, except that the webs are cut to remove the upper portion. In many cases, the block units are provided with knock out sections that are removed when the units are used in the bond beam.

The notch in the top of the webs creates a space into which the horizontal reinforcing is placed in a reinforced masonry wall. The vertical reinforcing extends through the cells in the bond beam unit. A wire mesh or fabric is placed in the bed joint just below the bond beam unit at any of the cells that do not contain vertical reinforcing as the walls are erected. This mesh prevents the grout from flowing down the vertical cell space so the horizontal bond beam can be filled without filling all the vertical cells.

Lintel or channel blocks are U-shaped concrete masonry units used above openings to create lintels. Since lintel block units are solid along the bottom, the underside can be exposed at openings. However, because of this feature, these block units do not allow vertical reinforcement to extend through them.

Therefore, lintel blocks would typically not be used in wall systems that have a combination of vertical and horizontal reinforcement. If lintel blocks are used to create bond beams in walls with vertical reinforcing, a portion of the bottom of the units would need to be removed in order to extend the vertical reinforcing through.

Lintel blocks also are often used within bearing walls that do not contain vertical reinforcing below the ends of steel joints. In this application, a steel bearing plate for the bar joists can be embedded in grout.

Cull lesson

Q: I have heard masons refer to broken or defective masonry units as “culls, ” yet I have seen many brick installed in the wall that have small cracks in corners or other defects. Wouldn't these also be “culls?”

If so, who decides which ones are acceptable to put in the wall and which ones are not?

A: A cull is defined as a unit that does not meet the ASTM standard specification requirements. For clay or shale face brick, the standard specification is ASTM C216.

ASTM C216 has several appearance standards that include tolerances for warp-age and chipping of the unit. Regarding chips, ASTM C216 states in Section 8.4 that “The face or faces that will be exposed in place shall be free of chips that exceed the limits given in Table 2.” The size of the permissible chippage at corners and edges depends on the type of brick (FBX, FBS -Plain, FBS - Textured, or FBA), and the percentage of such chips.

The term “Textured” is defined in this table to mean “extruded brick with the face sanded, combed, scratched, scarified, or broken by mechanical means such as wire-cutting or wire-brushing, or molded brick.” “Plain” is defined as “extruded brick with an unbroken natural die finish face.”

 

For FBS brick, which make up the majority of brick, 10% of the edges can be chipped ¼ in. to 5/16 in. and the remainder of the edge can be chipped from 0 to ¼ in. for “Plain” units and 15% of the edges may be chipped 5/16 in. to 7/16 in. and the remainder of the edge 0 to 5/16 in. for “Textured” units. Permissible chippage from corners for “Plain” units is 10% from 3/8 in. to ½ in. and the remaining 90% from 0 to 3/8 in.; for “Textured” units is 15% from ½ in. to ¾ in. and the remaining 90% from 0 to ½ in.

For cracks and other imperfections, ASTM C216 states that imperfections shall not detract from the appearance when viewed at 15 ft for FBX brick and 20 ft for FBS and FBA brick. The lighting requirements under which these samples are to be viewed is not defined. Although there are no mandatory requirements provided, it may be reasonable to use the lighting recommendations of 50-ft-candles or more illumination as identified in Note 8 in Section 8.1, which describes observation of changes to the finish of brick units before and after freeze-thaw testing.

Most shipments of brick masonry units include some that fail to meet the specification. ASTM C216 permits up to 5% of the units in a shipment to fail to meet these requirements. ASTM C216 is a product specification. If the shipment contains excessive defective units, the amount in excess of the 5% allowed in the specification should be replaced by the manufacturer.

However, once the brick units are placed in the wall, they have effectively been accepted by the installer and therefore are not the responsibility of the producer. This is clearly stated in Section 8.6 of ASTM C216.

The mason installing the units into the wall must use judgment in determining which units to discard. In many cases, units with small defects that are within the tolerance for cracks or chips may be discarded if the mason feels that they will be particularly noticeable in the wall.

This determination typically depends on the location of the unit in the completed masonry. For example, units placed near eye level at a major entrance would typically be held to a higher standard than units placed on the interior face of a roof parapet. Ultimately, acceptance of the wall will be up to the owner and the design architect of record.

Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.