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

Q What is the difference between ground block, ground face block, burnished block, and honed block?

A All these terms deal with concrete masonry units produced using essentially the same finishing technique. They describe a process where roughly the top 1/16 in. of the exposed face is ground using an abrasive wheel to create a smooth surface exposing the concrete matrix (cement and aggregate.) This process brings out the natural colors of the aggregates. Many manufacturers also include integral colors to tint the cement as well.

The ground surfaces are very absorptive. As a result, in applications where they are exposed to weather, walls constructed with these units are typically coated with a clear water repellent or sealer to prevent moisture penetration through the face. Acrylic sealers are sometimes applied to the face to enhance the appearance of the colored aggregates used in the mix, which makes the surface look similar to terrazzo.

I recommend using units with integral water repellents to further reduce moisture penetration and absorption through the face. Integral water repellents can provide greater control of efflorescence and other moisture problems than those with surface applications of clear water repellents and sealers.

Flashing concerns

Q Our firm designed a school building in the Chicago area. We specified plastic masonry through-wall flashing where a lower roof intersects a taller masonry wall. The through-wall flashing was to have a metal drip edge that extended out beyond the face of the wall. This drip edge was to be located just above the retainer bar for the roofing.

Our detail showed sealant between the underside of this metal flashing and the top of the retainer bar. Any water penetrating the masonry would reach the flashing and flow over the retainer bar and onto the roof.

The masonry contractor, however, did not use a metal drip edge. Instead, he extended the plastic flashing out 3- to 6-in. beyond the face of the wall and it hangs over the retainer bar for the roofing. The contractor said he planned to install sealant between the underside of the plastic flashing and the top of the retainer bar. After the sealant cured, he planned to trim the plastic flashing to the exterior face of the sealant.