I've worked as a mason for 31 years and have kept things
interesting by taking on the most unconventional and
technically challenging projects I can find, whether in repair,
remodeling, or new construction. Recently I did a massive
high-end residential job that fit that description well.
The project called for two gable-end reinforced masonry walls
that would perform an unusual structural function: provide the
support necessary to offset the centerlines of two large
masonry chimney flues that traveled as far as 8 feet
horizontally from the fireplaces they served. This unique
arrangement allowed the chimneys to pass through the roof
centered on the ridge. The floor and roof framing would also
depend on the masonry for structural support, from both sides
of the walls.
These functions would place the walls under not only vertical
compression, which unreinforced masonry can handle in spades,
but also lateral stresses, which it cannot. Another major
concern was wind-loading, which took on added importance
because of the walls' size and height.
Regardless of its specific application, reinforcing a block
wall to resist lateral and tensile stresses follows the same
general, prescriptive steps, which I will cover in this
The blockwork was on the critical path; the floor and roof
framing could not be completed without it in place. After we
built one wall to the next floor height, we removed our
scaffolding to allow floor framing to proceed. Meanwhile, we
moved to work on the other gable. We started work in the middle
of winter and ultimately performed about half of this job under
We began by staging both sides of the wall with pipe scaffold
and enclosing the perimeter with translucent, fireproof tarps.
(Regular poly sheeting, like the blue tarps used everywhere
else and commonly used to enclose work areas, is highly
flammable and poses a serious risk to workers. Back when I was
an apprentice, I witnessed a blue poly tarp enclosure go up in
a flash, ignited by an open-flame heater. The crew was
fortunate not to have been inside at the time.)
We bridged the span between the staging with "putlogs"
— steel scaffolding beams made for this purpose
— then decked it over solid with planks, plywood, and
more fireproof tarps. This temporary roof completely covered
the work area and prevented the tarps from sagging under rain
and snow loads. We kept the enclosed area comfortable with
propane heaters, which run clean and are easy to
To protect the sand and cement — not to mention the
laborers mixing the mortar and grout — from extremely
cold weather, we also set up a heated mortar shack adjacent to
the enclosure. Because the construction specifications
prohibited all use of winter additives in the mortar and grout,
we heated the sand and water prior to mixing. We used an
electric-powered 400-pound-capacity Imer mixer (800/275-5463;
www.imerusa.com) to eliminate the fumes
otherwise produced by a gas-powered mixer. Thus set up, the
masons worked through snowstorms and below-freezing
temperatures in safety and comfort.
Building a Monolith
Block lay-up began at basement level over a poured concrete
footing that measured 36 inches wide by 12 inches thick. Since
the footing was placed by the foundation contractor over both
undisturbed grade and compacted fill, it was reinforced with
various sizes of horizontal rebar. We set L-shaped
18-by-36-inch #5 rebar ties every 16 inches o.c. to coincide
with the block cells, connecting the block wall to the footing.
The rebar became locked in later, when we grouted the block
cells solid (see Figure 1).
Figure 1. Completely
grout-filled block cells are revealed in a cut made to widen an
opening following a change order.
Concrete masonry unit (CMU) walls are relatively simple
assemblies that rely on mortar to bond the blocks together and
light-gauge welded wire that runs horizontally every other
course to control the development of shrinkage cracks. A long
run of masonry wall should also include a vertical expansion
joint every 20 to 30 feet, to absorb the aggregate wall
shrinkage and resultant cracking.
Solid grouting of block cells and steel rebar is normally used
only in limited areas — vertically at building corners
and at large wall openings, and horizontally in bond beams
along the top of a wall. A bond beam helps tie the wall
together as a structural unit and spread the floor or roof load
evenly throughout the blockwork. This basic method creates a
reinforced concrete "post-and-beam" framework that's filled in
with unreinforced hollow masonry units.
The basic expectation for a conventional, minimally reinforced
CMU wall is that it will support a distributed load with
redundant compressive strength. But such a wall has virtually
zero tensile strength and therefore can't resist lateral loads
or serve as a shear wall. To perform as a shear wall, concrete
masonry must be not only built as described above, but also
systematically reinforced with steel rebar, with all cells
fully grouted. That way, it becomes in effect a monolithic
whole, similar to a reinforced cast wall (Figure 2).Figure 2. This
reinforced masonry wall has each of its cells completely filled
with high-strength grout surrounding vertical rebar. At floor
junctions, a horizontal bond beam encasing continuous rebar
pairs provides lateral tensile reinforcement.