Restoring a Concrete Façade

21 June 2019

Matching the
appearance of 60-year-old concrete proves to be a challenge

The Smith Campus Center at Harvard University after
completion of restoration work.

Harvard University, America’s oldest institution of higher
learning, has a diverse set of buildings dating back to the 17th century. Yet
at the dawn of the 21st century, a true physical hub where Harvard students,
faculty, staff, visitors, and the Cambridge community at large could come
together and feel at home had yet to be realized. In 2013, in pursuit of this
goal, the university decided to repurpose the three floors of Harvard Square’s
Holyoke Center into the new Smith Campus Center.

Josep Lluis Sert’s Holyoke Center in the 1960s shortly after

Designed in 1958 by Josep Lluis Sert, a Spanish architect
who served as dean of Harvard’s Graduate School of Design and chairman of the
Cambridge Planning Commission, the Holyoke Center reflects the principles of
modern architecture established by Sert’s mentor, Le Corbusier. For its
transformation into the new campus center, Harvard selected London-based Hopkins
Architects as design architect; our firm, Boston-based Bruner/Cott Architects
as executive architect; and Michael Van Valkenburgh Associates as landscape
architect. The project called for the renovation of the first, second, and top
floor of the 10-story, 100-foot-tall, city-block-square concrete building. The
other seven stories, which included the university’s health services center
remained occupied throughout design and construction.

Bruner/Cott Architects The façade has two distinct textures,
lightly exposed aggregate precast and board-marked, cast-in-place concrete.

Façade study
One of Bruner/Cott’s roles was to restore the building’s façade. Our specialist
subconsultant, Simpson, Gumpertz + Heger (SGH) conducted a study of its exposed
concrete elements and single-glazed storefront enclosures. This included
hand-sounding in selected spalled areas of cast-in-place and precast concrete.
Based on the results in these 10-story sample locations, we predicted the
likely extent of necessary repairs and concrete replacement.
Hopkins’ design team designated which façade portions should remain, which
should be removed, and how the newly designed façade sections should relate to
the original architecture. SGH surveyed all exterior concrete through
binoculars from street level and roofs while Bruner/Cott catalogued
deteriorated glazing and sealants. The resulting estimates of the number of
proposed concrete repairs helped the university decide to limit concrete
repairs to locations where detachment and spalling, PCB contamination, or water
penetration endangered people or the building’s physical integrity.

Boston University Photography Boston University’s Law-Ed
Building and George Sherman Union as seen from the BU Bridge over the Charles
River in 1966.

Bruner/Cott’s work on the façade was facilitated by a
similar project with SGH at the Boston University (BU) School of Law Tower, the
first structure in a concrete complex that Sert designed in 1959. At BU, we
identified and replaced all the failing concrete, including exposed
board-marked cast-in-place concrete and precast spandrels, mullions, and sills
with architecturally significant textures. Completed in 2015, that project
directly informed the repair work we did for the Smith Campus Center.

Bruner/Cott Architects Thirty-seven different cleaning
methods were tested on plain, board-marked, and precast surfaces for

Cleaning the surfaces
At Harvard, the building first had to be cleaned, and the right mixes of
cement, lime, and aggregate determined, for each distinct architectural
surface. Early in the design process, 37 different cleaning methods were tested
on plain, board-marked, and precast surfaces for comparison. The low pressure
dry-blast methods that initially seemed most effective created problems when
the mixture was wind-blown onto the busy streets below. Water-borne cleaning
techniques require the collection and disposal of their resulting wet waste
because of potential PCB-contamination caused by early sealants leaching into
the concrete, but create fewer conflicts with people on the streets below.

Ultimately, the most effective cleaning method for the main
cast-in-place façade areas employed garnet sand and medium water pressures with
an EPDM collection membrane on every scaffold platform, which then drained to
tanks for disposal. At street level, some walls had a very smooth concrete
surface compared to the rougher textures on upper stories. The imprint of the
original birch plywood formwork is still visible on these walls after cleaning.
For those sections, a mild chemical application with fiber brushes removed the
worst soiling while leaving the original surface intact.

Bruner/Cott Architects The concrete mix used to repair the
cast-in-place surfaces emerged after 24 samples representing four cycles of
cure were tested in-situ across three months.

Matching the old concrete
The number of trials for cleaning proceeded rapidly compared to the time
required for concrete mixes to be placed and cured across several weeks for
comparison to the existing surfaces. Even when clean, 45-year-old concrete does
not look like it did when it was new. Carbonation slowly shifts its tonality
from gray to tan, and the thin, brittle, cementitious laitance wears away to
expose more particles of aggregate, further changing both texture and hue.
Casting against smooth formwork surfaces creates textures that differ from
troweled applications, making it difficult to replicate the original attributes
of historic concrete façades, even with skilled workmanship and detailed
specifications. It doesn’t help that most concrete facades have many changes in
texture due to variations in placement where aggregates and paste settle
differently within and against their original formwork—not to mention fifty
years of exposure.

SGH and Bruner/Cott decided to begin sample trials at
Harvard to match the building’s existing concrete based on the mix that fit
best at our recent BU project. That mix emerged after 24 samples representing
four cycles of cure were tested across three months. This was a good starting
point, but it was far from a perfect match, so the cycle of test samples began
anew for cast-in-place patch areas. The white and gray limestone aggregates for
the precast sills and vertical panels turned out to be a closer match between
the BU and Harvard buildings, partly because matching aggregate was visually dominant.

Bruner/Cott Architects Workers cut away to sound concrete in
a rectangular format, chipping to a uniform depth that allowed new concrete to
be applied over existing rebar or mesh, coated steel with zinc-based paint, and
inserted cathodic protection devices.

Bruner/Cott Architects This precast fin was cut-out full
depth and then a cathodic protection device and reinforcing steel was installed
prior to concreting.

After cleaning and matching the mix, the final challenge was physical repair.
The most frequent cause of spalling and fractures in concrete that is exposed
to rain and freezing temperatures is too shallow rebar placement. Inadequate
depth of cover causes cracks mechanically that admit water to increase carbonation,
which reduces concrete’s alkalinity that protects steel against corrosion. The
frequency of rebar locations with inadequate cover is greater in cast-in-place
walls and exposed slabs than in more precisely fabricated precast pieces.

The center’s 3-inch-thick brise soleils and
story-high mullions suffered more frequently from inadequate structural
restraint than inadequate cover to steel. Deflection caused fractures that
brought water into contact with reinforcement. These large, thin, precast
elements did not suffer from poorly located steel mesh.

SGH sounded the entire façade by hand to identify hollow
areas with incipient detachment. Workers cut away to sound concrete in a
rectangular format, chipping to a uniform depth that allowed new concrete to be
applied over existing rebar or mesh, coated steel with zinc-based paint,
inserted cathodic protection devices, then replaced the cut-away concrete to
visually match the original in both dry and wet conditions.

Bruner/Cott Architects Matching the texture and color of the
60-year-old cast-in-place concrete with vertical board marks was challenging.

Twenty five years ago, our firm’s extensive façade
restoration of Harvard’s Peabody Terrace married student housing complex was
one of the first projects to recognize the importance of treating patches
visually as “Dutchmen,” in much the same way that rectangular pieces of new
stone are inserted during the restoration of stone masonry. Clear rectangular
cut-outs with narrow joints make slight variations in tonality or texture more
visually acceptable while avoiding the feathering and smeared geometry that would
result from simply chipping back to sound material. Spalls are one reason for
employing this geometric replacement technique; poor past repairs are another.
The depth of chipping required to allow effective thickness of concrete behind
reinforcing steel and cathodic protection devices determines how much concrete
to remove at individual locations. The depth of concrete cut-outs may vary
widely in a single façade, and different thicknesses of new patches may cure to
different tonalities. Stone restoration does not pose this problem.

Bruner/Cott Architects Some spalling of the precast concrete
was due to structural failure at anchors.

Bruner/Cott Architects The white and gray limestone
aggregates for the precast sills and vertical panels turned out to be a closer
match, partly because matching aggregate was visually dominant.

Cracks are also a crucial aspect to address in concrete
restoration. Patterns of board-marked façade textures were highly important to
Sert’s architectural vision for exposed concrete. The building features slab
edges that were downturned for apparent thickness and marked with the rough
texture created by vertically oriented, sawn form-boards. The shadows cast by
the rough texture made it difficult to see hairline cracks in early binocular
surveys. When scaffolding made close examination possible, a larger number of
hairline cracks became evident, even where hand-sounding indicated no
detachment. Crack repairs remain the most visually distracting category of
concrete restoration as they are usually long and roughly diagonal. Repair was
accomplished by applying a penetrating liquid corrosion inhibitor then filling
with dry-pack concrete mix or injected epoxy depending upon width (no
enlargement by cutting). Finally, all the concrete was coated with a
silane-siloxane water repellent to seal the material and slow carbonation.

Construction Noise
Noise is a key consideration in concrete projects. At BU, the construction site
was relatively contained, but at Smith Campus Center, not only did we have to
contend with Harvard Square’s busy streets but also had the additional
challenge of cleaning and repairing the façade as the building’s upper floors
remained fully occupied. The sound of the façade-cleaning pumps was loud, and
the vibration and noise associated with chipping proved even more disturbing to
the building’s occupants and to people nearby. This meant that cutting and
chipping was restricted to times when the disturbance was least
disruptive—weekends at Harvard, and nights at BU. The logistical implications
of these restrictions for a construction manager can be profound, especially
when coordinating the façade work with the schedule for interior
rehabilitation. Assigning relative disruption ratings to the noises associated
with different restoration activities can be helpful (for example, the shrill
mechanical removal of old polyurethane sealants may actually be worse than
cutting and chipping concrete). Pumps for water-borne sand cleaning also
produce high-amplitude, high-frequency noise levels both inside and out.

Bruner/Cott Architects Cutting and chipping was restricted
to times when the disturbance was least disruptive.

Building scale should also be considered relative to noise
concerns. At over 96,000 square feet, the Smith Campus Center is a large
building. The time required to complete façade repairs and restoration on
buildings of this size can be several years. Inherent disruption should be
factored into project planning from the earliest stage, especially during
pre-construction cost estimates that include schedule assumptions about time
restrictions for noisy work and consequential impacts on continuity of work for
the trades of sealant removal and abatement, masonry, and glazing. During
construction, complaints from occupants and neighbors can affect schedule and
construction costs significantly.

Concrete architecture
Despite these challenges, mid-century modern architecture that features exposed
monolithic concrete is a vital aspect of our built heritage, and it should be
appreciated when combined with good architecture. During this period, concrete
was celebrated as a splendid realization of the ethic of contemporary
aesthetics. It allowed for expression of structure and surface finishes on both
the interiors and exteriors of buildings, and was considered to be waterproof,
physically continuous, and inert.

Architecture based on monolithic concrete was relatively short-lived
as a movement, in part because of the public’s aversion to its non-traditional
appearance and the growing realization that effective energy performance and
moisture management necessitate separation between structure and cladding in a
building envelope. The problems posed by the best examples of these buildings
are not insurmountable however, and they deserve solutions consistent with
their historic and aesthetic value. The recently completed concrete restoration
at Harvard’s Smith Campus Center illustrates informed responses to the true
challenges posed by this special category of concrete repair.

About the Author

Henry Moss

Henry Moss, AIA, LEED AP, is a principal at Bruner/Cott
Architects. Expert in rethinking existing buildings to create new places, Henry
has a distinguished career in historic preservation and adaptive reuse.
Recipient of the 2015 Boston Preservation Alliance Codman Award for Lifetime
Achievement in Preservation, he is a founding member of the DOCOMOMO New
England chapter and has led technical preservation workshops for the Boston
Society of Architects for more than 25 years. Henry’s seminal work includes
MASS MoCA, the Boston University School of Law, and the Hemingway Estate
preservation near Havana, Cuba. He attended the Harvard Graduate School of
Design and practiced for 18 years in England before joining Bruner/Cott.


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