Cement at the Capitol:
a non-destructive investigation
Brick is a highly
durable material which can survive as well buried in wet foundations as
it can where it is exposed to the weather – until it is mistreated. It
is now well established that the strength of a brick structure is partly
derived from the inherent strengths of the component materials of brick,
mortar and their bonding, and in part from friction between the components.
In this, lime mortars, with both a self-healing capacity and a degree
of plasticity and flexibility, have shown themselves to be better suited
to the needs of historic buildings over time than the stronger but more
brittle mortars based on Portland cement.
The undesirable effects
of introducing these Portland mortars’ during repointing are also well
known, but Portland mortar can change the building dynamics in more subtle
Capitol in 1860
The history of the
1783 design by Thomas Jefferson for a new Capitol Building at Richmond
in the heart of Virginia is typical of such effects. The building forms
an iconic heart of American democratic consciousness – during the Civil
War, Richmond, a Southern town, held out against the Union’s bombardment
until finally the Confederate army razed what was left of the town and
left. No matter the destruction caused in this period, Jefferson’s Capitol
remained on the Hill above the town, unscathed and untouched.
The principal design
was a simple brick masonry plinth supporting brick piers topped with a
timber roof: the external facades were rendered with a lime stucco, scored
to give the appearance of ashlar, and finished in limewash.
In the 1880s the exterior
had a major overhaul and the lime stucco replaced with a (then) newfangled
Portland cement-based mortar with an oil based paint finish for durability:
early concrete technology unwittingly made many of the mistakes we still
make today, and the render suffered from cracking, induced by both shrinkage
and thermal movement. Patch repairs and over painting kept the weatherseal
There was a major
fire in 1903, and a wide programme of rebuilding, refurbishment and reroofing
took place, retaining historic fabric wherever possible. The Portland
render was also retained and extended to include the new extensions. The
modern impervious weathershield paint finish is still regularly maintained.
The current programme
of repairs was prompted in part by troublesome and disfiguring damp, extending
from the ceilings of the chambers to the committee rooms in the basement.
A new master plan is being developed, following a complete review by architects
Hilliers, engineers Robert Silman, and non-destructive testing, materials
and inspection practice GBG. Given the iconic nature of the building,
probe and exposures were necessarily limited, and as the home of the State
Legislature, the building is in continual use. Inspection had to be both
non-intrusive and non-disruptive.
brickwork immediately beneath the cement render.
brickwork sampled from the core of the wall
penetration in the basement
and past repairs revealed by thermal image
original ground floor is a half basement built over clay, with footings
sunk deep into it. The 2.4m thick retaining wall rises about 1.8m to a
belt course of dense stone that acts DPC, and then continues up some 1.8m
thick. The exterior is rendered above ground level in Portland cement,
and the interior is lime plastered.
The footing sits on
clay which forms a basic aquaclude (a geological formation which contains
water but does not readily transmit it), and the workspace outside the
wall forms trench which captures surface water. Salt crystallisation (efflorescence)
on the inside indicated that the walls were damp, and radar showed quite
clearly that the brickwork was saturated through the full thickness exposed
to the trench: the source of the efflorescence was obvious. Moisture in
the wall was trapped by the cement render on the outside and the belt
course above, and its only escape route was through evaporation inside.
Here the lime scratch coat was weak and friable and the surface of the
inner bricks were soft and failing – a relatively simple case of ground
salt damage to the plinth masonry.
As this damp enters
the building, it adds to the humidity brought in by the people who work
in the building and by the humid Virginian summers, and is returned to
the brickwork above the belt course in a natural equilibrium of relative
humidity. Lack of ventilation traps all moisture until it condenses on
cool surfaces including air-conditioners, the attic soffit and masonry
Within the exterior
walls, the point at which moisture condenses – the ‘dew point’ – varies
between the external stucco and the bricks, governed by the Virginian
continental climate: as the temperature of the walls falls, whether daily
or seasonally, the dew point moves further in. Where, as here, dewpoint
is mobile within brickwork, any soluble salts present a risk to the masonry
as they migrate seasonally, recrystallising as the dew point moves on.
The sources of the salts were the bricks themselves and the lime mortar,
together with the Portland cement of the render.
The immediate effect
of a Portland cement render is to reduce the porosity compared with lime
and to reduce the vapour permeability or in other words, the building
cannot breathe, and has difficulty in shedding any liquid water in that
area. The addition of weather proofing paints may reduce rainwater penetration
but does nothing to reduce the problem of trapped moisture.
Thermal imaging showed
surface temperatures rising in direct sunlight from summer ambients of
over 300C to over 500C after four hours; winter ambients are less than
-100C. In such conditions, the dew point will lie somewhere between the
render surface and the outer leaves of brick, and any salts will tend
to migrate to the boundary between the render and the bricks, and will
range regularly in form from solution through to crystallised, with rapidly
changing moisture content. Such phase changes are accompanied by dimensional
shifts, generating sufficient force to break up softer bricks, and cause
debonding of a render surface from them.
The thermal images
identified cracks and repairs under the weathershield paint, but judging
by their form and position, these were almost certainly primarily caused
by shrinkage of the render. Over the years, the cracks tended to be repaired
rapidly to keep the building looking pristine, so the process of salt
crystallisation and any action of frost on saturated masonry was contained
within an isolated environment.
radar (GPR) was used to map the moisture content profiles through both
the full width and height of the structure, and found high moisture throughout
the wall below the belt course, with the highest moisture content toward
the outer face of the brickwork, but with no apparent coherence to any
rainwater penetration. GPR also identified debonding of the render wherever
these higher moisture contents were found.
Brick exposure immediately
behind the facing render corroborated the non-destructive testing assessment
of the cause of failure, with the render being taken off easily in sheets,
with the first 20-30mm of brick frequently still attached or crumbling
away immediately the render was removed.
Deep inside the 1.8m
thick walls the brick work was still pristine with both mortar and bricks
sound and crisp.
High sulphate concentrations
were found in the plinth brickwork below the belt course, with efflorescence
and bond failure of the internal plaster finish.
were also found on the internal face of the external render above the
Alongside such structural
considerations, the biological activities associated with damp should
not be forgotten: wood-boring beetles were found to have been active wherever
dew points were associated with timber and offered them a sufficiently
attractive environment for population growth. Timbers in the uninsulated
attic spaces were completely destroyed by beetles.
All the Portland render
is to be removed and a new lime render is being designed to re-introduce
the original design’s breathable exterior, and water penetration through
the basement walls brought under control, together with complete monitoring
and control of the internal humidity.
article is reproduced from The Building Conservation Directory, 2004
BALLARD is the managing director of GB Geotechnics Ltd, a company specialising
in the non-destructive investigation with particular expertise in the
application of impulse radar, acoustics and thermography to structural
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