Cement at the Capitol:
a non-destructive investigation
|The Capitol today|
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 ways.
|Jefferson's 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 intact.
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.
|Spalled brickwork immediately beneath the cement render.|
|Sound brickwork sampled from the core of the wall|
|Water penetration in the basement|
|Cracks and past repairs revealed by thermal image|
The 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 in particular.
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.
Ground-penetrating 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.
Sulphate concentrations were also found on the internal face of the external render above the belt course.
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.