Masonry Structures
Structural Investigation and Assessment
Ian Hume
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Figure 1: Roof thrust or foundation problems? Investigation is required. |
There are essentially two approaches to the investigation and assessment of masonry structures. The first involves basic, low-tech methods, while the second uses much more sophisticated, and therefore more expensive, techniques. This article looks mainly at the less expensive methods available. It also looks briefly at the questions raised by investigative work and where to go for further help.
THE AIM OF INVESTIGATION
Appropriate investigation and assessment will help ensure that repairs to historic masonry are carried out where necessary and that unnecessary work is not undertaken, minimising the loss of original fabric and ensuring that money is wisely spent. Not everything is as bad as it sometimes seems but, conversely, structures which might initially seem sound can have hidden problems.
Considering the condition of individual stones or bricks to estimate their remaining lifespan is a fairly straightforward part of masonry investigation and assessment. The condition of the small elements that make up a masonry wall rarely affects its overall condition. Figure 2, which shows stones at Dunstanburgh Castle in Northumberland, illustrates this point. The much decayed stone in Figure 2a, below, cannot have much life left in it, but it could probably erode away entirely without threatening the stability of the wall. However, if a whole series of stones decays then problems will arise, which is why the repair shown in Figure 2b was carried out.
The same argument does not apply where loads are greater. In a column, for example, the serious decay of one stone can affect the stability of the whole column (figure 3), with potentially serious consequences for the entire building.
SIMPLE INVESTIGATIVE TECHNIQUES
The basic techniques described below are used for the initial investigation and assessment of most masonry structures. Further, more sophisticated assessment techniques may also be useful, but when the assessment is of a local church, a vernacular house in private ownership or a small commercial property, the funds available may only be sufficient for the basic techniques.
| 1 | Historic research |
Historic research Assessing the date of the structure is a sensible place to start as this will give clues to the likely construction of the masonry. For example, medieval masonry is likely to be very thick but may consist of two well constructed skins of cut masonry or random rubble with a rubble and mortar core. The wall core might be well built but, to quote the Roman architect Marcus Vitruvius Pollio, it is more likely to include ‘a lot of broken stone and mortar thrown in anyhow’.
Georgian and Victorian walls may not be as well constructed as they might at first appear because they may contain ‘snap headers’ (bricks with the short face visible as if bonding inner and outer leafs, but which are in fact half-bricks). This was a cost-cutting measure which enabled cheaper bricks to be used for the whole of the inner leaf, at the expense of reducing the effectiveness of bonding between the face and the main body of the masonry. If the building is a substantial one built around 1800 it may contain bonding timbers, which are prone to decay.
| 2 | Visual inspection |
This will involve looking for cracks, signs of settlement, leans and other distortions as well as inspecting the condition of the individual stones that make up the walls or columns. It is important to look at and along the face of the wall seeking out signs of bowing or bulging. This may indicate movement of the body of the whole wall or, more likely, ‘face separation’ where the outer skin parts company with the core, often known as delamination.
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| Figure 2a (above left): a decayed stone, as yet causing no structural problems. Figure 2b (above right): the stones replaced in this repair were undermining the courses above. | |
| 3 | Hammer inspection |
Tapping (not hitting) the face of the structure with a lump hammer or similar tool may reveal evidence of deterioration. If the wall is sound there should be a good ‘ring’ but a dull thud can indicate delamination. However, like so many things in conservation this sort of test relies on experience. The sound of a hammer striking a wall can also depend on the state of the mortar and the size of the individual brick or stone elements. This particularly affects the sound of a flint wall, where there is a much higher ratio of mortar to stone than in a brick or stone wall. Being a very hard material set in soft mortar it is sometimes possible to inadvertently drive flints into the wall. The wall shown in figure 5 looked very sound and a hammer inspection confirmed this but experience and the presence of a bulge suggested otherwise.
| 4 | Monitoring |
Where there are fractures and signs of movement it is vital to ascertain whether these are active and ongoing movements or merely indications of historic problems that settled down long ago and only need some local pointing. The structure may need to be monitored by a structural engineer experienced in conservation to finally establish whether or not it is moving. Figure 4 shows a classic example of historic movement. This brick gable wall belongs to a Georgian house but the major crack does not pass through the Victorian plaster cornice, indicating that the crack existed before the ceiling was installed and has not moved since.
| 5 | Excavation |
Removal of small elements of the wall allows the assessor to properly consider the condition of the core of the wall. This may just involve cutting out a few bed joints here and there (using a pointing chisel or quirk) or perhaps carefully removing the occasional brick or stone. It is important to remember that this sort of intrusive inspection may well require listed building consent.
| 6 | Borescope inspection |
Consider using a borescope (endoscope) to inspect the core of the wall. These instruments, which come in a variety of lengths and may be rigid or flexible, can be inserted into existing cracks in the wall or it may be necessary to purpose-drill small holes.
MORE SOPHISTICATED, NON-DESTRUCTIVE METHODS
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| Figure 3 A column threatened by the corrosion expansion of iron cramps: no serious masonry decay but more than enough to threaten the stability of the column and, in turn, that of much of the structure |
There is a range of more sophisticated (and therefore more expensive and time-consuming) methods of investigating masonry structures. These methods are not examined in detail here as they are highly technical and the outcome of any survey using them can depend on the type of structure being investigated. It is important to seek further information, initially by searching the internet (see Further Information below) and then, most importantly, by discussing the work with experienced practitioners before embarking on an expensive and possibly futile procedure. Will the method proposed work on the type of structure concerned? Are there better methods available? What access is needed? How much will it cost? These questions will need to be answered and there may be other concerns to be addressed that are specific to each technique.
Nevertheless, the results of good non-destructive testing can be extremely useful. For example, It is often useful to ascertain whether there is any metal in the structure either in the form of cramps (these will usually give themselves away by breaking the corners off stones due to the expansion of rust) or buried metal ties (see figure 6). The latter may be present as wrought iron rings around towers and domes to resist outward thrust or as hoop iron used as bed joint reinforcement.
OPTIONS
- Radar An echo-sounding method which uses radar (radio waves) to detect voids or buried metals, ground penetrating radar (GPR) offers greater penetration than metal detectors but is more expensive. GPR can also determine something of the corrosion level, whereas a metal detector will not respond successfully to a pile of corrosion product which is
- perhaps all that remains of buried metal.
- Ultrasonics An echo-sounding method which uses ultrasound, this technology is also used for seeking voids in masonry.
- Thermal imaging The use of thermal imaging cameras can highlight delamination and moisture penetration due to fine variations in surface temperature, and may be carried out remotely.
- Video surveys A wide range of video capture services are available for remote inspections and detailed surveys, from drain surveys to aerial inspections. Video survey cameras have been mounted on dirigibles and helicopters, wheeled robots and telescopic masts, and hand-held cameras can be used by steeplejacks and abseilers to record otherwise inaccessible faces of buildings.
It may well be useful to have the benefit of input from more than one method to supplement what your eyes and experience tell you.
HAVING USED THESE METHODS, WHAT THEN?
The next step is to determine why the structure has moved or deteriorated in the way it has, as there is little point in repairing a defect if the cause is not addressed. In the case of structural movement, typical issues to consider are outlined below.
- Which part of the structure is settling and which is essentially stable?
- Could the cause be ground related? This may seem unlikely in an old building but movement can be related to inadequate foundations, recent changes in water table levels or the root systems of nearby trees.
- Does the whole of the wall lean from foundation level or does it emerge from the ground essentially plumb but then curve?
- Is it roof-thrust? Has there been any impact, storm or bomb damage? Have nearby building works affected the structure?
- Has excessive water ingress weakened the structure? Do the gutters leak?
- What changes has the structure undergone and were any of these so poorly designed as to cause movement?
- Have thermal and or climatic changes affected the structure? Is there sulphate attack or have flue gasses caused expansion of mortar bed joints? The list of possible causes of movement is almost endless and no amount of detailed or sophisticated investigation can identify all the issues in every case, but efficient investigation adds enormously to the body of evidence that can provide the answers.
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| Figure 4 (above left): A classic example of old movement in a wall of a Georgian house. Figure 5 (top right): Face delamination of a flint wall that seemed, on first inspection, to be in very sound condition. Figure 6 (lower right): Metal ties embedded in stonework can cause damage when they rust. | ||
SOURCING EXPERTISE
It is clearly advisable to speak to architectural and engineering consultants who specialise in historic building work and who are accredited in conservation, and to other specialist investigation consultants. A directory of building conservation professionals is available at buildingconservation.com and the conservation accreditation registers (see Further Information) list accredited architects, surveyors and structural/civil engineers. These registers list practitioners who have a proven track record in conservation, have been judged by their peers to have kept up-to-date with continuing professional development and have the right background of a sound conservation philosophy.
A number of courses in the UK teach the basic conservation skills needed by craftsmen and professionals. There are several excellent post-graduate conservation courses run by various universities that cover the skills discussed in this short article. More targeted short courses are also provided by a small number of specialist organisations such as West Dean College in Sussex. The college runs short courses on masonry work and structural repairs to historic buildings that cover aspects of investigation and assessment. West Dean courses can also make use of a ‘ruinette’, built around 20 years ago using an eclectic mix of medieval, Tudor and Victorian masonry materials and techniques. Trainees are given the opportunity to carry out repairs such as pointing and grouting, rebuilding and repairs without risking damage to authentic historic fabric.
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Further Information
More articles on non-destructive investigations
Historic Scotland’s Technical Advice Note 23, Non-Destructive Investigation of Standing Structures (GB Geotechnics Ltd, 2001) is available online here
Accreditation Registers
Architects
- The Register of Architects Accredited in Building Conservation
- The Royal Incorporation of Architects in Scotland, Conservation Accreditation
- The Royal Institute of British Architects, Conservation Register
Surveyors
Structural engineers
FULL LIST OF ACCREDITING ORGANISATIONS
