What Lies Beneath

Acoustics, Radar, Thermography and Vibration Analysis in the Investigation of Masonry Defects

George Ballard

Ambition beyond budget? Various ages of remedial intervention from c1680-1980 visible in the fabric of a village church near Juliaca, Peru

Parish churches are frequently the most ambitious architectural projects in any small community. Put up in the first place by generous benefaction, they often become an onerous burden for that same community. Lack of maintenance, or rather intermittent and inadequate maintenance, shows in most historic ecclesiastical structures, whether it’s a Victorian church in the heart of London or a Baroque church in Peru.

The financial constraints that underlie maintenance problems remain the dominant constraints of any modern maintenance or conservation project. We need to recognise that both engineering and materials sciences can help to minimise global or whole-life costs, but that these can only succeed if based on good information.

The first task for any conservator is to assess what needs to be done. Often we must rely on the use of visual clues and surface expression to indicate what lies beneath the surface. However, in recent years the range of information available has been greatly extended by ‘non-destructive’ techniques as more instruments have become available which enable us to see beyond the surface without damaging it. These tend to be used for only the larger, more important and (consequently) better-funded buildings.

Top: leached oolitic limestone at St Pancras, London; above: water falling over a cornice erodes stonework and promotes rusting of a buried cramp at St Martin in the Field, London; right: inadequate intervention at Trinity Church, Southwark

There is an inevitability to this. Every pound spent on testing is a pound unavailable to the restoration work. Furthermore, testing is double-edged: although better knowledge can lead to cost savings through tighter specification of the contractor’s task, costs will increase if tests show that more re-construction and repair is needed than expected. The only possible justification for more rigorous testing is, of course, that every pound spent on a course of action which fails to achieve its object is a completely wasted pound.

The ability to understand what is going on beneath the surface is particularly valuable where masonry walls are concerned. The most common cause of their failure is poor rainwater control as rain which is not shed enters the surface of masonry, either into the stones or bricks, or else into the mortar between them. This can lead to a wide variety of problems far from the source, making the cause and its effect seem unrelated. At a simplistic level, any soluble material may be leached out, opening pathways for more moisture to penetrate, a process which may be accelerated when the water is acid. The opening of pores as soluble material is removed, weakens the structure of the stone or mortar, and may allow frost damage to take place. Passing through the wall, a pathway is opened for the transportation of oxygen and water to any embedded ironwork, and over long periods of time, internal mortar and rubble may weaken, compact and settle – no longer providing the load capacity of the original construction. Moisture levels in timbers embedded in the walls may also rise to the level at which insect activity and fungal growth are encouraged. When water reaches the interior of the building the risk of damage to internal surfaces is increased.

The mechanisms of deterioration are wider, but these examples demonstrate the importance of rainwater control, and the need to understand exactly what processes have occurred within the structure.

Visual inspection by a trained and experienced eye of one or both sides of a wall may find surface expression of water retention and damage. This can be assisted by thermography which enables differences in the surface temperature of the structure to be recorded photographically (see illustration) from the ground, without the need for scaffolding. However, examination of the damage to the interior structure can be considerably harder, particularly where the surfaces are obscured by fittings and finishes or where features like galleries, floors and partitions add to its complexity.

Thermography (inset) showed differential thermal gain in the polychrome tiles of the dome of St Vincent de Paul, Los Angeles, indicating either detached tilework or damp. Tapping the tiles showed that they were well secured, indicating that this was a damp problem, in spite of the California sunshine

ACOUSTICS AND RADAR

The most useful non-destructive means of determining the condition of stone, brick and concrete is ultrasonics and acoustics generally. These involve the transmission of a sounding pulse to create a range of ultrasonic or acoustic waves which penetrate the masonry and are
affected by its properties. Changes in their velocity and amplitude can be measured to provide information concerning the physical (and engineering) properties of the material. The difference between the various groups of waves (principally primary and shear waves) is related to the shear and flexural moduli of the material.

Vibration array on a creaking 17th century staircase with clear indication of the loose treads

In UPV (Ultrasonic Pulse Velocity) for example, a transducer sends a pulse of acoustic energy through the material and a receiver placed on the same unit nearby measures how the pulse has been affected by the material. As sound and defective materials affect the signal in different ways, a picture of the quality of the material immediately below the surface can be quickly established. In UPE (ultrasonic pulse echo) on the other hand, it is the strength of the echo and the time lapse between transduction and echo-reception that is recorded on a graph to provide an indication of the thickness of an element of structure and the depth of defects.

Guidance on non-destructive techniques is widely available, but on the use of these techniques the literature is unfortunately vague. UPV and UPE are described, but this is limited for UPV to the determination of general integrity and crack depth measurement, and for UPE to layer thickness. Mention is made of quantitative ultrasonics (QU) as used to investigate distributed damages such as microcracks and porosity variations, usually warning that the method is poorly tested in the field. Yet this is precisely the parameter of greatest interest when planning the conservation and remedial needs of an historic structure.

Radar is another valuable tool which can be used to map the arrangement of the masonry before attempting to use ultrasonic methods. It also provides the most rapid means of scanning for buried ironwork (a covermeter reaches about 125mm into stone or bricks), and can give an alternative method of obtaining accurate thicknesses.

In essence radar is a radio echo-sounding system. As radio waves and acoustics respond to changes in medium in very different ways, they are particularly useful when used in conjunction with each other. Whereas acoustic waves are stopped by a void, radio waves from the radar pass through – but at three times the speed at which they travel through stone. Moisture trapped in mortar also slows the transmission of a radio wave, making stone and mortar appear thicker than they really are, so when correlated with the acoustics the discrepancy provides a direct measure of moisture content. Comparison of the acoustic and radar transmission paths through the mortar beds and rubble core thus allows continuity, moisture content and void ratio to be estimated with sufficient accuracy.

Testing depth of stone deterioration and mortar damage in wall using UPE

Here again the available literature is inadequate. Many descriptions of tests and methods involve only one instrument and attempt to treat their output as wholly self-sufficient and stand-alone. This is to treat an instrument as if it were not part of the skill-set of the trained investigator, and ignores the interaction in analysis between different instruments responding to different physical characteristics of the same object. For this reason many systems are recognised in standards such as BS1881, which sets out to cover every form of concrete testing. However, such literature fails to recognise and appreciate how much more can be achieved by a complementary approach using a range of instruments to answer specific questions. Used together, a combination of techniques including radar, ultrasound and thermography allows the extent of damage in individual stones to be assessed. The combination may enable the investigation to put a figure on the depth of surface liable to frost damage as a result of water being trapped within the pores or cracks, or define the capacity of a stone within the load path down a building where bearing has been reduced or made eccentric.

VIBRATION ANALYSIS

No discussion of modern methods to aid church maintenance would be complete without considering the value of vibration analysis, which is an extension of acoustic analysis. Most buildings, particularly heavy masonry structures such as churches, can be analysed and understood in purely static terms, but a bell tower is another matter. The dynamic forces induced in the frame by a ring of eight bells are comparable to a minor earthquake or the pounding of heavy goods vehicles running over rough roads. As the bells swing, forces normal to the axis provide thrusts along the bell frame which are resisted by the tower walls. The sockets into which the frames fit are usually critically loaded, and damage in this zone leads rapidly to water penetration and loss of mortar. The whole tower also swings as an inverted pendulum in response to the forces generated by the bells. When everything is tightly fixed together this is of little issue, provided that the tower is adequately massive and strong. However, as the structure deteriorates, conditions change rapidly: if the bell frame works loose it acts as a battering ram on the walls, rapidly causing disintegration of the masonry. If the overall masonry starts to deteriorate then the inverted pendulum motion may become asymmetric or bimodal, with a rapid switch between modes. This causes additional stress in the tower fabric and accelerates the deterioration, often appearing as horizontal cracks which admit more water into the fabric.

An array of accelerometers on a bell tower can measure the effect of the bell's swing on the structure

The vibrational mode of a tower may be analysed by attaching accelerometers or velocity meters, small instruments weighing no more than a few grams and connected by wire to a central recording station, to the tower. These are set out as an array, with up to 32 instruments set over the surface of the wall – either inside or out. Measurements taken during a normal ring allow a simple comparison of the free vibrations in the tower with the driven vibrations. Early warning of asymmetric movement and frame looseness means repairs or strengthening can be carried out long before any serious damage occurs. The same applies to timber floors, stairs and roofs in which a small vibration can be introduced to locate failing joists and weakened timbers.

A range of sophisticated tests developed in mainstream structural inspection are now available to ensure that the maximum value can be obtained from the inevitably small budgets available to parish churches to maintain the glories of their architectural heritage.

COMPLEMENTARY METHODS
(1) THERMOGRAPHIC SCAN identifies an area of potential moisture retention
(2) VISUAL EXAMINATION finds depleted stone	(3) SURFACE UPV measurements find the velocity in the weathered stone and (4) PULSE ECHO gives whole body velocity, which includes remaining good stone, then sounds the thickness of the damaged stone (5) RADAR (not shown) checks the moisture content and locates damage around dowels