Experimental studies of near-surface temperature cycling and surface wetting of stone and its implications for salt weathering

It has long been accepted that thermal and moisture regimes within stonework exert a major influence upon patterns of salt movement and, subsequently, the type and severity of salt-induced decay. For example, it is suggested that slow drying is more likely to bring dissolved salts to the surface, whereas rapid drying could result in the retention of some salt at or near the frequent wetting depth. In reality however, patterns of heating, cooling and surface wetting regimes that drive them – are complex and inconsistent responses to a wide range of environmental controls. As a first step to understanding the complexity of these relationships, this paper reports a series of experiments within a climatic cabinet designed to replicate the effects of short-term temperature fluctuations on the surface and sub-surface temperature regimes of a porous Jurassic limestone, and how they are influenced by surface wetting, ambient temperature and surface airflow. Preliminary results confirm the significance of very steep temperature/stress gradients within the outer centimetre or less of exposed stone under short-duration cycles of heating and cooling. This is important because this is the zone in which many stone decay processes, particularly salt weathering, operate, these processes invariably respond to temperature and moisture fluctuations, and short-term interruptions to insolation could, for example,
trigger these fluctuations on numerous occasions over a day. The data also indicate that there are complex patterns of temperature reversal with depth that are influenced in their intensity and location by surface wetting and moisture penetration, airflow across the surface and ambient air temperature. The presence of multiple temperature reversals and their variation over the course of heating and cooling phases belies previous assumtions of smooth, exponential increases and decreases in subsurface temperatures in response, for example to diurnal patterns of heating and cooling

Micro-Environmental monitoring of temperature and moisture changes in building stone

The monitoring of temperature and moisture changes in response to different micro-environment of building stones is essential to understand the material behaviour and the degradation mechanisms. From a practical point of view, having a continuous and detailed understanding of micro-environmental changes in building stones helps to assist in their maintenance and repair strategies. Temperature within the stone is usually monitored by means of thermistors, whereas wide ranges of techniques are available for monitoring the moisture. In the case of concrete an electrical resistance method has previously been used as an inexpensive tool for monitoring moisture changes. This paper describes the adaptation of this technique and describes its further development for monitoring moisture movement in building stones.
In this study a block of limestone was subjected to intermittent infrared radiation with programmed cycles of ambient temperature, rainfall and wind conditions in an automated climatic chamber. The temperature and moisture changes at different depths within the stone were monitored by means of bead thermistors and electrical resistance sensors. This experiment has helped to understand the thermal conductivity and moisture transport from surface into deeper parts of the stone at different simulated extreme climatic conditions. Results indicated that variations in external ambient conditions could substantially affect the moisture transport and temperature profile within the micro-environment of building stones and hence they could have a significant impact on stone decay.

The use and meanings of 'time of wetness' in understanding stone decay

Moisture is a well documented, and crucial, control on the nature of stone decay. The term time of wetness has frequently been adopted to describe how long a stone block is wet, with a view to understanding the impact of this on decay processes. Although this term has proved conceptually useful, it has been used in different ways, by different groups to mean mean quite different things. For example, the time of wetness for a stone block surface (the traditional understanding) may be quite different from that of a block interior, controlled by the different dynamics of wetting and drying in those zones. Thus, surface wetting will occur regularly (sometimes swiftly followed by drying, depending on the time of year), with block interior wetting requiring the accumulation of surface moisture to penetrate to depth (more likely in autumn and winter months), and drying out much more slowly. This relatively new but important perspective, framed in the context of climate change, is crucial to understanding the length of time stone may remain damp at depth following a period of prolonged precipitation. The nature and speed of drying is also relevant in quantifying time of wetness of both surfaces and the interior of building stones.
These ideas related to time of wetness have implications for decay processes, specifically how a prolonged time of deep wetness may re-focus the emphasis of salt weathering in natural building stones toward chemical action. Literature on chemical change is discussed, suggesting that chemical change occurring during periods of prolonged wetness is likely to be significant in itself, with implications for weakening the stone (in terms of, for example, cement dissolution or grain boundary weakening) and exacerbating physical damage from salt crystallisation when blocks finally dry out.

Shifting Ground:Beyond National Architecture

The Irish Pavilion at the Venice Architecture Biennale 2012 charts a position for Irish architecture in a global culture where the modes of production of architecture are radically altered. Ireland is one of the most globalised countries in the world, yet it has developed a national culture of architecture derived from local place as a material construct. We now have to evolve our understanding in the light of the globalised nature of economic processes and architectural production which is largely dependent on internationally networked flows of products, data, and knowledge. We have just begun to represent this situation to ourselves and others. How should a global architecture be grounded culturally and philosophically? How does it position itself outside of shared national reference points?
heneghan peng architects were selected as participants because they are working across three continents on a range of competition-winning projects. Several of these are in sensitive and/or symbolic sites that include three UNESCO World Heritage sites, including the Grand Egyptian Museum in Cairo, the Giants Causeway Visitor Centre in Northern Ireland, and the new Rhine Bridge near Lorelei.
Our dialogue led us to discussing the universal languages of projective geometry and number are been shared by architects and related professionals. In the work of heneghan peng, the specific embodiment of these geometries is carefully calibrated by the choice of materials and the detailed design of their physical performance on site. The stone facade of the Giant’s Causeway Visitor Centre takes precise measure of the properties of the volcanic basalt seams from which it is hewn. The extraction of the stone is the subject of the pavilion wall drawings which record the cutting of stones to create the façade of the causeway centre.
We also identified water as an element which is shared across the different sites. Venice is a perfect place to take measure of this element which suggests links to another site – the Nile Valley which was enriched by the annual flooding of the River Nile. An ancient Egyptian rod for measuring the water level of the Nile inspired the design of the Nilometre - a responsive oscillating bench that invites visitors to balance their respective weights. This action embodies the ways of thinking that are evolving to operate in the globalised world, where the autonomous architectural object is dissolving into an expanded field of conceptual rules and systems. The bench constitutes a shifting ground located in the unstable field of Venice. It is about measurement and calibration of the weight of the body in relation to other bodies; in relation to the site of the installation; and in relation to water. The exhibit is located in the Artiglierie section of the Arsenale. Its level is calibrated against the mark of the acqua alta in the adjacent brickwork of the building which embodies a liminal moment in the fluctuating level of the lagoon.
The weights of bodies, the level of water, changes over time, are constant aspects of design across cultures and collectively they constitute a common ground for architecture - a ground shared with other design professionals. The movement of the bench required complex engineering design and active collaboration between the architects, engineers and fabricators. It is a kind of prototype – a physical object produced from digital data that explores the mathematics at play – the see-saw motion invites the observer to become a participant, to give it a test drive. It shows how a simple principle can generate complex effects that are difficult to predict and invites visitors to experiment and play with them.

Rude stone monuments of Apulia.

Malone , C., Popular Archaeology, 1981. 3(2): p. 33-7.