Comparative assessment of stone weathering intensities and bioclimatic conditions was conducted at four temples located in cleared and forested sites of the Angkor Park, based on similar protocols. Four thousand sculpted lotus petals carved in the same grey sandstone were categorized by using two customized scales of weathering intensity, and climate monitoring was conducted from December 2008 to November 2009. Whereas 70% of the sandstone lotus petals are almost completely destroyed by mechanical weathering in cleared areas, 74% of petals located in forested environments appear to be totally free of mechanical weathering and are only affected by superficial biochemical weathering. Ambient conditions are also contrasting, with the magnitude of the diurnal surface temperature and relative humidity ranges being three times higher at cleared sites than in wooded areas. As wetting–drying cycles are the driving force of sandstone decay at Angkor, causal links are suggested between weathering and climate regimes. In wooded areas, the microclimate is buffered by the forest and the associated lithobionts, which maintain constant humidity levels, reduce thermal stresses at the stone surface and induce a slow biochemical weathering regime. In cleared areas, direct exposure to sunshine and monsoon rains induces pronounced wetting–drying cycles conducive to swelling–shrinking movements and other potential processes, provoking the rapid mechanical decay of the sandstone. Even if local damage can be caused by tree roots, the forest cover and the associated lithobionts obviously play an overall protective role. Additionally, microtopographical factors related to architectural designs and post‐building events probably explain intra‐site and between site minor differences in the amount of sandstone decay, by influencing key factors such as the water residence time at the stone surface. Last, the contrasting weathering regimes in forested and cleared sites are but a trend, for besides overwhelming mechanical weathering, chemical weathering is also operative at cleared sites, as indicated by salt efflorescences and ferric oxidation. Copyright © 2012 John Wiley & Sons, Ltd.

International audienc

Three Romanesque churches of the Bourbonnais region in the French Massif Central have been investigated. These date back to the twelfth century, and are mainly built of red and white sandstones. Their ashlar blocks were extensively repointed in the mid-twentieth century with cement-rich mortar. In order to quantitatively assess the impact of repointing on the decay rates of the sandstones, the following methods have been used: dating of reference surfaces (‘zero datum levels’) based on stone-dressing marks, stone-by-stone and terrestrial LiDAR (Light Detection And Ranging) surveys, and petrographical analyses including on-site non-destructive tests. This study reveals that, between the twelfth century and the mid-twentieth century, intrinsic geological factors have been a key control on weathering rates. Stone surface recession has proceeded, on average, 500 times faster in the poorly cemented soft white sandstone compared to the iron-cemented red sandstone (4 mm/century v. 0.008 mm/century). Since the mid-twentieth century, the replacement of the original lime mortar with a cement-rich mortar has resulted in a marked increase in weathering rates and resulting surface recession of the poorly cemented white sandstone: this increase is fivefold where cement-lime mortar (‘bastard mortar’) has been used and 10-fold where harder cement-rich mortar has been applied. By contrast, the application of cement-lime mortar between the ashlars composed of iron-cemented red sandstone had no deleterious effects. This study confirms that the use of cement-based mortar should be avoided on soft and porous masonries. It also stresses the fact that accelerated stone surface recession rates are systematically associated with heritage sites that are subject to significant anthropogenic effects caused by inappropriate interventions or excessive pressure on the monuments or their environment.

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