Another interesting site worth saving, part of our history, but also an interesting study of the resistance of different cements to the ravages of environmental exposure.
The site comprised the remains of a WWII Prisoner of War Camp, a Working Camp, built during 1942. The camp remained in operation until 1948 when it returned to agricultural use and storage. The site was scheduled as a National Monument in April 2002.
CRL Surveys were commissioned to undertake a document review and a diagnostic structural investigation, to assess the material condition of the concrete and other elements forming the structures concerned and to evaluate repairability.
The buildings on the site were mainly “Ministry of War Production (M.o.W.P) Huts (18’6” span).” As illustrated, these structures basically comprised sheds or huts, with concrete bases, reinforced concrete portal frames, various infill panels, including pre-cast ”Nashcrete” panels, hollow clay-pots, clay brickwork and asbestos fibre-cement sheets, in both flat and corrugated formats.
The Huts were variously lined with hardboard, impressively decorated by the PoWs.
Compositionally, the concrete forming the portal frame units was of two types, a Portland cement in combination with mixed natural aggregates and brick fragments, and a Portland / ground granulated blast furnace slag (ggbfs) cement in combination with mixed ‘artificial’ and ‘natural’ aggregates.
After nearly 80years, not unsurprisingly, the Bases exhibited widespread degradation, and the Portal Frame units commonly exhibited cracking and spalling, mainly externally but some internally, associated with corrosion of the reinforcement. In some instances huts had partially collapsed due to breaking or snapping of the Corner Posts, with a subsequent progressive failures.
There appeared to be a stark contrast in the condition of the concrete portal frame units, element to element. Some appeared to be variably damaged, but generally quite severely so, with others almost pristine, apart from surface weathering and discolouration.
Testing of the concrete indicated that depths of cover to the reinforcement were consistently low, at 5mm to 15mm, but with little scope in such narrow units to improve on this. However, some units were found to be fully carbonated, with high levels of chloride (0.4% to 2.2% by weight of cement), with others only slightly carbonated (depths of 5mm or less; extremely good for 80years atmospheric exposure), with much lower levels of chloride (0.1% to 0.3% by weight of cement). The chloride was almost certainly ‘ingressed’, derived from airborne sea salts.
Closer examination of the concrete revealed that the lesser carbonated and chloride contaminated units comprised the Portland / ggbfs cement / mixed ‘artificial’ and ‘natural’ aggregate concrete, described above.
From research undertaken as a part of my dissertation for my Master’s Degree, many years earlier, I found, using scanning electron microscopy and micro-analysis, that concrete containing ggbfs, as a cement replacement, exhibited a ‘densification’ with age, due to the slower, secondary, activation or hydration of the ggbfs. This process essentially sealed the concrete and minimized the ingress of potentially deleterious reagents such as CO2 and chloride. My dissertation was entitled “The Hydraulic Reactivity of Ground Granulated Blast Furnace Slag”, when used in concrete as a partial cement replacement. “A right riveting read” (my mum).
Nineteen different repair scenarios were developed for the various portal frame units. However, these were generally for small areas of distress, with larger areas, particularly with respect to the thinner elements, considered probably irreparable.
As an alternative and considering the dilapidated state of some of the huts, together with the ease with which they could be dismantled, undamaged and repairable units from collapsed huts could be salvaged and used to replace irreparable units in other huts.
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