The Building Conservation Directory 2024

PROTECTION & REMEDIAL TREATMENT 4.1 127 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 better. The panels repointed with NHL and cement mortar and the control panel performed less well. Main phase Having proved the experimental concept, it was now possible to refine the work and create a more realistic construction composite to address the unexpected rapid wetting of the thin walls. Larger scale walls, 420mm thick, were built onto high-capacity weighing scales to track changes in wall weight during wetting and drying cycles, and a two-year testing program was initiated to understand the variable performance of the individual walls by comparing performance before and after intervention. The first two walls were built as ‘best practice’ using the same setts as the pilot work. Later, two additional walls were built with voids to mimic features seen in historic towers. One had large cavities constructed using temporary forms to create both continuous and discrete voids in the core. The other had areas of gravel in the core, to mimic mortar washout without significant cavity formation. All four walls were then tested to determine their baseline performance, after which they were treated as follows: • an external NHL 3.5 render (walls 1, 3, and 4) applied • a non-hydraulic lime putty render (wall 2) applied • an external render plus an internal lime putty plaster (for walls 1 and 2 only) applied • render removed but the voids grouted (walls 3 and 4) using the recorded location of voids to guide the process. Both rendering and plastering consisted of two 10mm coats, with the lower coat being 1:3 and the top coat 1:2½ binder-toaggregate by volume. All the walls were then tested again. At the end of the experiment, dye penetration testing followed by dismantling was used to better understand water transfer pathways through the walls. Results The more realistic walls performed in a markedly different way to the panels used in the pilot. In all walls an initial period of rapid uptake of water was followed by reduced rates of absorption. While changes in total weight confirmed that rain was entering the wall, there was no free water in the central collecting trough at the base of the wall, nor did water appear on the rear internal face of the wall. The rendered and plastered walls (walls 1 and 2) remained well below saturation, even after 18 days of simulated WDR. While water uptake between these two walls was similar, it was not identical. Application of renders significantly reduced the rate of uptake, although it did not completely prevent it with the lime putty render having a slightly larger water uptake than the NHL. The walls acted as sponges, preventing the relatively small volumes of water from appearing at the rear of the wall as free water. An unexpected finding was that the voided walls appeared to hold a smaller volume of water than the good-quality walls. This can be attributed to the voids breaking the liquid transfer pathways, coupled with the redirection of some of the penetrating water. For the purposes of the research, this meant that the solid and voided walls could not be directly compared with each other. In the field, over time it is likely that washing out of cores would create internal water storage and additional drainage pathways in walls, which would then act to channel volumes of water into the internal skin; a previously sound tower might begin suddenly to show significant dampness. Through voided walls, the ingress paths are extremely complex: water can pond and saturate capillaries and subsequently flow through the wall; there is also effectively less continuous wall thickness. This led to the same situation as in the initial pilot construction and all walls of this type were found to leak to some extent. Applying render to the voided walls reduced water uptake significantly but did not entirely eliminate it. Similarly, grouting was demonstrated to reduce the uptake of water in the voided walls, but not to eliminate it completely. Moreover, even at this small scale and with the void locations known, grouting proved difficult. Post-test dismantling revealed that while most of the large void spaces had been filled, a significant number of smaller voids were left without grout due to a lack of connectivity with the main areas treated. It was also clear from observation that rain was tracking through the holes drilled to insert the grout. Drying tests were inconclusive but suggested that the wall with lime putty render dried marginally faster than the Result of dye penetration testing of Panel 5. The black colouration is along the plane where mortar perpend meets block and not evenly distributed throughout the joint (Photo: SHU and Historic England) Building the cored walls for Stage 2 of the laboratory tests. (Photo: SHU and Historic England)