Furnas test blocks: cracks measured at 0.15 mm per pixel
For Furnas’ research programme on concrete affected by expansive reactions, face-by-face measurement of the cracks of 80 cm test cubes: 26 blocks in 2022 at 0.25 mm per pixel, 14 blocks in 2023 at 0.15 mm.
Background
At its ageing station in Aparecida de Goiânia, Furnas exposes 80 cm concrete cubes in the open air, instrumented and mixed to develop the alkali-aggregate reaction, delayed ettringite formation, both, or to stay sound; twelve more blocks of the programme are in Curitiba, at LACTEC. These reactions make the concrete of many dams swell and crack. The appearance, lengthening and opening of cracks had to be measured on every face without contact, in a way that could be compared from one campaign to the next.
What we did
From 29 August to 5 September 2022, the 26 blocks were photographed from 60 cm with the camera of an RTK drone fitted with a one-inch sensor: at least 25 georeferenced photos per face, 3,200 in total, faces scaled to 80 × 80 cm. The 128 orthomosaics at 0.25 mm per pixel gave 974 measurements, classified by opening (0.2 to 1 mm, 1 to 2 mm, over 2 mm) with an error of around 1%; formwork marks, small blowholes, microcracks under 0.2 mm and the joint between two concrete lifts are left out. From 18 to 20 September 2023, the 14 Goiânia blocks were photographed again from 40 cm with a one-inch-sensor compact camera: about 60 photos per face, 4,200 in total, 68 orthomosaics at 0.15 mm and 1,184 measurements, with every crack wider than 1 mm measured and the finer ones within a limit of 30 measurements per face. As nothing fixed surrounds blocks that swell themselves, the comparison uses cumulative lengths per class and per face: only two blocks showed no cracks at all, and on one of the most affected the cracks wider than 2 mm went from 8 to 15 and those of 1 to 2 mm from 17 to 40. The method was presented to Furnas’ teams on 18 September 2023, then described in an article co-written with researchers of the programme in IBRACON’s journal Concreto & Construções.
In the mission log
Second crack-monitoring campaign on 14 concrete test blocks
From 18 to 20 September 2023, at Furnas’ concrete ageing station: 4,200 photos 40 cm from the faces, 68 orthomosaics at 0.15 mm per pixel and 1,184 crack measurements, compared with the 2022 campaign.
- 0,15 mm per pixel
- 1 184 measurements
Gallery
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97310 Kourou
French Guiana
Route des Trois-Îlets
97229 Les Trois-Îlets
Martinique
20030-041 Rio de Janeiro – RJ
Brazil