PSI - Issue 77

João Custódio et al. / Procedia Structural Integrity 77 (2026) 447–456

454

8

João Custódio, et al. / Structural Integrity Procedia 00 (2026) 000–000

The Pracana dam, built in 1951, was the first Portuguese dam to undergo rehabilitation due to internal swelling reactions (ASR and DEF) (Fig. 2). Pracana dam is a 60 m high buttress structure, completed in 1951, built with a concrete mix including quartzite and granite aggregates (Gomes et al. , 2021; Batista, 2022). The dam exhibited an abnormal behaviour since the first filling of its reservoir, in 1951, characterised by progressive vertical and axial horizontal displacements (upstream-downstream direction), widespread cracking of the facing and significant water leakage downstream, mainly through horizontal cracks in the concrete joints, which led to the need to empty the reservoir in 1980 (Gomes et al. , 2021; Batista, 2022). From 1988 to 1992, the dam was subjected to large-scale rehabilitation works, which consisted of dam structural rehabilitation and the execution of complementary works, namely the construction of an auxiliary frontal spillway and the updating of the powerhouse, including the construction, in the dam body, of the intake for the new generation unit. The structural rehabilitation of the dam included: (i) concrete regeneration; (ii) installation of an impermeable membrane on the upstream surface, including the construction of an upstream foundation plinth; (iii) construction of two sets of concrete struts for locking the buttress webs at the foundation level; and (iv) consolidation of the foundation and execution of new waterproofing and drainage curtains (Gomes et al. , 2021; Batista, 2022). The buttress concrete regeneration was achieved by treating cracks with an opening greater than 0.5 mm through the injection of cement-stabilised grout, and mass treatment with the injection of epoxy resin in the thinner cracks (which corresponded to approximately 80 % of the mapped cracks). The waterproofing system of the upstream surface consists of a non-adherent synthetic membrane (impervious flexible PVC geomembrane 2.5 mm thick, heat-coupled during extrusion to a non-woven, needle-punched 500 g/m 2 geotextile). The observation system was also improved to comply with the latest Portuguese regulations for dam safety, and to allow better monitoring of the structural behaviour and the swelling process evolution (Gomes et al. , 2021; Batista, 2022).

(a) (b) Fig. 2. (a) View of the impermeabilization of the Pracana dam’s upstream face; (b) Downstream view of the Pracana dam.

The Alto Ceira dam was constructed between 1940 and 1949, with the first impounding of the reservoir occurring in March 1950. The main structure was a thin arch, defined by circular arches of a constant thickness, with a maximum height of 37.0 m, a crest development of 120.0 m, and a thickness varying from 1.5 m at the crest to 4.5 m at the bottom. The reservoir had a maximum capacity of 0.98 hm 3 . The dam had a surface spillway with a maximum capacity of 100 m 3 ·s -1 on the right bank and a shunting line tunnel with a maximum capacity of 8.9 m 3 ·s − 1 on the left. The Alto de Ceira dam was built with concrete produced with quartzite aggregates. The ASR was first detected in the 1980s. Evidence of this phenomenon included widespread cracking of the structure, water passing through cracks in the kidney area, and progressive displacements, both radially upwards and vertically. The dam was decommissioned and partially demolished in 2014 due to significant cracking and leakage as a result of the ASR, and a new dam (Alto Ceira II dam) was built in a downstream zone near the demolished dam (Fig. 3) (Custódio et al. , 2025). The Alto Ceira II dam is a double-curvature arch dam with gravity abutments. It has a crest length of 137.5 m and a total height above the foundation of 41 m. The total concrete volume of the dam is approximately 18,500 m³. The reservoir created by the dam has a capacity of around 1.3 hm 3 for the full storage level (at 665.90 m) and around 1.5 hm 3 for the maximum flood level (at 667.00 m). The alkali-reactivity demonstrated by the aggregates made it necessary to adopt mitigation measures, and thus, the concrete mixture incorporated fly ash (Custódio et al. , 2018).