Francisco Ortega, Cerqueira Bastos and Paulo Alves discuss the site planning, construction methods and alternative design features of Pedrógão dam, the first RCC dam to be constructed in Portugal

IN THE Alentejo region of southeast Portugal, the Alqueva arch dam has recently been completed on the Guadiana river, with the main purpose of generating electricity. The project will be completed with the construction of the Pedrógão RCC dam 23km downstream. The work was awarded in July 2002 to Mota-Engil, Engenharia e Construção, S.A. of Portugal, which retains Germany-based FOSCE as its RCC consulting engineer.

Pedrógão dam will be a straight gravity structure 450m long, including a 300m long free surface spillway. It will be constructed from 400,000m3 of concrete at a maximum height of 43m above foundation level. The RCC section will be divided in seven blocks, each separated by vertical joints. The traditional concrete section will have six independent sections and will make up the left side support and the area where the auxiliary safety and exploration systems are located. Also in this area are the power house and two blocks located in the surface spillway area.

Conforming design

Pedrógão was initially designed as an RCC retaining dam, with free surface spillway, but was later altered to incorporate a power station into the blocks of the left bank, a fish lock and bottom and intermediate outlets. As a result, the plan was changed to a composite structure, made from 250,000m3 of conventional vibrated concrete (CVC) in the blocks on the left bank and from 150,000m3 of RCC in the rest of the dam. Cementitious materials specified for the RCC are cement type I-42.5 and fly-ash from the Sines power station.

The contractor opted to take aggregates from borrow areas in the river bed of the site. Aggregate processing would involve: passage through a secondary crusher, reducing all material to a maximum size of 38mm; and sieving, leading to two aggregate sizes – 0–9.5mm and 9.5–38mm.

The RCC batching plant specified in the conforming design was a continuous type, with a capacity of at least 640t/hr or 270m3/hr. The concrete pour is done in 30cm layers. The vibrating rollers must have no less than 10t static weight.

Joints between layers are classified in the conforming design as ‘hot’ and ‘cold’ depending on whether the time between pouring of layers is less or more than 4hr. All ‘cold joints’ will be covered with a bedding mix prior to spreading the RCC of the next layer.

Two vertical joints are proposed: ‘structural’ and ‘induced’, which will be executed in the same vertical plane in every RCC layer.

Structural joints appear on a vertical plane that separates two adjacent blocks of the dam, with joint spacing from 40–60m.

Induced joints are placed in between at a maximum distance of 20m and can be executed by the introduction of a polythene sheet that covers a mould that is removed afterwards. For structural joints, an acrylic resin should be left at the joint plane.
In the original design, the upstream face was lined with precast concrete panels. The face of the panels in contact with RCC was covered with a sealing geo-composite made from a PVC geo-membrane with a minimum thickness of 2mm, coupled by fusion during the production stage to a geotextile with a weight of over 200g/m2.

A bedding-mix was placed in all layers against the faces in both the upstream and downstream area. In addition a fillet of bedding mix was extended to the downstream forms and to the upstream precast panels, covering the height of the layer under construction.

Alternative proposal

In March 2003, Mota-Engil submitted a proposal to modify some aspects of the dam. The client gave its approval in June 2003 and that summer a complete set of trial mixes were developed in the laboratory followed by a full-scale trial in September. After the successful evaluation of the fresh and hardened properties of the RCC and joints, RCC placement started in January 2004 in the area under the spillway flip bucket.

The main aim was to design RCC with better in-situ properties to avoid extensive treatment of the horizontal joints and to obtain enough impermeability in the structure to avoid the use of an upstream impervious membrane.

Improvement of aggregate
The aggregate source in the Guadiana river has an excess of fines passing through a #200 (0.075mm) sieve and in certain areas they show some plasticity.

From the beginning, it became clear that some washing was required to get a more uniform and clean aggregate. The original specification did not call for wet sieving and the fines could not be clearly separated into 0–9.5mm and 9.5–38mm size classes. Also the combined grading curve of the RCC was harder to achieve within the expected limits.

The number of size classes was therefore increased to four: two coarse aggregates (5–19mm and 19–38mm); a fine aggregate (0–5mm) – all three obtained from crushing and wet-classifying in a newly designed aggregate plant – and lastly a limestone filler that was available in the area and that has been used to improve the lower part of the gradation curve.

To meet the requirements of the new coarse and fine aggregates, both water jets on the screens and a wheel classifier have been incorporated. The final coarse aggregate is now a well-controlled product in terms of gradation and loss by washing is below 1%. The fine aggregate has a maximum percentage below sieve #200 of about 1–4% that, in combination with the limestone filler (95% below sieve #200), guarantees the quality of the fines. This has doubled the strength of the RCC for similar cementitious material content.

Cementitious materials
The original mixture proportions had an average cement content of 135kg/m3 and a fly-ash content of 25kg/m3. This high proportion of cement was required in order to achieve design strength with poor-quality aggregate.

Once the aggregate quality had been improved, a preliminary trial mix programme was undertaken to find the best combination of cementitious materials for the new mix. In addition, a mix with a higher cementitious content was proposed in order to improve the in situ properties of the RCC; (impermeability and strength at horizontal joints). A minimum paste/mortar ratio of 0.42 and at least 55% fly-ash were suggested.

Even a mix with a total cementitious material content of 200kg/m3 (65% fly-ash) would easily meet the strength requirements, as well as being cheaper. The cost of the extra fly-ash was by far compensated by reduced cement costs.

Further laboratory trials showed the introduction of a set retarder admixture with properties of water reducing agent, could bring the water, and the cement content, to a lower level for the same strength.

Finally, the main mix selected for the last stage of the trials contained 55kg/m3 of cement and 165kg/m3 of fly-ash. Water content was fixed for a consistency equivalent to a loaded VeBe time (surcharge of 10kg) of 15±3sec.

Other advantages
More fly-ash and less cement brought additional advantages. For example, due to restrictions in the thermal analysis, the main aggregates originally should have been produced and stockpiled during the winter.

In addition no pre-cooling means were foreseen for the RCC and placement was forced to take place exclusively during the winter. Less heat generated with the new RCC mix (heat of hydration at 28 days is 60% lower with the new mix) would ease the temperature restrictions meaning that more could be produced during the summer. Two consecutive overtoppings of the temporary cofferdams has pushed RCC placement to a warmer season. Recent thermal analysis indicates that the dam can be constructed during this period.

Another advantage is improved workability of the mix. Water demand for a similar consistency has been reduced more than 20% with direct implications for strength. The RCC can be transported, spread and compacted over a longer period of time, keeping its fresh properties and consistency required for good density and surface finish. And finally, less cement has helped to reduce the potential for alkali reactivity that had been anticipated with the conforming design mix.