Prevention is better than a cure21 April 2004
Peter U Volkart and Frank Jacobs report on the results obtained following a broad and comprehensive study on abrasion of concrete hydraulic structures in alpine regions of Switzerland
In industrial countries damage due to abrasion on concrete surfaces, including channels, conduits, weir structures and bottom outlets of high dams are frequent, often resulting in high costs to the structure’s owner. Abrasion here is understood as surface erosion produced by water-driven rigid particles. In hydraulic engineering practice cavitation erosion, mass vibrations or chemical attack often act simultaneously to abrasion as destroying processes, but are not dealt with in this article.
In the early 1990s, Technical Research and Consulting on Cement and Concrete Switzerland (TFB) and the Institute of Hydraulics, Hydrology and Glaciology (VAW) of the Federal Institute of Technology (ETH), Zurich, together with the Fund for Projects and Studies of the Swiss Electric Utilities (PSEL) initiated a comprehensive study on abrasion of concrete hydraulic structures in alpine regions.
The governing parameters for the hydro-abrasive impact are presented in table 1. The momentum of a solid particle is mainly defined by its mass, velocity and angle of impact. Velocity and angle result mainly from the water flow field.
In practice the complete set of governing parameters is unknown. For better understanding figure 4 systematically shows a simplified set of impact situations. There, the important distinction between ‘hitting’ impact and ‘sliding’ impact of rigid particles is made.
In 1991, PSEL, TFB und VAW, together with the North-Eastern Power Company (NOK), Switzerland, decided to start with a broad investigation on abrasion of hydraulic structures. As a result of a systematic inquiry, including most of the Swiss water power companies and many visits to construction sites, it became clear that a few large bed load components (> 0.1 m) cause much higher damage than order of magnitude higher amounts of the fine particles (< 0.01 m). As a consequence, the initial idea of a laboratory abrasion test became unrealistic and field tests on operating structures were chosen.
Field tests were performed over six years at the Runcahez diversion tunnel situated at the catchment area of the Vorderrhein Power Company (KVR) in Switzerland (fig 5). The 572m long Runcahez diversion tunnel is a flood by-pass built in the 1960s. The tunnel’s cross-section is 3.8m wide and 4.74m high. The longitudinal slope of the test fields is 1.5%. The tunnel inlet section is gated. In case of flood and high flow discharges, the sector gate is fully opened. Consequently, all the water, as well as the bed load and part of the wash load, are transported.
The discharge capacity of the tunnel lay within a range of 20m3/sec to 120m3/sec; the corresponding mean flow velocities are between 6m/sec and 13m/sec. The transported sediment is characterised by the following standard grain diameters :
d50 = 0.16 m
dmean = 0.23 m
d90 = 0.53 m
dmax = 1.20 m
The critical flow to initiate bed load transport had been calculated to be 30m3/sec. During the six year period of field investigations a total amount of bed load of about 40 x 103t has been transported through the tunnel.
Five concrete test fields were built on the floor of the diversion tunnel, each of them having a length of 10m, a width of 3.8m and a thickness of at least 0.30m. The characteristic data of the concrete in the test fields are given in table 2.
The abrasion rate of the five concrete specimens has been measured periodically and was additionally visually controlled.
The following conclusions can by deduced following the field tests :
• 1. An increasing longitudinal slope of the flow boundary combined with a relatively high water depth increases the water flow velocity and therefore more than proportionally the transport capacity of sediments; hence the abrasion rate is also increasing as well as the probability for the appearance of bigger sediment particles.
• 2. In general hitting impact is more dangerous than sliding impact. Hence, hydraulic engineers should prevent locations with increasing slope at relatively high flow depth.
• 3. Concentration of the streamlines of the flow towards a certain point of the concrete boundary increases the abrasion rate markedly.
• 4. From the viewpoint of concrete quality the following measures to prevent the surface from abrasion can in general be recommended: a high flexural strength and a high fracture energy; a sufficient bond between cement paste and aggregates; in case of predominant sliding impact : a high hardness of the concrete, determined e.g. by a revolving disk test; in case of hitting impact : a not too high modulus of elasticity.
• 5. Points 1. to 4. might already be estimated during the project design period and therefore allow for pre-qualification of a hydraulic structure.
• 6. Due to the fact that the abrasive erosion starts at the concrete’s surface the quality of the surface layer (top millimeters) determines the initial destruction. Therefore a good compaction, a smooth surface and a sufficient curing of the concrete are of major importance.
• 7. Lining with brittle slabs is not recommended for hitting impact. Field tests with slabs made from molten basalt slabs proved this clearly. One strong hit from a stone or large particle with high degree of hardness can result in the destruction of a single brittle slab. Once a slab is destroyed, the hydrostatic pressure underneath the adjoining slabs starts to increase and finally overcomes a critical value. In addition, the hydro-abrasive impact is growing and expanding more downstream. Within a short time in an extended lined area the slabs might be removed. Hence, lining with brittle slabs is for instance not recommended for any kind of stilling basins in case of intense sediment loaded flow creating hitting impact with particle diameters of more than some centimeters.
• 8. In case of particle diameters of 0.1m and more systematic abrasion tests in the laboratory are costly; the prediction of the abrasion resistance against large particles, determined with small particles, is far from being realistic.
• 9. The potential of abrasion resistant concrete seems not to be fully exploited until now.
• 10. Field tests in the alpine region of Switzerland resulted in the following classification of hydraulic structures: Water trapping by leaping weir structures can result in a very intensive abrasive erosion because of the extremely concentrated hitting impact; Sediment flow in diversion tunnels usually creates a combined hitting and sliding impact mechanism. The erosion rate has been measured as to be locally of 5 or more millimeters per year at the bottom at a non permanent discharge. High resistance concrete with a 28-days compressive strength of at least 70N/mm2 and a flexural strength of more than 10N/mm2 is still feasible. But under extreme conditions a local abrasion rate of some cm per hour can still happen; At torrent control works concrete with a 28-days compressive strength of 50N/mm2 and a flexural strength of about 6 N/mm2 reduces in general the annual abrasion rate to only a few millimeters; Concrete having a 28-days compressive strength of 40N/mm2 was successfully used for hydraulic structures at flatter rivers of max. 1 to 2% inclination. The average annual rate of hydro-abrasive erosion has been measured as to be limited in general to 1mm .
Dr. Peter U. Volkart, Head of the Hydraulics Laboratory at VAW, Federal Institute of Technology, Zurich, Switzerland : [email protected] .Dr. Frank Jacobs, Senior Consultant, TFB Switzerland, [email protected]<TablesTable 1 Table 2