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Searching for an optimal technical solution and concrete mixture for erosion prevention in dam slides

Conference Paper · September 2007

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1 INTRODUCTION Damage of concrete surfaces under water caused by abrasive action by sediment particles is

• ne of the major issues when designing the opera-

tion of hydraulic structures. The issue is especially severe in spillways and outlets of dams on rivers with significant torrential character. The term abrasion in hydraulic structures is used for the process of disintegration of exposed concrete surfaces, resulting from loads arising from sediment transport (Kryžanowski, 1991). The rate of disintegration of the concrete surface largely depends on the transport capacity of water and the ways of transport of solid matter (Mikoš 1993, Kim 2004). Accordingly, the abrasion of concrete can be divided into several phases. In the initial phase, the process of abrasion is caused by sediment transport. The damage to concrete struc- tures thus results from polishing/milling due to rolling or sliding of sediments (solid particles) against the surface. By increased transport capac- ity, the small particles start to move in suspension, and large solid particles move by way of bouncing. At this phase, the abrasion process depends on bed load transport or suspended matter. Besides the milling action of concrete surfaces, damage due to impacts of solid particles against the surface can be

• bserved. By increased transport capacity the size

and quantity of bouncing particles significantly in- crease, and, simultaneously, the pulsations of pres- sures in the water increase. This contributes to the intensity of abrasion. The damages on concrete surfaces that emerge are related to the increase of the size of solid particles and intensity of impacts against the bottom, where at first initial damages

• ccur, which represent, with progressing processes,

the core of progressive spreading of the damage in the direction of the water current (Jakobs et al. 2001). When designing concretes in hydraulic struc- tures it should be emphasised that there is no gen- eral criterion for defining abrasion resistance. Usu- ally, abrasion resistance of concretes is assessed based on a set of parameters that define the single mechanical properties of concretes, such as: com- pressive strength, tensile strength, aggregate strength, use of special cements, modulus of elas- ticity, water/cement (w/c) ratio, surface polishing,

Searching for an optimal technical solution and concrete mixture for erosion prevention in dam slides

• A. Kryžanowski & M. Mikoš & I. Planinc

University of Ljubljana, Faculty of civil and geodetic engineering, Ljubljana, Slovenia

• J. Šušteršič

IRMA Institute, Ljubljana, Slovenia ABSTRACT: In the paper the phenomenon of abrasion of concrete in hydraulic structures is analysed, which is caused by the abrasion process resulting from the sediment carried by water. In detail the adequacy of the laboratory procedure for definition of the level of abrasion of concrete in the standard ASTM C 1138 hydrau- lic structures was investigated. This was assessed based on a comparison between laboratory results and measurements of abrasion resistance of concrete under natural conditions by performing test plots in the still- ing basin of the Vrhovo HPP. The concrete built into the evacuation structures of the HPP`s on the Lower Sava River was tested for adequacy. The basic composition of concretes was modified by additives of the ce- ment binder, steel fibres and PP fibres, and rubber aggregate. The analysis has shown a qualitative similarity

• f the level of abrasion of concrete between laboratory simulation and measurements in the field. The quanti-

tative comparison of results has shown a good correlation between the laboratory measurements and meas- urements in the natural environment for concrete at 900-day age, while for concrete at 90-day age this correla- tion cannot be confirmed.

concrete cure, cement additives (fly ash, fibres), connected with investigating methods that more or less realistically simulate abrasive processes. The problem of studying abrasion resistance of concretes arises from the inability to create proper hydraulic laboratory conditions for the fully devel-

• ped abrasive action. The methods that enable the

modelling of tribology mechanisms of the water current with bed load come closest to the condi- tions present in the natural environment. Šetina (1969) has studied abrasion resistance of concrete with water current and an abrasive device (sili- ceous sand) in a circular flume. The total duration

• f the investigation was 25 hours, and the result of

measurements was the reduction of the cross sec- tion of the sample during the investigation. Liu (1981) has reported on the development of a test method, where the concrete test specimen is in a cylindrical container exposed to the abrasive action

• f steel balls. The method was standardized by the

procedure prescribed by ASTM C 1138. The test method is designed to duplicate the abrasive action

• f waterborne particles in the stilling basins. Circu-

lating water moves the steel grinding balls on the surface of a concrete specimen, producing the de- sired abrasion effects. The water velocity and agi- tation effect are not sufficient to lift the steel balls

• ff the surface of the concrete specimen to cause

any significant impact action against the surface. The test method can only be used to determine the relative resistance of the material to the abrasion action of waterborne particles. Based on the results provided by the method, the general findings are: abrasion resistance increases with the decrease of the w/c ratio, aggregate strength, addition of fibres, age of test bodies, and by increased compressive strength (Kryžanowski 1991, Liu 1981, Horszcza- ruk 2005, Šušteršič et al. 2004). Based on the in- vestigations following the same method and com- parative analyses with other methods of resistance analysis (cavitation, impact erosion) Scrivener et

• al. (1999) reports a considerably higher abrasion

resistance of concretes from calcium aluminous cements and synthetic aggregate. Bania (1989) and Horszczaruk (2004) have reported on the develop- ment and studies of a test method, which is com- posed of a fixed steel drum, which is partly filled with a mixture of water and aggregate, and a rotat- ing drive shaft with fixed tested concrete samples. The comparisons of results of abrasion resistance

• f tested concretes between the abrasion resistance

method (ASTM C 1138 and Bania) and abrasive method according to Boehme (DIN 52108) indi- cate a partial proportional linkage between the re- sults of the studies (Kryžanowski 1991, Jakobs et

• al. 2001, Šušteršič et al. 2004).

Common to the methods investigating abrasion resistance is that they provide only qualitative comparisons between the tested specimens, based

• n a proportional loss of mass or input of the abra-

sive medium during the investigation. The valida- tion of results and applicability of the methods for forecasting the behaviour of concretes in natural conditions can only be achieved by performing the test under the conditions similar to those in the ac- tual operation environment of the designed struc- ture, including the monitoring of all relevant hy- draulic/hydrological parameters (Kryžanowski & Šušteršič, 2003). Jakobs et al. (2004) studied the abrasion resistance of different types of concretes in the natural conditions, by preparing test plots in the by-pass gravel outlet of the Runcahez reservoir and in laboratory conditions, using the method ac- cording to Bania (1989). In the paper the correla- tion between the measurements in field conditions and those in laboratory conditions is discussed. This paper will analyse the suitability of the laboratory method for determination of the level of abrasion of concretes in hydraulic structures ASTM C 1138. This will be assessed by a com- parison between laboratory measurement and field measurements by introducing test plots in the Vrhovo HPP stilling basin. A comparison will be performed for concretes used in evacuation struc- tures of hydropower plants on the Lower Sava

• River. The basic concrete composition will be

modified by addition of the primary binder, steel fibres, polypropylene fibres and rubber aggregate. 2 PROBLEM DOMAIN An energy exploitation project that involves 6 run-of-river cascading HPP´s is underway on the Lower Sava river, while the first plant Vrhovo HPP has been operating since 1996. The Sava river has a typical torrential character with considerable hydrological extremes that are reflected in fluctua- tion of discharge with highly marked bed-load dis- charge (Colarič, 1985). Based on the monitoring of abrasive action in the existing dam structures it can be established that sediment loaded discharge represents a latent danger when trying to ensure

• peration security of objects and functionality of

evacuation structures (Šušteršič et al., 2004). For optimization of operation conditions, a pro- ject of installing further dissipation objects in the stilling basin of HPP Vrhovo was initiated, which coincided with the continuation of building HPP´s

• n the Lower Sava river, where because of safety

reasons the design of abrasion resistant concretes was an important segment. The issue of suitability

• f the materials used in protection of evacuation

structures was raised once again. The redevelop- ment project enabled the introduction of test plots in natural conditions with the following goals: (1) To define the adequacy of choice of con- crete composition on the Lower Sava river HPP´s spillways. (2) To enable quantification of results of labora- tory methods investigating abrasion resistance compared to measurements in the natural environ- ment. (3) To enable forecasting of behaviour of mate- rial during exploitation. 3 PREPARATION OF CONCRETE COMPOSITION In the study the Portland cement with 30% of slag was used, type: CEM II/A-S 42.5 R, which is in accordance with the SIST EN 197-19 standard. The aggregate was obtained by separation of the natural crushed gravel from the alluvial quaternary filling up of the Sava river on the site. Fractions 0– 4, 4–8 and 8–16 mm were used. The content of weathered grains, of grains with adverse effects to the properties of the concrete is less than 1%.

Table 1. Concrete mixture proportions (kg per m3 of con- crete). ________________________________________________ PC1 PC2 PMC1 PMC2 PMC3 PMC4 ________________________________________________ Cement 400 450 450 450 450 450 Water 112 120 75 90 75 102 Super plasticizer 3.6 4.05 Mineral supplement 20.0 22.5 Polymer 94 94 94 47 Steel fibers* 40 40 40 PP Fibers 0.5 0.5 0.5 1.0 0.5 0.5 PP Monofilament fibers 6.0 Gravel aggregate 0/4 mm 940 1280 1215 875 1210 1165 4/8 mm 285 535 505 510 505 485 8/16 mm 700 Rubber aggregate 0/4 mm 140 ________________________________________________ *L=16mm, Ø0.5mm

In this work, the C1 composition is adopted as control composition, which is basically the same as the composition of abrasion resistant concrete built in the spillways of the Vrhovo HPP (Tab. 1). In C2 composition and all further modifications the nominal maximum gravel of 8 mm was adopted. The C2 composite with smaller modifications was used with concretes on the spillway of the second plant of the cascade on the Lower Sava River, i.e. the Boštanj HPP. With the PMC1 composition, representing the initial composition for all further modifications, the mineral additive (SiO2>90%) and super plasticizer were replaced by polymeric binder (styrene-butadiene polymer with dry portion in dispersion 45.6±0.3%): (1) in the PMC2 compo- sition the proportion of the finest fraction was re- placed by rubber aggregate and the proportion of polypropylene fibres (L=10mm, Ø30~40µm) was doubled; (2) in the PMC3 composition the poly- propylene monofilament fibres (L=30mm, Ø0.5mm) were added; (3) in the PMC4 composi- tion the proportion of the polymeric binder was

• halved. The value of the w/c ratio in the compos-

ites did not vary considerably. The compositions of concretes were prepared in the laboratory mixer with a vertical shaft and with a volume of 75 dm3. Right after the mixing the fresh concrete properties, such as temperature, slump, air content and density were determined, following the Yugoslavian standard procedures - JUS (Tab. 2).

Table 2. Results of investigations of fresh concrete. ________________________________________________ Concrete Slump Air Density Concrete temperature content _____________________________________ composition JUS U.M1.032

JUS U.M8.050 JUS U.M1.031 JUS U.M1.030

_____________________________________ ºC cm % kg/m3 ________________________________________________

C1 23.2 6.0 1.5 2530 C2 20.1 23.5 1.2 2454 PMC1 19.3 16.5 3.5 2349 PMC2 19.5 5.5 3.7 2134 PMC3 17.6 8.5 3.6 2303 PMC4 19.4 7.3 4.2 2340

________________________________________________

The specimens for further investigations were prepared by building concretes into moulds. The building-in was performed with external vibra-

• tions. Then the moulds with specimens were kept

covered with plastic foil in controlled climate con- ditions at 20°C. At the 1-day age the specimens were taken out of the mould and kept, depending

• n the investigation, in an air-conditioned/air-

cooled chamber and in water until the date of per- forming the investigation.

Table 3. Results of investigations of hardened concrete. _________________________________________________ Compressive Density Modulus Wear strength

• f elasticity

__________ _________ ________ ________

JUS U.M1.020 JUS U.M1.020 JUS U.M1.025 ASTM C 1138

__________ _________ ________ ________ Concrete MPa kg/m3 GPa mm __________ _________ ________ ________ composition 28 90 28 90 90 90 900 __________ _________ ________ ________ day day day day _________________________________________________ C1 62.33 67.17 2428 2428 31.17 1.79 0.98 C2 73.09 79.17 2421 2440 35.43 1.64 1.16 PMC1 51.12 54.05 2331 2333 26.43 2.09 2.42 PMC2 22.45 23.81 2091 2099 16.37 0.61 0.60 PMC3 46.90 49.06 2266 2266 22.36 1.79 1.92 PMC4 54.79 58.4 0 2333 2343 25.75 2.81 1.84 _________________________________________________

The properties of the hardened concrete were proven with the standard investigation methods. The average values of investigation results of the hardened concrete are given in Table 3: (1) Com-

pressive strength and density were performed at the ages of 3, 7, 28 and 90 days, respectively, on cubicles of dimensions of 15cm, by taking three samples of each composition; (2) The static modulus of elasticity of the concrete was defined at the 90-day age on prisms of 10/10/40 cm, taking

• ne sample per each composition; (3) Abrasive re-

sistance test was performed at 90- and 900-day ages, on cylinders of Ø30/10 cm, taking one sam- ple per composition. The standard procedure of the investigation provides for the measurement of the wear of specimen surface at 12-hour intervals; the total investigation time is 72 hours. 4 MEASUREMENTS IN NATURAL CONDITIONS In 2001, a total of 9 test plots, dimensions of 2.5/2.5 m, thickness of 0.1 m at a distance of 1.0 m (Fig. 1), were built at the bottom of the Vrhovo HPP spilling basin. Out of which, 6 fields were equipped with concretes of laboratory composi- tions, and the rest were fitted with commercial high-strength concretes, which are not the subject

• f this paper. The concrete composites were trans-

ported to the construction site in the form of pre- fabricated dry mixtures. The preparation of con- cretes of the test plot was performed following the same procedure as for laboratory concretes. The concretes were built in manually, using a vibration pin, and afterwards they were manually finished. In the next 14 days an intensive wet curing with additional covering of the test plots with PVC foil was carried out.

Figure 1. Location of test plots in the stilling basin of the Vrhovo HPP spillway.

Before the overtopping of the stilling basin a levelling of all irregularities in contact points with concrete base was conducted together with a sur- veying campaign measuring the surface area of test plots with an accuracy ±10-4 m, and elevation was determined based on the existing bench-mark net- work on the dam. In order to exclude the influence

• f boundaries, the campaign was performed only in

the central part of test plots, in a raster of 30 × 30 cm, and in a total of 36 measuring stations. After the surveying campaign, the test plots in the still- ing basin were filled with water and operation of the spillway was blocked until the required 90-day age of the test concrete. The program of monitoring operational charac- teristics in the test plot plots commenced with set- ting-up of the spillway in function in February 2002, and ended in August 2004. The records on

• perational manoeuvres, discharges over spillway

and on the Sava river flow were held based on hourly records from the operational journal of the hydro-electric power plant. The transport of sedi- ments in the dam cross-section was not directly monitored, but an assessment was performed based

• n the known discharges of the Sava river during

the operational manoeuvres and sediment dis- charge curve in the dam cross-section, which was

• btained from years of measuring turbidity and

sediment transport in the Vrhovo storage reservoir (Mlačnik, 1999). During the time, the transport of

• ver 35,193 t of sediments was recorded in the

spillway area at a total discharge of 204.73 hm3 of water, which was on average 0.0172% of mass flow of water and solid particles over the spillway. The first control check was conducted in August

• 2004. After the visual check of the test plots it be-

came evident that the wear of the surface was uni- form, without any large irregularities or visible

• damage. Quality control of the concrete was per-

formed by sampling with a drill core. The visual investigation of samples revealed that the building- in of the test specimens was of high quality, of homogeneous structure and that a quality contact with the base concrete was achieved, except in PMC2 composition, where the fitting was poorer, which was expected, since the concrete with the rubber aggregate additive was harder to fit in labo- ratory conditions also. The geodetic surveying of wear of test plot sur- faces was performed in measuring stations that were height-referenced to the bench marks on the dam, following the same procedure and using the same equipment as in the campaign performed af- ter the introduction of test plots. Based on the conclusions of the modelling stud- ies it was assumed that, due to the changing veloc- ity of the water current, the wear of concrete sur- faces in the stilling basin cannot be uniform (Mlačnik, 2004). The estimate of the scale of the wear of the base concrete in the stilling basin was

performed based on the measurements of the size

• f wear of representative grains of the aggregate of

the base concrete, that is, in the belt between the pillar and the test plots. The measurements have shown that the wear of the base concrete increases linearly downstream, that is, from the assumed starting point right under the chute. The wear in the central line of the plots (C1, C2, PMC1) increases by a factor of 3.3 as to the initial value, and in the central line of the plots (PMC2, PMC3, PMC4) by a factor of 5.7, respectively. In processing of re- sults, the values obtained for test plots were multi- plied with the corrective factors, depending on the location, so that the measurement results were converted to a common denominator. Figure 2 shows the measurement results of abra- sion resistance following the procedure ASTM C 1138, and the corrected results of abrasion resis- tance in test plots, after approximately 2.5 years of

• peration. The following findings have been made:

(1) The depth of wear at all specimens was less than 3 mm, which ranks the investigated concretes among concretes that are highly abrasion resistant, which also provides a validation of proper compo- sition for abrasion resistant concretes on the Lower Sava river dams (Šušteršič et al., 2004). It can be drawn from the comparison that there is a similar- ity between the wear of the test concretes subject to laboratory conditions and those in the natural environment, which is especially significant in the comparison of laboratory tests at the 900-day age. (2) According to the control C1 composition in laboratory tests with the 90-day age of specimens, a considerable improvement of resistance to wear in composition with the added rubber aggregate PMC2 was achieved, and to a lesser degree in C2 composition, while in compositions with polymeric binder the resistance against wear, except in the PMC3 composition, was lower than in the initial composition. (3) In laboratory investigations, at the 900-day age, the achieved resistance in the PMC2 composi- tion was still higher than the control composition; however, the improvement of resistance with age with the C1 composition is considerably higher than in PMC2 composition. The abrasion resis- tance increased with age also in the C2 and PMC4 compositions, but it decreased in compositions with polymeric binders (PMC1 and PMC3), and in all compositions it remained smaller than the con- trol composition. (4) Comparison of the wear of compositions in test plots yielded similar results as laboratory in- vestigations at the 900-day age: compared to the control C1 composition, a better resistance is achieved only in the PMC2 composition, however there is a smaller resistance in the PMC1 composi- tion; in other compositions there are smaller differ- ences, however, they all indicate a smaller resis- tance than their initial composition.

0.00 0.50 1.00 1.50 2.00 2.50 3.00 C1 C2 PMC1 PMC2 PMC3 PMC4 depth of wear [mm] ASTM - 90 days ASTM - 900 days field

Figure 2. Comparison of wear between concrete composi- tions after ASTM C 1138 and measurements in the test plots.

5 ANALYSIS OF RESULTS The comparison of results between the labora- tory measurements and measurements in test plots showed the similarity between the wear samples of both investigations. The findings were validated with regression analysis, where the measurements in test plots were analysed with those under labora- tory conditions in the duration of: (1) total investi- gation time and (2) by selected cycles. Figure 3 shows the results of the analysis, where the ordi- nate axis indicates the results of wear in laboratory tests following the ASTM C 1138 procedure, and the abscise axis shows the corresponding values of wear measured in test plots at the 90-day age, Fig- ure 4 at the 900-day age.

0.2 0.5 0.8 1.1 1.4 1.7 2.0 2.3 2.6 2.9 0.2 0.4 0.6 0.8 1.0 1.2 1.4

PMC2 PMC3 PMC1 PMC4 C1 C2 PMC1 C2 PMC4 PMC3 C1

depth of wear - ASTM [mm]

▲ test time - 72 h

♦ test time - 24 h

depth of wear - field [mm]

Figure 3. Comparison of wear between the ASTM C 1138 re- sults, at 90-day age (after 24-h and 72-h duration of investi- gation) with measurements in the test plots.

The comparisons with the results of laboratory measurements in the entire duration of investiga-

tion and measurements in test plots have shown that based on the coefficient of correlation, at the 90-day age (R2 = 0.37), it is not possible to confirm the dependence between the measurements; at the 900-day age of specimens (R2 = 0.83) a fairly good correlation between the measurements is con-

• firmed. By taking into account the results of labo-

ratory measurements according to the single cy- cles, at the 90-day age, the dependency could not be confirmed – the correlation coefficient was the highest after 24 hours of investigation (R2 = 0.34). At the 900-day age, however, after 36 hours of in- vestigation excellent correlation was achieved (R2 = 0.92).

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 0.2 0.4 0.6 0.8 1.0 1.2 1.4

PMC2 PMC3 PMC1 PMC4 C1 C2 PMC1 C2 PMC4 PMC3 C1

depth of wear - ASTM [mm] depth of wear - field [mm]

▲ test time - 72 h

♦ test time - 36 h

Figure 4. Comparison of wear between the ASTM C 1138 re- sults, at the 900-day age (after 36-h and 72-h duration of in- vestigation) with measurements in the test plots.

6 CONCLUSIONS The paper provided an assessment of the suitability of the ASTM C 1138 laboratory method for the assessment of abrasion resistance of con- cretes in hydraulic structures by performing a comparison between laboratory measurements and measurements in the natural environment. The fol- lowing has been established: (1) Suitability of the materials and compositions used in the dam constructions on the Lower Sava river, in order to protect the construction against abrasive action of the water current. (2) Good correlation between the results of investigation of abrasive resistance after the ASTM C 1138 procedure and measurements in the natural environment for concretes of 900-day age; how- ever there was no correlation for concretes of 90- day age. (3) High abrasion resistance of concrete with addition of the rubber aggregate. It is therefore ad- visable that further research would be directed into

• ptimization of the composites with the rubber ag-

gregate, building-in technology and durability of the concrete. 7 REFERENCES Bania, A. 1989. Bestimung des Abriebs und der Erosion von Betonen mittels eines Gesteinsstoff-Wassergemisches, Dr. Sc. thesis, TH Weimar. Colarič, O. 1985. Technical report No. 831: Esti- mation of river bed and water level rise in the Vrhovo storage reservoir (in Slovenian). Ljubl- jana: VGI. Horszczaruk, E. 2004. The model of abrasive wear

• f concrete in hydraulic structures, Wear 256:

787-796. Horszczaruk, H. 2005. Abrasion resistance of high- strength concrete in hydraulic structures, Wear 259: 62-69. Jakobs, F. & Winkler, K. & Hunkeler, F. & Vol- kart, P. 2001. Betonabrasion im Wasserbau, VAW, 168. Zuerich: ETH. Kryžanowski, A. 1991. Analyses of concrete resis- tance to deterioration due to water action (in Slovenian). M.Sc. thesis. University of Ljubl- jana. Kryžanowski, A. & Šušteršič, J. 2003. Perform- ance of concrete exposed to long-term underwa- ter abrasion loading, 21st Congress ICOLD Montreal, Proceedings Q.82-R.13: 207-218. Kim, J.K. 2004. Control over bedrock channel in- cision, Dr. Sc. thesis, University of Glasgow. Liu, T.C. 1981 Abrasion Resistance of Concrete, ACI Journal 78 5: 341-350. Mikoš, M. 1993. Fluvial abrasion of gravel sedi- ments, Acta hydrotechnica 11 10: 107 pp. Uni- versity of Ljubljana. Mlačnik, J. Technical report No. 806: Measure- ment of turbidity and sediment transport in the Vrhovo storage reservoir (in Slovenian). Ljubl- jana: IHR. Mlačnik, J. 2004. Technical report No. 857: Hy- draulic model research of Boštanj HPP (in Slovenian). Ljubljana: IHR. Scrivener, K.L. & Cabiron, J.-L. & Letourneux, R.

• 1999. High-performance concretes from cal-

cium aluminates cements, Cement and Concrete Research 29 8: 1215-1223. Šetina, B. 1996. Technical report: Determination abrasion resistance of various materials exposed to abrasion erosion (in Slovenian). Ljubljana: VGL. Šušteršič, J. & Kryžanowski, A. & Planinc, I. & Zajc, A. & Dobnikar, V. & Leskovar, I. & Er- cegovič, R. 2004. Technical report: Perform- ance of concrete exposed to underwater abra- sion loading (in Slovenian). Ljubljana: IRMA.

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