application of polyurea coating on concrete lining of intermediate

APPLICATION OF POLYUREA COATING ON CONCRETE LINING OF INTERMEDIATE LEVEL OUTLET AT TEHRI DAM PROJECT – A CASE STUDY

Satish C. SHARMA Director(Technical)

R.K.VISHNOI

Dy General Manager

Tehri Hydro Development Corp., Pragatipuram, Rishikesh (UA) India – 249 201

ABSTRACT

Tehri Dam Project Stage-I has been constructed on river Bhagirathi, a tributary of Ganga, with an installed capacity of 1000 MW. Spillway structures of Tehri Dam Project in India are unique & complex which have been provided for the first time in its design and construction on any water resources project in India. The maximum velocity at the heel of spillway structures is about 45 m/sec. Adequate measures were implemented in the design of structures to ensure the safeguard against the adverse effects of high velocity. Concrete protection is one of these measures generally im-plemented in such cases. The field of concrete protection by way of any suitable coating is rapidly changing and its limits are constantly being stretched. The intro-duction of polyurea coating on the structures of intermediate level outlet of Tehri Dam Project, in order to achieve a host of structural and functional features, is one such attempt. The paper discusses the composition and the mechanical and func-tional properties of polyurea coating used on concrete lining of ILO at Tehri Dam Project. The experiments conducted for obtaining the desired properties, made it possible to decide for the application of Polyurea in abrasive conditions instead of the conventional epoxy mortar due to Polyurea’s 5 times abrasion resistance capacity as compared to epoxy.

734 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT

This paper provides sailient features of the spillway scheme of Tehri dam pro-ject, the considerations made in selecting the concrete protection system and a broad overview of polyurea coating application in abrasion erosion environment with a case study of Tehri Dam Project. The application of polyurea coating in abrasive conditions of the structures of hydro project, has been made on such a large scale for the first time in India.

1. BACKGROUND

Tehri Dam Project is the first multipurpose project on river Bhagirathi in order to harness the immense water and power potential of the mighty river. The 1000MW project envisages the construction of an 260.5m high earth and rockfill dam, spillway structures and an underground power house. Various innovations were imple-mented in respect of the planning, design and construction of flood discharge works at the Project. The spillways have been designed for a Probable Maximum Flood (PMF) of 15540 cumecs computed for a return frequency of 1 in 10,000 years at the project site. The routed flow through the spillway structures is 13200 cumecs at the maximum level of the reservoir. Optimization of the spillway design keeping in view the narrow valley and for passing high flood discharge, extensive model testing for both chute and shaft spillways were carried out to select the geometry and hy-draulic design of the spillway structures.

Four diversion tunnels, two on the left bank (T-1 & T-2) and two on the right bank (T-3 & T-4), were constructed for diversion of river Bhagirathi during the con-struction of the Project. Almost 70% length of these diversion tunnels has been used as horizontal portion for the shaft spillway system thereby, economically utilizing the already constructed structure which normally after plugging would have been abandoned. Each shaft spillways comprises of 14 meter dia excavated vertical shaft joining the horizontal tunnel of 12m dia eccentrically for energy dissipation. This induces a swirling motion at the junction by joining of the vertical shaft spillway with horizontal tunnel tangentially, thereby, preventing the possibility of extensive cavitation of concrete surface and also erosion of the heel of vertical shaft. The total transformation of the flow from vertical motion to the circular motion results into the maximum energy dissipation.

BASIN WATER MANAGEMENT 735

View of Completed Tehri Dam

Spillway structures are required to negotiate a head of about 220 meters, the terminal velocities being of the order of 45-50 m/sec. The main problems arising out of such high velocities are erosion due to the cavitation and abrasion and locked air shock pressures. For guarding against the cavitation damages, aeration/de-aeration arrangements have been envisaged in both chute and shaft spillways based on hy-draulic model studies. For taking care of possible damages due to abrasion, it was envisaged that a High Performance Concrete is required.

2. SALIENT FEATURES OF SPILLWAY STRUCTURES

Tehri Dam Project, has been designed and constructed as the first storage dam on the river Bhagirathi in Himalayas. The project comprises of a 260.5 m high earth and rockfill dam, chute and shaft spillways and an underground power house. The top of the dam is at El. 839.5 m. The Full Reservoir Level (FRL) and the Maximum Water Level (MWL) are at El. 830.0 and 835.0, respectively. The gross storage of the reservoir is 3540 MCM and the live and dead storages are 2615 and 925 MCM respec-tively. The reservoir area at El. 830 m is 42 sq km and the length of reservoir along the Bhagirathi valley is 44 km and along Bhilangana it is 25 km. Four diversion tunnels, two on the left bank (T-1 & T-2) and two on the right bank (T-3 & T-4), have been constructed to facilitate the construction of the project. A specified sequence for


the closure of these diversion tunnels was followed for taking up the impounding of the reservoir. The Stage-I of the project is 1000MW (250MW X 4) and almost all the works at the project for Stage-I have been completed. The general plan of the project works is enclosed as Fig.-1.

The spillway arrangement for Tehri dam Project has been designed for a PMF of 15540 cumecs and the spillway capacity has been provided to cater for a routed peak flood of 13200 cumecs at the MWL. The spillway configuration of the project consists of the following structures:

♦ Gated chute spillway on right bank designed for a discharge capacity of 5500 cumecs.

♦ Two nos. un-gated, right bank shaft spillways (RBSS) designed for a dis-charge capacity of 3900 cumecs.

♦ Two nos. gated left bank shaft spillways (LBSS) designed for a discharge capacity of 3800 cumecs.

♦ An Intermediate Level Outlet (ILO) at El. 700 which is the lowest level out-let for the Tehri reservoir.

The valley of Tehri Project site is very narrow and enough space is not available for accommodating a large open chute spillway structure adjacent to the main dam. The layout of spillway structures at the project has been finalized in a way to mini-mize the excavation of abutment slopes. In case of one chute spillway alone, the required width of the chute spillway control structure for passing the PMF, was not available on right abutment and a major part of the abutment needed to be excavated for creating the required space. As a result, the concept of introducing the shaft spillways in the spillway scheme was deliberated so as to create the spilling capacity through the tunnels and thus, reduce the requisite width of chute spillway thereby, reducing the mass abutment excavation. Four nos. of shaft spillways designed for a capacity of 7800 cumecs, were then envisaged to be provided utilizing the four exist-ing diversion tunnels. The scheme of spillways at the Project was finalized based on the extensive hydraulic model studies. Above spillway scheme is briefly described as below:

2.1 Chute Spillway:

A conventional chute spillway designed for a drop of 220 m is supposed to pass a maximum flow of 5500 cumecs at MWL. The total width of the chute spillway is 39.5 m at the control structure and 50 m at the Stilling Basin. The approach channel of the chute spillway is provided at El. 810.0 m while the crest of control structure is at El. 815 m. The discharge carrier consists of two parts; one of 1:20 slope and other of 1:1:9 slope. Three aerators have been provided on the sloping glacis to avoid the problem of cavitation on concrete surface. A baffle wall with crest at El. 618.0 m is


provided at the end of stilling basin, in order to provide adequate water depth for formation of hydraulic jump. The energy in chute spillway is proposed to be dissi-pated in the Stilling Basin of size 140 m X 50m wherein the high strength concrete consisting of micro silica has been used at the floor and side walls for providing sufficient resistance against the abrasion. Adequate river protection works in the form of anchored concrete blocks have been carried out for avoiding the erosion of river bed.

2.2 Right Bank Shaft Spillways (RBSS):

The right bank shaft spillways comprise of two number vertical shafts of 12.0m diameter, connected to the existing diversion tunnels T-3 and T-4. The right bank shaft spillways are un-gated spillways, having their crest at El.830.20m and intake structure in the form of morning glory type structure. At the crest level, the diameter of the funnel is 34m, the maximum outer diameter being 36m.

The vertical shaft joins the horizontal tunnel eccentrically through the tangential junction, which imparts swirling motion to the flow in the horizontal tunnel to achieve energy dissipation. On the up-stream side of the junction of vertical shaft and tunnel, a concrete plug has been constructed. The air accumulated at the center of the flow has been taken out with the help of a de-aeration system comprising of a de-aeration shaft (D-section of 3m x 6m) which runs parallel to the main spillway shaft above the level of separation chamber (El 660.0 m). An Intermediate Level Out-let (ILO) tunnel, after the transition zone of 42.0 m, joins the vertical shaft T-3 at El 687.40 m on the right side and at El 676.70 m on the left side. The layout of Right Bank Shaft Spillways is shown in Fig–2.

2.3 Left Bank Shaft Spillways (LBSS):

The left bank shaft spillways are two number gated shaft spillways connected to existing tunnel T-1 and T-2 on the left bank. The intake structure of LBSS is provided in the form of 10.5 m wide Ogee crest having radial gates for regulation of flow. The crest of the control structure is located at El 815.0 m. The discharge carrier tunnel is a sloping D-shaped tunnel joining the vertical shaft at El 778.78m. The vertical shafts of LBSS also join the tunnels T-1 and T-2 through a swirling junction for energy dissipa-tion. The de-aeration arrangement for LBSS is through a de-aeration tunnel located about 40m above the horizontal tunnels. Concrete plugs have been provided up-stream of the junction of vertical shafts with the tunnels T-1 and T-2. The layout of Left Bank Shaft Spillways is shown in Fig–3.

2.4 Intermediate Level Outlet (ILO):

Intermediate Level Outlet (ILO) of 8.5 m dia has been provided at El. 700 in or-der to operate the same during initial filling of reservoir or emergency evacuation of


reservoir. The outlet is also proposed to be used for releasing the irrigation dis-charges in case the power house is shut down for some reason. The tunnel of the ILO is 275 m long is joining one of the shaft spillways T-3 at El. 687.40 m. The layout of ILO is shown in Fig-4.

3. OPERATION CONDITIONS AT INTERMEDIATE LEVEL OUTLET

The Intermediate Level Outlet, which is located at an Elevation of 700m, having a diameter of 8.5 meters and the length of 275 meters, has been provided for releas-ing the water in case of any emergency or during the initial filling. The outlet has two distinct parts, one upstream of control gate shaft whereas the other is downstream of control gate shaft. Since this outlet is the bottom most outlet at the Project, the first part is always going to be submerged in water as it is directly connected with the reservoir. This part of the outlet was constructed with 1 (one) meter thick concrete of M-30 grade with a view to perform its designed function throughout the dam life. The service conditions here were very stringent that for the whole life of the Project, it would be submerged under water. Therefore, there would be no opportunity for any maintenance. Moreover, since the outlet has to operate under a maximum head of around 140m, the water is expected to flow at a very high velocity of around 25 m/s. The flowing water is expected to carry boulders, rock-pieces, sand, silt and other eroding materials and high abrasion resistance of the concrete surface was therefore, called for.

The Himalayan rivers carry large amount of fine sand and silt which makes abrasion-erosion of concrete as one of the main cause of deterioration of hydraulic structures. It is estimated that the abrasion erosion damages have occurred in as many as 68% spillways structures of hydropower stations of the world. Therefore, experts in the field have been trying to find an answer to protect the concrete with suitable coating and make it resistant to abrasion and erosion. Main reasons of the abrasion and cavitation damages in hydraulic structures are due to the faulty design, inferior construction materials and improper operation of the project. Extensive research on abrasion and cativation damages of the hydraulic structures has taken place in the past few years. It has been now widely considered that safeguarding hydraulic structures from abrasion-erosion problems cannot be done by improving the designs of the structures alone and the use of special concrete protection tech-niques involving the use of polymer coating, is necessary. The performance of the polymer coating on concrete surface, applied on these structures has, to date, gener-ally confirmed the favourable results predicted by many laboratory investigations.

As a result, it had become mandatory to protect the concrete, prevent it from abrasion erosion with better abrasion & wear resistance protective coating, apart from providing the treatment to fill-up the porosity of concrete. When water re-


sources engineers call for a concrete protection system, it is required to specify a multitude of variables, such as its functional properties, durability, application tech-niques, hardening characteristics, ease in application and much more. High perform-ance concrete of M-60 grade (with 8.5% micro silica) has been used in all the critical parts of spillway structures in order to take care of abrasion related complexities except in the upstream part of ILO where the lining with plain M-30 grade concrete had already been done before the high performance concrete (M-60 grade) could be developed at the Project. The protection of this part of the concrete lining against the abrasion erosion thus, became necessary due to the use of plain M-30 grade concrete.

Extensive studies and analysis was done to determine the best solution for pro-tecting the ILO lining under most aggressive environment and service conditions. Various options in terms of protective materials were looked into and one such op-tion was lining with Epoxy-mortar, which has been conventional and the most popu-lar application tried so far in Hydro and Water Resources Project. Functional re-quirement from the coating was very high abrasion resistance and hardness, without the limitations of brittleness. Further, it should possess high impact resistance, flexi-bility and elongation properties. Another property considered was that of its fast drying/curing time so that the application becomes smooth mainly in the crown portion of the tunnel. Moreover the process of applying the coating systems around the tunnel lining in such a big area, should be mechanised so that it should not take very long time.

4. POLYUREA COATING AND ITS PROPERTIES

The widely and conventionally used epoxy coating system in water resources projects, has a tendency to crack, break or get abraded under severe environment over a period of time when applied on such structures and therefore, has a limitation with regard to the durability of the concrete, causing erosion, corrosion or spilling of the concrete. In past few years, a new spray-applied technology has been devised by the technocrats for protecting concrete, steel and other industrial substances from degrading effects of corrosion and abrasion. This is a new innovative polymer coat-ing technology called Polyurea elastomeric coating, which is applied under high pressure and temperature with special equipment.

A polyurea coating/elastomer is derived from the reaction product of an iso-cyanate component and a resin blend component. The iso-cyanate can be aromatic or aliphatic in nature. It can be monomer, polymer, or any variant reaction of iso-cyanates, quasi-prepolymer or a pre polymer. The pre polymer, or quasi-pre poly-mer, can be made of an amine-terminated polymer resin, or a hydroxyl-terminated polymer resin.


The resin blend must be made up of amine-terminated polymer resins, and/or amine-terminated chain extenders. The amine-terminated polymer resins will not have any intentional hydroxyl moieties. Any hydroxyls are the result of incomplete conversion to the amine-terminated polymer resins. The resin blend may also contain additives, or non-primary components. These additives may contain hydroxyls, such as pre-dispersed pigments in a polyol carrier. Normally, the resin blend will not contain a catalyst.

Polyurea elastomeric coating is 100% solids, contains no volatile content and it generates extremely fast gel time which is ideal for moist environments like those in the hydroelectric projects and irrigation dams. Due to its specific curing profile and exceptional film properties, the polyurea spray coating technique developed into various areas, including corrosion protection, containment, membranes and linings. Due to its improved properties, Polyurea can be used in extreme conditions also. It has the capability to cure even at very low temperatures. Polyurea coatings combine high flexibility with hardness, thus resulting in high durability.

Although Polyurea elastomeric coating is often times considered a relatively new material, its development has been gradual and continual over the last few years. During this period, many notable changes have occurred and continue to oc-cur in the area of elastomeric compounds technology. With the increased knowledge that has been gained with respect to material availability, compound compositions and construction techniques, the feasible realm of Polyurea coating applications has grown substantially.

Polyurea can be considered a suitable coating in case of the following require-ments.

♦ Strong bond with concrete substrate ♦ Fast curing time ♦ Application under high humidity and/or at low temperatures ♦ Extreme abrasion resistance ♦ Impermeable membrane ♦ High elongation Polyurea is considered to be one of the world’s toughest polymer available for

uses in water conveying structures as it has excellent erosion and corrosion resistance apart from very high elongation property and excellent adhesion with concrete. Polyurea thus, appears an ideal coating for these Structures because of its good resil-ience and tough, yet flexible.

It is therefore, a solution for a kind of situation like in Tehri ILO. Due to ultra fast drying property, it can be applied on crown of the tunnel and also side surfaces without any sagging at 3 mm or more thickness. Because of very high elongation of the order of more than 300%, Polyurea applied on the joints would be able to take the


required elongation without breaking and thereby arrest leakage. However, the most significant property required from a coating to be applied on the ILO lining, was its abrasion resistance under wet conditions. Accordingly, Polyurea coating was evalu-ated on this criterion for its application in ILO lining. Following comparison has been reported for the above coating, which were verified by laboratory tests.

Test Concrete Epoxy Steel Polyurea

Wet Abrasio 137 grams weight loss as per ASTM C 1138

Weight loss 28 grams as per ASTM C 1138

0.0825 grams weight loss as per cavitation erosion test

0.0747 gms as per cavitation erosion test and 8 grams as per ASTM C1138

Dry Abrasion NA 0.023 gms as per ASTM D4060

NA 0.003 gms as per ASTM D4060

Polyurea, based on the above considerations, was found to be suitable for use in ILO tunnel, where the environment is highly aggressive due to water with abrasive silt flouring at high velocity of about 20 meter per second. Further, the product ‘Polyurea’ has been tested in the laboratories before accepting it for application in ILO.

5. TESTS ON POLYUREA COATING AND ITS PERFORMANCE

Polyurea Coating which is said to be the latest coating technology and is the choice of many Engineers in the American and European countries for similar kinds of applications. Having explored it further, it was discovered that Polyurea is today one of the world’s toughest polymer available. However, in order to verify these characteristics and to be very sure of its performance, a number of tests, as listed below, were got conducted from independent laboratories, before accepting for its application on concrete lining of ILO.

♦ Bond strength in tension with Concrete (after abrasive blasting and Primer application)

♦ Abrasion Resistant (Both Dry and Wet Abrasion) ♦ Impact Resistance ♦ Elongation Property ♦ Tensile strength of the coating ♦ Hardness test ♦ Flexibility ♦ Setting/Curing time


♦ Demonstration of drip or sag during application on overhead structures in order to simulate the conditions of the crown portion of tunnel and other vertical areas.

Based on the various tests and demonstration, it was revealed that the Polyurea coating is a suitable coating for uses in water resources structures. It has excellent bonding with concrete. Further, the abrasion resistance of the coating was found to be good as compared to steel but substantially high as compared to concrete and epoxy. The results of various tests conducted are given in Table-1.

Table-1

TEST RESULTS OF POLYUREA FOR PRE – DISPATCH OF LOT -1, LOT-2

Sl. No.

Description of Test For Lot-1 For Lot-2

1. Flexibility As Per ASTM : D-1737

No effect (After bending over 12mm Mandrel 1000 Micron)

No effect (After bending over 12mm marble 1000 micron)

2. Abrasion Residence (Taber Abraser, CS-17 Wheel, 1000 gms for 1000 Cycles) ASTM : D-4060

Weight Loss =0.01125 gms.

Weight Loss =0.0130 gms.

3. Percentage Elongation ASTM : D-638(Type-IV)

i) 315.46% ii) 281.11% iii) 271.02%

i) 386.75% ii) 366.57% iii) 365.67%

4. Young Modulus ASTM : D-638(Type-IV)

i) 3.3136 ii) 3.5439 iii) 3.5619

i) 2.5949 ii) 3.1755 iii) 3.2270

5. Adhesion Pull-off Test As per ASTM : D-4541

More than 25 N/mm2 More than 25 N/mm2 for Samples No 1 & 2

6. D-Hardness Test(SHORE-D HARDNESS) ASTM : D-2240

Value around 50 D

50D - Trial 1 49D - Trial 2 50D - Trial 3

6. METHODOLOGY / APPLICATION OF POLYUREA COATING

The concrete surface should be absolutely dry and moisture free. In order to achieve these conditions in ILO, all the leakage points were identified and measures were taken to stop the leakages. Main leakage preventive measure adopted at the site was extensive consolidation grouting. At the places where severe leakage was ob-served, the consolidation grouting in three-four stages was done to achieve the abso-lutely dry surface.

Polyurea coating process started with the application of primer which was a low viscosity, high solids(95-100%), two component epoxy system and acted as a pene-trating concrete primer for polyurea applications. The concrete surface on which the


primer was to be applied, was made free of oil, grease, paints etc. A surface finish of 60-70 microns was ensured for proper bonding, which was achieved either by sand blasting or other appropriate means. Mixing ratio of the primer components was as recommended by the manufacturer. Primer was applied over a wide range of sub-strate temperatures by brush, roller or spraying. Application of primer was done multi-directionally (north-south/east-west) to ensure uniform thickness. The recom-mended dry film thickness (DFT) was about 100 microns. It was advised not to do any primer application when temperature fell down below 100C or, rose above 500 C and when relative humidity rose above 90%.

The primer ensured maximum adhesion to the prepared substrate. The polyurea coating was sprayed after the primer coat was fully applied and the surface was near tack free. The spray of Polyurea coating was done with the help of a special gun having two separate nozzles. These two components were then drawn from two containers by way of suction passing through the heated coils (temperature ranging from 50-750 C. The Polyurea coating was then applied through the spray gun under very high pressure (1000psi). It was observed that the heating of the materials re-duces the system viscosity which improves the mix, flow and levelling of the applied material and finally leads to smooth coating film.

View of Polyurea Coated Concrete Lining of ILO


7. FINDINGS BASED ON EXPERIENCE AT TEHRI

At Tehri Project, Polyurea elastomeric coating resistant to the abrasion was used and abrasion resistance tests on the coating were carried out as per the ASTM stan-dards. Such tests carried out for assessment of abrasion resistance, have indicated that the coating when applied on concrete of M-30 grade can increase the abrasion resistance of the concrete substantially as measured by ASTM C-1138. Another point to be kept in view while finalizing a coating for concrete lining where it has been prescribed is that under service conditions, it is really not the strength but soundness, i.e. freedom from cracking, that plays the most important role. Hence, in a severe environment, to reduce the chances of coating peel off, necessary bond of coating with the concrete substrate, is to be examined.

It is believed that Polyurea coatings provide the best solution to protect water conveying structures viz., spillways, tunnels, aqua-ducts, chutes etc. from abrasion erosion. Polyurea coating is impervious and does not allow water to ingress from concrete porosity, hence suitable for arresting mild seepage also for protecting steel reinforcement in the concrete from corrosion. Because of high elongation, it can be used to cover expansion joints in concrete structures. It has good resilience and is tough yet flexible. It therefore, appears to be superior to other types of coating being applied so far. Application of Polyurea coating not only facilitated easy placement on concrete through the special equipment but also resulted in a packed and smooth concrete surface. The tests carried out on the work sites of Polyurea coating, revealed that specified bond strength was achieved almost on all the occasions.

8. CONCLUSION

Application of Polyurea coating has been considered on concrete lining of ILO at Tehri dam project to meet the functional demands of the structures designed to per-form in harsh abrasion conditions. Polyurea has been used in the Intermediate Level Outlet (ILO) tunnel, where the environment is highly aggressive due to water with abrasive silt flouring at high velocity of about 25 m/s.

Based on the findings of the tests and its performance, the application of 3mm thick Polyurea coating on concrete lining of ILO was recommended and the work was successfully executed in a period of around 3 months covering an area of around 5500 sq m. The coating was applied successfully under high pressure and high tem-perature with a special gun brought by the applicator. Few tests for functional prop-erties such as bond strength in tension, hadness etc were also carried out at the site. Due to ultra fast drying property, Polyurea can be applied on overhead and vertical surfaces without any sagging at 3 mm or more thickness.


The entire work of application of Polyurea on ILO concrete lining was found to be in consonance with the quality control parameters as per the ASTM standards. After the application of Polyurea coating, ILO was put into operation by closing the last diversion tunnel for impounding the Tehri reservoir.

9. REFERENCES

Journal of Protective Coatings and Linings.

Technical Reports of Central Water Power and Research Station Pune on the tests con-

ducted on epoxy compounds.

Case Study of Application of Polyurea coating on 230 vertical foot concrete chimney lo-

cated west of salt lake city, UT.

Test Reports of Corrosion Science & Engineering, Indian Institute of Technology Bombay

on the tests conducted for Solventless Elastomeric Polyurethane samples.

Articles and Information provided by Polyurea Development Association.

ASTM Codes No D-2240, D-4060, D-1737, D-638, D-412.

ASTM-1138 for the wet abrasion test.

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application of polyurea coating on concrete lining of intermediate