rcc dams

ROLLER COMPACTED CONCRETE (RCC) DAMS

By

Rajiv BhagatEngineer (Civil Design)

MARCH, 2011

Aquagreen Engineering Management Pvt. Ltd.

CONTENTS

BRIEF INTRODUCTION

CONCEPT AND SIGNIFICANCE

PRINCIPLE

MATERIALS

METHODOLOGY

SEQUENCE OF PLACEMENT & COMPACTION

MERITS

DEMERITS

DESIGN ASPECTS

RCC DAMS COMPLETED/UNDER CONSTRUCTION THROUGHOUT THE WORLD

FIRST RCC DAM DEVELOPMENT IN INDIA


INTRODUCTION

Roller-Compacted Concrete (RCC) or rolled concrete is a special blend of concrete that has the same ingredients as conventional concrete but in different ratios.

It has cement, water, and aggregates, but RCC is much drier and essentially has no slump. RCC is placed in a manner similar to paving, often by dump trucks or conveyors, spread by bulldozers or special modified asphalt pavers. After placement it is compacted by vibratory rollers.

RCC is typically used for concrete pavement, but it is increasingly used to build concrete dams because the low cement content causes less heat to be generated while curing than do conventional massive concrete pours.


CONCEPT AND SIGNIFICANCE

The development of Roller Compacted Concrete (RCC) caused a major shift in the construction practice of mass concrete dams and locks.

The traditional method of placing, compacting, and consolidating mass concrete is at best a slow process. Improvements in earth-moving equipment made the construction of earth and rock-filled dams speedier and, therefore, more cost-effective.

The maximum placement of 18,000 m3 of RCC in one day, which is still the world’s record, was a clear evidence of the potential of this new construction method.


PRINCIPLE

The edges of the dam are formed first, by making low concrete walls on the upstream and downstream faces of the dam.

Concrete is then transported by dump trucks to the area between the walls, and spread in layer/lift (typically 300 mm thick) using bulldozers.

Rollers are then driven over the concrete to compact it down.


MATERIALS

CementThe consolidation by a roller does not require special cements; however, when RCC is to be used in mass concrete, the recommendation of selecting cements with lower heat generation should be followed.

Mineral AdmixturesMineral admixtures are used extensively in RCC mixtures. The use of large amounts of mineral admixtures reduces both the adiabatic temperature rise of concrete and costs, and improves durability. In the United States, Class F fly ash is the most common mineral admixture used in dams, however, in other parts of the world Class C fly ash , slag , and natural Pozzolan have also been used.

Contd…..

Chemical Admixtures• Air-entraining and water-reducing admixtures are used in RCC compositions that

contains higher volume of paste.

• Set-retarding admixtures can extend the time up to which the concrete lift should remain unhardened, reducing the risk of cold joints with the subsequence lift. In RCC mixtures of dry

consistency, however, chemical admixtures show rather a limited effectiveness.


MATERIALS

Aggregates•Aggregates greater than 76 mm in diameter (3 in.) are seldom used in RCC because they can cause problems in spreading and compacting the layer.

•The size of coarse aggregate has a significant influence on the degree of compaction in small layers. This influence is less marked in relatively thicker layers specially when large vibratory rollers are employed.

•The use of material finer than 75 mm (No. 200 mesh sieve) produces a more cohesive mixture by reducing the volume of voids.


METHODOLOGY

The overall planning of a RCC dam is conceptually different from a gravity dam. To minimize thermal stresses, traditional mass concrete is built in separate, monolith blocks.

This process is slow but allows great flexibility; if a problem develops in one of the blocks, the construction front moves to another block. RCC dams do not have such luxury.

The operation is continuous, building one horizontal lift at a time.

There are no special requirements for batching and mixing of RCC which can be produced using the same equipment as for conventional mass concrete.

Contd…..

Ready-mixed concrete trucks cannot be used to transport RCC because the zero-slump concrete is too dry and cannot be discharged.

To obtain significant economical benefits, special care must be taken in the selection of equipment and construction methods for fast placement and consolidation of RCC.

Conveyor systems can be an efficient method of transporting RCC.

Contd…..


METHODOLOGY

The success of a RCC dam is often contingent on the correct selection of lift thickness, which depends on the mixture proportions and on the equipment available.

If the lift is too thin, the placement rates will be small, thereby reducing the advantages of using RCC.

If the lift is too thick, the compaction will not be adequate, creating horizontal layers of higher porosity, thereby compromising the strength and durability of the structure.

Normally, the thickness of the lifts ranges from 0.15 to 0.90 m; in the U.S. a lift thickness of 0.3 m is often used.

Contd…..

Compaction of the lift is achieved by using a vibrating steel-wheel roller.

Compaction of the lift should be performed as soon as possible, typically within 10 minutes after spreading and no more than 40 minutes after mixing.

Once adequate compaction is achieved, good curing conditions for the finished surface are essential; the surface should be kept in a moistened condition until the next lift is placed.

Contd..


METHODOLOGY

The dry consistency of RCC results in difficulty in bonding fresh concrete to hardened concrete.

This bond can be improved between the lifts by reducing the time of casting the lifts or by increasing the paste content in the mixture.

Typically, bedding mixtures contain 360 to 460 kg/m3 of cement, 170 to 220 kg/m3 of fly ash, and 4.75-mm maximum size aggregate.


SEQUENCE OF PLACEMENT


COMPACTION


ADVANTAGES

Costs: Depending on the complexity of the structure, RCC costs 25 to 50% less than conventional concrete.

Rapid Construction:For large projects, RCC dams can be finished 1 to 2 years earlier compared to regular mass concrete dams.

Cement consumption is lower because much leaner concrete mixtures can be used.

Formwork costs are lower because of the layer placement method.

Pipe cooling is unnecessary because of the low temperature rise.

Cost of transporting, placement, and compaction of concrete is lower, because concrete can be hauled by end dump trucks; spread by bulldozers and compacted by vibratory rollers.


DISADVANTAGES

Protection of Surface of RCCBecause of the relatively low workability of RCC, formed surface are typically characterized by significant voids which reduces the strength & durability of RCC. So type of protection work need to be framed.

There are several methods which includes1.Cast in place concrete2.Precast Concrete 3.Membrane Systems has been used individually and in combinations to construct framing of RCC dams which provides a durable surface and minimize the permeability of structure.

RCC may not be appropriate when Aggregate material is not reasonably available, the foundation rock is of poor quality or not close to the surface, or where foundation conditions can lead to excessive differential settlement.

The supply of fly ash and cement can be a bottleneck. This is because the quantity of these input materials is large: much bigger than the conventional input requirements. Hence sufficient storage on site is vital. There is also need to optimise on the size of the bulker for cement and fly ash transport to reduce traffic on site roads.

Stability analysis of RCC dam is done as per IS-6592:1984.

Approach to stress and the stability analysis for RCC dam are similar to those used for conventional concrete structure . However ,for RCC there is added emphasis on the tensile strength and shear properties of horizontal lifts joint and on non-linear stress-strain behavior

methods of analysis-FEM ,Trail load twist method


RCC DAMS COMPLETED

The countries that had completed RCC dams at the end of 1996 are shown below:


RCC DAMS COMPLETED OR UNDER CONSTRUCTION

RCC dams completed or under construction as shown



Ghatghar Pumped Storage SchemeGhatghar Dams, at Ghatghar, near Shahapur in Thane district of Maharashtra, are the country's first RCC (roller compacted concrete) dams. The upper dam is in Ahmednagar district. The dams are successfully built by the water resources department of the Maharashtra government. The project is also a success of the RCC technology in India. The Ghatghar project is the 10th fastest construction dam in the world. TCE, Mumbai, J. Power of Japan and Dr. M.R.H. Dunstan, UK, were the technical advisors.

The RCC method involves placing the concrete on a continuous basis, in layers of 300 mm thickness.

Fly ash: The RCC construction has two critical requirements: (i)The concrete should be compatible with the compacting rollers and (ii)Control the heat of hydration. These requirements are fulfilled excellently at Ghatghar. The Ghatghar project uses fly ash produced at the coal-fired thermal power plants, in substantial proportion. Normally, 35 per cent fly ash is kept in fly ash + cement mixture. For the first time, the Ghatghar project used 60 per cent fly ash in the mixture. The results were better than expected. Then the fly ash proportion was increased to 70 per cent.



Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases

The Ghatghar project makes tremendous contribution to environment conservation. It uses a 'CCP' (coal combustion product), the fly ash. Use of fly ash gives other benefits and improves the quality of concrete. Fly ash generates less heat of hydration and therefore thermal stresses are lesser. Also, the fly ash gives a denser concrete and that increases the life of the structure


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ALL THE BEST FOR YOUR BRIGHT FUTURE

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