The Adaptation of Roller Compacted Concretein the Construction of Alpe Gera DambyJeally V. Arabos

A report submitted in partial fulfillmentof the requirements forENGL 4E Communication and Presentation Skills

College of Engineering and ArchitectureUniversity of the Cordilleras

ABSTRACTRoller Compacted Concrete, one of the significant breakthroughs had gain great respect in the field of engineering. This paper generally provides information about the said improvement and how it was adapt in the construction of Alpe Gera Dam.

INTRODUCTION

Innovation in the field of engineering is a non stop progress. The causes are primarily consists of the considerations on new published ideas based on researches as well as accommodations to the needs of society. One of these is the use of Roller Compacted Concrete in many constructions. RCC continues to gain recognition as a competitive material for construction of new gravity dams and for rehabilitation of existing dams. During the past two decades, many design details and construction methods have been implemented to improve this relatively new dam construction method with the goal of enhancing the product while maintaining the RCCs competitive edge associated mainly with speed of construction. It also has many differences on a usual concrete in terms of quality, economy, and methodology that makes it well-known since its first adaptation on Alpe Gera Dam (Chengdu, 1999).

DISCUSSIONS

Characteristics of Roller Compacted ConcreteA recent online article titled New Innovations in Engineering, says Roller-compacted concreteor RCC takes its name from the construction method used to build it. It is a special blend ofconcretethat has essentially the same ingredients as conventional concrete but in different ratios, and increasingly with partial substitution offly ashforPortland cement.RCC is a mix of cement/fly ash, water, sand,aggregateand common additives, but contains much less water. The produced mix is drier and essentially has noslump. RCC is placed in a manner similar topaving; the material is delivered bydump trucksorconveyors, spread by small bulldozersor specially modifiedasphalt pavers, and then compacted byvibratory rollers. But unlike conventional concrete, it's a drier mixstiff enough to be compacted by vibratory rollers. Typically, RCC is constructed without joints. It needs neither forms nor finishing, nor does it contain dowels or steel reinforcing (par. 2 6).

Image 1: Dam made of Roller Compacted Concrete

RCC is frequently used for the construction of mid-sized to large mass concrete applications, including dams, because of its faster placement rate, lower heat of hydration, and reduced cost compared with conventional concrete. RCC is a no-slump concrete that is typically spread in lifts as thick as 12 inches with earthmoving equipment and compacted with a smooth vibratory roller. Early experience on RCC dam applications in the 1980s showed a tendency for seepage to develop along the 1-foot-thick lift lines due to the tendency at the time to use a lean RCC mix. Therefore, many RCC dam designers started including an upstream facing system as a watertight barrier in combination with careful attention to lift joint preparation and treatment. The upstream barrier is typically constructed of a zone of conventional concrete, an exposed liner system, or precast concrete panels with or without a liner system (Kbler-Ross, 2000).The American Concrete Institute (ACI) defines RCC as concrete compacted by roller compaction; concrete that, in its unhardened state, will support a roller while being compacted. Properties of hardened RCC can be similar to those of conventionally placed concrete. However, RCC can also be made with hardened properties that are outside the range of typical properties of conventionally placed concrete. The term roller compaction is also defined by ACI as a process for compacting concrete using a roller, often a vibrating roller.

The Alpe Gera Dam

Image 2: The Alpe Gera Dam in Italy

The Alpe Gera Dam ( known as Diga Alpe Gera in native localities) is a gravity dam on the Cormor River, 17kilometers northeast of Sondrio in the Lombardy region of Italy. It is 174meters tall, 528 meters in length, and has a crest elevation of 2,128 meters. It has 1,700,000 cubic meter of volume, active capacity ranging from 62,700,000 to 68,100,100 cubic meters and a spillway capacity of 62 cubic meters per second. It also supports 35 megawatts of hydroelectric power station (Smith 2000). An online article Alpe Gera Dam reports that the dam was constructed between 1958 and 1964 and is best known for the concrete placement techniques used during its construction. Instead of concrete being poured into conventional monoliths, it was poured in layers with a lean mix. Next, the concrete was settled with immersed vibrators and then contraction joints were cut into the layer. These methods were not only cost-saving but instrumental in the development of roller-compacted concrete in dam construction (par 1 4).Alpe Gera Dam is the highest concrete gravity dam in Italy, and one of the worlds highest of this type. Since, in the solid-gravity dam, the concrete strength is generally higher than that prescribed by Italian specifications, the dam body was built using a low water-cement ratio concrete. In this way, it was possible to place the concrete by mean of trucks, in horizontal layers of limited thickness, covering the whole horizontal section of the dam. The dry, non-flowable nature of concrete, allowed to be sure that its characteristics would in no way be affected by trucks being driven over it only few hours after its placement. Transversal joints were made by cutting the concrete layers, while still soft, with a vibrating blade (Kbler-Ross, 2000).

The Process in the Placement of RCC

Image 3: The Placement of RCC

The following processes are acquired on an online article Advancement on Roller Compacted Concrete.

Transportation and placement of RCCThere are various types of transportation and placement plants used in RCC dams. But the one used in Alpe Gera Dam is the picking up of concrete from the batch plant by a belt conveyor and placing it with a jack posts system with a tripper conveyor, line conveyor and crawler placer/ crater crane. Surface cleaning before concrete placementThere is always some time-gap in concrete placements between every two successive lifts and it is usual that the horizontal surface gets contaminated during this system-bound operational process. Contamination is injurious to the surface of the lift and must not be allowed. It is essential before concrete placement that horizontal construction surface is made perfectly clean, and any laitance, loose material or free water on the surface is removed by vacuum pickup car. High pressure air or water jets should not be used because these will damage RCC surfaces that are less than 72 hours old and prevent bond development. During cleaning process, if some loose material comes out and holes appear, these should be properly refilled, leveled and thoroughly compacted.The contact surface aggregate should be perfectly clean and in condition to bond with the mortar layer, which is generally laid before starting the next lift. Placement rates of less than one lift in 24 hours usually require some surface preparation. Material loosened by equipment travel should also be removed. A vacuum truck driven over the old lift immediately prior to placement of the next lift works well. Water cleaning or sandblasting of green concrete tends to remove more material than necessary and tends to leave a washed sand coating on the surface, reducing bond. However, lift surfaces that are older than approximately 72 hours may be washed with low-pressure water, if necessary. Placement rates of three lifts or more per 24 hours generally do not require any surface preparation. The cleanliness of horizontal lift surface is quite critical for an RCC dam and should not be neglected; otherwise it will be permanent seepage problem in the structure affecting its long-term durability.Mortar spreadingThis has relation to maturity factor of concrete. This becomes necessary for proper fusion of two layers of concrete when the time element between two successive layers of concrete is more than permissible.Joint cuttingVertical contraction joints are provided in a dam body to avoid the occurrence of random cracks due to temperature stress. Construction of joints should be done with every lift after spreading and proper finishing but before compacting the lift by vibratory roller. In case of any problem, vibratory cutter can also cut the contraction joints immediately after completion of compaction. The task is completed by the use of a vibratory joint cutter to which is attached a vertical heavy-duty jack. The jack forces its joint cutting blade while applying vibration to make a narrow groove reaching down to a specified depth and installing a joint filler board in this groove. If any difficulty is experienced in inserting the joint filler board, a 3 mm thick galvanized iron sheet can be placed inside the groove in place of filler board. Care should however be taken to ensure that groove is cut vertical and the joint filler is also placed in a perfectly vertical position. Joint cutting, though simple and easy, requires proper supervision. Only trained and experienced operator should carry out this job.CompactionBest results have been obtained by use of 15 T double drum vibratory rollers. Concrete discharged by belt conveyor or dumper-scrappers is dozed to the required line and level,and thereafter compacted by use of vibratory rollers. Trials are normally done to decide the number of passes of vibratory and non-vibratory rollers to get proper density. Placement of correct thickness of concrete is achieved by automatic laser guided system connected to the dozer blades. Normally 10 passes of vibratory compactors have been found to be adequate.CuringUnder no circumstances RCC must be allowed to dry for first 21 days of placing. Curing of concrete on slopes is achieved by use of wetted hesian cloth of appropriate specifications. Curing the lift area is done by a water spray operated intermittently depending upon ambient temperature prevailing at site.Water slopesThese are placed as close to the upstream face as possible. These are fixed to the correct locations by use of mechanical fixtures since there is no radial joint shuttering provided in RCC dams. In conventional concrete dam, water-stops are fixed by the help of radial shuttering.

CONCLUSIONS AND RECOMMENDATIONS

Economy and speed of construction continue to be the main reasons designers select RCC for new gravity dam construction. The Alpe Gera Dam discussed in this paper has been completed and is currently in service.It will be seen that the future of expedient construction of RCC dams to save costs and earn revenue lies in adopting the present world technology and improving it further. This will also help in a big way the utilization of fly ash, which is becoming increasingly necessary.The following design features, acquired in the same online article Advancement on Roller Compacted Concrete, resulted in significant effects on Alpe Gera Dam speed of construction and total cost: Increasing the dam size in order to reduce the required RCC strength provided an opportunity to use marginal on site aggregates. Building the dam without having to form the RCC on the downstream face and covering the unformed RCC with soil to provide protection against freeze-thaw action. Designing the dam to resist full hydrostatic uplift pressure eliminated the need for foundation drains and drainage gallery. Eliminating the construction of a stilling basin.

REFERENCESBooks

Chengdu, (1999), Concrete Dams, Nashville: Vanderbilt UP.Kbler-Ross, (2000), Hydropower and Dams, Volume 7, Issue 3, NY:McGraw-HillSmith, (2000), Dams and Construction, Vol 1. 6th ed. Philadelphia, PA: Elsevier

Websites

New Innovations in Engineering The Age [online], Available: http://www.hydroworld.com/etc/medialib/platform-7/hydroreview/whitepapers/dams-__civil_structures.Par.37178.File.pdf [Accessed 3 July 2012]Alpe Gera Dam The Age [online] Available: http://www.usace.army.mil/inet/usace-docs/(Accessed 3 July 2012)Advancement on Roller Compacted Concrete The Age [online] Available: http://civilengineerweb.com/rcc-roller-compacted-concrete-history/ [Accessed 3 July 2012]