Placement and Compaction of Roller-Compacted Concrete in Dam Construction

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Roller-compacted concrete (RCC) is a type of concrete that can support a roller during its unhardened state while it is being compacted. 

The placement and compaction of RCC is a rapid construction process and therefore its success relies on the pre-construction planning and coordination of all interrelated activities.

The roller-compacted concrete should be as fresh as possible at the time of placement to compact it adequately and achieve maximum density.

Generally, static rollers are initially used to compact RCC lifts, and then a vibratory roller is used to complete the compaction process. 

Inaccessible areas in dam projects, like the area near abutments and around conduits, need to be compacted with small-sized compaction equipment because large-sized machinery cannot access the area or may damage the conduits.

Planning and Preparation

• Using equipment that is adequate in size and number to meet the construction requirements.
• Provide backup equipment at the construction site.
• Sequence construction operation, for instance, access and routing for equipment, air and water support system, foundation preparation and joint treatment, setting of forms or precast work, setting of line and grade control, placement of conventional concrete at contacts or in facings, and placement of bedding mortar. These operations should be carried out with minimum interference with the placement, spreading, and compaction works of RCC.

Placement of RCC

At the time of compaction, roller compacted concrete mixture should still be fresh to get the best compaction results. 

The bond between different lifts of roller-compacted concrete should be ensured by performing lift surface cleaning, maintaining surface moisture, and utilizing a bonding mixture like fluid bedding mortar or concrete.

The concrete mixture should be compacted when placed and spread in a uniform lift thickness. Normally, most rolling procedures start with a static pass to even out the loosely placed concrete, and then the vibratory roller is applied.

Number of Passes of Rollers

The required number of passes must be specified before the construction work through the test section. The concrete placement should correlate the number of passes to obtain the designated density for the mixture within a given moisture content range.

Compaction Process

The compaction of roller-compacted concrete should begin 15 minutes after placing and 45 minutes after mixing. 

Concrete placement should proceed from one abutment of the dam to another continuously. The next lift should be placed on the previous lift.

Compaction of High Consistency RCC

RCC with Vebe consistency of 15 to 20 s can be compacted with roughly 6 passes using a 10-ton dual drum roller. At this consistency, a 30 cm thick loose lift of RCC will lose around 2.5 cm thickness.

The density of the RCC will increase drastically after four to six passes but drop slightly with extra passes. This is because of the rebound off the top surface behind the roller. The surface can be tightened with static roller compaction one hour after the initial compaction.

Compaction of Low Consistency RCC

RCC mixture with low or no measure Vebe consistency requires more than six passes vibration roller to compact adequately. The density of the concrete may level out without a distinct peak. 

The roller may bounce off the surface at the final stage of the compaction process and fracture aggregate on the top surface. This shows contacting particles instead of cement paste between them. 

Sequence of Compaction

The roller operator needs to establish a rolling sequence based on the width of the RCC lift and placing sequence. 

When a loose lane is placed and spread, the roller should keep a minimum distance of 15 cm to 30 cm from the edge of the lane. 

The uncompacted part will be compacted with the adjacent lane. A recommended rolling sequence is shown in Figure-3.

Construction Joints

If the placement of RCC stops before completion of a lift due to plant or equipment breakdown, roll down and compact on a slope the loose material along the edge of the lift. 

This compacted material is treated as a construction joint and should be cleaned from any loose material before covering it with RCC to complete the unfinished lift.

Application of Small-Sized Compaction Equipment

Some places of dam structure cannot be compacted with sizable compaction machinery due to inaccessibility, lack of restraint, and safety of unformed places like upstream facing, etc. 

In this circumstance, small compaction equipment such as a tamper, small roller, or a backhoe-mounted plate vibrator is appropriate for compaction of RCC. Areas in the dam project that require small-sizes compaction equipment are:

1. The area along downstream and upstream facings
2. Adjacent to diversion works, outlet works, and other conduits
3. Areas closed to dam foundation and abutments.
4. Localized compaction for the repair of lift surface damaged by equipment operation
5. Around instrumentation or other embedded item

FAQs

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