Evolving correct mix design for RCC

This is one of the most time consuming activities and requires a period in excess of six months. We need to evolve a design mix to optimize gradation and workability of RCC so that cohesive RCC could be obtained with little potential for segregation. Consistency tests by use of Vebe apparatus is used to measure consistency of no slump concrete. Consistency is defined as “ability of fresh concrete to flow”.

The design philosophy of medium/ high paste RCC dams is that the concrete should be watertight. Thus the RCC has to be designed to bond layer to layer to have an in situ permeability equivalent to that of traditional concrete dam. In the same way as in Roller Compacted Dam (RCD), contraction joints are formed through the dam but these are at large spacing. After observing the performance of RCC dams all over the world, the present trend is to construct medium/ high paste RCC dams for medium/ large heights, realizing the fact that durability is important in achieving long term economy to owner.

The consistency of RCC can vary from stiff to extremely dry. Some mixes may have even less workability than extremely dry and Vebe procedure may not be applicable. A Vebe consistency in excess of 2 minutes is considered the extreme limit of the test procedure. Normally, RCC mixes will have a consistency of 10 to 30 seconds when the Vebe test is used. Mixes with a Vebe consistency of about 15 seconds will consolidate in about 6 passes with a 10 ton dual-drum vibratory roller. Fresh RCC with a 15 second Vebe time will compress about 25 to 50 mm after the first pass with the roller. Drier mixes will require greater compactive effort and will leave a less noticeable depression with the roller during compaction.

The consistency of RCC will vary with changes in water content, aggregate grading and entrained-air. The procedure takes approximately 15 minutes to complete, including the density test. Besides this, the in-place density tests are taken by use of nuclear density gauge.

Concrete produced from the batch plant after using the mix design evolved by various trials to determine +90 days strength needs to have addition of water reducing agent/ superplasticizer for better workability. It is necessary to estimate the maximum quantity of concrete placement required to place one lift of 300 mm to design capacities of all plants viz. (1) aggregate production; (2) concrete production; (3) concrete placement; (4) concrete compaction; (5) shuttering; (6) GEVR concrete (Grout-enriched Vibratable RCC).

Another aspect which is also important for sizing of plants is the initial setting time of concrete. Trials with various retarders need to be made to freeze the time required for completing a 30 mm thick layer comfortably and starting the next layer which is to be placed on the top of earlier lift without the first lift getting hardened.

Having worked out this time, the total plant required to execute the work effectively can be worked out. In large number of areas, provision needs to be made for inclement weather, monsoon period or ambient temperature in excess of 45 C degrees. Necessary provision of overall efficiency of plant is also required to be made. In order to cater for breakdown of aggregate supply plant or raw material required for aggregate production, a stockpile of aggregates equivalent to 15 days peak requirement needs to be made to have uninterrupted placement of RCC.