Mix design proportioning methods for Roller Compacted Concrete RCC:
Mix design proportioning methods for no slump concrete applicable for Roller Compacted Concrete.
Materials Used in RCC mix design
Cementitious materials used in RCC typically have a low cement content, ranging from 70 to 130 kg/m3. Any kind of Portland Cement can be used, although low heat cement is preferred when available. The use of Class F pozzolans as fly ash can replace up to 80% of the cement content.
In selecting aggregates for RCC, quality control is essential. Coarse aggregates with a Nominal Maximal Size Aggregate (NMSA) between 37.5 and 75mm are commonly used, with the thickness of the placement layer being at least three times the NMSA. Fine aggregates strongly influence paste requirements and compatibility of RCC, affecting water and cementitious material requirements needed to fill the aggregates voids and coat aggregate particles. Fines (material passing 0.075 mm sieve), typically comprising 5% of total aggregates, are required as supplements in low cement content mixtures.
Admixtures for RCC are mainly needed for their set retardation and plasticizing effects. Water Reducer Type D (WR + retarder) is a beneficial admixture for strength gain and retardation, which may be required for an extended period. The main product used for RCC is Plastiment TM 25, specifically designed for RCC.
Mix design considerations:
The workability or consistency of concrete is an important characteristic that can be measured using the Vebe vibrating table according to ASTM C 1170. The test involves placing the concrete into a cylindrical mold and measuring the time it takes for the concrete to be consolidated through vibration. A longer consolidation time indicates that the concrete is drier and less workable. Typically, the Vebe times for RCC mixtures range from 10 to 40 seconds.
To compare the consistency of concrete measured by different methods, ACI 211.3R recommends comparing the results obtained from the Vebe apparatus and the slump test. Both methods provide useful information about the workability of concrete, but they measure different aspects of consistency. The slump test measures the ability of the concrete to flow and deform under its own weight, while the Vebe apparatus measures the time required for consolidation through vibration. By comparing the results obtained from both tests, a more comprehensive understanding of the workability and consistency of the concrete can be obtained.
| Consistency | Slump (mm) | Vebe (s) |
| Extremely dry | – | 32 to 18 |
| Very stiff | – | 18 to 10 |
| Stiff | 0 to 25 | 10 to 5 |
| Stiff plastic | 25 to 75 | 5 to 3 |
| Plastic | 75 to 125 | 3 to 0 |
| Very plastic | 125 to 190 | – |
Concrete is a widely used material for construction, and its strength is an important factor for designing structures like RCC dams. The long-term strength of RCC is usually based on a period of 90 days, 120 days, or even one year. The strength of RCC is influenced by the water-cement (w/c) ratio, especially for mixtures with a Vebe time of approximately 15 to 20 seconds. For drier mixtures, the strength is more dependent on the moisture-density relationship. If the water content is less than optimal, there can be voids in the structure, poorly compacted concrete, loss in density, and strength. The compressive strength of RCC is typically measured by testing cylinders with a diameter of 15cm and a length of 30cm, and strength measured on cores is also possible.
Segregation of concrete is a common issue that needs to be addressed to ensure quality. Aggregates, especially those with a nominal maximum size aggregate (NSMA) greater than 37.5mm, should be well graded to minimize segregation. Mixtures with higher cement content are generally more cohesive and have less tendency to segregate.
Permeability is another important factor to consider when using concrete. Mixture with a paste volume of 18 to 22% by mass is recommended for achieving suitable impermeability. Higher cementitious content or high workability mixes that bond well to fresh lift joints will produce water-tightness. For lower cementitious content mix, and/or low workability mix, a bedding mortar between lifts may be needed.
Heat generation is an important consideration for massive structures made of RCC. Limiting cement content and/or using fly ash can help reduce heat generation. Additionally, the durability of RCC must be ensured by avoiding damaging effects of alkali-aggregate reactivity.
Quality control of RCC concrete is essential to ensure the desired strength and properties. Tests such as the Vebe Consistency Test, density and air voids tests, moisture/water content tests, and cement content evaluation should be conducted regularly. Temperature monitoring is also important to ensure that the concrete does not overheat during curing. However, it is important to note that concrete specimens are fragile due to low early strength.
Lift joints are an essential part of RCC construction and should be kept moist before placing the next lift. In some cases, a “flowable” bedding mortar may be required, which should be retarded with admixtures to ensure proper sealing.