Guidelines For RCC Mix Design

Mix Design for RCC

Mix design proportioning methods for Roller Compacted Concrete RCC

ACI 207.5R (chapter 2.4) and ACI 211.3R extensively described the mix design proportioning methods for no slump concrete applicable for Roller Compacted Concrete RCC

Materials Used in RCC mix design

a) Cementitious materials:

Cement content is generally low (typically 70 to 130 kg/m3). Any kind of Portland Cement (but low heat cement is a preferred type when its is available)

Fly Ash: Class F pozzolans are preferred. When used pozzolans, may replace up to 80 % of cement content.

b) Aggregates:

Selection of aggregates and control of aggregates quality is essential. Aggregates shall meet ASTM C33 standard.

Coarse aggregates:
Most of project use a coarse aggregates with a Nominal Maximal Size Aggregate (NMSA) between 37.5 and 75mm. The thickness of placement layer shall be more than 3 times the NMSA.

Fine aggregates:
The grading of fine aggregates strongly influence paste requirements and compactibility of RCC. It also affects water and cementitious material requirements needed to fill the aggregates voids and coat aggregate particles.

Other aggregates: Fines (crusher fines…):
In low cement content mixtures, supplements fines (material passing 0.075 mm sieve) are usually required. Fines quantities is generally 5% of total aggregates

c) Admixtures:

Admixtures for RCC needed mostly for set retardation (and plasticizing) effect. Water Reducer Type D (WR + retarder)
WR use is beneficial for strength gain, and retardation which can be required very extended. Main Sika Product for RCC: Plastiment TM 25 specifically designed for RCC.

Mix design considerations

a) Workability / Consistency:

Measured with a Vebe vibrating table (ASTM C 1170).
The test principle is to measure the time for the concrete to be consolidated by vibrating in a cylindrically shaped mold. The longer the time, the drier/lesser workable is the concrete. Vebe times of RCC mixtures is generally from 10 to 40 seconds. Comparison of concrete consistency measured by slump and Vebe apparatus. ACI 211.3R

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

b) Strength:

Design strength of RCC dam is often based on long term strength (90 days, 120 days or even 1 year!). The strength of RCC is a function of w/c ratio only for mixture with Vebe time of ~ 15 to 20 sec. range. For drier mix, the strength is more controlled by the moisture-density relationship.

If water content is less than optimum => voids in structure, poorly compacted concrete with loss in density and strengths.

Compressive strength of RCC is usually measured by testing cylinder (15cm diameter, 30cm long).

Strength measured on cores is also possible.

d) Segregation:

It is necessary to produce mixture with minimum tendency to segregate => Aggregates (especially with NSMA > 37.5mm) shall be well graded.

Higher cement content mixture are generally more cohesive => less tendency to segregate.

e) Permeability

Mixture with paste volume of 18 to 22% by mass shall provide suitable level of impermeability. Higher cementitious content or high workability mixes that bond well to fresh lift joints will produce watertightness.

For lower cementitious content mix, and/or low workability mix, a bedding mortar between lifts may be needed.

f) Heat Generation:

It is an important factor considering the massive structures. Limitation of cement content, and/or use of fly ash is helping reduce heat generation.

g) Durability:

RCC should be free of damaging effects of alkali-aggregate reactivity

Quality control of RCC concrete

  • Vebe Consistency Test
  • Density and air voids tests.
  • Moisture/water content tests.
  • Cement content evaluation
  • Temperature monitoring
  • Test (cylinders) specimens:

– Using modified Vebe apparatus (generally for mix with Vebe up to 20 seconds),

– Using of special vibrating hammers

Beware, because of low early strength of concrete specimens are “fragile”.

Lift-Joint sealing systems

Lift joints shall be kept moist before placing next lift and normally do not require special treatment. In some cases, it is required to apply a “flowable” bedding mortar which need to be retarded with admixtures.