Roller compacted concrete, also known as RCC, is a type of concrete often used when large quantities of concrete are required.
Unlike normal concrete, it does not contain any reinforcement to support the load exerted on the structure. Furthermore, RCC is mainly used in gravitational structures or in structures that are not subject to significant tensile stresses.
Typical examples of structures are as follows.
- CCR dams
- Substructure for road construction
- Road construction
What is RCC concrete
Laminated concrete is a type of concrete that contains fine aggregates, coarse aggregates, cement and water. Additionally, additives such as fly ash can be added depending on the project needs.
Therefore, the types of materials used in RCC are the same as other types of concrete.
Generally aggregate size is higher than normal concrete and can be around 75 mm . Depending on the mix of projects, the unit size may vary.
Portland cement or combinations of cement with fly ash or other materials are generally used in larger concrete placements as a measure to reduce the heat of hydration.
Advantages of laminated concrete
For larger project scales, significantly larger concrete pores and comparatively large element sizes, it is common to increase the size of aggregates – especially coarse aggregates.
Let's discuss the advantage of RCC,
construction costs
The construction cost is one of the main factors to be considered when finalizing the conceptual design. A significant amount of money can be saved by using innovative methods that deviate from traditional approaches.
RCC is also a method that brings savings to the project. It is known that the cost reduction of laminated concrete would be in the range of 25% – 50% compared to conventional concrete.
Quick build
Construction can be completed within 24 hours if there are no physical obstacles. Additionally, the only work that needs to be done in most areas is pouring and compacting the concrete. Therefore, the time required is less compared to conventional concrete.
Minimize diversions and cofferdams
Significant costs will have to be spent on construction-related work, such as rerouting the river. For most projects, the construction of a cofferdam and diversion channel/tunnel is essential.
Depending on the floods expected during the construction period, cofferdams and tunnels are constructed in such a way that the works are not affected by floods.
By constructing the RCC dam, the size of the cofferdam and tunnels could be reduced as the return period could be shortened.
A certain amount of overflow may be permitted in rolled concrete dams as this maintains their strength after the concrete has hardened and can support the load.
Unlike other types of dams, this is a very useful feature available in RCC dams.
Furthermore, the high speed of construction also allows the main dam to be used before the works are completed.
How to define laminated concrete
Two methods can be used to specify RCC concrete. These are based on the method used to specify the mixture.
- Soil Compaction Approach
- Concrete technical approach
Soil Compaction Approach
When soil mechanics principles are used to specify rolled concrete, it is based on maximum dry density. We check whether the maximum dry density of concrete at the construction site is achieved through compaction. With this method, the usual tests can be used to check soil compaction.
The optimum moisture content is checked and consequently the maximum dry density can be determined. The greater the compaction, the greater the strength of the concrete.
Concrete technical approach
This method uses the traditional concrete specification method. The concept is based on the water-cement relationship and defines the characteristic resistance of concrete.
This method is more familiar and easier to check compared to the previous method. We use general quality control and quality assurance procedures to ensure that construction is acceptable.
Role of the RCC
Now let's list some of the main technical aspects related to laminated concrete.
- RCC is not the same as traditional concrete and is compacted with rollers.
- A low degree of slump is maintained and therefore a lower water-cement ratio can be maintained.
- As the material is decomposed and compacted in layers, there are no high costs for the formwork.
- One layer is approximately 300 mm. and performs compression. The layer thickness depends on the maximum size of the aggregates, the type of vibrating equipment used for compaction, etc.
- Nonstop. The work can occur on a 24×7 basis.
- To reduce the heat of hydration, cementitious additives such as fly ash are generally used. Class F fly ash, which contributes to low heat of hydration, is most commonly used in construction.
- Up to 50% of the cement volume can be replaced by fly ash.
- Typical unit nominal size is 75 mm.
Mix design by RCC
Mixing proportions must be chosen so that economical construction is possible while maintaining durability and design requirements.
When deciding on the concrete mix, several factors must be taken into consideration.
- durability
- Strength
- Processability
- Heat of hydration
- Aggregates
- water content
durability
Durability is the key factor for modern construction. Unlike the old regulations, the new regulations establish shelf life requirements as the basis of the design.
Item Factors Affect Concrete Durability and Concrete Shelf Life Requirements discuss more about the durability of normal concrete.
When designing, the designer must first consider durability requirements and determine the structural materials and compositions to be used.
For example, when we design a building, we first determine the exposure class and determine the concrete grade, minimum cement content, water-cement ratio, etc.
The durability of RCC concrete depends on many factors such as strength, cement content, quality of aggregates, degree of compaction, etc.
Strength
The strength of concrete reflects the structural performance of the element. As with traditional concrete, construction requires ensuring that the concrete has the specified strength.
Processability
The workability of RCC concrete is not as pronounced as that of traditional concrete, which requires greater workability to be able to pour the concrete between the reinforcing bars.
However, there is a need to place and compact the concrete without segregation. Furthermore, this is The workability of RCC concrete is not specified based on slump because there is no slump.
The consistency of the rolled concrete is measured using a Vebe device.
Heat of hydration
RCC is not reinforced and does not have any elements that can absorb the tensile stresses created during the curing process.
Thermal cracks in the initial phase, that is, cracks due to the high heat of hydration, must be controlled by the concrete itself.
The low water content allows the use of a low cement content and the use of cementitious materials such as fly ash also reduces the heat generated in the hydration process.
Furthermore, the ability to replace up to 50% of the cement volume is another advantage in reducing the increase in stress in the concrete during curing.
Aggregate
Aggregates play an important role in RCC concrete as they contribute significantly to the strength and durability of the concrete.
Using optimally sized aggregates results in a better mix. Larger aggregates increase the likelihood of separation, spreading, poor compaction, etc.
In general, 75 mm is considered the maximum size for coarse aggregates.
water content
A low water content is used for rolled concrete.
The water content to be used in the mix depends on the consistency and strength of the concrete. The water content is specified in EM 1110-2-2006 and is based on the consistency of the Vebe.
Furthermore, the larger the size of the aggregate, the lower the water content to be added.
Properties of laminated concrete
Although rolled concrete is also a type of concrete, there are some significant differences compared to traditional concrete.
Let's discuss each one in detail.
Compressive strength
Compaction of RCC concrete is considered acceptable when the percentage of air voids is less than 1.5. If 5% pores remain after compaction due to insufficient/incomplete compaction, this can result in a loss of strength of approximately 30%. Furthermore, air voids of about 20% can reduce the strength of RCC concrete by 80%.
The compressive strength of RCC is comparatively lower compared to conventional concrete. As stated in EM 1110-2-2006, RCC can be designed with a minimum strength of 13.8 MPa for greater durability.
Furthermore, the development of concrete strength does not take as little time as in the case of normal concrete. Strength can be achieved in 90 days, a year, etc.
tensile strength
Unlike normal concrete, which has reinforcement that can absorb tensile stresses, laminated concrete has a much greater risk of developing tensile stresses.
The type of classification can be seen based on strength development.
- Direct tensile strength
- Direct tensile strength of lifting connection
- split tensile strength
- Bending resistance
- Dynamic tensile strength
Drag
Creep is a time-dependent stress due to sustained loading.
This increases the stress in the concrete over time. The rate of increase decreases over time.
Creep depends on the material properties. Concretes with a high modulus of elasticity and high strength generally have relatively low creep deformations.
Volume change
As concrete hardens, its volume changes.
There could be two main reasons for this
- Dry shrinkage
The reduction in volume of concrete as it dries is called drying shrinkage.
It mainly depends on the water content and properties of the aggregates.
- Autogenous volume change
There is also talk of autologous shrinkage, that is, a type of concrete shrinkage.
The volume reduction is caused by the hydration of the cementitious material without the concrete absorbing or losing moisture.
For large volume concrete, volume changes can be significant. Furthermore, the duration of autologous shrinkage can be significantly longer than drying shrinkage.
permeability
The permeability of RCC concrete is very important in the construction of water retaining structures such as dams.
This depends on the mixing proportions, placement method, degree of compaction, etc.
Furthermore, the higher the cement content, the lower the permeability.
Generally, well-compacted concrete has permeability in the range of 1.5 to 150×10 -8º mm/sec.
Crack Controlled Methods in RCC Structures
- Temperature control
Temperature control is carried out in the same way as for conversion concrete. To reduce the rise in temperature, the following measures are taken.
-
- Limit the temperature rise in the mixture.
- Limit installation temperature
- Use of insulation
- Concreting at night
- Pre-cooling techniques
To reduce heat in concrete, the following measures can be taken.
-
- Use ice instead of water
- Use liquid nitrogen to cool the RCC mixer
- Transverse contraction joints
The construction of vertical transverse joints upstream of RCC dams is carried out taking thermal movements into account. A water barrier is placed at these joints to prevent water movement through the joint.
The article Types of dams More information about the design and construction aspects of different types of dams can be found here.
