ROLLER COMPACTED CONCRETE
Roller Compacted Concrete (RCC) is a mixture of aggregates, with or without cement supplementing cementing materials, water, and in some cases a water-reducing mixture, proportional to supporting external compression equipment.
This hard (dry and straight), zero-slump concrete mix has the consistency of wet gravel. Low water content requires this in a continuous flow system, mixing, usually in a pug mill mixer, is spread with a modified asphalt paver and compacted with a roller.
The resulting product is a building material with the strength and characteristics of conventional concrete.
A pug mill plant on or near the site is used because it is easily portable and allows a constant and continuous flow of material for greater productivity.
The pug mill provides a vigorous mixing process, which is required to disperse small amounts of mixed water throughout the RCC. The process does not require any forms, finishes, surface texture, or joint jigsaw and sealing.
Compared to conventional concrete, RCC only requires 60 to 75 percent cement and avoids undesired conditions. From the high heat of hydration in the mass concrete.
The water content is so low that the freshly mixed RCC looks like wet gravel, and if the site conditions are guaranteed, it can be transported to the construction site in dump trucks or pavers or loaders and spread in 200−300 with bulldozers or conventional asphalt spreaders or graders. Flakes of mm thick are called lifts.
Lifts are then compressed using vibrating steel-wheel and pneumatic tire rollers, as shown in Figure 14.4, to 95-100 percent of the specified density. Immediately after the compression is completed, the water is sprayed into a fine mist to cure the concrete.
Pavement roller-compacted concrete can be sized and shaped with conventional paving machines; construction of gravity dams using roller-compacted concrete uses the methods and equipment used for earth dams, thus requiring significantly less labor.
The main concern in the design of roller concrete is to obtain adequate bonding between the lifts. The dry consistency of the RCC mixture particularly composites larger than the nominal size of 40 mm, creates problems for the fresh lift to the rigid lift.
Improved bonding can be achieved by restricting the time interval between the placement of the lifts, providing complementary joint therapy, or increasing the paste content of the mixture.
Construction Process
For effective consolidation, the concrete mix must be dry enough to withstand the weight of the vibration equipment, but moist enough to allow proper distribution of the paste binder throughout the mass while mixing and vibration process.
The techniques for the construction of compacted concrete with a roller for mixing, transporting, and laying are controlled primarily by the characteristics of the material in terms of moisture retention, segregation, and compaction.
In the wet phase, it shows the material handling properties of moist, granular soil; hence standard earthmoving equipment can be used for transport, placement, and compaction.
After hardening of compacted concrete with a roller, it is concrete with physical properties and the finished appearance of conventional concrete. The construction process for the RCC is a three-step cycle that results in the continuous placement of the mixture to eliminate delays in the placement of subsequent elevators.
The three steps are:
(I) mixed proportion,
(ii) transport to the site and
(iii) spreading, compaction and hardening.
The following are the factors that affect the proportion of the mixture:
1. Cement The content of cement and cementitious material in RCCs usually varies between approximately 12 and 15 percent of dry materials, ie. from 250 to 350 kg / m3 during carrying, and in the base layer ranges from 100 to 150 kg/m3.
2. Pozzolana Modern RCC is lean concrete with modified fly ash that contains a large volume of fly ash and a very low water content. The use of fly ash in RCC mixtures is an effective technique for increasing paste volume and improving compatibility.
It also results in low heat of hydration, less shrinkage, delay in the beginning and completion of the setting process, ease of spreading, and compaction operations.
Flying ash improves long-term strength and durability under adverse conditions and the chemical action of chlorides and sulfates.
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3. Aggregate Aggregate makes up approximately 75 to 85 percent of the volume of the RCC mixture and therefore has a significant effect on both fresh and hardened concrete.
The use of continuously evaluated aggregates is desirable for the best results, as it affects the relative compatibility of the concrete and affects the minimum number of vibrating rollers required for the complete consolidation of a given lifting thickness.
It also affects the water and cement requirements of the aggregate to fill the gaps in the aggregate and to coat the aggregate particles to obtain a solid volume of concrete.
The ideal rating for minimum paste requirements would be one that would produce the maximum density of a dry rod with the smallest area.
Depending on the thickness of the arrangement, coarse aggregates with a maximum nominal size in the range of 10 to 225 mm can be used.
To avoid the risk of segregation and loosening of the elevators and to obtain a better quality of the pavement surface, aggregates with a maximum size of 20 mm are preferred.
However, for low traffic applications, an aggregate with a maximum size of up to 40 mm can be used and it is necessary for two-thirds of the aggregate used to be of the crushed type.
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4. Additives Air intake, water reduction, and installation control mixes are effective in reducing the vibration time required for full
consolidation of SRS.
Complete consolidation reduces the content of trapped air in the cavity, increases the hardness, and reduces the permeability of the concrete.
The proportions of the RCC mixture differ primarily from those of the conventional concrete mix due to its lower water content; lower paste content; higher fine aggregate content to produce a combined aggregate that is well rated and stable under the action of a vibrating roller; and maximum aggregate size of no more than 20 mm to minimize segregation and obtain a relatively smooth surface texture.
RCCP concrete is continuously thinned with a special emphasis on thin aggregates to achieve high density and smooth texture of the slab surface.
Moreover, unlike ordinary cement concrete (PCC), the law of water-cement ratio is not good in the case of RCC which is non-perishable concrete, it only applies in the case of arable mixtures.
In the RCC mix design, water is selected from the optimum moisture content (OMC) at the maximum dry density (MDD). The chemical reaction of water with cement is a secondary effect. By adding cementitious mineral additives (pozzolanic), the RCC mixture can be redesigned and modified according to performance requirements.
Compared to conventional concrete, typical RCC mixtures contain less cement paste and cementitious material (250-350 kg / m3) and a significantly higher proportion of fly ash ranging from 25-70 percent by weight of cementitious material.
RCC mixtures typically achieve strengths of 28 days similar to or even greater than conventional concrete. The trend of increasing the bending strength of the RCC pavement with a hardening period is compared to a typical conventional concrete pavement in Figs. 14.5.
Applications
Construction of roller-compacted concrete Pavement (RCCP)
RCC is weighted-batch and mixed in a continuous mixing pug mill or ordinary mixers, such as a clay-cement treated base or asphalt concrete used for construction.
The sidewalk was initially constructed in elevators of 150-200mm with a duration of 30 minutes to 2 hours between lifts for a pavement thickness of more than 400mm. A 150mm lift of 95 percent of the specified density can be placed and compressed in two or three passes of the regular paver roller shown in Figure 14.6.
Low lift requires more compact effort with more passes of the roller to achieve the desired density. If a smooth pavement surface is not obtained, an asphalt layer can be used to seal the surface and smooth the roadway.
The Vee-Bee Constometer tool can be used with some modifications to evaluate RCC stability. The curing of the RCCP is accomplished by keeping the pavement surface wet for seven days. Water spray or fine mist is the most suitable and is commonly used.
Wet sand can also be used for RCC curing. Due to the very low drying shrinkage of the RCC, the compression joints are provided 10 to 20 m apart compared to the PCC but the distance between the compression joints in the PCCP is 4.5 m.
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The RCCP can be immediately opened to traffic as a result of the high stability achieved by the mineral skeleton formed by aggregates after contraction. In addition, the stability of the mineral structure permits the use of a greater number of mineral binders than that used on compacted concrete pavements.
The total thickness of the RCCP structure is less than that of the same load-carrying capacity asphalt/gravel pavement.
Due to its advantages as a relatively inexpensive and durable paving material, the RCC is emerging as a common ground for conventional highways. And street pedestrians.
Thus RCC is a material that has the longevity of concrete at the cost of asphalt.
Dam Construction
Applications of RCC technology to dam construction have allowed concrete to play a more important role in the construction and rehabilitation of dams. Typically, in the construction of the RCC Dam, 300 mm thick lifts are spread with a slump-free concrete mix.
The RCC mix is usually mixed in a makeshift plant built near the dam site and transported to the placement site by a conveyor belt, front-end loader, or dump truck.
The RCC’s newly placed lift is compressed with a vibrating roller. Continuous deployment of RCC in dam projects is desirable to reduce the cold joints between horizontal lifts which prevent the interconnection of concrete lifts.
RCC can be used to overlay the downstream slope of an existing embankment to protect the dam from erosion if the structure overflows with water.
RCC construction is less expensive than conventional methods of dam construction because RCC can be completed faster than embankment projects because they require less material.
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