What is Cell-Filled Concrete Pavement?
IIT Kharagpur developed the technology which is known as cell-filled concrete, it has been proven to be successful as a solution to problems such as overloaded vehicles, inadequate drainage facilities, and waterlogging. The cell-filled concrete is made of a formwork of plastic cells over the compacted subgrade or sub-base and then filled with concrete or stones.
Listed below are the components of Cell filled Concrete Pavement:
1. Plastic Cells
The plastic cells are able to provide both the form and the reinforcement for the pavement. High-density polyethylene (HDPE) sheets are used to make the plastic cells which are around 0.22mm to about 0.25mm. The plastic cells can be supplied in the form of rolls of strips which are 50mm to 100mm wide which depends upon the required depth. Cells can be formed by heat-welding or stitching the strips.
At 300mm intervals a pair of strips can be welded. To make the stitch lie at the center of the previous stitching the third strip is welded to the first pair at 300 mm intervals. Similarly, the third and the fourth ones are again welded like previous two.
The top 300 mm thick portion of the embankment is what forms the subgrade. Usually the embankment subgrade is compacted in two layers which are usually to a higher standard than the embankment. In case of poor embankment soil the top 300mm of the subgrade can consist of better-quality material from borrow pits with the CBR exceeding five. With least 100 percent of Maximum Dry Density per IS:2729 (Part 7) the subgrade might be compacted. With the expansive black cotton soil (BCS) the minimum compact must be at 95% of the maximum dry density along with moisture content 2% higher than the optimum.
Due to traffic the subgrade soil of the existing roads is expected to have attained a stability which is required. It is important to conduct the CBR test at in situ dry density and moisture content after four days of soaking. It is also important to conduct a dynamic cone penetration test so as to determine CBR values of subgrade quickly, from records of similar soil the CBR values corresponding to soaked condition can be obtained.
Along with these locally available aggregates for example murrum and kankar mixed with lime fly ash can also be used. The subbase can also be the locally available materials such as natural gravel/soil aggregate or when blended with suitable aggregate fractions of stone, gravel, moorum, sand, or a combination of these materials all of which depends on the grading required.
In case of conditions where the number of commercial vehicles is more than fifty per day then it is advised to use 150mm of cementitious sub base with a minimum 7-day strength of 1.5 MPa. The subbase must be provided with stone or with concrete block, or Brick on edge there must be laid on either side of the carriageway projecting 50 to 100 mm above the subgrade or subbase for the need to confine and protect.
To fill up the cell conventional pavement concrete with 28-day strength of 30 MPa with a slump of about 30 to 50mm can be used. To reduce water requirements from chosen slump Super-plasticiser should be used. The Roller Compacted Concrete (RCC), as specified in clause 1502 of Specifications for Rural Roads, can also be made to use for filling up the plastic cells after which it is compacted with a roller.
Construction Procedure of Cell-filled pavement concrete.
Specifications of roads must be kept in mind when the construction of embankment, subgrade, and subbase takes place.
To rural roads a proper camber as applicable must be provided. There must be a setup for drainage layer in high rainfall areas (annual rainfall exceeding 1000 mm) as is laid down in Road Manual.
For the confinement and protection of cell-filled concrete stone or even concrete block or Brick on end edge must be laid on either side of the carriageway projecting 50 to 100 mm above the subgrade or subbase.
It is important to provide a hard shoulder with proper cath for the concrete blocks' stability as there can damage to the unconfined concrete blocks present in the edge caused by trucks traveling close to the edge.
0.85m width of the hard shoulder must be maintained on either side of the pavement.
Across the compacted subbase there must be Formwork of plastic cells which are then put under tension with iron spikes so that cells are close to squares in plan.
To prevent the cells from collapsing while filling the cells with concrete nylon threads must be passed at 10 mm below the top of the cells.
In case of any opening of stitch of the cells during tensioning it must be made sure that it is stapled near the top, middle, and bottom.
The concrete should be filled 20mm higher than the cell depth where the depth of cells is 120 mm.
Once the cells are filled with concrete the iron spikes shall be removed. In case of RCC, two passes of the roller in static mode followed by two passes in vibratory and another pass in static mode must be sufficient for compaction and a good finish.
The texture of aggregates and moisture content determines the number of passes in static and dynamic modes.
To compact the conventional concrete having a slump of about 30 to 40 mm conventional vibrator can be used.
For the determination of trial run the number of rollers passes, the amplitude of vibrations, the depth of loose concrete in cells, and the amount of water to be added must be taken to account.
To prevent drying in the hot weather the concrete's surface should be covered with wet jute mats or paddy straw.
Just like the camber of 3 to 3.5 % is provided for rural roads, wet jute or coir mats and wet paddy straw shouldbe provided for better water curing option and only light traffic must be allowed to move on the surface. In the water ponding method, the water accumulates on the lower side of the mud enclosurewhereas the higher part becomes dry soon.
Advantages of Cell Filled Concrete Pavements
- Recycled plastic can be used.
- Since there is no requirement of the expansion or contraction joints, therefore, the maintenance of joints is eliminated.
- Compared to conventional cement concrete pavement the cost of construction is considerably reduced.
- There is a reduction in the consumption of aggregates to 50% when compared to normal CC pavements.
- In case of individual block failure, it can be easily replaced without putting in much effort and maintaining a low cost.
Disadvantages of Cell Filled Concrete Pavements
- The preparation process of the cells is tiring.
- It requires proper care as there are high chances of cells getting disturbed while trying to place the concrete.
- As the process is slow, the men and machinery's efficiency is less than the normal construction.