Construction Methodology of PQC
PQC stands for “Pavement Quality Concrete” and it is also referred as Cement Concrete Pavement. It is a type of concrete that is usually utilized for construction of roads and highways. This type of concrete requires slightly higher content of cement than the normal standard concrete. It is mostly designed for M45 or M50 grade concrete.
A well designed and well constructed cement concrete pavement is a rigid pavement that can provide a trouble free high quality riding surface for high traffic volumes and large traffic loads for up to 50 years. I is best suited for development of National Highways.
Moreover, the major disadvantage of this type of road is its high initial cost, despite the fact that it is economically good during its design period.
- Excavator or JCB
- Transit Mixers/Dumpers
- Concrete Batching Plant
- Paver with Sensor
- Water tanker
- Needle / Screed Vibrator
- Survey kit etc.,
Cement shall be procured from reputed manufactures in cement Bulkers. The cement shall be stored in covered cement silos and transfer to silo by means of pumps which are already fitted in cement bulkers for shifting cement.
Aggregates that are used for PQC shall be natural material complying with IS: 383 and shall not be alkali reactive.
Coarse Aggregate shall consist of crushed stone. Stone aggregates shall be produced at integrated stone crushing and screening plants established on site. The crushed aggregate shall be checked for physical properties such as Aggregate Impact Value, Water absorption etc., before accepting for incorporation in works. The maximum size of coarse aggregate shall be 30 mm.
Fine Aggregate shall consist of clean, natural sand shall conform to IS: 383. Fine aggregate shall be free from soft particles, clay, shale, loam, cemented particles, mica, organic and other foreign matter.
Admixtures conforming to IS: 6925 and IS: 9103 will be used to improve workability of the concrete or extension of setting time, on satisfactory evidence that they will not have any adverse effect on the properties of concrete with respect to strength, volume change, durability and have no deleterious effect on steel bars. Satisfactory performance of the admixture shall be proved both on the laboratory concrete trail mixes and in trail paving work.
Water used for mixing and curing concrete shall be clean and free from injurious amounts of oils, salt, acid, vegetable matter or other substances harmful to the finished concrete. It is proposed to use underground water and through deep bore wells.
The water shall be tested to check its suitability for use in concrete work before actual start of work. Large underground water storage tanks have been constructed at locations of all batching plants with storage capacity sufficient to produce single days concrete requirements.
Impermeable plastic sheeting, 125 microns thick shall be laid flat without creases on top of previously laid DLC. Before placing the separation membrane, the DLC surface shall be swept clean of all the extraneous materials using air compressor. The separation membrane shall be nailed to DLC layer with concrete nail at suitable interval so as to ensure that it remains in positive during paving.
For four lane road, Polythene sheets of about 5m width shall be used in two lanes with overlap of 300mm.
Pre-moulded Filler Board
20mm thick compressible filler boards are to be used at the location of expansion joints. Depending of the manufacture, the material should be non-degradable and have long life. These shall be tested for compression as per IS-1838. The height of these boards shall be 25mm less than thickness of slab within a tolerance +3mm. Holes to house dowel bars shall be accurately bared or punched out to give a sliding fit on the bars.
Dowel bar of 38 mm diameter, 500 mm length are to be used at every construction joint spaced at 5 m and at expansion joint for 300 mm thickness pavement. The dowel bars shall be cut smooth with power saw / Bend saw. These shall be cut from full length MS rods in advance and stocked in reinforcement yard.
Middle 150 mm length of dowel bar shall be painted with bituminous paint for protection from corrosion. The bituminous paint shall be done well in advance so that the paint gets dried before use in the works.
Tie bars of 12 mm / 16 mm diameter HYSD steel are to be provided at longitudinal joints at different spacing’s as specified in approved drawings. These shall be cut to the required minimum length of 640 mm / 800 mm towards deformed bars and stacked in reinforcement yard.
Mils steel bars for dowels and tie bars shall conform to the requirements of IS: 432, IS: 1139 and IS: 1786 as relevant. The dowel bars shall conform to grade S 415.
PVC Sleeves of minimum 334 mm length are required to be provided on Dowel bar for construction joint and 300 mm length shall be provided for expansion joint.
For Expansion joint, a closely fitting cap of 100 mm long PVC shall be placed over sheathed end of Dowel bar. The expansion space of 20 mm shall be formed between end of cap and at the Dowel bar by using compressible sponge. To block the entry of cement slurry between Dowel bar and cap, it shall be taped.
The joint sealing compound shall be cold polysulphide type conforming to BS: 5212 having flexibility, resistance to age hardening and durability. Polysulphide sealant is generally available in 2 parts and shall be mixed before application.
Synthetic Back-up Rod of 6mm / 8mm diameter shall be laid in contraction / Expansion / Longitudinal Joints.
De-bonding strips of following sizes are to be used for various types of joints:
- Contraction Joint – Compressible de-bonding strip with paper hook of size 10mm x 5mm.
- Expansion Joint – Compressible de-bonding strip with paper back of size 10mm x 1-2mm.
- Longitudinal Joint – Compressible de-bonding strip with paper back of size 8mm x 1-2mm.
Storage of Materials
Fine and Coarse aggregates shall be stored at plant area in stack yards separately identified for different sizes of aggregates in such a manner so as to prevent their identification and contamination by foreign matter and to ensure their satisfactory quality and fitness for use in the work. The stack yards for these materials shall have accessibility for withdrawal and inspection.
Materials such as PVC Sleeves, de-bonding strip, sealant etc. shall be procured in bulk and stored in central store so as to prevent them from any damage due to harsh weather etc.
Mix Design and Quality Control
Minimum M45 grade of concrete has to be specified for rigid pavement. The trail mixes shall be conducted in the field laboratory to establish design mix for M45 grade of concrete. The important parameters that shall guide the design mix are:
- Consistency of Mix
- Low slump of 25 + 15mm
- Flexural Strength
- Compressive Strength
When crushed sand is proposed as fine aggregate, Design mix shall also be conducted with crushed sand blended with rived sand to ensure continuity of work in case of shortage of crushed sand.
The quality control of PQC pavement is very critical and important for the long lasting rigid pavement. The quality control shall be exercised as per clause 903.5.2 of MOST (Ministry of Surface Transport) specifications.
Sampling & Testing of Beam and Cube Specimen
Atleast two beams and to cube specimens, one each for 7 and 28 day strength testing shall be cast for every 150 m3 of concrete placed at site. Atleast 3 pairs of beams and cubes shall be made for each day’s work. Each pair shall be different from a batch of concrete.
The ratio between 7 and 28 day strengths shall be established for the mix to be used in slab in advance by testing beams and cubes on atleast six batched of trail mix. The average strength of 7 day specimens shall be divided by average strength of 28 day specimens for each batch and ration “R” shall be determined.
If during construction of trail length or during normal working, the average value of any four consecutive 7 days results falls below the required 7 days strength as derived from value of “R”, then cement content of concrete shall be increased by 5% in consultation with Engineer and work shall be executed with increased cement content until four corresponding 28 day strengths are in conformance with target mean strength.
The workability of concrete shall be checked both at point of production and length. The slump test shall be conducted as per IS-1199 and its value shall be maintained in the range of 25 +/- 15 mm. However, the slump at batching plant location shall be controlled depending upon distance between paving site and batching plant. Initially slump test shall be carried out on every dumper. However, the frequency shall be reduced once the consistency of slump value is established.
The density of the concrete laid at site shall be checked by taking cores at site and the same shall be compared with theoretical maximum dry density.
The thickness shall be controlled by taking levels of the paved concrete. Thickness of the slab at any point shall be maintained within a tolerance of -5mm to +25mm of the specified thickness.
Pull out Test
The pull out test as specified under clause 602.6.5 of MOST specifications shall be done to check the average bond stress between PVC Sleeves and Dowel bars. The test shall be conducted in the field laboratory ensuring the bond stress between PVC Sleeve and Dowel bar movement in allowable tolerance.
The position and alignment of Tie bars shall be checked by drilling cores. The position of Tie Bars shall be checked for the center of each bar within 50mm of the intended position and minimum cover of 30mm below joint groove.
The concrete for PQC shall be produced in fully automatic concrete batching plants of required capacity that have been installed at plant site locations. The plants should have the capability of proportioning the materials by weight and providing a uniform homogenous mix. Cement from silos shall be weighed separately from the aggregates.
To keep the temperature of concrete produced within 30°C, Concrete batching plant has separate water tank in which ice will be poured so as to provide chilled water to ensure that the final temperature of concrete is within 30°C. Since the maximum temperature in the project are is generally less than 35°C, maintaining temperature of concrete shall be be a constraint.
Plant mixed lean concrete shall be transported from plant to site in transit mixers / dumpers covered with tarpaulin.
Spreading and Compaction
Freshly mixed concrete shall be brought in transit mixers / dumpers in reverse and the driver shall be carefully guided to unload concrete. In the PQC patch the approximate quantity required for laying i.e. large heaps of concrete are not formed.
Initially concrete shall be dumped. concrete shall be spread in each corner of the PQC patch with the help of vibrating needle. For starting PQC paving for the first time, a starter beam shall be positioned that tit should have facility to hold Dowel bar at specified spacing of 300 mm. The started beam shall be positioned in ground by spiking.
The mix is laid in PQC patch from transit mixers / dumpers an by means of needle vibrators it is placed at every part of the patch. Proper care will be taken while using vibrating needle due to which the shuttering may be loose that can disturb longitudinal alignment of PQC patch.
The slump of the concrete shall be kept as close to 25 +/- 15 mm as possible so as to ensure that sides of paved concrete do not collapse. In case, the slump obtained at site is high and it is felt that sides may not remain vertical, in such a case, the sides shall be supported temporarily with steel shuttering to allow initial setting.
Dowel bars are placed in PQC patch as per drawing and also spacing distance locations all will be as per given in drawing.
The depressions in the concrete paving caused due to Dowel bar and Tie bar insertion shall be levelled by oscillating beam and super smoother, which follow Dowel bar and Tie bar inserters.
Texturing and Curing
Texturing and Curing shall be done with wire brush having steel bristle not less than 450mm wide the brushed shall be made up of 32 gauge tape wires grouped together in tufts spaced at 10mm centers and 100 mm long with 2 rows of tufts having 20mm apart distance.
The brush shall replaced when the shortest tufts wears down to 90 mm long. The texture depth will be determine by sand patch test as in clause 602.12.
Immediately after surface texturing the surface and sides of the slab shall be cured by the application of approved resin based aluminized reflective curing compound which hardens into an impervious film or membrane with help of mechanical sprayer.
The curing compound shall be kept in agitated condition with agitator provided in curing compound tank. The spreading rates shall be as per specification, which is 0.1 to 0.2 ltr/m2. Further, curing of PQC pavement shall be done by keeping the surface moist with Hessian cloth and water. Fresh concrete surface shall be protected for altleast 3 hours by covering with tent in case of adverse weather conditions.
Temperature of Concreting
In order to reduce shrinkage, cracks etc. in concrete PQC shall be produced at batching plant by using chilled water. Maintaining the temperature of concrete within 30°C shall not be a problem.
In case, it is felt that it is advisable to avoid direct sunlight, concrete paving shall be started in the evenings and continued through out the night for maintaining better quality of pavement. For such a situation sufficient lighting arrangement shall be made at site for night working.
Initial cutting of the joints, which shall be 3mm wide and 100mm deep (Contraction joints) shall be done as soon as it is possible to do so without damaging concrete surface after paving. Normally initial cutting of the joints shall be started about 10 hours after paving of concrete.
The concrete slurry formed after joint cutting shall be removed by washing the joint with separate water. The final cut of joint shall be done at about 15 days after the date of paving. The final cut shall be 8 – 10mm wide and 20 – 22mm deep.
The widening of initial 3mm joint to 8 – 10mm width shall be made by making one or two passes of cutting blade on wither side of the initial cut. Joint cutting shall be done with petrol driven joint cutting machines with diamond pointed cutting discs.
The joint shall be cleaned with water and back up rod shall be provided in the groove so as to prevent ingress of any duct particles. Subsequently de-bonding strip shall be provided after final cut.
The back-up rod, then shall be pushed below the widened cut. Polysulphide Sealant, which is available in two parts shall be mixed in a mixer at site and shall ne filled in groove with a sealant gun.
The top surface of the sealant shall be shaped with a shaping tool. The same process shall ne followed for expansion and longitudinal joints.
- Clean the DLC surface with compressor / Mechanical Broom or manually.
- Mark the boundaries of laying patch width.
- Spread separation membrane with overlap of 300 mm.
- Fix pegs @ 10 mm center to center on both sides at a fixed distance from pavement edge.
- Fix string line at a fixed height above proposed pavement surface.
- Put the ice blocks in tank well in advance to have chilled water available for concrete production.
- Check the batching plant for all its functions.
- Keep requisite number of transit mixers / dumpers ready after cleaning at batching plant.
- Check the PQC screed vibrators, an needle vibrators for all functions.
- Check mechanical sprayer nozzles for proper spray of curing compound.
- Fix position of Dowel bars and Tie bars.
- Position starter beam fitted with Dowel bars at start point
- Mark position of contraction joints at 4.5 m center to center from starting point on DLC with paint.
- Check Mason, Skilled workers for their tools including 3 m straight edge.
- Position a trained person in front of transit mixer / dumper to guide driver during reversing.
- Keep Quality control tools such as slump test apparatus, cube moulds, Beam moulds ready at site and batching plant.
- Fill the Dowel bar tray with Dowel bars and Tie bar inserter with Tie bar.
Procedure at site
- Start production of concrete, transport in transit mixer / dumper and on reaching site reverse the transit mixer / dumper at starting point of PQC patch.
- Unload concrete in PQC patch at two locations.
- Push concrete with the help of vibrators.
- Use screed vibrators to spread concrete in full width.
- After laying of first 2-3 m paved concrete shall be compacted externally with needle vibrator and surface finished manually. Thereafter manual finishing is not required.
- Continue laying
- Insert Dowel bars at marked position of contraction joint.
- Tie bar shall be placed at specified spacing and locations.
- Check the paved layer for any finishing required and verticality of side edges. In case of extra slump, support edges with side shutters of 300 mm height.
- Finish the top surface wherever required by using working bridge.
- Start texturing of concrete surface when it is fit to apply brush.
- Curing shall follow immediately after texturing by spraying curing compound.
- Monitor temperature, slump of concrete received at site.
- Cover the paved PQC with weather protection shed.
- Start initial cutting of joints after 8 – 10 hours of paving.
- Finish paving by putting end stopper at the end finishing last 1-2 m stretch manually using needle vibrators.
- Keep the surface moist with wet hessian cloth.
- Do final cutting of joints after 15 days and fill the joins with sealants.
- Open to traffic after joints are completed.
- To ensure regular uninterrupted supply of materials.
- If night work is to be performed, the area must be well illuminated.
- On-site safety measurements must be carefully maintained such as Hand gloves etc.,
- IRC : 15-2011 – Standard Specifications and Code of practice for Construction of Concrete Roads.
Above all, this is the general procedure. The procedure may vary depending on the type of pavement, design, characteristics and various other factors.
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