Concrete Roads: PQC, Methods of Construction and How it is Made!
Introduction to Concrete Roads:
Concrete roads fall under the category of high quality/superior type of roads built with cement concrete. These pavements may or may not be provided with sub-base/base courses, and they may be constructed directly over a well-compacted soil subgrade. They derive their strength to support the wheel-loads of traffic from their flexural strength and are capable of bridging any weak spots in the layer over which they are placed.
However, provision of a sub-base/base course under cement concrete pavements will enhance their performance significantly; thus a well-designed and well-constructed cement concrete pavement is a rigid pavement capable of providing trouble-free high quality riding surface for high-volumes and heavy traffic loads for as long as 30 to 50 years. Portland cement concrete is well understood in its engineering behaviour and hence, a concrete pavement can be designed on a rational basis.
The primary drawback of this type of road is its high initial cost, although its economic cost (initial cost plus maintenance cost) over its design life is attractive. Further, it calls for the highest level of quality control at all stages – material selection, mix design, placement, compaction, joint-provision and curing.
Pavement Quality Concrete (PQC):
ADVERTISEMENTS:
Pavement Quality Concrete (PQC) is cement concrete made with large size aggregates in accordance with IRC specifications and laid over a dry lean concrete sub-base.
This construction is used specifically for highway concrete pavements and for airport runway pavements as it can take heavy loads.
Construction of Cement Concrete Roads:
The steps in the construction of cement concrete roads are the following:
ADVERTISEMENTS:
1. Preparation of subgrade/sub-base
2. Placing of formwork.
3. Batching of materials
4. Proportioning of the mix
ADVERTISEMENTS:
5. Preparation of the mix
6. Transporting and laying the mix
7. Compaction
8. Joints
ADVERTISEMENTS:
9. Screeding and finishing the surface
10. Curing
11. Opening of traffic
The relevant IRC Specifications are:
ADVERTISEMENTS:
(a) IRC: 15-2002 (Third Revision): “Standard Specifications and Code of Practice for Construction of Concrete Roads”, IRC, New Delhi, 2002.
(b) IRC: 61-1976: “Guidelines for construction of concrete pavements in hot weather”, IRC, New Delhi, 1976.
(c) IRC: 91-1985: “Guidelines for construction of concrete pavements in cold weather”, IRC, New Delhi, 1985.
Methods of Construction:
There are two distinct ways of construction of cement concrete pavement:
In this method, the construction of cement concrete pavement is taken up in alternate bays; the bays left in the first instance are done after the concrete already laid gets hardened adequately-one week in the case of ordinary Portland cement and two to three days for rapid hardening cement.
In this method all the bays of one lane are laid continuously; however, construction joints are provided at the end of the day’s work.
Alternate bay method is said to have the following advantages:
(i) Availability of more working space for laying a bay of the slab.
(ii) Joint-provision is considered to be easy.
(iii) The bay laid earlier can stand the adjacent bay laid later.
However, the method also has the following disadvantages:
(i) More number of transverse joints.
(ii) Rain water pooling in rainy season in the bays left to be laid in the second stage.
(iii) Since no single lane is ready for traffic, the diversion of traffic during construction is a disincentive.
(iv) Equipment has to be moved back and forth.
Although provision of joints involves more work in the continuous method, this method is preferred because traffic diversion needs are minimal.
Salient Features of IRC: 15-2002 (Third Revision):
This code covers the requirements of fully mechanised construction as well as partly mechanised and partly labour-oriented techniques.
It deals with various aspects of cement concrete road construction such as materials, proportioning, measurement, handling and mixing, subgrade and sub-base preparation, formation, joints, reinforcements, concrete mix laying, finishing, curing, and plant and equipment.
Cement:
Ordinary Portland Cement, 43 Grade (IS: 8112) should be preferred. If the soil around has sulphates in excess of 0.5%, the cement should be sulphate-resistant.
Admixtures:
Admixtures conforming to IS: 6925 and IS: 9103 may be used to improve workability of the concrete or to extend setting time. If air-entraining admixture is used, the quantity of air-entraining agent shall be 5 ± 1.5 percent (for 25 mm nominal size aggregate) by volume of the mix. This counters the freezing and thawing effect, if any, and also improve the workability.
Aggregates:
IS: 383:1970-Specifications of coarse and fine aggregates from natural sources for concrete, BIS-New Delhi.
Coarse Aggregates:
Coarse aggregates shall consist of clean, hard, strong, non-porous pieces of crushed stone/crushed gravel. The maximum size shall not exceed 25 mm. Continuously graded or gap-graded aggregates may be used depending on the grading of the fine aggregate.
Fine Aggregate:
Shall consist of clean natural sand and shall conform to IS: 383. It shall be free from organic matter.
Water:
Shall be clean and meet the requirements stipulated in IS: 456. Portable water is considered to be satisfactory for mixing and curing.
Steel:
Shall conform to IS: 432, IS: 1139 and IS: 1786 as relevant. Dowel bars shall conform to Grade S-240 and tie bars to Grade S-425 of IS. If steel mesh is used, it shall conform to IS: 1566. Steel shall be coated with epoxy paint to prevent corrosion.
Proportioning of Concrete on the Basis of Strength:
The design should be based on the flexural strength of concrete. The mix shall be designed in the laboratory so as to ensure the minimum flexural strength on-field with the permissible tolerance level. The mix shall be designed in the laboratory for a marginally higher strength that what is needed on-field.
To achieve the desired minimum flexural strength, S’, known as the characteristic strength, the mix is designed for a target strength, S, which is always higher making due allowance for the extent of quality control feasible in the field.
This relation is-
Ja-values for different tolerance levels are given below:
In the absence of facilities for testing of beams for flexural strength, the design may be based on equivalent compressive strength values and the following formula may be used –
The expected standard deviation values of compressive strength are given in Table 8.28.
For design of cement concrete mixes, guidance may had from IRC: 44-2008: “Guidelines for Cement Concrete Mix Design for Road Pavements, IRC, New Delhi, 2008”, “IS: 10262-1982, Recommended Guidelines for Concrete Mix Design, IRC, New Delhi, 1989”, and “IS: SP: 23 – Hand-book on Concrete Mixes.”
The minimum cement corresponding to 4.5 MPa flexural strength in the field at 28-days shall not be less than 350 kg/m3 of concrete. (The upper limit is 425 kg/m3 of concrete).
The maximum water-cement ratio shall be 0.50. Adjustments for workability shall be made by varying the ratio of the coarse-to-fine aggregate or improving their grading without change in cement content or water-cement ratio.
Slump 0-25 (low) and compacting factor 0.78 for vibration by power operated machines. Lower slump and higher compaction factor are needed for hand-operated machines.
Tools, Equipment and Appliances:
These are specified separately for semi-mechanised construction and for fully mechanised construction, covering the different phases of work such as subgrade and sub-base compaction, concreting and formwork, concrete preparation, transportation, laying and compaction of concrete, finishing, joints, curing, and cleaning and sealing of joints. Wet-mix macadam lower sub-base and dry lean concrete sub-base may also be provided over which the paving concrete may be laid for producing a superior type of concrete road.
Weather Limitations:
Concreting shall not be done when the temperature is below 5°C or below 30°C.
Forms:
Side forms shall be of mild steel channel sections. Wooden forms are permitted only on curves with radii less than 45 m.
Joints:
Expansion joints, Contraction joints, Warping or Longitudinal joints are the three general types. In addition, construction joints may be provided.
Fig. 8.11 shows sealing details of contraction joint groove.
Fig. 8.12 shows the sealing details of a longitudinal joint groove.
Fig. 8.13 shows the sealing details of expansion joint groove.
Construction:
The plant and equipment shall be planned in such a way that the rate of paving is at least 60 m/hr.
Storage and Handling of Cement:
Large-capacity vertical silos are used for storage. In the case of small projects involving manual or semi-mechanised paving, cement in bags may be used.
Rapid hardening cement should not be stored for more than three months.
Storage and Handling of Aggregates:
Aggregate stockpiles should be made on leveled and well drained ground adjacent to the carriageway.
Batching of Materials:
Batching of materials shall be by weight. The weight of cement is to be taken as 1440 kg/m3. Water may be measured by volume.
Batching Plant and Equipment for Fully Mechanised Construction:
Fully mechanised construction needs the following:
(i) Bins and hoppers
(ii) Automatic weighting devices
(iii) Mixers
(iv) Control cabin
Uniform mixing shall be achieved in the mixers. Minimum speed of the drum shall not be less than 15 revolutions per minute. Minimum mixing time is 1½ minutes.
Hauling and Placing of Concrete:
Freshly mixed concrete from the plant shall be transported to the site by trucks/tippers. The time between preparation of the mix and final finishing by the paver on site shall not exceed 100 to 120 minutes.
Placement of Steel:
Displacement of the reinforcement during concreting operations shall be prevented. Dowels and tie bars shall be provided as designed at the joints.
Fully Mechanised Construction:
A fixed form paver or a slip form paver is used to spread, consolidate from the mould, screed and float-finish, and texture and cure the freshly laid concrete in one complete pass of the machine. The paver shall be equipped with electronic sensors to pave the slab to the required thickness, camber and alignment in the case of slip form pavers.
Dowels may be placed either by using a dowel bar inserter (DBI) or by prefixing the dowels on steel chairs on the sub-base. The DBI is normally fitted in the paver finisher to achieve better progress of work.
The fixed form paving train shall consist of separate powered machines which spread, compact and finish the concrete in a continuous operation. This needs rails on either side of road to move along the length.
The slip form paving train shall consist of a power machine which spreads, compacts and finishes the concrete in a continuous operation. The compaction is done by internal vibration and shaping between the side forms by either a conforming plate or by vibrating and oscillating finishing beams. The slip form paver has its own support rails, along with which it can move longitudinally; it does not need fixed rails.
Surface Texture:
The surface of the slab shall be brush-textured in the transverse direction.
Wire brushes are used for this purpose. The texture depth is usually 1.00 ± 0.25 mm.
Some plant and equipment are considered essential even in semi-mechanised and labour- oriented construction.
A longitudinal float is used, operated from a foot-bridge to smoothen the surface. For testing the trueness of the surface for the designed alignment, a 3-m straight edge is used.
Curing of Concrete:
Curing may be done by spreading wet hessian over the newly laid concrete and moistening it regularly. Curing compound may also be used.
Removal of Forms:
Forms shall not be removed from freshly placed concrete until it has set, or at least for 12 hours, whichever is later. No damage shall occur to the edges of the pavement. If any pointing of minor damage is needed, it shall be done with 1:2 cement mortar (1 cement and 2 sand).
Sealing of Joint Grooves:
Sawing the small width of the groove initially provided is necessary to widen it for proper filling with the sealant.
Opening to Traffic:
Unless rapid hardening cement is used, the concrete pavement shall be thrown open to traffic only after 28 days of curing. In case rapid hardening cement is used, seven days of curing is adequate.
Quality Control:
At least two beam and two cube specimens, one for testing after 7 days and the other after 28 days, shall be cast on each day’s work. They shall be tested and their appropriate tolerance limits checked.
Plant, Machinery and Equipment for Cement Concrete Road Construction:
The equipment necessary for the construction of concrete roads range from large plants and machinery to small pieces of equipment—for batching, mixing, laying, finishing, surface texturing, curing and quality control.
Concrete Mixer with Batching Device:
For batching by volume, boxes of known capacity are used. Concrete mixer of at least 0.2 m3 capacity is used. A mixer with a water-measuring device and equipped with automated locking devise to control the mixing period is preferred. For large jobs, batching and mixing plants with automatic control devises are employed.
Wheel Barrows:
These are small containers with two wheels, used for hauling concrete for short distance; they are useful only for small jobs.
Vibrating Screed:
This is usually a mild steel screed driven by vibrating units mounted on it, travelling on forms at the sides of the road.
Internal Vibrators:
These are driven by compressed air or electric power and are part of fully mechanised construction.
Float:
A longitudinal float is made of wood and fitted with one or two handles depending upon the width.
Straight Edge:
This is used to check the surface of the slab in the longitudinal direction. It is made of wood with MP plate at the bottom, 3 m long, 10 cm wide, with two handles.
Belts:
Canvas belts are used for finishing the pavement surface in manual method. This should be done before the concrete hardens. The belt is 25 cm wide and has extra length compacted to the width of the slab. Handles are fitted at the ends.
Brush Brooms:
Brooms with fibre brushes are used to provide anti-skid surface.
Miscellaneous Tools and Equipment:
Edging tools for rounding the edges at joints and the longitudinal edges, spades, shovels, pans, and other such miscellaneous items are also useful.
Fixed form Paver:
This is a type of equipment used for fully mechanised construction. The fixed form paving train shall consist of separate powered machines which spread, compact and finish the concrete in a continuous operation.
The concrete is discharged without segregation into a hopper of the spread, which is equipped with means of controlling the rate of deposition on the base. The spreader strikes off the concrete to the surcharge adequate to ensure that a vibratory compactor thoroughly compacts the layer. The final finisher shall be able to finish the surface to the required level and smoothness as specified.
The fixed form paver works by moving on rails provided at either edge of the pavement slab.
The slip form paving train shall consist of a power machine which spreads, compacts and finishes the concrete in a continuous operation. The slip form paving machine shall compact the concrete by internal vibration and shape it between the side forms with either a conforming plate or by vibrating and oscillating finishing beams.
The deposited concrete shall be struck off to the necessary average. Sensors are attached at the four corners of the slip form paving machine to control the level of the finishing beams and of the conforming plate.
Slip form pavers will have vibrators attached to it. The rate of construction shall desirably not be less than 60 m/hr. A schematic of a slip form paving train is shown in Fig. 8.14.
Since a slip form paving train has its own tracks over which it can move on either side of the road, there is no need to have additional fixed tracks.
Pre-Stressed Concrete Pavement:
Pre-stressing technique has been applied to cement concrete pavements for highways in the recent past. In fact, it has been applied for airport runway pavements more in view of the superior strength and load-bearing capacity of pre-stressed concrete.
One great advantage of this type of pavement is that a continuous length of 120 m of slab can be built without any joints using pre-stressed concrete; this leads to better riding comfort and reduces the maintenance cost.
The thickness of the slab also can be reduced compared to a cement concrete slab, plain or reinforced. A width of 3.6 m is a desirable, thus necessitating a longitudinal joint. Minimum thickness of 150 mm is considered desirable.
A minimum of 22 kg/cm2 of pre-stress is recommended in the longitudinal direction. Transverse pre-stress of about 3 to 4 kg/cm2 is considered adequate.
The pre-stressing cables should be 7 mm in diameter with ultimate tensile strength of 140- 170 kg/cm2.
The disadvantages cited are:
(i) It is a skilled job that needs a skilled work team.
(ii) Loss in pre-stress can occur due to subgrade restraint.
(iii) Quality control is required to be of high order.
(iv) It is difficult to construct these pavements on curves.
Pre-stressing may be either by pre-tensioning or post-tensioning; the latter is preferred for highway pavements.
Freyssinet or Gifford-Udall system may be used for highway pavements.