How are concrete roads made?
We are going to describe Processing and Construction of Concrete road
Weather and Seasonal Limitations in Concrete road construction
Unless special precautions as specified are taken, concreting shall not be done during extreme weather conditions, e.g., during monsoon months and when atmospheric temperature in shade is above 400C or below 40C. Guidelines for construction of cement concrete roads in hot weather, reference may be done from IRC : 61-1976.
Preparation of base for Concrete road construction
The base to receive the cement concrete shall be checked for line, grade and cros ssection as per the drawing. All irregularities beyond the permitted tolerances shall be rectified.
Where concrete is to be laid over an absorbent surface, the latter shall be kept moist in saturated surface dry condition or covered over by a water-proof kraft/Polyethylene sheeting is done to prevent absorption of water from the concrete mortar.
Fixing of formwork for Concrete road construction
The formwork shall be of correct shape, free from bends and kinks and sufficiently rigid to maintain its shape and position under the weight and working conditions of the laying and compacting equipment, It shall be set to true lines and levels and securely fixed in position to prevent any subsequent disturbance during compaction. Formwork is checked properly in line and level and any deviation greater than 3mm in 3m rectified. Deviation in formwork should be not allowed at the joints.
Placement of Concrete in road construction
Adequate surcharges of concrete shall be given over the desired finished level. The amount of surcharge shall be determined in the field by actual trial. The surcharge shall be uniform over the entire area and the concrete as spread shall be to the same camber and slope as the required finished surface.
The concrete is to be compacted fully using vibrating screeds and/or internal vibrators (Surface Vibrator or needle vibrators) . The vibrating screeds and internal vibrators shall conform to IS :2506 and IS:2505 respectively. Compaction shall be so controlled as to prevent excess mortar and water working on to the top due to over vibration.
After longitudinal floating is done but while concrete is still plastic, the slab surface shall be tested for trueness with a 3m straight edge in accordance with the procedure as per the clauses of CPWD specification 2000. Any undulations showing departure from the true surface should be immediately corrected. High spots shall be cut down and refinished. Holes or undulations formed of about 8-10 cm filled up with fresh concrete, compacted and finished.
All the above operations shall be completed within 75 minutes/ (60 minutes in hot weather) of mixing. After correcting the undulations before the concrete becomes non-plastic, the surface should be finished or corrected by belting, brooming and edging.
Where the concreting of roads is to be laid in two layers, the second layer should be placed within 30 minutes of compaction of the lower layer of the road.
Control of Concrete Strength
The strength of concrete should be checked from cube or beam specimens. For this purpose, during the progress of concreting work, cube (six samples) samples should be casted for testing at 7 and 28 days. Sampling and testing shall be in accordance with IS : 1199 and 516 respectively.
Quality of the work should not be based on a single test result but on statistical basis, such that the lower control limit calculated for a tolerance level of 1 in 15 test results, It should not be lower than the minimum strength of concrete required. The work should be done in the manner to meet the specification requirements; lower control limit is above the specified strength. Where the above requirements are not met with or where the quality of concrete or its compaction is suspected, the actual strength of the hardened concrete in the pavement shall be checked as set forth.
Joints in Concrete road construction
All materials required for the joints viz., expansion joint filler boards and joint sealing compound shall be checked for specification requirements before their incorporation in the work. The sealing compound shall conform to IS : 9.834.
All joints spaces and grooves shall conform to the specified lines and dimensions. After the complete curing period and before opening to traffic, the joint grooves should be cleaned thoroughly and sealed as per the IRC : 57-1974 guidelines . Care shall be taken to see that the sealing compound is not heated beyond the specified temperature.
Curing of concrete in road construction
Curing shall commence soon after the finished pavement surface can take the weight of the wet burlap, cotton or jute mats normally employed for initial curing, without leaving any marks thereon. The mats or jute bags should be extended beyond the pavement edges at least by 0.5 m and it should be constantly wetted. Initial curing should be for 24 hours or till the concrete becomes hard enough to permit labour work further without damage.
Final curing, after the removal of the mats, etc., shall be carried out by wet earth, ponding of water or other means specified. Where water is used for curing it shall be ensured that the entire pavement surface is kept well saturated throughout the specified curing period. Where water is scarce or pavement is on a steep gradient, impervious membrane curing shall be adopted as per details specified.
Checking the Quality of Hardened Concrete
Soon after the initial curing period, the surface of the hardened concrete shall be checked for surface regularity in accordance with the procedure set forth in the relevant clause of CPWD specification 2000. Surface irregularities formed during concreting work beyond the permissible tolerances should be rectified as indicated in IRC : 15-1970.
Where the strength of concrete tested vide Clause 10.8.4. falls below the specified limits or where the quality of concrete or its compaction is suspected, the actual strength of the hardened concrete shall be ascertained by carrying out tests on cores cut from the hardened concrete. Frequency of testing shall be as indicated in Table. Crushing strength tests of concrete works on cores should be corrected for height-diameter ratio and also for the age for obtaining the corresponding cube strength at 28-days as per the set procedure given in IRC :15-1970. The corrected test results shall then be analysed for conformity with the specification requirement on the lines of Clause 10.7.4.1.