Wet mix macadam (WMM) act as a base course just below the bituminous layer (bitumen base course or surface course).
The thickness of a single compacted Wet mix macadam(WMM) layer shall not be less than 75mm.when vibrating or other approved types of compacting equipment are used, the compacted depth of a single layer of the sub-base course may be up to 200mm.
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Physical requirements
- Material finer than 425 micron shall have plasticity index (PI) not exceeding 6.
To determine this combined portion, the flaky stone from a representative sample should first be separated out. Flakiness index is weight of flaky stone metal divided by weight of stone sample. Only the elongated particles are separated out from the remaining (non-flaky) stone metal. Elongation index is weight of elongated particles divided by total non flaky particles. The values of flakiness index and elongation index so found are added up.
If the water absorption value of the coarse aggregate is greater than 2%, the soundness test shall be carried out on the material delivered to site as per IS: 2386 (part-5)
Grading requirements:-
The aggregates shall conform to the grading given in table below-
The final gradation approved within these limits shall be graded from coarse to fine and shall not vary from the low limit on one sieve to the high limit on the adjacent sieve or vice versa.
To get the information about surface finish and quality control work of WMM, kindly click on below link 🔗
https://gyanofengineering.blogspot.com/2020/05/surface-finish-and-quality-control-of.html
The surface of the sub-grade /sub-base/base to receive the WBM/WMM shall be prepared to the specified grade and camper and cleaned of dust, dirt and other extraneous material.
When WMM/WBM is to be laid on an existing metalled road, damaged area including depressions and potholes shall be repaired and made good with the suitable material.
If wet mix macadam (WMM)/ Water Bound Macadam (WBM) is to be laid directly over the sub- grade, without any other intervening pavements course, a 25 mm course of screening (Grading B)or coarse sand shall be spread on the prepared sub-grade.it is advisable to lay 100 mm insulated layer of screening or coarse sand on the top of fine grained soil.
PREPARATION OF MIX:-
Wet Mix Macadam(WMM) shall be prepared in an approved mixing plant of suitable capacity having provision for controlled addition of water and forced/ positive mixing arrangement like pugmill or pan type mixer of concrete batching plant. The plant shall have following features:
i) For feeding aggregates- three/ four bin feeders with variable speed motor
ii) Vibrating screen for removal of oversize aggregates
iii) Conveyor Belt
iv) Controlled system for addition of water
v) Forced/positive mixing arrangement like pug-mill or pan type mixer
vi) Centralized control panel for sequential operation of various devices and precise process control
vii) Safety devices
Optimum moisture for mixing shall be determined in accordance with IS:2720 (Part-8) after replacing the aggregate fraction retained on 22.4 mm sieve with material of 4.75 mm to 22.4 mm size. While adding water, due allowance should be made for evaporation losses. However, at the time of compaction, water in the wet mix should not vary from the optimum value by more than agreed limits. The mixed material should be uniformly wet and no segregation should be permitted.
SPREADING OF MIX: Immediately after mixing, the aggregate shall be spread uniformly and evenly upon the prepared sub-grades/sub-base/base in required quantities. In no case shall be dumped in heaps directly on the areas where these are to be to laid nor shall their hauling over a partly completed stretch be permitted. The mix may be spread by a paver finisher.
In exceptional cases where it is not possible for the paver to be utilized, Mechanical means like motor grader may be used with the approval of Engineer.
The Engineer may permit manual mixing and /or Laying of WMM where small quantity of WMM is to be executed.
COMPACTION:-After mix has been laid to the required thickness, grade and crossfall/camber the same shall be compacted to the full depth with roller. If the thickness of single compacted layer does not exceed 100 mm, a smooth roller of 80-100 KN weight may be used. For a compacted single layer up to 200 mm, the compaction shall be done with the help of vibratory roller of min. static weight of 80 to 100 KN. The speed of the roller shall not be exceed 5 KM/h.
To get the details about Rolling pattern, number of rolling passes to compact the bed of WMM and more about Rolling please click on below link 🔗
https://www.gyanofcivilengineering.com/2019/02/standard-rolling-process-of-various.html
In portions having unidirectional cross fall/super elevation, rolling shall commence from the lower edges and progress gradually towards the upper edge. Thereafter, roller should parallel to the centre line of the road, uniformly overlapping each preceding track by at least one-third width until the entire surface has been rolled. Alternate trips of the roller shall be terminated in stops at least 1 met away from any preceding stop.
In portion in camber, rolling should begin at the edge with the roller running forward and backward until the edges have been firmly compacted. The roller shall then progress gradually towards the center parallel to the center line of the road uniformly overlapping each of the preceding track by at least one – third width until entire surface has been rolled.
Rolling should not be done when the sub-grade is soft or yielding or when it causes a wave- like motion in the sub-base/base course or sub- grade .
If irregularities develop during rolling which exceed 12 mm when tested with 3 m straight edge, the surface should be loosened and premixed material added or removed as required before rolling again so as to achieve a uniform surface conforming to the desired grade and crossfall.
Rolling shall be continued till the density achieved is at least 98 % of the max.dry density for the material as determined by the method outlined in IS: 2720 (part-8)
Setting and drying
After final compaction of WMM course, the road shall be allowed to dry for 24 hours.
Measurements for pavement
WMM shall be measured as finished work in position in cubic metres.
Thank you
Mukesh Kumar
Gyan of engineering
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