Construction Methodology for Dry Lean Concrete
1. Name of activity:
Dry Lean Concrete Layer
2. Highway Activities (Rigid Pavement)
3. Description
of Activity:
The work shall consist of construction of dry lean
concrete sub base for cement concrete pavement in conformity with the lines,
grades and cross sections shown on the drawings or as directed by the engineer.
Acceptance Criteria :
Min.97% of MDD and as per clause 601.7
4. References:
a) Ref. to Technical Specification (MORT&H Spec. & Amendments)
a.1) Clauses: 600, 903.2.2
a.2) Tables: 600-.
b) Ref. other related works prior to this activity:
b.1) Mix design approval, GSB 150 mm, sub grade and Level Recording.
c) Ref. to Drawing(s):
c.1) PQC Panel detail (Approved by Consultant)
d) IS/IRC Specifications
d.1) Nil
5. Scope:
This methodology shall be applicable for construction of
dry lean concrete sub-base with approved materials and mix, to the lines,
grades, camber and thickness as shown in the drawing for general roads. For
taxiway / roadway / Toll plaza the work shall be executed as per the special
technical specifications for pavements.
6. Materials:
a.
Cement:
These shall consist of Ordinary Portland Cement of 43 grade
from approved source, to achieve the desired average compressive strength of
10N/sq.mm at 7days.
b.
Coarse aggregate:
These shall be in confirmation with IS 383. The coarse
aggregate shall be dry, clean, hard, and durable of crushed stone. The water
absorption of aggregate shall not be more than 2% and the soundness shall not
be more than 12% with sodium sulphate or 18% with magnesium sulphate. The
maximum size of aggregate shall be 25 mm.
c.
Fine aggregate:
These shall be in confirmation with IS 383 and shall be free
from clay, loam, shale, mica, etc., and of natural sand or crushed stone sand
or combination. The dine aggregate shall not have clay lumps more than 4%,
material passing 75microns sieve more than 4%.
The blended aggregate of coarse and fine aggregate shall
confirm to the following gradation as per table 600-1 of MORT&H
|
IS Sieve (mm)
|
% of passing
|
|
26.5
|
100
|
|
19.0
|
80-100
|
|
9.5
|
55-75
|
|
4.75
|
35-60
|
|
0.600
|
10-35
|
|
0.075
|
0-8
|
d.
Water:
Water used for mixing and curing of concrete shall meet the
requirements of IS 456.
7. Procedure:
a.
Mix:
The mix shall be proportioned with a maximum aggregate cement
ratio of 15:1 and with minimum cement content of 150kg/cum. However the
proportions of aggregate to cement can be increased based on the mix and
machine performance during the trail patch. Trial mixes with moisture contents
of 5.0, 5.5, 6.0, 6.5 and 7.0% shall be made using the above stated cement and
blended aggregates. Optimum moisture and density were obtained by making cubes.
These cubes shall be made in three layers, each layer being compacted with
electronic/pneumatic vibratory hammer of 300-400N downward force. After the
determination of moisture a set of 6 cubes shall be cast for determination of
compressive strength on 3 and 7 days. If the required compressive strength is
not achieved, increasing the cement shall make trials and the above stated
procedure shall be repeated to achieve the required strength. Before production
of concrete, natural moisture in the aggregate shall be determined and adjusted
on day-to-day basis.
b.
Mixing:
The materials shall be mixed in a mechanized batching plant,
with weighing balances calibrated. While mixing, moisture content shall be kept
optimum+2%, to avoid the evaporation losses. The capacity of the batching plant
shall be at least 25% higher than the proposed capacity of the laying
arrangements.
c.
Trial length:
A trial length of 500 Sqm
shall be made for the full width of the pavement and rolled, by involving at
least on transverse construction joint involving hardened concrete and fresh
laid concrete. The in-situ density of the freshly laid concrete shall be
determined by sand replacement method with 200mm dia density cone, by making 3
density holes at equal distances that diagonally bisects the trial length,
these shall not be made in the strip of 500mm from the edges. The average of
these shall be considered as 100%, and with this the field density of the
regular work shall be compared. The hardened concrete may be cut for 3m width
in the trial length and reversed to check the bottom surface for any
segregation or honey-combing, if found necessary adjustments shall be made in
the mix design.
d.
Transportation and placing:
The plant mix concrete
shall be transported by sufficient number of tipping trucks to ensure
continuous, uniform supply of concrete to feed the laying equipment. The mix
shall be protected form the weather, by covering the tippers/dumpers with
tarpaulin during the transit. The lean concrete shall be laid to the required
widths, grades, camber and thickness with self-propelled, electronic sensor
paver. The equipment shall be capable of laying the materials in an even manner
without segregation, to the specified thickness after the compaction. The
paving machine shall have high amplitude tamping bars to give good initial
compaction. Transverse and longitudinal construction joints shall be staged by
500-1000mm & 200-400mm respectively from the corresponding joints in the
overlaying concrete slab.
e.
Compaction:
The compaction shall commence as soon as the concrete laid and
leveled to the full width till there is no further movement under the roller
and the surface is closed. Double drum smooth wheeled vibratory rollers of
80-100kN static weight shall be used for this purpose. In addition to the
number of passes required for compaction, there shall be a pass without
vibration as preliminary pass and another at the end as final pass. The minimum
dry density obtained shall be 97% of that achieved in trial length, and at a
distance of 500mm from edge it shall be 95% of that achieved in the trial
length. The final surface shall be inspected immediately, and all loose,
segregated or defective areas shall be corrected by using fresh lean concrete.
Honeycombed areas, any level/ thickness deficiency and surface irregularities
shall be corrected by concrete with 10mm and down aggregates. The final
finishing of the same shall not exceed 90 minutes (if concrete temperature is
above 25°C & below 30°C) and 120 minutes if less than 25°C. Work shall not
be proceed when the temperature of the concrete exceeds 30°C & Ambient
temperature is above 35°C.
Special care and attention
shall be exercised during compaction near joints, kerb, channels, side forms,
around gullies and manholes.
Longitudinal and transverse
joints are to be provided as shown in drawings. At these joints unless vertical
forms are used, the edge shall be cut back to a vertical face where correct
thickness of compacted layer has obtained.
Covering the surface with
gunny bags/ Hessian cloth, which shall be kept continuously moist for 7days by
sprinkling water, shall do curing.
8. Safety
and environmental activities:
a. All tippers / tankers shall be provided with reverse
horns.
b. Flagman shall be provided at all exit points of the
trucks / tippers.
c. The speed of vehicles plying in the work area shall be
restricted to 40kmph maximum.
d. If night work is to be carried out the area shall be
sufficiently provided with lighting.
e. The tipplers when being positioned in front of the
paver shall be move very slowly as per the directions given by the supervisor
or by the helper provided.
f. Check for the working condition of safety valves of
air compressors, if compressed air is being used for cleaning.
9. Manpower
Requirement:
1. Section Engineer
2. Site In charge
3. Site Supervisor
4. Surveyor team
5. Concreting team
10. Machinery
Requirement:
Batching
plant, Tipping trucks, Sensor Paver, Water Tanker, double drum roller of 80 KN
static weight, Shovels, Broom, Wire brush, Straight edge.
Setting out will be done prior to work demarking the
area of execution.
11. Testing
& quality Control:
Field density of compacted layer shall be determined
by sand replacement method with 20cm dia density cone. Cubes for Compressive
strength test at 7 days shall be made with vibratory hammer. Frequency of tests
shall be maintained as per section 900 of MORT&H. All the test reports
shall be recorded on relevant specified format.
A trial length will be constructed in advance of the
proposed date of commencement.
a)
The manpower
& machinery proposed is indicative only. The requisite qty. of resources
may vary based upon site condition.
b)
Any modification
in execution of activity shall be done with prior information with consultant.
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