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Cement treated soil and cement flyash treated sub-base /base (CTSB)[Detailed specifications and it's tolerance limits]

Cement treated sub-base (CTSB) / cement treated base (CTB) acts as a base course or sub base course depends upon purpose of use. CTSB/CTB course are capable to carry more loads than granular sub base (GSB).
CTSB/CTB performance is good in rutting and fatigue cracking as compared to GSB. When we compare CTSB/CTB courses with GSB course, CTSB/CTB gains strength as time passes even under traffic load. 

So to get detailed information, let's begin...............

1-Scope

This work shall consist of laying and compacting a sub-base/base course of soil treated with cement or cement-flyash on prepared subgrade/sub-base, in accordance with the requirements of these Specifications and in conformity with the lines, grades and cross-sections shown on the drawings or as directed by the Engineer.

To get the more technical specifications about granular sub base (GSB),kindly click on below link 🔗 



2-Materials

(I) Material to be Treated
The material used for cement or cement-flyash treatment shall be soil including sand and gravel, laterite, kankar, brick aggregate, crushed rock or slag or any combination of these. For use in a sub-base course, the material shall have a grading shown in Table -1. It shall have a uniformity coefficient not less than 5, capable of producing a well-closed surface finish. For use in a base course, the material shall be sufficiently well graded to ensure a well-closed surface finish and have a grading within the range given in Table -1. If the material passing 425 micron sieve is plastic, it shall have a liquid limit not greater than 45 percent and a plasticity index not greater than 20 percent determined in accordance with IS:2720 (Part 5).


(II) Physical requirements

If the water absorption of the aggregates determined as per IS: 2386(part-3) is greater than 2 %, the aggregates shall be tested for wet aggregate impact value (AIV) (IS: 5640). Soft aggregates like kankar, brick ballast and laterite shall also be tested for wet AIV.


(III) Cement

Cement for stabilization shall either be ordinary Portland Cement, Portland Slag Cement or Portland Puzzolana Cement and shall comply with the requirements of IS:269, 455 or 1489 respectively.

                       Table -1: Grading Limits of Material for Stabilisation with Cement

(IV) Lime
If needed for pre-treatment of highly clayey soils, the following treatment shall apply.
Lime for lime-soil stabilization work shall be commercial dry lime slaked at site or pre-slaked lime delivered to the site in suitable packing. Unless otherwise permitted by the Engineer, the lime shall have.purity of not less than 70 percent by weight of Quick-lime (CaO) when tested in accordance with IS:1514. Lime shall be properly stored to avoid prolonged exposure to the atmosphere and consequent carbonation which would reduce its binding properties.


(v) - Flyash
 Flyash may be from anthracitic coal or lignitic coal. Flyash to be used for cement-flyash treatment shall conform to the requirement given in Table- 2 and table -3.
                                        Table -2 Chemical requirements for fly ash as pozzolana


                                                  Table -3 Physical requirements for fly ash as a pozzolana

Pond ash or bottom ash, which do not meet the requirements of Tables -2 and table-3 can also be used for cement-flyash treatment. However, in all cases of cement stabilised fly-ash/ bottom ash/ pond ash, mix should develop adequate strength.

(VI) Quantity of Cement in Cement soil stabilised mix

The quantity of cement to be added as percent by weight of the dry soil shall be specified in the Contract. Also if lime is used as pretreatment for highly clayey soils, the quantity as percent by weight of dry soil shall be specified in the Contract. The mix design shall be done on the basis of 7 day unconfined compressive strength (UCS) and/or durability test under 12 cycles of wet-dry conditions. The laboratory strength values shall be at least 1.5 times the minimum field UCS value stipulated in the Contract.


(VII) Quantity of Cement in Cement/Fly Ash Treated Sub-base/Base
The quantity of cement shall be more than 2 percent by weight of cement/ fly-ash mix. The mix design shall be done to achieve a strength of 1.75 MPa when tested on cylindrical specimens compacted to the density at optimum moisture content, tested in accordance with IS:2720 (Part 8 as specified in the contract) after 7 days moist curing. The design mix shall indicate the proportions of cement and fly ash and the quantity of water to be mixed.

(VIII) Water

The water to be used for cement stabilization shall be clean and free from injurious substances. Potable water shall be used.

3- Construction Operations

(I) Weather Limitations

Stabilisation shall not be done when the air temperature in the shade is less than 10°C.

(II) Degree of Pulverisation

For stabilisation, the soil before addition of cement shall be pulverised, where necessary, to the extent that it passes the requirements as set out in Table -4 when tested in accordance with the appropriate  method.
                                            Table - 4 soil pulverisation requirements for cement stabilisation

(III) Clauses (α),(β) and (Φ) shall apply as regards spreading and mixing the stabilizer except that cement or lime plus cement as the case may be, shall be used as the stabilizing material in place of lime.

(α) Equipment for Construction
Stabilised soil sub-bases shall be constructed by mix-in-place method of construction or as otherwise approved by the Engineer. Manual mixing shall be permitted only where the width of laying is not adequate for mechanical operations, as in small-sized jobs.
The equipment used for mix-in-place construction shall be a rotavator or similar approved equipment capable of pulverizing and mixing the soil with additive and water to specified degree to the full thickness of the layer being processed, and of achieving the desired degree of mixing and uniformity of the stabilized material. If so desired by the Engineer, trial runs with the equipment shall be carried out to establish its suitability for work.
The thickness of any layer to be stabilized shall be not less than 100 mm when compacted. The maximum thickness can be 200 mm, provided the plant used is accepted by the Engineer.

(β) Mix-in-place Method of Construction 
Before deploying the equipment, the soil after it is made free of undesirable vegetation or other deleterious matter shall be spread uniformly on the prepared subgrade in a quantity sufficient to achieve the desired compacted thickness of the stabilised layer. Where single-pass equipment is to be employed, the soil shall be lightly rolled as directed by the Engineer.

The equipment used shall either be of single-pass or multiple pass type. The mixers shall be equipped with an appropriate device for controlling the depth of processing and the mixing b laces shall be maintained or reset periodically so that the correct depth of mixing is obtained at al times.
With single-pass equipment the forward speed of the machine shall be so selected in relation to tl-e rotor speed that the required degree of mixing, pulverisation and depth of processing is obtained. In multiple-pass processing, the prepared sub-grade shall be pulverised to the required depth with successive passes of the equipment and the moisture content adjusted to be within prescribed limits mentioned hereinafter. The lime shall then be spread uniformly and mixing continued with successive passes until the required depth and uniformity of processing have been obtained.
The mixing equipment shall be so set that it cuts slightly into the edge of the adjoining lane processed previously so as to ensure that all the material forming a layer has been properly processed for the full width.

(Φ) Construction with Manual Means
Where manual mixing is permitted, the soil from borrow areas shall first be freed of all vegetation and other deleterious mater and placed on the prepared subgrade. The soil shall then be pulverized by means of crow-bars, pick axes or other means approved by the Engineer.
Water in requisite quantities may be sprinkled on the soil for aiding pulverisation. On the pulverized soil, the lime in requisite quantities shall be spread uniformly and mixed thoroughly by working with spades or other similar implements till the whole mass is uniform. After adjusting the moisture content to be within the limits mentioned later, the mixed material shall be leveled up to the required thickness so that it is ready to be rolled.

(IV) Moisture Content for Compaction
The moisture content at compaction checked vide IS:2720 (Part 2) shall not be less than the optimum moisture content corresponding to IS:2720 (Part 8) nor more than 2 percent above it.

(V) Rolling

Clause(€) shall apply except that care shall be taken to see that the compaction of cement stabilised material is completed within two hours of its mixing or such shorter period as nay be found necessary in dry weather.
Clause (€) Immediately after spreading, grading and levelling of the mixed material, compaction shall be carried out with approved equipment preceded by a few passes of lighter rollers if necessary.
Rolling shall commence at edges and progress towards the centre, except at super elevated portions or for carriageway with unidirectional cross-fall where it shall commence at the inner edge and progress towards the outer edge. During rolling, the surface shall be frequently checked for grade and crossfall (camber) and any irregularities corrected by loosening the material and removing/adding fresh material. Compaction shall continue until the density achieved is at least 98 percent of the maximum dry density for the material determined in accordance with IS:2720 (Part 8).


Care shall be taken to see that the compaction of lime stabilised material is completed within three hours of its mixing or such shorter period as may be found necessary in dry weather.
During rolling it shall be ensured that roller does not bear directly on hardened or partially hardened treated material previously laid other than what may be necessary for achieving the specified compaction at the joint. The final surface shall be well closed, free from movement under compaction planes, ridges, cracks or loose material. All loose or segregated or otherwise defective areas shall be made good to the full thickness of the layer and recompacted.

(VI) Curing
The sub-base/base course shall be suitably cured for 7 days. Subsequent pavement course shall be laid soon after to prevent the surface from drying out and becoming friable. No traffic of ary kind shall ply over the completed sub-base unless permitted by the Engineer.


4 -Surface Finish 

The surface finish of construction shall conform to the requirements of Clause given below.

A- Horizontally alignments

Horizontally alignments shall be reckoned w.r.t. the center line of the carriageway as shown on the drawings.kThe tolerance for edges of the roadway and lower layer of pavements shall be ±25 mm.

B- Surface Level 
The levels of the subgrade and different pavements courses as constructed, shall not vary from those calculated with reference to the longitudinal and cross-profile of the road shown on the drawing or as directed by the engineer beyond the tolerance mentioned in table below(table 900-1).



For checking compliance with the above requirements for subgrade and sub-base. Measurements of the surface levels shall be taken on a grid of points placed at 6.25 m  longitudinally and 3.5 m transversely. For any 10 consecutive measurements taken longitudinally or transversely, not more than one measurement shall be permitted to exceed the tolerance as above, this one measurements being not excess of 5mm above the permitted tolerance.



C - Surface Regularity of pavement courses

The longitudinal profile shall be checked with a 3 metre long straight edge/moving straight- edge as directed by the engineer at the middle of each traffic lane along a line parallel to the centre line of the road.



The max. allowable difference b/w the road surface and underside of a 3 met.

Straight-edge when placed parallel with or right angles to the centre line of the road at points decided by the engineer shall be:-


For GSB/base courses                           
                                                                                                                                      8mm

For sub-bases under concrete pavement.                                                          10mm

For subgrade                                                                                                            15mm

5-Strength and Quality Control
Control on the quality of materials
and works shall be exercised by the Engineer.
Cement treated soil sub-base/base and cement/fly ash treated sub-base/base shall be tested for the unconfined compressive strength (UCS) value at 7 days, actually obtained in-situ. In case of variation from the design UCS, in-situ value being on lower side, prior to proceeding with laying of base/surface course on it, the pavement design shall be reviewed for actual UCS value. The extra pavement thickness needed on account of lower UCS shall be constructed by the Contractor at his own cost.

6 -Arrangements for Traffic
During the period of construction, arrangements for traffic shall be provided and maintained as directed by the engineer.

7- Measurements for Payment 
Stabilised soil sub-base/base shall be measured as finished work in position in cubic metres.

8-Rate
The Contract unit rate for cement treated soil sub-base/base with pretreatment with lime if required and cement/fly ash treated sub-base/base shall be payment in full for carrying out required operations including full compensation for all components listed below .
i) making arrangements for traffic as directed by the engineer in charge ;

 ii) supplying all materials to be incorporated in the work including all royalties, fees, rents where applicable with all leads and lifts;

iii) all labour, tools, equipment and incidentals to complete the work to the Specifications;

 iv) carrying out the work in part widths of road where directed; and

v) carrying out the required tests for quality control.

Thanks a lot
Mukesh Kumar
www.gyanofcivilengineering.com

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