A Case Study

On

Washable Aprons

By

S.SatyanarayanaAXEN/C,PLH,ECOR

Mob:09437262392

09441208330

welcome

Washable Apron

for

Passenger Platforms

INTRODUCTION

We are all aware of maximum nuisance by passengers in the station yards, particularly platform lines due to stoppage of passenger trains. It is obvious that these lines cannot be kept clean with conventional track with ballast. Apart from unhygienic conditions created on these line, entire track becomes slushy, water logged and track condition deteriorates. This also gives rise to flies and mosquitoes and whole circulating area looks shabby and unhealthy.

Only solution to above problem is perhaps providing a washable apron. In fact, washable apron are essential on environmental and hygienic considerations at all important stations where large number of passenger trains halt.

But due to its very nature of use and intense interaction with water, the concrete in washable apron deteriorates quickly unless extreme care is taken in quality and protection

The following types of washable aprons are being provided all over the Indian Railways.

1.Providing wooden blocks over the raised concrete bed and fixing the rails to the wooden blocks.

2. Providing Bituminous concrete below and all around the PSC Sleepers as per the RDSO Drg.No:T-4786

3. Providing 25mm to 50mm thick Bituminous Mastic Layer below the PSC Sleepers and filling the mastic mortar between the sleeper and filler concrete.(Adopted at SEALDAH station )

4. Providing 25mm to 50mm thick Bituminous Mastic below the PSC Sleepers and filling the filler concrete with nominal reinforcement around the sleepers ( Being adopted in S.C.Rly)

5. Providing plain cement concrete below and all around the PSC Sleepers.(Being adopted in Southern Rly)

6. Providing RDSO approved Grooved rubber pads –10mm thick below the PSC Sleepers and filling the mastic mortar all around the sleepers.

7. Providing ballast and pre-cast blocks below the Railway track popularly known as ‘Ratlam Design’.

8 Ballast less track as being provided in Metro Railways and Tunnels.

9. Providing Reinforced Bituminous felt below the PSC Sleepers and filling the mastic mortar around the sleepers as suggested by Sri R.N. Malhotra Member Engineering at Lucknow on 10.10.2000.

10. Providing Non-shrinkable concrete below the PSC Sleepers and filling the 10mm thick gap between Sleepers and filler concrete with carbonaceous sealing compound and non-biodegradable polymer based sealant. (Adopted at HWH station for repair of damaged washable apron)

The merits and de-merits of two types of washable aprons which are generally used are discussed below:

2. Provision of bituminous concrete of 75mm thick below and around the PSC Sleepers.3. Provision of PCC 1:2:4 or 1:1 ½ : 3 as a filler concrete in the balance portion.4. Provision of 10mm thick Epoxy mortar on the top of

bituminous concrete at the junction with PSC Sleeper and filler concrete.

A. Washable apron as suggested by the RDSO vide Drg. No.T-4781;The salient features are

1. Provision of RCC Slab of 350mm thick over a layer of sand filling and lean concrete of 100mm thick each.

Washable aprons executed based on the above method failed miserably within a short span of 1 to 3 years.Contrary to this With the experience gained from these failures by modifying the RDSO specifications washable aprons executed at RGDA & SBP are working satisfactorily even after 3 years of commissioning to traffic.

The construction mainly suffered from the following deficiencies.i) Lack of adequate bond between the bituminous concrete and the adjacent PSC Sleeper/filler concrete.This is because of the reason that the bitumen is not a cementacious material and do not have proper adhesion. The epoxy being used to hold these three things viz. sleeper, bituminous concrete and filler concrete intact is failing under vibration of sleepers during passage of traffic.So The cracks formed at these junctions giving way to the entry of water and it travels inside.

Accumulated water inside the concrete activated by the repeated vibrations on the passage of traffic causes deterioration to the bituminous concrete below and around the sleeper and pumped out, forming a gap.

B.Raft type construction:

This method is nothing but the long old process of making washable aprons. Embodying the PSC Sleepers with plain cement concrete without any flexible material around.

Salient features:

(1) PSC Sleepers are supported either on RCC base slab or PCC.(2) After linking the track to the required level and alignment, gaps all around the sleepers are filled with plain cement concrete 1: 2:4 or 1:1 ½ :3 to the slope of 1 in 40 towards the drain.

In This method also, though the sleeper and the filler concrete may not act monolithically but the bond between the pre-cast PSC Sleeper and the cast-in-situ filler concrete is some what sufficient to negotiate the moving loads in the station yard limits.

This is evident from the washable aprons observed at Chengalpat (commissioned in 1998) and Chennai Central Stations, where no single crack has been observed around the PSC Sleepers. The photograph taken in the month of Sept’03 are enclosed herewith.

Drawbacks: As no construction technique or adhesive had been used in placing the PSC sleeper on bearing concrete the water finds its way below the sleeper when the sleeper vibrates up and down as the trains move.This water accelerates the wear & tear of the concrete surface below the sleeper particularly at the rail seat.This situation creates center binding for the concrete sleeper and the sleepers start cracking from the centre

S.No Bituminous concrete & Epoxy Plain Cement Concrete ( Raft Type)

1 The packing of bituminous concrete around the PSC sleepers is cumbersome and difficult task as bituminous concrete is not workable and cohesive.

The packing of PCC below and around the sleepers can be easily done with the help of a vibrator.

2 This requires shuttering at two stages for packing the bituminous concrete.

No shuttering is required.

3 The cost of bituminous concrete with epoxy only constitutes 40% of the overall cost of the apron.

As these materials avoided completely, the cost of apron will be reduced by 40%

4 Stringent supervision and skilled work is required.

Ordinary supervision is sufficient.

5 As the bituminous emulsion and epoxy are not being used regularly in Railways, without proper knowledge, it is difficult to ensure the quality of the product.

Cement concrete is a known material and quality can be ensured easily.

6 Cracks around PSC sleepers are observed within a short span of 1 to 3 years.

No cracks are observed even after passage of 5 years.

7 More period of traffic block is required. Less period of traffic block

CO MPARISIO N

.

Suggestions: The life of the washable apron can be increased drastically by making the PSC Sleepers and filler concrete act monolithically. This can be achieved either by

(1) Embodying the reinforcement bars transversely at a spacing 250mm c/c in to the PSC Sleepers at the time of casting and anchoring the same with the filler concrete while executing the washable apron.

(2) embedding the MS Angle at the bottom face of sleeper at the time of casting. Subsequent welding of the reinforcement rods to the angle and anchoring with filler concrete.

PSC SLEEPER

MCC 1:3:6

350

PCC 1:2:4

ISA 65 X 65 X 6 FOR WELDING 8 MM DIA BARS.

RCC RAFT M25

8MM DIA BARS EMBEDDED IN PSC SLEEPER @ 200 C/C

(3) Laying of rails directly on the MCI inserts embedded in the RCC base slab. As the PSC Sleepers are avoided completely the cost of the washable apron can be reduced directly by Rs. 1200/Rmt. Though this method requires strict vigilance and careful supervision at

every stage of construction this method is cost effective and durable.

(4) The cost of RCC RAFT alone comes to Rs.5714/-per RMt. Where as the cost of ballast filled with sand comes to Rs. 850/-only per Rmt.so by replacing the RCC RAFT with the ballast filled with sand the cost of washable apron can be bring to Rs.2562/-per Rmt.By providing this flexible material as a dampening media below the Rly track the chances of formation of cracks around the PSC sleepers can be minimized when compared with rigid RCC base slab where reactions are much more.

(5) The central binding of the PSC sleeper can be avoided by tying the thermo Cole to the bottom of the

sleeper before placing the concrete.

(6)The quality of the material used in construction plays an important role in achieving the quality and durable apron. At the same time the agency to whom the work is awarded plays equal role in achieving the quality of apron. So much should be the stress in deciding the agency for execution of the work. Irrespective of the lowest tenderer the work should be awarded to reliable and quality oriented contractor who has good track record, at workable rates. (7) In many number of cases the quality is spoiled in a hurry to complete the work, at the last stage , with in the block period without allowing sufficient time for curing , setting and stabilizing.So from the initial stage itself the daily progress should as per the program planned meticulously by the department.The penalty clause should be strictly enforced if the agency fails to adhere to the program.A bonus clause should also be incorporated to encourage the contractors to complete the work with in time.

1 Bituminous concrete and Epoxy. Line No. 4 at VZM. 1998 59672 -do- Line No.1 at SBP 2001 121053 -do- GRC 2002 140004 Non shrinkable concrete with

biodegradable polymer based sealantRepairs to the Line No.8 at GRC 2002 9250

5 Bituminous concrete and Epoxy. BZA 2003 116676 Bituminous concrete and Epoxy. BBS 2003 107597 Bitumin mastic and filler concrete with

nominal reinforcementBZA 2004 12500

8 PCC New Line No.1 at VZM 2004 7426

Type of construction and location Location Year of executio

n

Cost per Rmt. (Rs.)

Remarks

CO ST CO MPARISIO N

Sl.

(

Conclusion :A damaged washable apron is worse than having no washable apron. This is more so for hygienic and environmental reasons. The sleepers get broken, the filler in between sleepers got damaged causing the night soil to stick and make it worse.

Reconstruction /repair of washable apron with in a short span of 5 to 15 years is not at all economical and not practicable as the traffic density is increasing day by day.Hence

For such an important work on which huge amount of crores of rupees is spent every year,careful study,testing and standardization of specifications and method of construction is very much essential by central institutes like RDSO.