Performance of Thin White Topping (TWT) Overlay for NH-848

Vikas V. Thakar, MD, Pavetech Consultants India, presents the uses and advantages of TWT as a cost-effective treatment and a sustainable solution for rehabilitating and strengthening of flexible pavements, as seen in the NH-848 Nashik Peth Section in Maharashtra State.

Introduction

IRC SP76-2015 defines Thin White Topping (TWT) as concrete overlay with thickness between 100 to 200 mm. The Thin White Topping treatment for the current project site of NH 848 is carried out on existing bituminous pavement with concrete overlay thickness of 200 mm after milling of existing bituminous surface for creation of bond. Proper Pre overlay treatment is carried out for the isolated failed patches before undertaking the concrete overlay work. The project was titled as “Concrete overlay for Nashik to state border of Thane-Nashik-Peth-Kaparda-Pardi Road NH 848 - commencing from km 16/000 to km 34/000, km 39/000 to 57/000, & km 62/000 to 65/600 in the State of Maharashtra”.

The project was executed on EPC (Engineering Procurement Contract basis by P.W. (NH) Maharashtra with funding through Ministry of Road Transport and Highways (MoRTH).

The existing carriageway of the Project Highway is two-lane with paved shoulder from km 16/000 to km 34/000, km 39/000 to 57/000 & km 62/000 to 65/600. The two laned paved shoulder configuration is maintained in the TWT concrete overlay project and concrete carriageway of 10 m width (7 m + 1.5 m wide paved shoulders on each side) are constructed. The existing carriage way in these sections was flexible pavement with fair crust thickness (100 to 120 mm bituminous layer and 300 to 600 mm granular base).

After completion of the defect liability period (DLP) for the said stretches with flexible pavement overlay, it was decided to concretize the 15 Km length (in various stretches) of the project highway. As the existing flexible pavement was in a fair condition, it was decided to use the technology of Thin White Topping (TWT) for conversion of flexible pavement to concrete pavement with a design life of 25 years. This was taken up as a pilot project by MoRTH for implementation of this cost-effective technology for high volume traffic roads.

Accordingly, the design was prepared for the said project by using guidelines of IRC SP 76 2015. Design thickness of 200 mm with M40 grade concrete was implemented for the white topping work with small size panels. The profile correction was undertaken with bituminous macadam layer for cross profile correction and removal of localized sags in the existing bituminous pavement before the overlay work. The pre overlay treatment such as rectification of the failed stretches and profile correction were undertaken before the white topping overlay treatment. The design for the TWT was reviewed and approved by CRRI New Delhi in 2020.

Reasons for White Topping

The white topping technology overcomes some of the problems in bituminous/ asphalt roads such as:

• Frequent failures of roads resulting in potholes and high recurring maintenance cost
• Formation of potholes and unsafe driving conditions in monsoon
• Need for resurfacing very frequently
• Amenable to damage due to drainage issues

These disadvantages are mitigated in White Topping by providing a durable and maintenance free concrete riding surface using TWT technology. Using the benefit of composite action and lower warping stresses due to small panel size, a significant reduction in the concrete layer is possible in TWT as compared to conventional concrete pavement. The TWT layer is also useful in strengthening of the existing flexible pavement considering the future traffic growth.

The advantages of a rigid riding surface with life above 25 years are offered by making complete use of the existing available bituminous road crust. In addition to these advantages, TWT is a cost-effective technology with savings of approximately 35 to 40% as compared to a new rigid pavement. TWT is also an eco-friendly technique which makes 100% use of the existing available material and eliminates the requirement of additional costly material for new crust layers, thus drastically reducing need for quarrying and crushing operations.

Types of White Topping

White Topping (Concrete Overlay) is categorized into three types based on thickness of concrete layer

• Thickness above 200 mm is termed as Conventional White topping
• Thickness over 100mm and less than 200mm is called Thin White Topping (TWT)
• Thickness between 50mm to less than 100mm are termed Ultra-Thin White Topping (UTWT).

White Topping Benefits

The obvious benefits of white topping include:

• Superior riding experience with designed service life of above 25 years
• Cost effective in maintenance cost and operating cost.
• Resistance to weather, oil spills etc.
• Better reflectivity specially during nights,
• Use of indigenous materials like cement, aggregates as against imported bitumen
• Use of eco-friendly products like Fly-ash, GGBS, Silica fume. The concrete after its service life is 100% recyclable.

Construction Stages–White Topping Road

The construction steps for white topping project are broadly summarized below.

• Milling of existing bituminous surface
• Cleaning of exiting surface & profile correction
• Conduit laying (if required)
• Concrete laying by Paving machines
• Surface finish by Bull float, Texturing
• Curing Compound spraying, Groove cutting, Lane Marking

White Topping Projects

White topping in its various forms has been used in the USA and Europe on Airports, Inter-state roads, Primary & Secondary Highways, Local Roads, Streets and Parking lots to improve the performance, durability, and riding quality of deteriorated bituminous pavement surfaces. Conventional White Topping, and Thin White topping (TWT) with 25 years of design life offer immense potential as a rehabilitation strategy for Indian roads and Highways.

Several successful low / medium volume traffic projects have been executed at Thane, Aurangabad, Pune, Mumbai, Nagpur, Jaipur,and Bangalore in the last decade or more. The rehabilitation and strengthening of flexible pavement using TWT has shown satisfactory performance over the past decade in these cities. However, all TWT projects in India have been undertaken for local and internal roads having low to medium volume traffic.

Literature Survey

White Topping is a type of concrete pavement overlay that is used to resurface old or deteriorated pavements. Some important literature on white topping is summarized in this section.

1. Thin White Topping: State-of-the-Art Review by B.V. Venkatarama et al., stated in this paper as the white topping technique with construction methodology and the construction process are explained along with pre overlay treatment.
   
   
2. A critical review of the PCA and IRC methods of thin white topping pavement design by Venkata Jogarao Bulusu et al., stated that TWT is the most economical solution for blacktop road strengthening. The paper studied the differences between IRC and PCA design methods for TWT by considering various parameters. In this study, both methods considered 8-ton single axle load and 16-ton tandem axle load for estimating the stresses in cement concrete layer. The stresses estimated with IRC equations are 40% to 50% higher compared to PCA equations for 8-ton single axle load and 0% to 22% higher for 16-ton tandem axle load.
   
   
3. Performance evaluation of thin white topping pavements under accelerated loading conditions by N. Vaishakh et al., stated that the performance of in-service white topping pavements as a rehabilitation technology, by means of pavement distress survey and falling weight deflectometer (FWD) test is represented. The performance of white topping pavements was characterized by both the Pavement Condition Index (PCI) and the Pavement Distress Index (PDI). KENSLAB was used to determine pavement response for fatigue analysis.

Case Study on TWT for NH-848 Project

The white topping treatment for urban roads with low/ medium volume traffic is successfully implemented on various roads in India. The white topping work on National highway having high volume commercial traffic has been undertaken and successfully completed for the first time on NH 848, Nashik to Peth Section in the state of Maharashtra. The project highway consists of existing two-lane paved shoulder flexible pavement (10.0 m wide). The width of the white topping overlay (with paved shoulders) is maintained as 10.00 m. The TWT overlay is constructed by using M 40 grade concrete with fibers and the thickness design is done as per guidelines of the IRC 76-2015. The total length of the project is 15.00 km which consists of 3 (three) sections between km 11/600 to 16/000, km 34/000 to 39/000 & km 57/000 to 62/000.

Data Collection for Thin White Topping Work

For the designing of the TWT overlay, a detailed field data collection and extensive investigation study are required. The investigation and data collection activities included the detailed pavement condition survey, existing crust and soil investigation, structural evaluation of flexible pavement, drainage study and traffic data collection for the design purpose.

i. Pavement Condition Survey

Pavement Condition survey was undertaken to assess the existing flexible pavement and identification of distresses in the existing pavement crust. Overall, the existing flexible pavement for the said project stretches was observed to be structurally sound without significant distresses such as fatigue cracking, rutting etc.

However, some localized failures were observed such as settlement, cracks etc. specifically at the locations with poor drainage and where the height embankment was inadequate resulting in inundation conditions. The distresses such as raveling, potholes, isolated settlement and cracking were also observed at few locations. The detailed distress survey was carried out for the existing pavement to assess the condition of existing pavement and to devise a treatment schedule. The photograph showing distresses on existing pavement are shown in Image 1 and 2.

ii. Traffic Data

The seven-day, 24-hour Classified Traffic Volume Count was conducted at Ch 32/400 and the traffic volume data is summarized below in Table 2. The axle load survey was also conducted near Karnjale Village. The axle load survey for determination of Axle load characteristics and loading pattern are required as inputs for the design of the concrete overlay. Table No 1 gives the details of the crust for existing road and Table No 2 provides the details of Average Daily Traffic (ADT) for the project highway.

The details of the axle load survey and percentage distribution of loads in various categories (Rear single axle, Rear tandem axle) are shown in Table 3. This data was used for the design of TWT overlay as per the guidelines of IRC SP 76-2015.

iii. Evaluation of Existing Bituminous Layer for K value estimation

The subgrade support for design of the concrete overlay is assessed in terms of Modulus of subgrade reaction (k- value). The modulus of subgrade reaction on top of the flexible pavement can be determined using the graph provided in Appendix III of IRC SP 75-2015. The K value is estimated based on correlation available between the characteristic deflections on the bituminous surface measured with a Benkelman Beam device as per Appendix III of IRC SP 75 2015. Deflection study using a Benkelman Beam deflection device was carried out and the K value on top of the flexible pavement was derived based on the correlation chart provided in IRC SP 76. The characteristic deflection from Benkelman beam (BBD) study was found to be as 1.16 mm and based on the deflection and correlation figure in IRC SP 76 the K value was estimated as 8.5 kg/cm3 . BBD survey can be seen in Image 3.

iv. Trial Pits for Existing Crust Thickness

The trial pits and trial cores were taken along the project highway to assess the availability of the existing pavement crust layer thickness. It was found that the average thickness of the bituminous layers was between 100 to 120 mm and the thickness of the granular base and subbase layers was between 300 to 600 mm. Details of existing crust can be seen in Images 4 and 5.

Pre-overlay Treatment for the Thin White Topping Design

The pre overlay treatment is vital for correction of all the damaged/ failure locations before undertaking the TWT treatment and ensuring a uniform support to the TWT overlay. At all such damaged locations identified during the distress survey were excavated upto subgrade level for full roller width of 2.5 m. The backfilling was done with 200 mm GSB and 250 mm Wet mix macadam and 75 mm bituminous macadam layer to match level with the existing road level. The typical cross section for correction of distressed portions is shown in Figure-1.

Profile Correction Course

The profile correction course as required on site was provided for some stretches for provision of cross fall and correction of the localized sags. The profile correction was done using bituminous macadam layer after taking proper road levels for the existing road cross sections and identification of the profile correction sections carefully. The average thickness of profile correction course was 60 mm. Profile correction work can be seen in Image 6.

Thin White Topping Design based on IRC SP 76 Guidelines

The thin white topping overlay was designed as a bonded overlay with small square panels to take advantage of the composite action of the overlay concrete and existing bituminous pavement. For the design, the guidelines of IRC SP 76 -2015 were used and the design thickness was derived as 200 mm using M 40 grade concrete and square panel size of 1.25 m X 1.25 m. The typical section of the TWT overlay treatment is shown in Figure 2.

Construction of the TWT Overlay

The TWT overlay was undertaken by using slip form paver for a paving width of 5.0 m. The pre overlay treatment for the failed/ settled and damaged portions was completed in advance and the traffic was allowed to pass on the rectified portions for a brief period of 2 to 3 weeks before concrete overlay work. The bituminous macadam profile correction was provided using paver finisher with sensor devices after proper identification and marking of the profile correction stretches on site. The milling of the existing bituminous layer was undertaken for 25 to 30 mm to create a bond between the existing flexible pavement and the new concrete overlay. Subsequently, the Thin White topping overlay treatment with M 40 concrete. Milling operation can be seen in Image 7 and concreting work with slip form paver can be seen in Image 8. Construction operation for groove cutting and curing can be seen in Image 9.

Grade concrete and slab size of 1.25 m X 1.25 m was provided by cutting of the concrete slab within 10-12 hours window with 3 to 5 mm initial groove cut. Complete curing of surface and sides with wet hessian cloths was provided for a period of 14 days after which the cleaning and sealing of joints with polysulphide sealants was undertaken. Tie bars were provided at the center of the carriageway with 12 mm tor steel diameter bars, 500 mm long and 450 mm c/c provided at the mid depth of the concrete overlay. Dowel bars 25 mm diameter, 400 mm long at 300 mm c/c provided at mid depth at the construction joints only.

Approximately 300 to 350 m concrete overlay in length with 5 m width was carried out daily with adequate quality control and supervision for the concreting work. The Wirtgen SP 64 Slip form paver with automated tie bar inserter was used for the paving operations with laying, compaction, finishing nd texturing done with the paver finisher machine. Ground Granulated Blast Furnace Slag (GGBS) at the rate of 310 kg was used in the concrete mix in accordance to the provisions of IRC 17 2018. The cement content was 360 Kg per cum and w/c ratio of 0.42 in controlled environment. The target mean strength of 52 MPa was intended in the concrete mix. Design slump was maintained at 25+/-15 mm on site. Adequate measures were enforced for concreting to maintain the temperature of the concrete below 27 deg cel during the concreting operations. The plant placement was such as to complete the finishing operations within 120 mins of concrete mixing at the RMC plant site. Fibrillated polypropylene fibers at the rate of 01 (one) Kg per cum were introduced in the mix to arrest shrinkage cracks. Adequate arrangements in the form of plastic sheets were provided to cover the fresh concrete to avoid rapid evaporation and subsequent shrinkage cracks.

Quality Testing & Monitoring

The TWT overlay work was monitored and supervised with the best quality standards as per MoRTH specifications and guidelines of IRC 15-2017. A comprehensive Quality Assurance Plan (QAP) was devised and implemented to ensure the highest quality of the concrete pavement used for TWT overlay treatment. A full-fledged laboratory was established by the contractor at site for conducting all routine tests for materials such as aggregates, sand, cement, GGBS, concrete cube and flexural beam tests, steel etc.


1. Cement:- The cement used in construction is of OPC 43 grade which was tested as per IS 8112:2013 Table-3 and the test result was found to be within permissible limit. The test on cement included:

• Fineness of Cement---2.08%
• Standard Consistency---28.75%
• Soundness of Cement---1.8 mm
• Initial setting time---100 min
• Final setting time---295 min
• 28 day compressive strength—46.21 N/mm2

2. Aggregate:- The routine aggregate use for the concrete were 20 mm, 12 mm, 6 mm coarse aggregate (Ramshej, Gawalwadi) and tested as per IS 2386:Part VIII-1963


3. Bitumen Testing: Bitumen use in this project for various purposes was VG-40 and viscosity test Penetration test, Ductility test, Softening point test and flash and fire point test done to check the quality of bitumen as per IS 73- 2013. The test results for bitumen samples were as follows.

• Penetration test---53 mm
• Ductility Test-----84 cm
• Sp. Gravity Test—1.01
• Flash Point---------2370
• Absolute Viscosity at 600C --4000 Poise
• Kinematic Viscosity at 1350C --432 cSt

4. GSB/WMM Test:- The samples collected from various chainages along the road for WMM and GSB material and tested under control condition for following test

• Sieve analysis
• Liquid limit
• Plastic limit
• Impact value
• Combined flakiness and elongation index

Test result was found acceptable.

5. Concrete Test: - Concrete cube and concrete beam were casted daily for testing to check the compressive and flexural test. 9 cubes and 9 beams of specific size were for M40 grade concrete batch wise. Average compressive strength above 44 MPa and flexural strength above 5.5 Mpa was achieved for the concrete in the laboratory tests with continuous monitoring and implementation of the approved concrete mix at the RMC plant.

6. Water Test: - The water used in the concrete was tested on as per IS 3025 (Part11, 15, 17, 18, 22, 23, 24, 32)-1983, and the values of all tests were found to be within permissible limit.


7. Steel Material:- 25mm dia dowel bars and 12mm tie bar used at construction joint and were tested as per IS 2062-2011 for its chemical composition. The steel bars were tested for weight per meter, tensile strength test, elongation test, bend and re-bend test, as per IS 432-1982 and the values of test result were found to be in permissible limit.

8. Slump testing: The slump for the concrete mix was carried out for each dumper and maintained at 25+/-15 mm. The material from vehicles with higher slump were rejected and used for minor site works.

Performance Monitoring for TWT Work of NH 848 Project Highway

The TWT overlay work was completed by the contractor in April 2021. The performance of the TWT overlay is being monitored for the past two (02) years by conducting routine inspections after every six (06) months for checking any observed distresses in the TWT work. The last site inspection was conducted in March 2023 for visual inspection of the concrete pavement and assessment of the distresses in the TWT work on this pilot stretch.


Some of the distresses observed on the TWT overlay work include multiple structural cracking in 1 -2 stretches at some locations which is confined to a day’s work thus pointing towards the deficiency or problems with the mix for that day. Additionally, shrinkage cracks are also observed on the PQC at some locations and full depth transverse cracks are observed at some locations. The complete stretch was visually inspected, and cores were taken at several locations in the concrete slab to ascertain the concrete strength at the locations of cracked concrete panels.

The cores were also taken at the contraction joint location to ascertain the depth of the saw cut and the propagation of the crack under the saw cut. The cores were also tested in the laboratory to cross verify the concrete strength as per the design requirement. The core strength of the concrete is found to be within the acceptance limit.


The total cracked panels observed on the project highway after 2.5 years are shown in Table 4. Out of total 96,000 panels casted for the TWT work, cracks and distresses are observed on 2,341 panels which is approximately 2.44 % of the total panels casted under the project. These include panels with full depth as well as partial depth and shrinkage cracking. Replacement of the full depth cracked panels (approx. 50 % of total cracked panels) is suggested and has been undertaken by the contractor in his defect liability period. Due to small size panels without tie or dowel bars the replacement activity is comparatively easier as compared to conventional PQC panels. The replacement of the cracked panels is conducted during the defect liability period. Details of distressed panels is shown in Table 4. The observed distresses on white topping work can be seen in Images 10, 11 and 12 and field cores of concrete can be seen in Image 13.

A. Primary Distresses observed in TWT Pavement

1. Plastic Shrinkage Cracks
2. Transverse Cracks (across 5 m width – 4 panels)
3. Multiple Structural Cracks interconnected across multiple panels
4. Ravelling and low severity scaling at some locations

The causes of the cracks and distresses observed in the TWT identified on site are as follows:

1. Plastic Shrinkage Cracks due to concreting in hot weather and due to high wind velocity as mostly concrete is done between February and March months.
2. Transverse Cracks (across 5 m width – 4 panels) – Due to non-formation of the panels of size 1.25 m X 1.25 m (as per design) due to inadequate joint cut depth and non-propagation of the crack underneath the saw cut thus resulting is slab size higher than the design size.
3. Multiple Structural Cracks interconnected across multiple panels-
   • Due to non-formation of the panels of size 1.25 m X 1.25 m due to inadequate joint cut depth and non-cracking underneath the saw cut.
   • Suspect material quality during the days paving/ higher w/c ratio, excess fines etc
   • Weak/ cracked BT surface not treated adequately before the TWT (WMM+BM layer)
   • Internal drainage issues for the existing pavement. The cracks are observed generally where the existing embankment is not very high and poor drainage conditions exist in the pavement.
4. Raveling/ scaling (low/ medium severity) at some locations- Due to segregation during paving, mix issues etc.

Brief Technical Information Concerning Distresses Observed on the Project is presented in this section.

a) Plastic Shrinkage Cracking – Plastic shrinkage cracks are tight, about 0.3 m to 0.6 m long formed in parallel group’s perpendicular to the direction of the wind, at the time of paving. Plastic shrinkage cracking is a result of rapid drying at the pavement surface. The cracks normally extend down to a depth of about 20mm-30mm. Adequate curing measures are necessary to prevent their occurrence. Experience has shown that these cracks rarely influence the overall performance of the pavement, therefore a superficial repair may be required.


b) Drying Shrinkage cracking: -Wider/deeper cracking is usually attributable to the drying shrinkage and restraint developed in the concrete due to inadequate joints pacing, improper saw cutting or misalignment of dowel bars.


Reasons for plastic shrinkage cracks:

• Drying shrinkage stresses in surface
• Poor curing
• Hot windy conditions
• Excessive water at surface (bleeding)

c) Multiple Structural cracking is often caused due to excessive loading, long joint spacing, and shallow or late sawing of joints, restraint at base or edge, due to joint lock-up, inadequate thickness, material related problems etc. Use of proper construction techniques and traffic load controls can reduce/ avoid such structural cracks. Often reasons for structural cracking could be pumping of fines from the sub-grade or the subbase, excessive warping of the slab, subsidence of utility trench, excessive temperature stresses and moisture content. Structural cracks unless repaired effectively reduce the load carrying capacity of the pavement and adversely impact the designed service life of the pavement.


d) Transverse and longitudinal cracks are also observed in the concrete pavement as seen in Image 14, which occur due to the following reasons:

• Excessive drying shrinkage stresses
• Inadequate depth of joint or late joint sawing
• Excessive joint spacing
• Sudden/abrupt thermal & moisture gradient changes
• Downhill paving; cracks perpendicular to the direction of super elevation
• Settlement of embankment which leads to subsequent settlement of slabs
• "Vibrator trails" caused by malfunctioning or improper adjustment of vibrators on the paving machine
  • Tensile stresses in concrete are more than tensile strength of concrete
  • Misaligned, corroded, locked, burred ends dowel bars
  • Crack at the end of the dowel bars; or locking of dowel bars
  • Delays or interruption of concrete placing for more than 30 minutes

e) Diagonal cracking is also observed in concrete pavement due to some of the above stated reasons and in some instances the corner breaks are observed due to poor load transfer, dowel restraint etc.

Conclusions

Based on the construction of TWT overlay for high volume traffic corridor as a pilot project, many insights are gained from the technology. TWT presents itself as a cost-effective and sustainable solution for rehabilitating and strengthening of flexible pavements. The experience gained from the construction of TWT overlay for high traffic volume NH848 section can be used to further encourage this technology to other NH/SH projects with improvements.

While the TWT work has some limitations, these can be mitigated by proper design, construction and close supervision and quality monitoring practices. Research on improving the performance and durability of TWT will further enhance its application and benefits. Some of the important findings and suggested improvement in thin white topping projects for high volume traffic corridors include the following points:

• Use of higher-grade concrete for overlay min M50 grade concrete with fibers should be preferred for durable pavement with service life above 25 years.
• Improvement in drainage measures for the project to avoid drainage related failures is critical and essential in the form of cross band drains, open drain augmentation etc.
• Dense bituminous macadam should be preferred for profile correction instead of open graded bituminous macadam layer.

The following good construction practices are recommended to be followed to achieve good quality and longer lasting white topping overlay pavements:

• All cracked portions in the existing flexible pavement should be completely removed and reinstated before the TWT treatment.
• Adequate provisions for pre overlay treatment and thorough identification and rectification of damaged portions before concrete overlay treatment is to be followed to ensure proper base support for concrete overlay.
• Provision of multiple concrete initial groove cutting equipment with backup to avoid delayed cutting as the cutting length of small panels is significant.
• Slightly higher depth of groove cut appox 10 to 15 mm more than 1/3rd slab depth to ensure proper square panel is essential.
• Continuous checking of initial groove cut depth is critical to avoid formation of large size panels resulting in random cracking in the slabs.
• Enforcement of strict quality control and supervision for concrete overlay works.