Why some seals work and others don’t

Why do some seals work and others don’t?

The headline is meant to be an attention grabber. In fact, all sprayed seals or chip seals work but some are just more appropriate than others. It all depends on the correct selection criteria, substantiated by the fundamental principles any design engineer must employ in order to ensure the success of any seal type specified. By Johan Muller

ABOUT THE AUTHOR:

Johan Muller, founder of Bituminist Consulting, has an MSc in Organic Chemistry and has worked in the roads industry for more than 27 years. He started to live his passion as an independent road binder specialist in 2018. Knowledge transfer and training form key components. When water gets into a road surface, it results in an unwanted pothole pandemic

The central theme for the roads feature in the April 2021 issue of IMIESA is repair and maintenance interventions. It is therefore imperative that when we discuss the most appropriate process for seal selection, important aspects are included in this discussion; however, it is inevitably impossible to include all the details involved in the design process of surface seals.

Two things that bituminous binders do not adhere to at all are water and dust. Therefore, if you want a successful seal, these two culprits must be dealt with prior to any construction attempt.

Obviously, road owners must assess the condition of the existing roads to be repaired and maintained. Often, we reseal over existing asphalt layers, but reseals can also be done on top of aged seals. Some, but not all, important criteria to consider are: • Cracks, and type of cracking: the magnitude, severity and extent are all important. When water gets into a road surface, it results in an unwanted pothole pandemic. Sadly, the costly result of insufficient attention to maintenance and repair strategies is all too visible in most municipalities. • Rutting/deformation creates another collection point for water, which results in unsafe driving conditions, with excessive mist spray. Worst-case scenarios cause aquaplaning. • Stone or chip loss is a result of construction errors or ageing over time.

This needs to be corrected before your next reseal intervention. • Bleeding and punching go hand in hand.

Care must be taken before attempting your next maintenance intervention. Technical Methods for Highways (TMH) 9 Manual for Visual Assessment of Road Pavements Part A and Part B, issued by the Committee of Transport Officials (COTO) in May 2016, describe the process to assess surface seals both visually and with instrumentation. Visual assessments include surface failures (de-bonding), surface cracking, surface ravelling (aggregate loss) and bleeding, whereas

surface macro-texture and skid resistance are measured with instrumentation.

Without going into detailed design processes, there are numerous high-flyers and some totally neglected surface seals worth mentioning:

Cold binder applications

Although rejuvenation sprays do not classify as a seal type, they are an important surface spray to remedy and prevent water ingress. Simple basic environmental conditions to perform fog sprays are optimum during the colder, dry winter months in summer rainfall areas. Under such conditions, cracks and pores in the road surface are wide open and the bitumen emulsion rejuvenator alternatives are capable of penetrating into the road surface.

Texture slurries are essential to ensure an even road surface before a subsequent seal is applied. Timely pre-treatment of the existing road needs to be planned and programmed well in advance to ensure curing and prevent punching of the subsequent seals. In cases of severe rutting, it is often necessary to perform rutfilling, which calls for a more sophisticated micro-surfacing slurry.

Conventional anionic slurries and cationic

micro-surfacing slurries are completely underutilised in the suburban environment. In Sabita’s Manual 28 – The Best Practices for the Design and Construction of Slurry Seals (June 2011), practitioners were assisted with valuable guidelines and this is now incorporated in Manual 40/TRH3. Slurry seals are cost-effective solutions and, furthermore, have the ability to create wonderful job opportunities for labourenhanced construction techniques.

Slurry-bound macadam construction is now included in the surface seals in the new COTO manuals and has beautiful application potential in road network improvements in previously neglected municipal infrastructure development areas.

Hot binder applications

Sand seals and grit seals are still implemented on a large scale in the Western Cape rural areas. Although grit seal construction has been used in the past as part of a municipal engineer’s arsenal, it has sadly gone almost extinct in most municipal environments.

In some urban environments, grits seals are excluded due to sensitivity to early rain before the binder has worked itself into the grit layer. Sand may also wash into subsurface drains. However, the best examples of durable bitumen rubber grit seals are probably found in the Roodepoort area in the City of Johannesburg. Here, these seals have performed exceptionally well under increased traffic volumes for more than 20 years.

Some of these grit seals were intended by Johan Hattingh from PHB Engineers as multilayered construction techniques with a 4.75 mm grit, which were supposed to have been built up in subsequent years. It is absolutely remarkable to examine the durability of the bitumen rubber grit seal binder that is still flexible after 25 years.

Many suburban roads are constructed with 7 mm (old 6.7 mm) road stones. Children could safely play in cul-de-sac zones and quiet suburbs in years gone by. These thin seals offer wonderful resurfacing alternatives in suburbs where asphalt surfacing would generally have been specified in recent times.

Municipal examples

This cost-effective seal type is totally underutilised, but if larger metros such as the City of Cape Town have grasped the benefit of a 7 mm and 10 mm bitumen rubber single seal, what prevents other metros from coming to the same conclusion?

The City of Tshwane adopted bitumen rubber and polymer-modified single seals in favour of asphalt in the late 1990s, which resulted in huge savings. Not only are bitumen rubber single seals functional and aesthetically pleasing, but they are also more durable and outlast any other binder type.

High traffic areas

In order to accommodate heavier traffic loads, the seal types selected employ larger road stone. These comprise single size aggregate such as 14 mm, 16 mm and 20 mm, which are used in single and multiple seal construction types. Gerrie van Zyl at MyCube indicated that 20 mm single seals in the urban environment are too noisy and 16 mm single seals should predominantly be used on slower and heavy traffic roads.

Historical perspective

About 20 years ago, many seal designs called for 20/10 mm (the old 19/9.5 mm) double seals. Sadly, misconceptions and the misinterpretation of what a ‘proper’ double seal or a one-and-a-half seal were

Slurry-bound macadam constructed with a labour-enhanced method on Abel Erasmus Pass in Limpopo

Cape seal before slurry application on a section between Swakopmund and Walvis Bay in Namibia. Due to the fog common to the area, hot-applied bitumen could not be employed. Cationic spray grade emulsion was used for the seal construction. Anionic stable grade emulsion was in turn used for the slurry construction

Cape seal between Swakopmund and Henties Bay in Namibia

resulted in these seals being prone to stone loss (stripping) in time – sometimes shortly after construction.

Nowadays, it is more common to see 20/7/7 mm triple seals being specified. The dry run of the second 7 mm layer is cleverly designed to absorb the motion of the road stone and binder under traffic, in situ, and over time. The durability of these triple seals has now been proven in many published studies to provide the best value for money for road owners of heavily trafficked road networks. It should, however, be noted that these triple seals still have sensitivity on very coarse crushed stone bases.

Cape seals

Cape seals of various stone sizes are probably the heroes of all sprayed seal construction, especially when it comes to seal life. The combination of various stone sizes from 10 mm to 14 mm to 20 mm, in combination with single or multiple slurry seals with these stone sizes, provides an interlocked system that is less pervious to water and air.

The result is reduced oxidation of the seal binder and it is generally accepted that the Cape seal may outlast most other sprayed seal types, as it is regarded as a stiff seal structure. The Cape seal is an excellent, lowrisk choice with regard to early aggregate loss due to the turning actions of traffic at intersections.

Cold-applied binders, like bitumen emulsions, were prone to run off the camber or at steep elevations in the past, but the new-generation, high-viscosity cationic bitumen emulsions make the use of these emulsions possible as tack binders in the Cape seal and other seal types.

In areas where the road temperatures could not be achieved with hot-applied binder, a new construction approach has now opened for the contractor. The emulsion, in combination with manually propelled chip spreaders, opens another area for new entries into the construction fraternity. A wider scope for more job creation in the roadbuilding sector, especially applicable for rural and suburban construction, is now further enabled.

Otta seal

The use of a graded stone seal, like the Otta seal, which is widely and successfully used on the rural road network in Botswana, has many application possibilities across Southern Africa on low-volume roads. Sadly, this construction technique has not received the necessary attention in rural community projects. Otta seals require soft binders such as MC3000 or 150/200; back-brooming takes about three months and extends the time before road marking can be completed. Additionally, Otta seals are sensitive to early rain. Mining permits are also required to screen material next to the road. Therefore, if the requirement is to purchase aggregate from a commercial source, it becomes an expensive seal. Here, one would much rather select a Cape seal as a more economical alternative.

Urban perspective

It is important to acknowledge that there are constraints with certain seal types in a built-up environment. Engineers must ensure that additional accommodation for the residents’ demands are catered for to address issues such as early traffic accommodation and the potential contamination of existing infrastructure.

Environmental factors

As the pressure increases to use more environmentally friendly and durable construction practices, two product types for use in sprayed seal construction are now receiving more global attention.

The use of end-of-life tyres in products such as bitumen rubber is nothing new to South Africans. Incentives in Australia and New Zealand to incorporate more tyres in road binders for seal applications are hot topics and receive high-level attention in these countries.

The promotion of the use of more cold-applied binders, such as bitumen emulsions, is also increasing in popularity. The probability and potential to employ these products in the municipal environment in sprayed seal construction are soon going to predominate the sprayed seal market in appropriate applications.

Industry preparation

Despite the negative impact the Covid-19 pandemic has had on the activities of global citizens during the last year, several

Slurry-bound macadam being applied on the Abel Erasmus Pass project in Limpopo

valuable industry documents have been revised and adopted as standard practice. Two worth mentioning are: • Design and Construction of Surface

Treatments – Sabita Manual 40/

TRH3 Second Edition –February 2021. (The previous revision Technical

Recommendations for Highways was

TRH3 2007). This document, available from Sabita, now also includes

Sabita Manual 10 related to the urban environment ‘Bituminous surfacings for low volume roads and temporary deviations’.

A micro-surfacing slurry project in eThekwini employing Ralumac coldapplied asphaltic material (Photo credit: Dickie Werner, Tosas) • COTO Manuals: Standard Specifications for Road and Bridge Works for South

African Road Authorities Chapter 10

Surface Treatments October 2020. (The previous revision of the Committee of Land Transport Officials (COLTO) was 1998). Chapter 8 deals with pretreatment such as texture slurries.

These documents are available on the

Sanral webpage. I think it is commendable that, after the many years required for the review process and consolidation of these best practice documents, they are now available for use. The responsible use of the manuals is now promoted through a series of webinars facilitated by the Society of Asphalt Technology (SAT) and the South African Road Federation (SARF). SARF and SAT organise these courses in the interest of engineers, and it is a resourceful way for engineering professionals to earn Continuing Professional Development (CPD) points.

From the above discussions, it’s clear that there are many solutions for rehabilitating and constructing South Africa’s road network. A key factor is selecting the right seal.