Enviro Shield

The kiwi dream is to own a house and it's even better if it has a nice smooth asphalt surface outside it. Any new subdivision in New Zealand will have an asphalt surface but what happens when that road needs to be resurfaced in the future?

This is a typical residential street in New Zealand. What was once a new subdivision with a nice smooth asphalt surface is now 15-20 years old. This means the asphalt is starting to degrade due to oxidation leaving the surface with micro-cracks, fretting and unravelling of aggregates from the surface allow the water to penetrate through the seal. This can then lead to an even more rapid deterioration of the surface when traffic drives on it.

This road needs to be resurfaced. As there are probably only 100 vehicles that drive this road daily, a roading engineer can't justify replacing the surface with asphalt for four times the cost of chip seal. We all know the residents thoughts on having the road converted to chip seal....

This is where Enviro Shield comes in. At its core it is a rejuvenation treatment. It is a cold applied surface treatment emulsion. It acts as a protective layer for aged asphalt and chip seal surfaces that are beginning to fret and unravel. Its low viscosity allows for greater penetration into the surface, and once cured reduces the effects of ageing by offering additional waterproofing and locking aggregate into the existing surface. This is accomplished by using a specifically designed bitumen with a low-tack finish. Enviro Shield is extremely hard-wearing and durable. It acts as a protective layer to reduce the rate of oxidation and the degradation of the treated surface.  

Enviro Shield’s industry-leading bitumen emulsion technology allows it to be applied quicker and more cost effectively than conventional surface treatments - by a standard sealing crew. Compared with other products, Enviro Shield is safer to handle, with a normal spray temperature of 80 degrees Celsius, which means less carbon emissions. 

For a fraction of the cost of asphalt, Enviro Shield alleviates the stress of changing an asphalt surface to chip seal. It is a viable alternative to chip seal, especially in residential areas where the current surface has become a safety and aesthetic issue. 

Effect of Aging on Asphalt and Chip Seal Surfaces 

Over time, the quality of the pavement naturally deteriorates and gives rise to safety and economic problems. Although the mechanisms of failure are principally traffic related, the environment’s impact remains significant as it leads to binder-aging. 

Binder-aging is mainly caused by oxidation of the binder at the surface of the seal when exposed to air and sunlight. The oxidation of the binder occurs at a quicker rate under the action of higher ultraviolet radiations and at higher temperatures. It leads to binder hardening for chip seal surfaces and stiffening of the bituminous mixture for asphalt. In other terms, the bitumen becomes brittle and the bond between chips and binder are weakened, causing the surface to start fretting and unravelling being detrimental to the road conditions.  

Weather conditions generate temperature variations, which cause the bitumen to expand and contract. Large temperature differences in one day, such as those typically experienced in New Zealand, induce stresses in the pavement that generate micro cracks. The risk of thermally induced micro cracks increases excessively for aged binders.  

The effects of worn surfaces can be quite serious in terms of safety and ride quality although the damage is restricted to the surface. Therefore, applying a rejuvenation treatment such as Enviro Shield is a cost-effective solution to prevent the binder from aging and reduce the rate of degradation of the treated surface. 

Why Enviro Shield works 

Road surfaces naturally get worn due to binder oxidation. Over time, surfaces start to fret and unravel, to lose chip and to present micro cracks allowing the water to infiltrate more easily into the surface. This gives rise to safety and aesthetic issues that are just the beginning of more serious problems. 

As a rejuvenation treatment, Enviro Shield offers to protect the seal against oxidation, fill the micro cracks, lock the aggregates into the surface and offer additional waterproofing. It provides an effective preventative maintenance with a non-negligible aesthetic benefit that makes it a great alternative to applying another chip-seal.  

For a rejuvenation treatment to be successful it needs to have the following qualities:

• Skid resistance - to stop the surface from being slippery to reduce potential car accident
• Low tack finish - so it doesn't pick up on car tyres, shoes and end up in residents homes (on their carpets)
• Water proofing - to stop further water related damage such as potholing
• Reduce the rate of aging - to extend the life of the existing surface
• and obviously look good

The following shows us why Enviro Shield makes for a great rejuvenation treatment.

Skid Resistance 

Skid resistance is the force developed when a tire that is prevented from rotating slides along the pavement surface (Highway Research Board, 1972). In other terms, it corresponds to the surface slipperiness. This parameter is of significant importance as inadequate skid resistance will lead to higher rates of skid related incidences.

For a rejuvenation treatment such as Enviro Shield to be successful, it needs to produce skid resistance similar to that of a new asphalt surface. 

The skid resistance was evaluated using the British Pendulum Method. Enviro Shield was applied to the surface of an asphalt (DG7) and black sand spread across the top as per the standard application methodology. The results were compared to a control sample of raw asphalt (DG7) untreated with Enviro Shield. A summary of the results can be seen in the Table 1 below. 

The testing showed that the skid resistance was the same when dry and the Enviro Shield with black sand was slightly better when wet. This proves that this Enviro Shield application methodology would not negatively affect the skid resistance of the road after application. 

Low tack finish 

A low tack finish is required for rejuvenation products to prevent any chance of pick up on tyres after application. With the increasingly hot summers New Zealand has been experiencing over the last few years, bitumen sticking to car tyres is becoming a real problem with some extreme solutions being used to alleviate this...

Enviro Shield has been designed to remain non-tacky even in the height of summer. To prove this, Enviro Shield was tested for tackiness and compared to a standard binder (130/150 grade bitumen) used in chip sealing. The tackiness of the binder was tested using a dynamic shear rheometer at three different temperatures (Gorsuch C, CTAA, 2016).  

The product tackiness was first assessed by visual observation, as per Figure 1 below. It shows that, at the same temperature, the Enviro Shield binder is less tacky as it detached more cleanly and easily from the upper plate than the standard chip seal binder.  

Figure 1: Tackiness comparison by detachment from the upper plate, at 25˚C  

In order to detect subtler differences between the samples, the data collected by DSR were used to calculate the normalised area parameter of each sample at each temperature; these results are presented in the Figure 2 below. A smaller area parameter correlates to less tackiness. The samples characterised by an area parameter lower than 1.5, such as the Enviro Shield binder at 25˚C and 35˚C are described as non-tacky. While area parameters higher than 1.5, characterise tacky samples.  

Figure 2: Evaluation of binder tackiness by DSR 

This method enabled us to determine that Enviro Shield has a lower area parameter than Enviro Seal at all temperatures. It highlights that Enviro Shield is overall a less tacky product than standard bitumen. This testing proved that Enviro Shield provides a high quality, low tack finish which will result in no risk of pick up on tyres even quickly after application and at high pavement temperatures. 

Waterproofing

In order to simulate an aged asphalt, where water permeability has become an issue, dense graded asphalt samples (AC14) were prepared in the laboratory with high levels of air voids to the point where they were quite permeable to water (8%). 

The permeability of the base asphalt was quantified and then 1L/m2 of Enviro Shield was applied to the surface and allowed to cure. The permeability was again tested using the equipment presented on the Figure 3 below. 

Figure 3: Asphalt samples on the permeability test. 

The permeability is expressed as the average amount of water able to pass through the asphalt block per 10 seconds, under 80 kPa vacuum. The permeability of the raw asphalt block was 9mL/10sec in average, while it was 0mL after treatment with the Enviro Shield.  

The application of Enviro Shield to permeable dense graded asphalt with air voids at the extreme end of what would be seen in the field fully sealed all ingress to water. Enviro Shield provides additional waterproofing and is, therefore, a suitable pavement restoration technique even when permeability has become an issue. 

Anti-aging protective layer

The Enviro Shield binder was designed to be less susceptible to aging when compared to standard binders.  

The Durability Test Method for Bitumen (NZTA T13:2011) was used to assess the long-term field durability of the binder as the resistance to oxidative hardening in New Zealand conditions. Films of the bitumen are oxidised at 60˚C under 2069 kPa of air for 80 hours. The binder dynamic shear modulus is measured before and after aging the binder, and the percent of change between these two values is calculated. The lower the percent change, the better the resistance to oxidative hardening and therefore the less the binder is susceptible to ageing. 

This test was performed on the Enviro Shield binder and compared to a 60/70 bitumen, a commonly used binder in asphalt; as well as to a commonly used alternative binder for rejuvenation. The results are presented in the Figure 4 below. 

Figure 4: Durability test results

The lower percent change between the binder before and after aging of the Enviro Shield binder compared to the other two reference binders shows that Enviro Shield has a better resistance to aging. The properties of the Enviro Shield binder are less susceptible to change between the mix design time and after its application on site. This proves that the Enviro Shield will perform more consistently on site and protect the road from oxidation for a longer period when compared to other standard binders.

Aesthetic Benefits

Enviro Shield is specifically designed to meet the challenges of residential areas where old residential asphalt and chip seal surfaces are due for renewal but do not qualify for asphalt surfacing; and when chip seal is not a preferred treatment due to residents’ complaints.

A rejuvenation programme using Enviro Shield guarantees to extend the life of the existing pavements by: 

• Slowing the oxidation of the surface
• Binding any loose aggregates 
• Filling cracks 
• Enriching the surface bitumen 
• Improving the water-tightness of the surface

Most importantly, Enviro Shield eliminates complaints from residents during and after construction and provides a significant aesthetic benefit. As can be seen on the Figure 5 below, Enviro Shield also offers a new-look with surfaces presenting a uniform appearance and deep black colour, with no signs of ravelling and no loose chip.

Conclusion

Although asphalt surfacing offers a good looking and long-lasting solution (up to 20 years), it cannot be economically justified due to the low traffic volumes in residential areas. Alternatively, for a quarter of the cost of asphalt, chip sealing is the most affordable solution in the medium term (6 to 10 years) but gives rise to countless complaints from residents during and after application.  

Enviro Shield falls between these two extremes. As a rejuvenation emulsion, it extends the life of the existing pavements by slowing the oxidation and the degradation of the treated surface while improving its water-tightness, thus delaying the onset of more costly damages. And for a fraction of the cost of asphalt, Enviro Shield alleviates the stress of changing from asphalt to chip seal keeping residents and councils in peace. 

Enviro Shield effectiveness has already been successfully proven in Whangarei, Auckland, Wellington and Papamoa Beach and is quickly becoming an undeniable alternative to chip seal. 

Written by Nina Barbier