Experience with modified binder single seals over twenty years has shown that considerably higher binder application rates can be tolerated in modified binder seals than in seals with conventional bituminous binders.
The previous version of the TRH3 (1998) suggested an approach towards the design of modified binder application rates, assuming less embedment of aggregate and an increase in voids due to less orientation of the aggregate. Based on specific studies, the ring and ball softening point and equivalent traffic volumes were used to quantify the increase in binder application rate. The results compared well with appropriate application rates determined from field experiments and ten years experience using 13,2mm and 9,5mm aggregates and mainly non-homogenous binders.
Where the non-homogenous modified binders were favoured from 1985 to 1995, the use of homogenous modified binders has increased dramatically over the past decade. Experience gained through design, application and performance monitoring resulted in the following important conclusions regarding the TRH3 (1998) modified binder designs namely:
• Design application rates for non-homogenous modified binder single seals are still appropriate. • Design application rates for homogenous modified binder single seals are too low (in the order of 10 -
15%).
• Design application rates for homogenous modified binder double seals are too high (in the order of
10 %).
Due to insufficient experience at that stage, no guidelines were provided regarding the design of seals making use of a combination of conventional and modified binders.
The purpose of this section is to address the mentioned shortcomings and to ensure more appropriate designs.
7.7.1.1 Single seals with modified binders
The majority of practitioners in South Africa are of the opinion that the appropriate application rate for a particular modified binder is a function of the appropriate conventional binder application rate and specific properties of the selected modified binder.
In addition to all factors incorporated in the conventional design application rate, experience in various parts of the country indicates that the shape of the aggregate, the spread rate, rolling type and effort as well as temperatures within a few days after construction could drastically impact on the initial performance of modified binder seals.
It is acknowledged that much research work is still required to perfect the theory of application rate design, taking into account all factors influencing initial and long term performance.
Therefore, the decision has been made, for purposes of this version of TRH3, to document best practice from experience without substantiating the methodology in terms of current theory.
In order to utilise the rational application rate design curves for conventional binders a single function is applied to increase the calculated application rates for each of the hot homogenous and non-homogenous modified binders. These functions are based on experience and opinions of a panel of current practitioners.
7.7.1.2 Conversion factors for hot modified binders
a) Non-homogenous modified binder (Bitumen Rubber)
Typical conversions from net cold conventional binder to net cold non-homogenous modified binder vary from 1.4 to 2.0 with a typical value applicable to 8,0 mm ALD aggregate of between 1.5 and 1.9 times the application rate appropriate for a conventional binder. Some practitioners hold the opinion that the lower conversion factor should apply for high heavy vehicle volumes.
This relates to approximately 1.65 and 2.1 times the minimum net cold conventional binder to obtain the appropriate hot application rate for Bitumen Rubber (Non-homogenous binder).
The absolute minimum practical application rate is 1.8 ℓ/m2 for the summer grade products and 1.6 ℓ/m2 for winter grade products (Suppliers generally recommend a minimum of 2.0 ℓ/m2)
Although non-homogenous binders have been applied at rates of more than 5 ℓ/m2 for 13.2 mm seals, the general consensus is than a 3.2 ℓ/m2 maximum hot application rate is sufficient, even for 19.0 mm seals to perform well.
Notes:
Due to the sensitivity of fattiness during the first hot season, the use of winter grade products are not recommended by several road authorities.
Due to the minimum application rate of 1.8 ℓ/m2, small aggregate e.g. 6,7 mm cannot be applied in a single layer without bleeding. The smallest aggregate recommended is 9.5 mm.
Aggregate used with non-homogenous modified binders are normally precoated to improve adhesion. Based on the experience of practitioners and in order to utilise the rational design procedure for single seals with conventional binders the following procedure is recommended:
• Calculate the minimum net cold application rate for a conventional binder
• Adjust the application rate by multiplying the net cold conventional application rate by the relevant
adjustment factor in Table 7-8
• Convert to hot application rate (Typical adjustment = 1.07 x net cold binder) • Allow for allowable tolerance (i.e. add 5% to minimum application rate) • Limit the maximum application rate to 3.2 ℓ/m2, regardless of traffic volumes
• Adjust to the minimum application rate (1.8 – 2.0 ℓ/m2 ) dependent on supplier, but take into account the risk of bleeding or that the surfacing life might be reduced due to insufficient texture depth, or
• Specify a larger aggregate size that would result in application rates higher than the practical minimum
b) Homogenous hot modified binders (Polymer modified)
Initial experiments with 13.2 mm single seals in the Northern parts of South Africa applying polymer modified binders at application rates between 1.2 ℓ/m2 and 1.8 ℓ/m2 indicated a low sensitivity to variation. The best results on medium to fine textured roads were obtained with application rates varying between 1.4 ℓ/m2 and 1.6 ℓ/m2.
Since then numerous roads have been resealed using both SBR and SBS polymer modified binders, mainly with precoated aggregate. Feedback from practitioners indicated that the application rates as calculated through the suggested process in TRH3 (1998) are too low, resulting in several cases of early aggregate loss.
Through back calculation of documented guidelines and rationalization of several opinions, conversion factors from conventional to homogenous modified binder of between 1.1 and 1.4 as recommended in Table 7-6 and Table 7-7 provided more appropriate design application rates.
The simplified process of calculating the design application rate for a modified binder single seal is as follows:
- Calculate the minimum net cold application rate for a conventional binder
- Adjust the application rate by multiplying the net cold conventional application rate by the relevant adjustment factor in Table 7-6 or Table 7-7
- Convert to hot application rate (Typical adjustment = 1.08 x net cold binder) - Allow for tolerance (i.e. add 5% to minimum application rate)
- Limit the maximum application rate to 2.6 ℓ/m2
- Should the calculated hot application rate for a 13,2 mm single seal be less than 1.3 ℓ/m2 adjust to 1.3 ℓ/m2
Note:
Application rates obtained through the multiplication factor are applicable only if the rational design method is applied.
T a b l e 7 - 6 A d j u s t m e n t f o r S - E 1 b i n d e r a p p l i c a t i o n