Paving, roofing, and industrial uses are the three key markets for bitumen materials. 10-15% of the market share for asphalts and bitumens is consumed by roofing materials. Residential steep-slope shingles and commercial modified waterproof membranes comprise the roofing segment of the bitumen market. This article discusses the household and industrial applications of bitumen roofing.
The roof of a house that is insulated with bituminous products

The bitumen industry has three primary areas for its products:

  • paving
  • roofing
  • industrial applications.

Roofing products consume approximately 10-15% of the market share of asphalts and bitumens. The roofing sector of the bitumen market is divided between residential steep-slope shingles and commercial modified waterproof membranes. This article addresses bitumen roofing applications in both residential and industrial construction.

Roofing asphalt flux manufacture

Roofing flux is a vacuum residue produced during the distillation of crude oil. It is called roofing flux because it is used to produce harder grades of Oxidized Bitumen (via a bitumen blowing unit) for use in roofing products. Over 2000 different crudes are produced worldwide, and they are all different in terms of both their physical and chemical properties. The most critical aspect of roofing flux is the source of crude oil; less than 20% of crudes make acceptable material. The main sources are Middle East and North and South American crudes.

Atmospheric distillation

Crude oil is a very complex mixture of hydrocarbons differing in molecular weights and boiling ranges. Manufacturing of residue involves several steps of physical separation and chemical treatment. The first step is fractional atmospheric distillation, where the crude is heated to temperatures in the range 350-380C. The column bottom stream is called atmospheric or ‘long’ residue. It requires further vacuum flashing before it can be used as roofing flux. The process is illustrated in Figure 2.1.

Vacuum flashing

The atmospheric residue needs further distillation at reduced pressure in a high vacuum unit. Typical process conditions are 10-100 mmHg pressure with a temperature in the range 350-425C. The vacuum column removes the non-condensable gas that enters the vacuum unit with the feed. The process is illustrated in Figure 2.2.

To prevent cracking of the heavy components, the residence time at temperatures above 350C has to be as short as possible. The bottom stream obtained by vacuum distillation of the long residue is called the vacuum or ‘short’ residue (also known as vacuum tower bottom (VTB)). This VTB residue can be used as roofing flux if the crude source is suitable.

Roofing products

The two general categories of roofing products are steep slope and low slope. Steep slope roofings are coverings installed on slopes exceeding 148, and mostly occur on residential properties. Low slope roofing includes waterproofing systems installed on slopes that are 148 or less. This type of roofing is usually used for commercial buildings. Steep slope roofing is the dominant product, representing over 75% of the total roofing market. Data on the usage of the available types are shown in Table 24.1 (ARMA et al., 2011). As can be seen from Table 24.1, in the USA, most steep slope roofing consists of shingles, an example of which is shown in Figure 24.1.
Note that built-up systems are those where several layers of roof felt are laminated together with bitumen. Bitumen 60/70 is utilized for roofing repairs.

  • Asphalt shingles are composed of the following components
  • fibreglass felt
  • coating asphalt (to provide weather resistance and adhesion)
  • mineral filler
  • mineral granules.
North American and European bitumen roofing production by roof type 2006
Shingles on a roof of the house

Related Article: Bituminous Membranes

Roofing coating specification

The American Society for Testing and Materials (ASTM) produces standard specifications for coating bitumens used in the roofing market in ASTM D312-00 (ASTM, 2000) (Table 24.2).

Roofing shingles specification

Standard specifications for asphalt shingles made from glass felt, and surfaced with mineral granules are given in ASTM D3462/D3462M-10a (ASTM, 2010d). The document requires that several properties are met by

Physical requirements for bitumen in roofing ASTM D312 00 ASTM 2000
Standard specification for roofing shingles ASTM D3462 D3462M 10a ASTM 2010d

bitumen shingles, as shown in Table 24.3. As can be seen in Table 24.3, two principal properties – penetration and softening point – are required that relate solely to the coating bitumen used in the shingle manufacture. These two properties provide some indication of the stiffness of the bitumen at intermediate and high temperatures. 

Roofing flux specification

There is no standard specification for roofing flux, either from the ASTM or from the industry itself. Thus, flux specifications vary between producers, manufacturers, and suppliers. Roofing industry companies have different requirements for flux quality depending on their specific needs.

The most critical criteria for roofing flux, as related to roofing manufacture, are the properties of the crude oil used in making the roofing flux, as the end product is the oxidized coating material resulting from the air blowing process. The chemistry of the crude oil contributes to the resulting performance. Even if a flux meets the required physical properties, it does not follow that it has the required coating properties. Therefore, in most cases, a new flux has to pass an approval process before it can be used in the manufacture of roofing products.
As there is no well established relationship between the flux and the coating performance, the flux requirement (not a specification) can only come from a pre-approved crude oil resource or from a pre-approved supply. Assay of the crude oil does not indicate whether it will result in a good roofing flux or a


Physical properties of four bitumens typically used in North American roofing

good shingle coating. Roofing manufacturers need good oxidized coating performance rather than good flux properties.

However, there are some basic requirements that can be used for flux: for example, for paving grade bitumens the flash point (Cleveland open cup (COC) method) needs to be a minimum of 550F (250C)); and, for reasons of safety, some roofing manufacturers require the closed cup flash point (Pensky Martens closed cup (PMCC) test) to be a minimum of 450F (232C). For a roofing flux made from a pre-approved crude oil, the viscosity range can be used to specify the properties. This can be the absolute viscosity at 60C, or the kinematic viscosity at 135C. The stain index is also critical, because it represents the volatile oil content and relates to the discolouration performance of the coating. The requirement for the stain index varies between different manufacturers (Tables 24.4 and 24.5).


Physical properties of four bitumens typically used in North American roofing

Product approval

Shingle manufacturers provide guarantees on their products with periods ranging from 15 years’ to a lifetime warranty. The durability of the coating (filled and unfilled) is critical. The accelerated xenon arc weathering test (ASTM, 2011) is the traditional method used to evaluate the long term ageing performance. The coating panel must show less than 10% of failure after a 2000 h cycle of exposure. The time needed to pass the weathering test is usually 3-4 months.

The coating material also needs to meet certain softening point/penetration relationship criteria, without the inclusion of any additives. Once the coating has passed the testing phases, a plant trial is carried out to evaluate the material’s performance in the process. A trial is typically one to three truck loads (approximately 25-75 tonnes). Following a trial, it usually takes around a month to complete the product evaluation.

Air blowing of roofing flux

Oxidation requires a complex series of reaction processes in order to produce the bitumen coating product from a roofing flux. The blowing process can be carried out in batch or continuous mode. The batch production mode is the one most commonly used in the USA. The reaction process introduces oxygen into the system; the temperature is typically in the range 480-500F (218-260C). A series of chemical thermal reactions takes place during production. There are mainly two types of reaction

  • dehydrogenation, which introduces oxygen to hydrogen at the molecular level
  • polymerisation, which converts most aromatic compounds to a condensed asphaltene structure.

It has been found that all the oxygen taken up by the bitumen can be accounted for by the formation of hydroxyl, carbonyl, acid and ester groups (ARMA et al., 2011).

The air blowing time depends on the properties of the flux, and the final oxidised product requirements. As a general rule, the softer the flux is at the start, the longer it will take to achieve the final product. This is illustrated in Figure 24.2: the higher the softening point to be achieved, the longer the time that is needed.
In terms of its chemical composition, the flux changes during the oxidation process, the proportion of saturates tends to remain the same, some aromatics convert to resins, some resins convert to asphaltenes and the asphaltenes themselves remain unchanged. The net result is that there is an increase in asphaltene content and a dramatic drop in aromatic content (Table 24.6).

Softening point oxidation time chart

After air blowing (Figure 2.4), the end product (typically called ‘bitumen coating’ in Europe and ‘asphalt coating’ in the USA) exhibits decreased penetration, an increased softening point, higher viscosity, improved weathering resistance and good temperature susceptibility.

Asphalt low slope roofing products

Low slope roofs have slopes of 148 or less. Most low slope roofs are used in commercial or industrial buildings. Bitumen is used in two different low slope roof systems: polymer modified bitumen membranes and built-up roofing (BUR) products. In the USA, these two systems account for 35-40% of the low slope market. The total share of low slope bitumen systems is even higher in western Europe, where polymer modified bitumen systems dominate the low slope bitumen market (Figure 24.3).

Polymer modified membrane systems

Polymer modified bitumen membranes are often used in commercial roofing, including low slope roofing applications. The most common base bitumens are European paving grades 160/220 and 100/150 pen, depending on the market and customer requirements.


Typical changes in SARA between the flux and the coating
North American European bitumen roofing production by application

Polymers typically used in this application are atactic polypropylene (APP) and styrene-butadiene-styrene (SBS). Different polymers require different bases. In most instances, a base with the highest saturate content would be suitable with an APP, whereas a base with a higher aromatic content would be appropriate for use with an SBS application.
An APP modified bitumen membrane felt typically contains

  • bitumen 52-63%
  • APP 15-25%
  • filler 20-30%,

while an SBS modified membrane felt typically contains

  • bitumen 60-70%
  • SBS 5-15%
  • filler 20-35%.

Specifications for APP modified bituminous sheet materials are given in ASTM D6223-02 (ASTM, 2009e), and specifications for SBS modified bituminous sheet materials using glass fibre reinforcement can be found in D6163-00 (ASTM, 2008) (Table 24.7).

Future trends for roofing bitumen shingles and coating evaluation

Although rheological testing of paving grade bitumens has become much more widespread over the last two decades as a result of the Strategic Highway Research Program (SHRP), the conventional empirical tests (e.g. penetration, ring and ball (R&B) softening point and viscosity) remain the


Polymer modified membrane roofing felt requirements

dominant tests and requirements for oxidised coating and BUR products in the bitumen roofing industry in North America. The rapid development of the dynamic shear rheometer (DSR), including the 4 mm parallel plate measurement for low temperature testing and the master curve application for paving grade bitumen, and the belief that the results can be linked to bitumen performance, has created an interest in employing these techniques for use in the roofing industry.

Due to proprietary issues in the roofing industry, some individual roofing companies have undertaken research based on rheological properties using rotational rheology tests. This is an attempt to link the rheological properties of the bitumen to the performance of coatings and shingles, and characterisation of roofing fluxes. However, little has been done to produce a unified or standard specification for bitumen fluxes or coatings. For the last 10 years, the Asphalt Institute has, in cooperation with the roofing industry, been very actively involved in research based on rheological properties (Asphalt Institute, 2014). The purpose of this research is to explore the possibility of developing a new standard based on the use of rheological parameters as a measure of oxidised bitumen coating, which could be linked to shingle performance with a view to replacing the conventional empirical testing. For example, a useful bitumen relationship has been developed based on the complex modulus (G∗) and penetration.

log(GT=25C,f=0.4Hz) = 2.923 − 1.9 log(penetration)


Relationship between G∗2.5 and penetration at 258C

The current consensus is to use the rotational rheology test to characterise the oxidised bitumen coating material at intermediate and higher temperatures

  • DSR at 25C, 8 mm plate, 1% strain at a frequency of 2.5 rad/s
  • DSR temperature sweep tests from 90C to 110C in 10C increments. 

Similar studies of predicting ageing behaviour using rheological testing have been carried out on paving grade binders (Farrar et al., 2013; Mike, 2013; Qin et al., 2014), and these suggest that a master curve can be generated on the basis of the temperature sweep test. The rheological parameter (G∗2.5 – the complex modulus multiplied by 2.5) was chosen to create a correlation with conventional physical properties such as penetration and softening point (Mike, 2013) (Table 24.8). The data suggest that there is a linear relationship in the penetration range 13-25 with a variability of 13.5%. Figure 24.4 shows the relationship between G∗ (complex modulus) and the softening point for oxidised coating materials.

The relationship from all data is

  • log G∗2.5 = −1.6281∗log(pen) + 8.6582

Penetration = 12 dmm


Relationship between the rheological parameter G∗2.5 Pa


  • G∗2.5 = 7.96E6 Pa Penetration = 25 dmm
  • G∗2.5 = 2.41E6 Pa Proposed specification values: 2.50E6 ≤ G∗2.5(25C) ≤ 8.00E6
  • Testing variability for T 315 (PAV DSR) Single operator d2s% = 13.8%.

The proposed use of the rheological parameter (G∗2.5) criterion to replace the softening point and penetration tests is as follows.

  • Proposed procedure to replace softening point

– perform a temperature sweep using a DSR
– 25 mm parallel plate, 10% strain, 1 rad/s
– start at 90C, increase to 110C in 10C increments
– plot h∗(1 rad/s) versus temperature on a semi-logarithmic graph
– calculate Tc when h∗(1 rad/s) = 1200 Pa.s.

  • Proposed procedure to replace penetration at 25C

– perform single point temperature test using a DSR
– 8 mm parallel plate, 1% strain, 2.5 rad/s, 25C
– determine G∗2.5 and compare with the recommended specification value.

Work is underway to evaluate the proposed criteria further.

Reference : The Shell Bitumen Handbook, compiled by the research team of PetroNaft Co.


Leave a Reply

Your email address will not be published. Required fields are marked *