Penetration index

The penetration index (I ) indicates the temperature susceptibility of the paving grade bitumens, for grades 20130 to 160/220. The penetration index is calculated from the values of penetration, at 25°C, and the softening point determined. 

The penetration index (7p), according to CEN EN 12591 (2009), Annex A, is calculated from the following equation  

The above equation, developed by Pfeiffer and Van Doormaal (1936), is based on the following hypotheses  

The limiting theoretical values of the penetration index are -10 for bitumens with very high susceptibility to temperature variations, up to +20 for bitumens almost independent of temperature variations. In practice, penetration index varies between -3 and +7 for paving grade bitumens and oxidised bitumens. The smaller the penetration index, the more sensitive the bitumen is to temperature variations. 

It is noted that penetration index value equal to zero (/p = 0) is attributed to a bitumen with a penetration of 200 dmm at 25°C and a softening point of 40 C. 

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This equation is based on the approximation that the penetration is 800 at the softening point, but the approximation fails appreciably when a complex flow is present (80,81). However, the penetration index has been, and continues to be, used for the general characteristics of asphalt for example asphalts with a P/less than —2 are considered to be the pitch type, from —2 to +2, the sol type, and above +2, the gel or blown type (2). Other empirical relations that have been used to express the rheological-temperature relation are fluidity factor a Furol viscosity P, at 135°C and penetration P, at 25°C, relation of (H—P)P/100 and penetration viscosity number PVN again relating the penetration at 25°C and kinematic viscosity at 135 °C (82,83). 

Phipps, P.R. Gonda, I. Bailey, D.L. Borham, P. Bautovich, G. Anderson, S.D. Comparisons of planar and tomographic gamma scintigraphy to measure the penetration index of inhaled aerosols. Am. Rev. Respir. Dis. 1989, 139, 1516-1523. 

The effect of altering the penetration index is poorly understood. 

Perhaps more importantly, it is freqnently unclear what the significance of a good penetration index is. For a drug like pentamidine, good peripheral deposition is obviously desirable, as its site of action is the alveoli. For drugs such as steroids and antibiotics in cystic fibrosis, their site of action is likely to be the... 

The modification of bitumen with SBS copolymer powder was done using the mechanochemically devuicanized GRT (Zhu et al., 2009). The penetration index, softening point, 5°C ductility, aging behavior, and rheological properties of bitumens modified by 8,10, and 12 wt.% GRT/SBS mixtures were measured. In comparison with the bitumen modified by incorporation of 5.5 wt.% SBS alone, the majority of properties of the blends were improved, except their penetration. Rheological properties indicated that at high temperatures 10 wt.% GRT/SBS-modified bitumen was better than SBS-modifled bitumen. The SEM observation of the fractured surfaces showed that bitumens mixed with the GRT/SBS powder had a better interfacial adhesion with matrix than with SBS alone. 

Paving grade bitumens are characterised by their consistency at intermediate (determined by penetration test), consistency at elevated service temperatures (determined by softening point or viscosity test) and durability (determined by resistance to hardening test). Their brittleness at low service temperature (determined by the Fraass breaking test) and temperature dependence of consistency (determined by the penetration index) may also need to be determined to meet regional requirements for specific conditions such as extreme cold or wide ambient temperature variations. Flash point is also determined as well as, optionally, the density. 

When Severity 2 is selected, it shaii be associated with the requirement for Fraass breaking point or penetration index or both measured on the unaged binder (seeTable 3.3). 

Reference to normative AnnexA of CEN EN I2S9I (2009) dealing with the calculation of penetration index. 

During fractional distillations (atmospheric or with vacuum), the lightest volatile ingredients of the bitumen are removed, which results in an increase in asphaltene concentration. Asphaltene concentration also increases during the oxidation (air-blowing) process. This fact makes the bitumen harder and less susceptible to temperature variations (increase of the penetration index value). 

By adding an elastomer to the bitumen, the penetration value decreases, the softening point increases and the penetration index increases. These changes indicate that the bitumen becomes harder and less susceptible to temperature variations. 

Multigrade bitumen is a special binder that, for the same level of penetration as conventional bitumen, is not as susceptible to temperature changes. This means that it is more consistent at high temperatures and less fragile at low temperatures. In fact, multigrade bitumens have higher penetration index values than conventional bitumen. 

The Heukelom chart also provides the facility to estimate the penetration index (PI) of paving grade bitumen. This is achieved by drawing a parallel line form the focal point, located on the chart, to the straight line obtained when plotting penetration and softening point. The penetration index is read from the intersection of the parallel line with the PI scale positioned on the chart. 

The stiffness modulus is also affected when the grade of bitumen and its penetration index change. This has been demonstrated in Shell Bitumen (1990, 2003). 

Lefebvre J.A. 1970. A modified penetration index for Canadian asphalt. Proceedings of the Association of Asphalt Paving Technologists, Vol. 39, p. 443. 

Some laboratories and organisations have developed various methods/techniques (equations or nomograms) from which the fatigue performance of asphalt can be predicted more easily with reasonable accuracy. The predictive methods are based on asphalt stiffness, bitumen properties (penetration index, volume of bitumen or softening point) or air voids. 

A method for predicting the fatigue life of asphalts by the use of a nomograph has been developed by Bonnaure et al. (1980). The required data are percentage volume of bitumen, V, penetration index of the bitumen (PI), stiffness modulus of the mixture, d initial strain... 

The in situ determination of the strength of the compacted unbound layer is carried out by the Dynamic Cone Penetrometer (DCP) or by the plate bearing test. The DCP test is gaining grounds because of its simplicity in use. It must be reminded that the penetration index, determined by DCP, or k value, determined by the plate bearing test, can be correlated to the CBR value (see Sections 1.6.3 and 1.7). 

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