He penetration test of bitumen quantifies the hardness or softness of bitumen by measuring the depth of penetration of standard loaded needle in five seconds while maintaining bitumen sample temperature at 25 °C. The more viscous the bitumen, the less distance needle is able to penetrate. Hence, the penetration value for viscous bitumen is less.
For instance, if the needle penetrates between 6 and 7 mm in 5 seconds, we will have 60/70 bitumen.
The penetration is measured to specify the bitumen type in accordance with the climatic conditions of the area of bitumen consumption.
This penetration depth is empirically (albeit only roughly) correlated with asphalt binder performance. Therefore, asphalt binders with high penetration numbers (called “soft”) are used for cold climates (like 85/100) while asphalt binders with low penetration numbers (called “hard”) are used for warm climates (like 40/50).