Cumulative standard axle load; rigid pavement advantages

Cumulative standard axle load

Cumulative Standard Axle load (CSA, in msa) is the total number of repetitions of the standard 80 kN single axle that the pavement carries in its design life. Pavement thickness is read against this value.

Steps:

1. Find the commercial vehicles per day (CVPD, laden weight above 3 tonnes) at the last count, $P$.
2. Grow it to the year of completion: $A=P(1+r)^x$, where $x$ is the number of years from the count to opening.
3. Find the vehicle damage factor $F$ from an axle-load survey (fourth power law).
4. Find the lane distribution factor $D$ (IRC:37).
5. Compute

$$N_s=\frac{365\,[(1+r)^n-1]}{r}\,A\,D\,F$$

with $n$ the design life (15 years for NH/SH, 10 to 20 for other roads) and $r$ the growth rate (if no data are available, 5 percent as per IRC:37-2012; the older IRC:37-2001 used 7.5 percent).

The legal axle load is the maximum axle load permitted by law on public roads (as per DoR / Vehicle and Transport Management rules in Nepal; about 10.2 t for a single axle with dual tyres, in India 10.2 t single and 19 t tandem). The standard axle is the reference 80 kN (8.16 t) single axle to which all axles are converted by the fourth power law (AASHO Road Test). It is important because one common unit allows mixed loads to be added, damage rises about as the fourth power of load (a 10 t axle is about 2.3 times as damaging as 8.16 t), and all design charts (IRC:37, AASHTO, Asphalt Institute) use it.

Advantages of rigid pavement over flexible pavement

1. Longer life - 30 to 40 years against 10 to 15 years, so lower life-cycle cost.
2. Low maintenance - no rutting, potholes or bleeding; only joint resealing is needed, so maintenance cost and traffic disruption are lower.
3. Distributes load widely - the slab spreads the wheel load, so weak subgrades (low CBR) and moisture are tolerated better and fewer layers are needed.
4. Better performance in wet conditions - not damaged by standing water, oil or fuel spillage; suitable for waterlogged and flood-prone areas, and for bus stops and junctions where braking and slow loads harm bituminous surfaces.
5. No effect from temperature softening - does not deform under hot weather and heavy trucks.
6. Good visibility at night - the light surface reflects light and reduces street-lighting requirement.
7. Local materials - uses cement, sand and aggregate; in Nepal cement is produced in the country while bitumen is imported, so there is saving of foreign exchange and local employment.
8. Durability against overloading (with proper design) and resistance to ageing; the surface does not wear as quickly.