Question

Design of Rigid pavement for new construction - USING AASHTO METHOD

Answer 1

Design procedure

The design procedure for rigid pavement by AASHTO involves solving an empirical equation of similar behavior with that of the one used for flexible pavement. Here the thickness of the concrete slab is the only value that is obtained at the end of the iteration. The empirical equation has the following form:

Where the variables are as follows:

W18 = predicted number of 18 kip traffic load application (ESAL)

So = combined standard error of traffic prediction

ΔPSI = serviceability change during the design period

pt = final serviceability of pavement

Sc = modulus of rupture of PCC

Cd = drainage coefficient

J = load transfer coefficient

Ec = elastic modulus of PCC

K = modulus of sub grade reaction

D = slab thickness

Final Serviceability Index (pt): It refers to the final serviceability index when the pavement is thought to have finished its service period. Typical values range from 1.5 to 3 depending on the use of the road.

Modulus of Rupture of PCC (Sc): Modulus of Rupture of concrete is a measure of the flexural strength of the concrete as determined by breaking concrete beam specimens. The value of Modulus of Rupture is obtained from flexural test results.

Drainage Coefficient (Cd): Drainage coefficient characterizes the quality of drainage of sub base layers under the concrete slab. With a good drainage provision water cannot saturate in the under lying layers; thus pumping is not likely to occur. Generally quick-draining layers that almost never become saturated have a drainage coefficient as high as 1.2 while slow-draining layers that become saturated quite often are given a drainage coefficient as low as 0.8. Table 4.2 shows the recommended values of drainage coefficient according to AASHTO, 1993.

Load Transfer Coefficient (J): The load transfer coefficient is used to indicate the effect of dowels, tied shoulders, tied curbs and reinforcing steel in reducing the stress on the pavement structure. The use of load transfer devices and tied shoulders increase the amount of load transferred and decrease the load transfer coefficient. The lower this value is the better the load transfer. Table 4.1 shows the recommended load transfer coefficients for various pavement types and design conditions.

Elastic Modulus of PCC Modulus of elasticity is a measure of how well a material returns to its original shape and size. It is defined as the ratio of stress to strain when it is loaded by a certain load. In other words it can also be expressed as the slop of stress-strain curve within the elastic range of a material under a constant loading. For such high stiffness materials such as PCC, AASHTO recommends that elastic modulus should be measured according to the procedure described in ASTM C 469 (AASHTO, 1993). The elastic modulus of PCC may also be determined using correlation developed by the state’s department of transportation or by some other reputable agency. The following is a correlation recommended by the American Concrete Institute for normal weight Portland cement concrete.

  
Where,

Ec = PCC elastic modulus (in psi)

fc’ = PCC compressive strength (in psi)

Effective modulus of Sub-grade Reaction: The AASHTO guide allows the designer to take in to account all the layers to be placed under the concrete slab in the rigid pavement design. It also allows designers to consider the effect of loss of support of the underlying materials due to erosion or deterioration. This is done by determining effective modulus of sub-grade reaction (Keff). Like the effective roadbed soil resilient modulus for flexible pavement design, an effective modulus of sub-grade reaction (Keff) is developed for rigid pavement design. The method presented in AASHTO (1993), takes in to consideration several factors besides the road bed soil resilient modulus. These include the type of sub base, the thickness of the sub base, loss of support and the depth to rigid foundation. The first step is to identify the combination of the above factors to be considered. Once all factors are identified the next step will be identifying the seasonal roadbed soil resilient modulus values. The second step is assigning elastic modulus for the sub base. The thickness for the sub base is a pre determined value. By using a chart presented in AASHTO (1993), it is then the final step which is determining the composite modulus of sub-grade reaction (Keff). This process assumes a semi-infinite sub-grade depth (i.e. depth to bed rock greater than 10 feet). If the depth to a rigid foundation is less than 10 feet, again the Keff value is should be modified. This is done by using a chart presented in AASHTO (1993), for correction by loss of support. In case the design is to be performed without the presence of sub base, the composite modulus of sub grade reaction is defined the following theoretical relationship.

Where MR is the elastic modulus of the roadbed soil and Keff is the effective modulus of sub grade reaction.

After all these adjustments are done, the Keff value is used in the main design equation to determine the required slab thickness. Finally the Keff value should be adjusted to account for the potential loss of support arising from sub base erosion. A chart presented in the AASHTO (1993), is used for this adjustment. It is beyond the scope of this work to deal with the above process of computing the Keff. Therefore, the user of this program is required to determine the final value of Keff and input it in the program