Base course modification through stabilization using cement and bitumen

American Journal of Applied Sciences, Jan, 2009 by Marandi S.M., Safapour P.

To determine the modulus of elasticity and to control the quality of the sections performed, a FWD machine was used. The structural condition of a multi-layered traffic lane during utilization can be determined by the deflections (deformations) caused by heavy traffic. With these known deflections under a controlled load and with the assumption of a suitable model, it is possible to back-calculate the material-constants and the stress and strains in a multi-layered pavement structure. They are basis for further design calculations.

The applied dynamic impulse load will cause the biggest deflection at the center of load and corresponding lower deflection values at distances away from the center of load. Connecting these single deflection values graphically a so-called deflection basin is determined. If the resulting stresses and strains resulting from a controlled load in a single structural layer are compared with the allowable material constants for materials used in connection with allowable load-repetitions, the structural condition of a multi-layer pavement can be evaluated and the service live can be estimated.

To enable the back-calculation of the elastic modulus of the single (individual) layers of a pavement, the thickness of this layer must be known (e.g., determined by geo-radar or by means of extracting cores drilled) as well as the deflection-basin.

The use of FWD machine enables the operator to determine the deflection-basin caused by a controlled load. The load application occurs with a defined mass, dropped from a defined height on a spring system with a base plate located on the surface of the pavement. The falling mass (weight) produces a dynamic impulse load with load duration of 25 mille second into the pavement. The loading time simulates a moving wheel load (vehicle) with a speed of 50 km [h.sup.-1]. Depending on the falling weight; a maximum load impulse of 250 kN can be applied. The selected size of the load impulse for the measurement depends on the actual loads on the pavement caused by the traffic. If in fact shorter load times occur, they can be taken into consideration during evaluation.

The measured deflections are basis for the calculation of the E-Modules of the individual layers of the structure. Based on the deflection values, homogeneous sections are formed depending on the construction.

For the evaluation the initial assumption is made, that for each homogeneous section of the construction the mean and Standard deviation are the characteristic values.

The back-calculation of the E-Modules is done with special software e.g., PAVERS or PADS. The basis for calculation of flexible multi-layered pavements, a linear-elastic, homogeneous and isotropic material behavior is assumed. Based on a first assumption of E-Modules, the deformation for a load similar to the one applied during load bearing capacity measurements for the structure is calculated and it is varied for so long until it agrees with the actual measured values. The approximation to the actual values is done by means of iteration starting from the lowest laying layer up to the top layer and from the view of the situation of the geophones from the remotest to the nearest, meaning from the sub-grade up to the unbound layers and then to the top bound layers of the structure.