Assessment of the evenness of cement concrete pavements and study of the means to achieve high quality evenness

Pavement evenness is one of the characteristics that are most directly felt by the road user. It determines not only driving comfort and safety for obvious reasons, but also the dependability and operation costs of vehicles. Evenness is measured using longitudinal profile surveys produced by the A.P.L. (longitudinal profile analyzer). This apparatus continuously measures the variation in level of each scanning wheel of two measuring trailers towed in the normal wheel tracks of traffic by a vehicle travelling at constant speed. Using a scanning speed of 6 mIs, the A.P.L. can detect irregularities ranging from.:! 0.3 mmto i 50 mmand extending over 0.3. to 15 m. An evenness coefficient is calculated from the mean surface area of the deformation with respect to a line given by smoothing the measured longitudinal profile. The basic lengths chosen for this smoothing process in the case of a measuring speed of 6 m/s are 15 m and 2.5 m. Thus the method makes it possible to discriminate the effects of long-range and short-range deformations and consequently to characterize the detected irregularities. The parameters that play apart in achieving good evenness are connected with the mixing operations in the concrete plant and the laying of the concrete on the building site. Control on the mixing process consists of continuously recording the power consumed by the motor of the mixer, which characterizes the consistency of the fresh concrete, and of weighing concrete ingredients in order to rapidly detect and remedy any defect in the operation of the proportioning devices. As for the laying of the concrete on the building site, the methodology that has been developed consists in observing the following parameters: - the evenness of the underlying layer and especially of the running paths for the crawler belts of the slipform paver - parameters connected with the adjustment of the guide' wires and the various other parts of the paerer , the possible influence of which must be demonstrated; - parameters connected with the organisation of the work, e. g. concrete supply tempo and paver progression speed. The method has been used on several sites and particularlyon a section of the N 4 state road in Namur where strengthening works have been carried out. The following conclusions can be drawn from this survey: - the controls performed in the concrete mixing plants have generally led to the production of concrete with a more or less homogeneous consistency. This factor has undoubtedly contributed to the achievement of the high quality surface evenness observed on this site; - in general, the processing of the results of evenness measurements by spectral analysis points out that the evenness of the concrete deteriorates with respect to the evenness measured on the running paths when irregularities of shorter range are considered ; - the evenness measurements have made it possible to correct certain defects in the adjustment of the guide wires and to observe the performance of the paver's controlling devices; - the recordings of the parameters connected with the laying of the concrete have made it possible to define the optimum progression speed of the paving machine and to assess the influence of concrete supply conditions on the evenness of the pavement. These findings should be refined in the future with a view to establishing organisation rules for the execution of concreting works under various site conditions. In this respect, recent Belgian specifications require contractual acceptance of evenness based on the measurements performed with the longitudinal profile analyzer.