A paver is a machine by means of which unbound and bound layers or pavements can be produced. As soon as a laid pavement is made available for its intended use, subsequent improvements entail substantial efforts, such as the closure of road sections or parts of buildings. Hence, quality control is of great importance in the field of asphalt laying. A system which allows the quality of asphalt laying or the quality of the laid asphalt to be measured is shown in WO 2004/034351 A2. In particular, it is possible to manually check the properties of the asphalt, and, in so doing, individual measurement processes can be linked with site data.
An important process variable in road construction is, in particular, the processing temperature of the pavements laid, such as asphalt or bitumen. The processing temperature substantially influences the usage properties, such as stability, layer adhesion and service life of the laid pavements. Pavers typically distribute the paving material and carry out precompacting of a surface of the paving material with a screed, which is attached to the rear end of the paver and is drawn thereby. The thus laid pavement is subsequently compacted still further by rollers. Like other factors, such as environmental and weather conditions during laying, the temperature of the material in different stages of the laying process influences the efficiency and the success of a paving job.
Processing e.g., paving material under optimum temperature conditions has long been recognized as important, but such processing often entails manual control measurements on the part of the support and operating staff. Paving material is typically obtained at a comparatively high temperature at an asphalt or bitumen plant. Depending on the distance a supply machine has to travel so as to reach a work site as well as on the traffic and the ambient temperature, the asphalt may cool to a certain extent prior to delivery. In addition, progress of the paving machines and of the compacting machines or rollers may vary. The extent of cooling, once the paving material has finally reached the paving machine or paver, may vary depending on the temperature of the paving material at delivery, environmental factors, etc. In some cases, paving material may segregate within the paving machine, and thus relatively cooler and relatively hotter pockets or accumulations of material within the machine may exist, leading to unexpected, mostly punctual, temperature gradients in the paving material once the latter is distributed on the work surface. In a typical laying process, the paving material is discharged, distributed by the paving machine or paver, and subsequently precompacted by means of the screed, and is then ready to be compacted still further by the various compacting machines. In the course of this process, the material temperature can deviate significantly from an expected temperature. In addition, the material temperature may be non-uniform from one paved region to the next due to changing weather conditions or due to unintended segregation or poor mixing.
Due to the importance of the laying temperature of the pavement in the laying process, measuring of the laying temperature becomes increasingly important and, within the last few years, various solutions have been developed, which satisfy the demand for metrological proof of the laying temperature and thus facilitate also subsequent improvements. Known systems measure for this purpose the laying temperature behind the paver, in particular behind the screed. The systems available range from a pyrometer array to thermal or infrared scanners and also to pivotable pyrometers. These systems are used for obtaining a more or less areal impression of the temperature profile behind the paver.
Other systems for obtaining laying temperature information are based on data obtained from an infrared camera whose image data are arithmetically converted into scanning lines by means of a suitable software. These lines of an ascertained thermal image show the temperature profile at a certain distance from the paver or the trailing edge of the screed. Each line may here represent the temperature profile of the paved layer transversal to the travelling direction of the paver after the individual lines have been combined so as to form a planar image, a so-called temperature map or two-dimensional temperature profile. These images can subsequently be used for assessing the temperature distribution of the laid asphalt. As mentioned above, a temperature distribution having the highest possible uniformity is here a quality characteristic, since this will provide uniform preconditions for subsequent compacting by means of rollers.
WO 00/70150 A1 discloses a temperature monitoring system of the type in question, which is secured in position on a paver and which scans the temperature of the laid pavement line by line. The temperature data obtained are either directly used for controlling the screed or communicated to other machines of the paving train in the laying process.
DE 10 2008 058 481 A1 describes an evaluation of such a temperature profile during the paving process. In particular, the adaptation of a paving process to the individual machines of the paving train is described. Especially on the basis of the obtained temperature information of the laid asphalt, the distance between the compacting rollers and the paving machine or paver is adapted such that the asphalt will not be processed by the subsequent compacting rollers within a temperature range referred to as “tender zone.” To this end, a comparison between predicted temperatures and actually measured temperatures is suggested. If there is a difference between the predicted temperature and the actually measured temperature, the machines are decelerated or accelerated accordingly. For predicting the temperature, a model is used, which makes use of the external weather conditions, such as the ambient temperature. If the model predicts comparatively slow cooling due to high ambient temperatures, the machines can travel at a comparatively lower speed or such that the distance between them is enlarged in a suitable manner.
The temperature measurements of the known methods for monitoring the laying process do, however, not permit any direct conclusions with respect to the actual cooling behavior of the laid pavement. In the case of the known measurement systems making use of pointwise or line-by-line scanning, it is attempted to simulate the cooling behavior by models. The models take into account external factors, such as the weather. Hence, these solutions require additional sensors, e.g., wind gauges or rain gauges, and the user has to manually enter into the system the cloudiness at the time in question. For improving the quality of the measurement results and for simplifying the method, it is therefore desirable to be able to do without such additional sensors and manual inputs.
A direct measurement or determination of the cooling behavior of the laid pavement is, however, not possible by means of the known system. Even systems in which the laying temperature is continuously detected so as to obtain an areal temperature image of the laid pavement, it is impossible to draw conclusions with respect to the cooling behavior or the cooling rate of the laid pavement, since the lines of the thermal image were recorded during a continuous laying process at the same moment in time and at the same distance from the screed.