Development of the technological concept for roadbases made of hydraulic bound granular materials in the Federal Republic of Germany

A number of research projects relating to the cementbound sub-base (CBSB) are currently in progress in the Federal Republic of Germany. Their object is to develop an optimized technological approach with a view to further improvement and even greater economy in the use of this well proven form of construction. This approach comprises: (I) The use both of natural mineral aggregates with different maximum particle size and of suitable waste materials and recycled materials. (2) Durable resistance to climatic influences. especially frost action. (3) Optimizing the strength and deformation behaviour of CBSB mixes under service conditions in terms of mix compositions. (4) Avoiding the occurence of wide open cracks in the CBSB and of reflected cracks in the overlying courses. (5) The long-term loadbearing behaviour of road structures with CBSBs in actual practice. Research on the resistance of the CBSB to climatic influences is being carried out by Springenschmid and Paulmann. Springenschmid is concerned with initial tests on the cracking behaviour of the CBSBs subjected to deformation restraint. The long-term behaviour of these sub-bases in practice is being studied by Eisenmann. Comprehensive laboratory lnvestigations into the strength and deformation behaviour of the CBSB have been undertaken at the Research Institute of the Cement Industry with the object of optimizing the composi cion of the CBsa mixes and the loadbearing behaviour of the sub-bases. The investigations show that CBSBs with the compressivestrengths specified in the Federal Republic will as a rule also have sufficient flexural strength to withstand the service stress conditions and will have appropriate deformation behaviour if such compressive strength is achieved by a sufficiently high cement content (duly suited to the nature and composition of the aggregate mixture in any particular case) - not mainly by other means of increasing the strength. e. g •• lower water content or a higher degree of compaction. If the latter situation exists, it may occur that, despite adequate compressive strength. the long-term loadbe~ring behaviour is impaired by large permanent deformations of the sub-base, e.g., as a result of particle redistribution and subsequent consolidation ( under traffic). The results of the research have been taken into account in the new "ZTVT" code for cement-bound sub-bases. Complementing the above research, the Research Institute is now investigating the question as to what materials may be used and structural measures applied in order to control and favourably modify the cracking already in the young CBSB.