Road gulleys are exposed to extreme loads, particularly below cast-iron or steel covers. Due to the small distance from road surface and permanently high moisture their base consisting of brick masonry or concrete, as a rule, suffers from repeated freezing and thawing. It is exposed to chemical effect of de-icing salts and acid rains. Moreover, the base is exposed to extreme mechanical loads of mostly dynamic character. The result is the base deterioration and the well-known sinking of gulleys so deplored by the motorists. The physical and chemical causes of deterioration of continuous-porosity materials of the type of unglazed clayware or concrete are clear and incorrigible. The service life of wooden wedges often used for levelling the covers flush with the pavement surface, naturally, is even lower. The only solution of the problem is the placing of the covers on an unabsorbing, high-strength material which, however, must satisfy a number of other criteria: it must be easily workable in any atmospheric conditions, serviceable shortly after manufacture and must enable vertical fixation within a wide range of the order of 10 mm to 100 mm.
The sought solution are the compensation rings of polymer concrete, placed in a polymer mortar bed. Thanks to different thickness within the range of 20 mm to 100 mm stepped at 5 mm they enable any vertical levelling either by the selection of a ring of adequate thickness or by a combination of several rings. The combination of polymer mortar of high tensile strength and high bond with concrete, polymer concrete and metal surfaces ensures a safe transfer of brake forces without complicated rebates, customary with concrete rings. Polymer concrete used for the manufacture of compensation rings has discontinued porosity, i.e. is absorption-resistant and, consequently, inert to freezing and thawing cycle, chemically resistant to both salt and acid solutions and to petroleum products. Its compressive strength attains as many as 100 MPa, as a result of which it is practically unbreakable by road traffic. Last but not least the reinforcement of the rings, significantly increasing the tensile and flexural strength of polymer concrete, consists in glass-fibre strands entirely inert to the effects of moisture or acids which could penetrate to them by diffusion.
The compensation rings are excellently suited for the reconstruction of sunken gulleys, because they enable full traffic resumption in a few hours after the repair. They prove equally well in new construction. In either case they ensure that once installed the gulleys need not be repaired in a short period and their reconstruction or costly maintenance are entirely dispensed with. The application of polymer concrete rings as entirely new components of road construction is spreading continuously thanks to excellent experience with them in a number of Czech towns and cities. Also German firms have shown interest in the import of these products and their technology to replace the systems used in the FRG at present.
For the manufacture of compensation rings a production line of a capacity of some 3 000 standard rings of mean 50 mm per day thickness was designed and installed. Apart from those a simple exchange of moulds enables the manufacture of any other types according to the client´s requirements as well as the rings of other shapes. At present the manufacture includes the rings for various standard sewerage gulleys and water supply manholes of circular and square cross sections, such as dia. 610/790 mm circular rings, dia. 470/790 mm circular reducing rings, dia. 650/790 mm circular sealing rings, dia. 125/235 mm rings for water supply closing valve chambers, dia. 165/290 mm circular rings for hydrant closing valve chambers, elliptical hydrant rings, circular rings for dia. 390/650 mm sewerage gulleys, 560/760 mm and 400/550 mm square components.