Jiri Grosek

The short material service life of concrete is currently a frequently discussed topic, especially for newly constructed roadways with cement concrete pavement. This early degradation of cement concrete pavement slabs is a major problem that threatens the service life of sub-sections or the entire road. As some partial results demonstrate, a possible cause of this degradation can be seen in the surrounding road environment, namely the possibility of transferring aggressive substances to the concrete pavement slab. This paper is concerned with the verification of the possibility of the penetration of alkalis penetration from the surrounding environment into the concrete pavement slab by simulating the effect of different external conditions and environments.

Over the recent years the amount of fast destruction defects of cement paste matrix has increased markedly for exposed concrete structures of highway and airport pavements. The accelerated destruction of silicate structure is unusual due to its irregular occurrence. It only occurs at 30 to 50% of newly built pavements (in continuous segments). The issue is applicable to loaded as well as unloaded pavements. Detailed diagnostics found that the probable cause is based on concurrently occurring expansive reactions in capillaries, cracks and pores in the cement paste matrix. In defect spots the presence of several expansive substances was proven, wherein the biggest question poses the unexpectedly high potassium concentration. Potassium is only contained at best in trace amounts in road concrete input components in the Czech Republic. Neither existing regulations nor standards sufficiently deal with this type of defects of cement concretes. If they mention the existence of this issue at all, they are just limited to recommendations. Since the modification of the existing commercially produced cements to pavements is still improbable, the solutions to this issue seem problematic.

Monolithic as well as precast Portland Cement Concrete pavement have shown a gradual increase in quantity and severity of defects and failures in recent years. Especially, the radical increase in the auto-destructive deterioration of the concrete structure, which is most often described as the expansion reaction, is alarming. The consequences may lead to extreme reduction of the lifetime of concrete on loaded or unloaded surface courses. The main causes are due to the combined effect of environmental effects (climate, chemical pollution), drainage of roads and the behaviour of the existing cement binders, provided that we do not reflect the modification of pavement structure layers. Only the impact of climate effects can be effectively reduced on the existing roads. The paper presents a summary of results obtained from the current portfolio of secondary concrete protection products. The testing was performed in terms of resistance to water infiltration and durability against the common effects of freeze/thaw and salt. The results show a dependence or independence according to the material basis when exposed to secondary protection products. Oils, silicate emulsions and a group with other material bases, so called other products, behave quite differently.

This paper describes the testing of effectiveness and behavior of dowels placed in transversal joints of concrete pavements, while focusing on dimensions and quality of commonly used materials. The analysis uses experimental tests in laboratory conditions which were performed independently in the Czech Republic and Slovak Republic. The comparison of quality as well as potential use of alternative materials of dowels is made with the use of developed tests focusing on main requirements, such as longitudinal displacement in cement concrete, resistance of coating to damage, and reduced potential to concrete damage. Furthermore, the paper describes and compares loading results of the relative concrete deformations around dowels by strain gauges that were analyzed. Results of deformations on beams with an inserted dowel and the findings that were observed during the measurement are presented.

The basic assessment of cement concrete pavement condition is performed by a visual inspection, when the surface faults which are macroscopically distinguishable are documented. Detailed data on concrete condition require further tests performed on cores. Apart from common standard tests, such as layer thickness, compressive and tensile strength, resistance against water and chemical defrosting agents, other macroscopic and microscopic analyses have become useful. Some of the most important ones include petrographic assessment of aggregates (based on the fraction and type of rocks), quantification of cracks (expressed by e.g. cracking index or damage rating index), analyses of air void characteristics in concrete (expressed by e.g. spacing factor test), indication of type and extent of expansion reactions with the help of chemical reagents that visually mark the affected concrete parts. The information can be complemented and adjusted through a suitable combination of microscopic methods. Prepared thin sections are used to determine types of aggregates including the specification of its harmfulness. In combination with concrete fragments taken at least from three height levels of the core (surface-centre-bottom), differences or conformity of concrete porousness, and level and type of mineralization of pores and cracks created by secondary phases, are determined. The identification of created phases and their correlation with applied aggregates can be used for prediction of potential risks. The paper describes additional test methods that can be performed on cores from concrete structures and that are based on macroscopic and microscopic analyses.

Dowels are located in transverse joints of Jointed Plain Concrete Pavements (JPCP) and they are used to provide load transfer between individual slabs, reduce faulting and improve performance. Dowels and the concrete itself are under the highest stress in the vicinity of joints; thus, in terms of pavement design, the joints are the weakest points of the whole structure. This study dealt with the drawbacks of JPCP with dowels. The evaluation was based on direct measurements on real airport and motorway pavements and highlights insufficient efficiency of load transfer and its possible causes. The authors present a successful outcome with validation by using the finite element method where high tensile stress values of the surrounding concrete were found.

Concrete pavements are designed for heavy loaded road structures. Their usage brings a number of specific issues. It is necessary to solve them all to ensure that concrete pavements will fulfil their function along the whole design period. One of these issues concerns dowels, which are located in transversal joints. Modelling of load, caused by heavy vehicles, with the use of the finite element method, provides valuable information about the stress condition of concrete pavement. The results of modelling can be verified by measurements or experiments in practice. Dowels and tie bars in jointed unreinforced concrete pavements and the importance of their correct placement, dimensions and material quality on pavement behaviour and lifespan were studied as a part of R&D projects of Technology Agency of the Czech Republic Nos. TA02031195 and TE01020168. The paper presents the experience from the modelling and performed experiments and makes conclusions which are important for the use in practice.