AUTOGENOUS SHRINKAGE OF COMPOSITES BASED ON PORTLAND CEMENT

Autogenous shrinkage of cement based composites is important property influencing number of their engineering application. Its ultimate value is predominantly determined by mineralogical composition of cement and its particle size distribution. Present paper introduces experimental study focused on the evaluation of various cements of grade CEM I 42.5 produced in Czech Republic in terms of shrinkage under autogenous conditions. Selected cement type is currently the most frequently used cement. Conducted study confirmed essential differences in ultimate values of shrinkage, which is partially determined by its specific surface area. Accompanying tests of mechanical properties indicate the influence of particle size distribution, which controls initial phases of cement hydration

SHRINKAGE OF VARIOUS TYPES OF PORTLAND CLINKER-BASED CEMENTS WITH RESPECT TO THEIR HYDRATION DEGREE

The volume changes of cement based composites are significantly exhibited in the hardening process. Initial phases of the hardening are complemented by the expansion due to the heat evolution that is subsequently alternated by the shrinkage. Both could cause a crack initiation. It is evident that ultimate volume changes of cement based composites are a complex process, because the final shrinkage is determined by the binder used, exposition and also by the previous history. The paper focuses on the evaluation of the main types of cements based on the Portland clinker by using a conventional procedure for the determination of the shrinkage on the standard cements mortars. These mortars were exposed to drying after 1 and 3 days of curing, related to the actual degree of hydration, which was estimated on the basis of compressive strength development. The hydration process was additionally monitored using thermogravimetry on the accompanying paste specimens during one year. The performed experimental program confirmed the essential sense of the curing regime especially for blended cement systems, which exhibited very low values of the hydration degree at applied curing intervals. Despite the slightly higher values of shrinkage of blended cements, the obtained results signalize the crucial effect of prolonged curing for these types of binders. The conclusion highlighted the necessity of taking into consideration the hydration degree during cement testing by using the conventional contact method. Otherwise, the simple interpretation leads to an overestimation of the less-suitable material solution

Influence of Surface Treatment of Fresh Concrete on its Resistance to Drying Shrinkage

The volume changes of cement-based composites are significantly exhibited during the hardening process. Initial phases of the hardening are complemented by the expansion due to the heat evolution that is subsequently alternated by the shrinkage. Both could cause the crack initiation causing significant loss of the durability and service-life shortage. The present paper focuses on the experimental investigation of the surface treatment of fresh concrete, which is applied to prevent sudden loss of the moisture during a hardening process, especially during the concrete highway construction. The technology of concrete highway production is extremely costs demanding, but its efficiency is determined by the longer lifespan in comparison with the asphalt pavement. However, negative impacts of the drying shrinkage could significantly reduce the durability. The efficiency of used treatment was investigated in terms of restrained shrinkage tests, water adsorption and mechanical testing. In addition, there were studied two types of conventional Portland cement. Performed program confirmed great sense of the curing on the concrete durability; in addition, there was well illustrated the efficiency of the utilization of blended binder systems in the paper

SIZE EFFECT ON THE ULTIMATE DRYING SHRINKAGE OF CEMENT MORTAR: 1-YEAR EXPERIMENT AND NUMERICAL MODELING

The magnitude and time evolution of shrinkage are influenced by numerous factors which are implemented in the design codes often in a different way. The time-dependent behavior of concrete in structures sensitive to creep and shrinkage should be verified by means of short-term laboratory measurements. Extrapolation of drying shrinkage from short-term measurements is an illposed problem. The process is extremely slow but can be accelerated by reducing the specimen size. The knowledge of the size-effect on drying shrinkage is a necessity to establish the transition from the laboratory to the structural size. In the literature, the experimental data on such size-effect are insufficient. For this reason a new experiment was developed to study this phenomenon on small-scale specimens made of cement mortar and the results from the first year are summarized in this paper. The measured data are validated by coupled FEM hygro-mechanical simulations

Estimation of Hydration Degree of Blended Cements with the Help of k-Values

The growing utilization of various mineral additives in the building industry has caused concern worldwide to reduce the emissions of carbon dioxide from Portland cement (OPC) production. The present paper is focused on the determination of the degree of hydration of blended binding systems based on Portland cement. Blast furnace slag, fly ash, and ceramic powder are used in the study; they are applied by 12.5 wt.% up to 50% of OPC replacement. The evolution of the hydration process is monitored using thermogravimetry in selected time intervals to determine the degree of hydration; its ultimate value is obtained from numerical estimation using the Michaelis-Menten equation. However, due to the application of active mineral additives, the correction in terms of equivalent binder is conducted. Corrected values of the degree of hydration exhibit good fit with compressive strength