During the construction of concrete structures, it is often useful to know compressive strength at an early age. This is an amount of strength required for the safe removal of formwork, also known as stripping strength. It is certainly helpful to determine this strength non-destructively, i.e., without any invasive steps that would damage the structure. Second only to the ultrasonic pulse velocity test, the rebound hammer test is the most common NDT method currently used for this purpose. However, estimating compressive strength using general regression models can often yield inaccurate results. The experiment results show that the compressive strength of any concrete can be estimated using one’s own newly created regression model. A traditionally constructed regression model can predict the strength value with 50% reliability, or when two-sided confidence bands are used, with 95% reliability. However, civil engineers usually work with the so-called characteristic value defined as a 5% quantile. Therefore, it appears suitable to adjust conventional methods in order to achieve a regression model with 95% one-sided reliability. This paper describes a simple construction of such a characteristic curve. The results show that the characteristic curve created for the concrete in question could be a useful tool even outside of practical applications.

This paper deals with the influence of periodic freezing on lightweight concrete characteristics. Sets of lightweight concrete prismatic specimens are cyclically frozen in range from +20C to -20C and non-destructively tested after every 25 cycles. Freeze-thaw resistance is determined from measurement of the frost-attacked and non-frost-attacked (referential) specimens. The referential specimens are air/water-cured. Non-destructive methods, especially ultrasonic impulse method and resonance method are used for determination of specimen's degradation. Experiments are finished with destructive test in order to determine the static modulus of elasticity.Příspěvek se zabývá vlivem cyklického zmrazování na vlastnosti lehkého betonu. Sady vzorků ve tvaru hranolu jsou cyklicku zmrazovány v rozmezí +20C do -20C a nedestruktivně zkoušeny po každém 25tém cyklu. Mrazuvzdornost je určena z meření provedených na zmrazovaných a nezmrazovaných (referenčních) vzorcích. Referenční vzorky byly ošetřovány střídavě na vzduchu a ve vodě. Pro určení degradace vzorků byy použity dvě nedestruktivní metody - ultrazvuková a rezonanční. Experimenty jsou ukončeny destruktivními zkouškami za účelem určení statického modulu pružnosti betonu

The modulus of elasticity is a property of concrete, which is known for its high variability of values. A high enough value of the modulus of elasticity is a critical requirement in complicated concrete structures and in order to attain it, a number of factors must be taken into account, as it is subject to many. Curing is one of the most important influences. It can be described as an effort to maintain the temperature of concrete within the correct limits, prevent loss of water and the shrinkage connected with this. This paper describes an experiment focused on the observation of the development of the dynamic and static modulus of elasticity in dependence on the method of curing. The experiment worked with air-entrained as well as non-air-entrained concrete of the C 30/37 strength class. The results show that the curing method strongly affects not only the development, but also the final values of the concrete modulus of elasticity

Freezing and thawing resistance is very important characteristic also in the case of lightweight concrete. The paper deals with the influence of periodic freeze-thaw cycles on lightweight concrete fracture parameters values. Sets of lightweight concrete specimens (prismatic shape) are cyclically frozen in range from +20C to -20C. The main aim of the experimental part is to determine and compare the mechanical/fracture parameter values of a composite both frozen and non-frozen. The non-frozen specimens are air/water-cured. A three-point bending test of beams with a central edge notch is used in order to determine the fracture parameters values. It particularly covers effective fracture toughness and fracture energy. The notch is cut after 200 freezing cycles before fracture tests. The other parameters values - compressive and splitting tensile strength on the specimen fragments - are determined.Mrazuvzdornost je také v případě lehkých betonů velmi důležitou charakteristikou. Příspěvek se zabývá vlivem cyklického zmrazování na hodnoty lomových parametrů lehkého betonu. Sady vzorků ve tvaru hranolu jsou cyklicku zmrazovány v rozmezí +20C do -20C. Hlavním cílem peovedených experimentů je určit a srovnat hodnoty mechanických/lomových charakteristik kompozitu na zmrazovaných a nezmrazovaných vzorcích. Nezmrazované vzorky byly ošetřovány střídavě na vzduchu a ve vodě. Pro určení lomových parametrů je použita metoda tříbodového ohybu trámce se zářezem. Jedná se zejména o určení efektivní lomové houževnatosti a lomové energie. Zářez byl proveden po ukončení 200 zmrazovacích cyklů. Ostatní hodnoty parametrů - pevnost v tlaku a pevnost v příčném tahu - byly určeny na zlomcích trámců získaných po lomových zkouškách

The paper deals with the experimental determination of shrinkage development during concrete ageing. Three concrete mixtures were made. They differed in the amount of cement in the fresh mixture, 300, 350, and 400 kg/m³. In order to determine the influence of plasticiser on the progress of volume changes, another three concrete mixtures were prepared with plasticiser in the amount of 0.25% by cement mass. Measurements were performed with the goal of observing the influence of cement and plasticiser content on the overall development of volume changes in the concrete. Changes in length and mass losses of the concrete during ageing were measured simultaneously. The continuous measurement of concrete mass losses caused by drying of the specimen’s surface proved useful during the interpretation of results obtained from the concrete shrinkage measurement. During the first 24 hours of ageing, all the concrete mixtures exhibited swelling. Its magnitude and progress were influenced by cement, water, and plasticiser content. However, a loss of mass caused by water evaporation from the surface of the specimens was also recorded in this stage. The measured progress of shrinkage corresponded well to the progress of mass loss.