Properties Investigation Of Young Concrete

The investigations done in this paper is one of the projects aided by National Natural Science Foundation of China (NSFC 5017 8063).Generally, the 28-day compressive strength of concrete determined by a standard uniaxial compressive test is accepted universally as a general index of concrete strength. So the tests done before are to investigate the properties of 28-day concrete and its long-term characteristics, less about young concrete. Further investigation on concrete asks us to learn more about young concrete. Addressing the question of how early is early age, S.GBergstrom of the Swedish Cement and Concrete Research Institute said:Time is not a very good measure of "early". The time when the concrete has reached a certain maturity, is dependent on so many factors: cement type, reactivity of the cement, temperature, admixtures, etc. The time factor is not significant in the general case if you are not specifying the case very carefully. Then of course the degree of hydration gives a much better indication, which however is not always available if we deal with the practical side. You can also use another more practical definition perhaps, giving the time the property you are interested in has reached the level you need. All times earlier than that level are evidently early ages; which means that the definition depends on the way you will use concrete. The form stripper would say that he needs about 15MPa, whereas a slipformer does not need as much as that. These two have quite different concepts about early age. The answer is that there is no universal answer. When we try to define the area where we are going to work, we will, as a rule of thumb and for the normal concrete, in normal situations, say about 24hours, some say about 48hours, but that is just to indicate the order of magnitude...While, as S.GBergstrom said: "Then of course the degree of hydration gives a much better indication, which however is not always available if we deal with the practical side." In my testing process, I still use the time as indication to investigate the properties of concrete at various early ages in order to give an explicit definition to "early age".I did four experimentations in lab of HKUST civil engineering department.(1) Compressive strength, tensile strength, complete stress-strain curves analyzing of NCS and HSC at early age.(2) Fracture parameter investigation (Ks Ic , CTODc) of young concrete for NSCand HSC using TPFM.(3) Research on porosity, pore size and pore size distribution of young concrete.(4) Observation on the function of "self-healing" of concrete by preloading it. . Testing results show that each property of young concrete has its own developingrule and has certain relationship with each other. Some conclusions can be drawn from this experiment:(1) Either NSC or HSC has its own rule of development, simply speaking, HSC has a high mature rate.(2) Strength and fracture parameters, as well as other parameters of concrete have similar developing rule. Stress-strain curve analyzing is a content method to study the behavior of concrete against external load.(3) There are certain relationships between fracture parameters, pore structure and strength development of concrete. They are all the representations of hydration process of concrete. Concrete has capacity of self-healing in proper conditions.