PSI - Issue 43

Vlastimil Bílek et al. / Procedia Structural Integrity 43 (2023) 107–112 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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occurrence is an intrinsic characteristic of concrete, Aïtcin (2008), Aïtcin (2014). The microcrack occurrence can affect the mechanical properties of concrete, especially bending strength, but the effect is not very conspicuous. Fracture properties – fracture toughness and fracture energy – are more sensitive to the microcrack content; see e.g. Bilek (1999). The most important characteristic of concrete is the water to cement ratio, w/c , which is the ratio of “mass of water / mass of cement”. The value of the water to cement ratio is very important from the point of view of microcracking. A concrete with a high water to cement ratio has a high porosity and its strength is reduced. The reason is a long distance between the cement grains and the necessity of hydrated products bridging these distances. Another consequence of high porosity is also easy water evaporation from concrete, which significantly affects the shrinkage and microcracking. On the other hand, curing concrete with water is relatively easy; an appropriate curing can significantly enhance the properties of the concrete. Low water to cement ratio means closely arranged cement grains and a denser microstructure. Porosity decreases, and strengths increase. However, if the water to cement ratio is lower than approximately 0.40, the concrete starts to be so dense that curing water is not able to penetrate inside the concrete. This effect has two consequences: 1) the cement inside of the concrete cross-section is not able to fully hydrate; and 2) thanks to hydration, the original (mixing) water inside the concrete is consumed and the concrete dries and shrinks. This is reflected in the arising of microcracks, Aïtcin and Mindess (2011). This is the reason for searching for an optimum self-curing of concrete. The self-curing can be performed in various ways, Talaiekhozan et al. (2014), Mahmoodi, S., Sadeghian, P., (2019), Qureshi, T., Al-Tabbaa, A., (2020). This paper represents a base for the self-curing use of internal water contained in saturated porous aggregates (calcinated clay of shale). For the evaluation of the self-curing effect, some concretes without self-curing were prepared, and their properties were studied during the 28 days of curing in water and without the exchange of water with the environment. As fracture mechanics seems to be the best tool for the evaluation of the properties of concrete with respect to microcracks, it is also necessary to characterize microcracks. This paper deals with the evaluation/quantification of the dependence between the change of the fracture mechanical characteristics and the characteristics obtained using CT-tomography. Nomenclature E ef effective modulus of elasticity – modulus of elasticity in three-point bending on the notched beam f b bending strength – three-point bending, prisms 40 × 40 × 160 mm f c compressive strength performed on the fragments of prisms 40 × 40 × 160 mm f r modulus of rupture – the flexural strength on the notched beam G F fracture energy K IC ef effective fracture toughness S span – the distance of support in a 3PB test  volume density w/c water to cement ratio – mass of water / mass of cement

2. Materials and Experimental Details 2.1. Materials

Three concretes were mixed, the compositions are shown in Table 1. The concretes differ in their w/c = 0.3, 0.4 and 0.5, the dosage of cement was kept constant, and the volume of the mixture up to the volume unit (1m 3 ) was completed by aggregates. Two fractions of aggregates were used – 0/4 mm sand and 4/8 mm crushed granite. The batches of 20l volumes were mixed in a laboratory mixer. After mixing, the concrete was put into steel molds and intensively vibrated. The consistency was adjusted with a superplasticizer, but the workability was low, especially for the mixes with w/c = 0.40 and 0.30. After compactation, the molds with fresh concrete were covered with a PE-foil and left in the laboratory. The specimens were unmolded the next day. Some of them were stored in water with the temperature of (20 ± 2) ° C, and the other specimens were carefully enveloped by the PE-foil to prevent them from