Some Technological Parameters of Construction Joints Formation Using the Self-Compacting Concrete Mixtures

Introduction. The important condition ensuring the monolithicity of the erected massive monolithic reinforced concrete structures, divided in compliance with the standards into the heat-shrinkable blocks, is the quality of construction joints in terms of the new-to-old concrete bonding. The research on the bonding strength dependence on the technological parameters of the construction joints formation using the widely spread in recent years self-compacting concretes is a relevant task, because the number of publications on the respective topic is insufficient. The aim of the present paper is to develop a scientific concept regarding the influence of the certain technological parameters on the new-to-old concrete bonding strength during the construction joints formation.

Materials and Methods. The research was carried out using the self-compacting and ordinary heavy vibratory-compacted concretes with the compressive strength ranged from 34.5 to 69.2 MPa. The object of the study was the dependence of the new-to-old concrete bonding strength on the method of standardised surface pre-treatment of the “old” concrete in a joint and the compressive strength of the grade of the “new” self-compacting concrete. The composite Mörsch specimens in which the “old” concrete was cured for up to 3 days were used as the main samples. The construction joint relative strength was considered to be the joint quality evaluation criterion equalling to the ratio of the shear strength of concrete in a composite Mörsch specimen to the minimum axial tension strength values of the “new” and “old” concrete. The quality indicators of the concrete were determined by applying the standard methodology. The specimens were cured under conditions simulating the production of works in the spring-summer season.

Results. During the research, the construction joint quality criteria in terms of the “old” to “new” concrete bonding were analysed along with the changes they undergo depending on the studied recipe (concrete grade) and technological (length of the technological break, surface treatment method) factors. The dependences of the proposed construction joint quality criterion on the “new” concrete compressive strength (grade) were obtained and the inefficiency of the surface treatment with a metal brush was shown, especially if accompanied with the technological break extension. The required bonding strength values for B25 ... B35 grade concretes can be for sure provided by the surface pre-milling, whereas the extension of the technological break has negative affect on the bonding strength.

Discussion and Conclusions. Due to the revealed low bonding strength of the “old” to “new” concretes, regardless the technological factors, it is expedient to look into the ways of ensuring the construction joint quality by implementing structural solutions such as keyed joints and (or) additional reinforcement.

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