Maria Kagan, Senior Lecturer at the Department of Construction Technology, conducted a research project titled "Development of Technology of Construction Joints and Connections in Reinforced Concrete Structures" as part of the 2026 grant competition for research activities by SPbGASU academic staff.
The relevance of this research topic stems from the widespread use of cast-in-place, precast, and composite reinforced concrete structures in modern construction. A reduction in strength and density is frequently observed at construction joints—where freshly placed concrete mix ("new" concrete) meets previously placed concrete ("old" concrete)—thereby impairing the structures' performance characteristics. Existing standard methods for preparing joint surfaces are labor-intensive and do not always ensure the required bond quality.
The objective of the research was to improve the technology for forming joints and seams to ensure reliable contact of equal strength between concretes of different ages. To achieve this goal, the following tasks were addressed:
- technological factors influencing the strength of the concrete contact zone have been identified;
- technological solutions for improving the quality of the bond between concretes of different ages were proposed, and a techno-economic assessment of the proposed methods was conducted;
- methodological approaches have been developed to substantiate the technological parameters for creating concrete joints of equal strength.
It has been established that the quality of the bond is significantly influenced by the physicochemical properties of the freshly placed mixture—specifically, the type of binder and the pH level of the mixing solution. It has been demonstrated that a strong bond forms when slag-alkali concrete mixtures (with a mixing solution pH exceeding 12) come into contact with a concrete substrate ("old" Portland-cement-based concrete). Furthermore, the use of such mixtures eliminates the need for labor-intensive mechanical removal of the cement laitance from the joint surface, thereby significantly simplifying the technological process.
A method was proposed for constructing monolithic, horizontally oriented reinforced concrete structures with a construction joint under practical conditions, yielding strength and crack resistance characteristics comparable to those of a monolithic structure.
By modeling the construction processes involved in casting a monolithic floor slab with a construction joint, the technical and economic performance of various methods for forming such joints in reinforced concrete structures was evaluated. Compared to the baseline standard (SP 70.13330.2012 "Load-bearing and Enclosing Structures"), the proposed technology reduces the construction schedule by up to 15 percent, with a negligible increase in labor input (up to 1 percent). Furthermore, the production cost of the slag-alkali mixture is up to 42 percent lower than that of its Portland cement-based counterpart.
The scientific novelty of the work lies in the proposed methodological approaches that enable the selection of technologies for constructing concreting joints and monolithic connections in reinforced concrete structures based on criteria ensuring the compressive, shear, and tensile strength of the concrete in the contact zone, as well as the determination of technological parameters for the concrete mix being placed, taking into account the age and surface treatment methods of the "old" concrete. The scientific novelty also extends to the advancement of theoretical understanding regarding the selection of technological parameters for constructing working joints in reinforced concrete structures using alkali-activated, clinker-free concrete mixes.
In the context of implementing resource-conservation policies—both national and global—developments involving the use of industrial by-products and secondary material resources are of particular importance. This is highly relevant for all regions of the Russian Federation, aligning with the strategic development concept for Russia’s construction and housing/utilities sectors (covering the period up to 2030, with a forecast extending to 2035) through proposed technological solutions that utilize energy-efficient, alkali-activated, clinker-free concretes made from metallurgical industry waste.
The technology has undergone pilot-scale testing and is recommended for use by construction companies. The results obtained should be utilized when developing work execution plans and method statements for the construction of cast-in-place horizontal reinforced concrete structures (such as floor slabs and foundation slabs), as well as for the execution of joints between precast reinforced concrete elements.
The research findings are of interest to design and construction organizations, enabling the incorporation of effective and cost-efficient solutions—which ensure high quality and operational reliability in reinforced concrete structures—as early as the design stage. The research materials will also be utilized in the educational process at SPbGASU for the training of construction specialists.
The news is published based on the results of the SPbGASU 2026 grant.