During a fire, ordinary Portland cement (OPC) systems lose their mechanical properties. For this reason, it is important to find a way to protect it. This study suggested alternative uses of slag and phosphogypsum to produce coatings for fire-resistant applications. Five compositions of 10 mm thick alkali-activated slag coatings were investigated. In these compositions, different amounts of phosphogypsum (1%, 3%, 5%, 7%, and 10%) were used. In the first stage of this study, the residual compressive strength of samples with the coatings based on alkali-activated slag was compared to the results of OPC concrete samples without coatings. The experimental results showed that a higher residual compressive strength of 33.2–47.3 MPa OPC concrete was achieved for the samples with coatings. Meanwhile, the residual compressive strength of the uncoated samples was 32.37 MPa. In the second stage, OPC concrete samples were reinforced with fiberglass polymer (FRP) rods, and they had a similar positive effect on alkali-activated coatings. After exposure to higher temperatures, the pullout tests of the glass FRP bars showed that the adhesion strength was (9.44 MPa) 43.9% higher for the samples with coatings compared to the samples without coatings (6.56 MPa). Therefore, a higher bond strength can be maintained between concrete and FRP bars. Alkali-activated slag with 3% phosphogypsum can be used for the production of fire-resistant coating. These coatings could protect OPC concrete and reinforced concrete with glass FRP bars from fire.

In this study, alkali-activated mortars were prepared using two different types of fine aggregates: natural sand and biomass bottom ash. These mortars were used as a repair material for structures constructed using old reinforced concrete structures based on Ordinary Portland cement (OPC). Experimental studies have shown that the alkali-activated slag mortar with biomass bottom ash (BBA) from the bubbling fluid bed meets the repair mortar class R1 according to EN 1504-3. The suitability of such repair mortar is determined by the good adhesion properties of the alkali-activated slag binder to old OPC concrete. The adhesion after 28 days was 0.31 MPa and the samples broke off at the repair matrix. The AAC/BBA repair mortar had a compressive strength of 18.69 MPa, the shrinkage due to drying deformations consisted of 0.1903% after 28 days. Alkali-activated slag mortars are effective in repairing, renewing and rebuilding damaged OPC concrete structures.

A time-dependent stress-strain state of reinforced elements induced by gradually developing shrinkage of concrete is analysed. Depending on environmental conditions the early age concrete shrinkage can produce a significant tensile stress in concrete that reduces the cracking resistance of the cross-section. To analyse this phenomenon a numerical model is proposed. For the exact analytical prediction of the time-dependent stress-strain state induced by restrained shrinkage, the key parameter is the coefficient of aging which is strongly dependent on the time-evolution of tensile stress of concrete and modulus of elasticity. The values of this coefficient obtained in the current analysis are suitable for the proper evaluation of the reduction of cracking resistance of the cross-section induced by restrained shrinkage evolving during concrete hardening process in a period prior to element loading. DOI: http://dx.doi.org/10.5755/j01.mech.24.1.17901

The use of tire recycling waste in construction composites reveals sustainable opportunities in civil engineering. Research has shown that shredded tire rubber, used as a partial replacement for fine aggregate, reduces the compressive strength and modulus of elasticity of concrete, but has little effect on flexural and splitting strength, while significantly improving frost resistance. For example, replacing 2.5% of fine aggregate with rubber decreases compressive strength by about 13.6%, yet the flexural strength differs by less than 5% compared to control concrete. Steel cords and wires from tires, applied as dispersed reinforcement, demonstrate high tensile strength (1871–2491 MPa), exceeding that of certified steel fibers (~1200 MPa), and can therefore be rationally used as substitutes. Test results indicate that concrete reinforced with such cords achieves 8–25% higher flexural strength than control concrete reinforced with certified fibers. Overall, tire recycling waste—rubber and steel cords—can be successfully applied in sustainable concrete production, contributing to solving ecological pollution issues. However, the amount of rubber in the mixture must be limited to avoid deterioration of the main mechanical properties of concrete.

Studies on Wooden Constructions in Kaunas