Ultra-high performance fiber reinforced concrete (UHPFRC) is a high-performance cementitious material with enhanced tension, compression, and toughness, strengths in the post crack region with high ductility, toughness, and durability. The companies prefer to use it to construct highly durable structures such as high-rise buildings, towers, and bridges. In addition, the thickness of the flat slab produced by UHPFRC might be thinner than the conventional concrete. One problem that has always been a concern in a flat slab is the punching shear failure since this failure is brittle and occurs suddenly without any previous notice. Besides, the position of the critical section for punching shear could be changed based on the thickness of the drop panel and the inclusion of fiber in the concrete. This paper highlights the effect of drop panels dimension on the punching shear resistance in UHPFRC flat slabs. The four two-way interior UHPFRC supported flat slab panels, consisting of one control flat slab without drop panels and three-flat slabs with different sizes of drop panels (10.5%,14.5%, and 19%) of the total area of slab drop panels, tested under punching load. Results indicated that the covered area of flat slabs by drop panel around 10.5% improved punching load up to %20 and 37% at the crack and ultimate loads. Furthermore, the test results show that the efficient covered area for resisting punching was 10.5% of the total area of the tested slab. Besides, the deflection values, strain in reinforcement and concrete, rotation at supports, and the inclination angles of cracks were improved due to the stiffness enhancement in the flat slabs.

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In the last few years, the flat slab system has become widespread for constructing multi-story buildings in many countries due to its simplicity and fast construction. However, the demand for the strengthening of reinforced concrete is a critical element in existing structures due to design or construction errors, and changes in the building function become a challenging area to develop and extend the durability of structures. In the reinforced concrete structure, flat slabs are the most critical element that requires strengthening because of exhibition punching shear failure. There are many materials and techniques used in the strengthening field. Due to well-known superior mechanical properties in strength and durability, ultra-high-performance fiber-reinforced concrete becomes the early relevant substance in the strengthening field. This study highlights the behavior of ten small-scale specimens of reinforced concrete flat slabs with different concrete grades, strengthened against punching shear by ultra-high performance fiber reinforced concrete strips in different distribution patterns at punching shear critical sections. The strips are jointed to the tensile slab surface through adhesive epoxy material. The outcomes indicated that the ultra-high performance fiber reinforced concrete strengthening strips enhanced the punching shear resistance of the normal strength concrete slabs, up to 53.1%, 16.63%, and 16.5% for different concrete grades examined in the study for flat slabs, which are 20.8, 32.6, and 43.3 MPa, respectively. This improvement in punching shear resistance was obtained by enhancing slab thicknesses and widening the resisting area for punching shear at critical sections. In addition, the strengthening technique transforms the failure's mode of slabs from brittle to ductile.

Concrete deficiency is a very common issue and has several forms, including cracks in a plastic or hardened state. Various materials and methods are used nowadays for repairing because repairing is cost-effective, can rehabilitate deteriorated structures, and increases the overall structural life cycle. Epoxy injection is one technique used for repairing cracks with 0.05mm width. The main objective of this review is to evaluate the efficiency of this technique for normal and high-strength concrete beams and investigate the factors that affect the improvement ratio of repaired samples, such as; the depth of the beam, the number of drilled holes for injection, reinforcement distribution, the viscosity of the epoxy and width of the crack. The results proved that epoxy injection is an effective method for restoring the original structure and stiffness if sufficient repairing procedures are followed. A low viscosity epoxy is preferred as it can penetrate easily into the crack depth and restore strength. On the other hand, beams with smaller depth had a higher percentage of improvement, and the number of drilled holes for injection directly affects the results, fewer holes better the strength improvement.

The demand for the construction of high-strength concrete in the civil engineering zone is growing, particularly in the last couple of years, due to the construction of sustainable and economic buildings with an extraordinary slim design. Concrete curing in construction is an operative manner and essential to provide that concrete structures meet future performance and durability. High-strength concrete has a low water-to-binder ratio; proper concrete curing is important to ensure its planned performance and durability. Conventionally, exterior curing applied after placing and casting concrete stays warm and moist to provide continued cement hydration. Lately, theoretically and experimentally comprehends that internal curing is an important tool to provide additional moisture in the concrete to enhance cement's hydration. Internal curing of high-strength concrete is an active technique to lessen or even remove autogenous shrinkage and effects on chemical shrinkage, dry shrinkage, etc. Most studies recently have emphasized that a reduction in high strength concrete mixtures' shrinkage is due to internal curing, and the compressive strength can increase higher in mixtures with LWA or SAP than in mixtures without this agent rising degree of hydration by providing extra water in the hydrated cement paste. However, the use of internal curing leads to improving the durability of high-strength concrete.