UHPC opens the door to new ideas and exciting opportunities for redesigning buildings. Some research works on the utilization of UHPC in proposed hybrid structures are summarized below.
Using UHPC with glass fiber reinforced polymer (GFRP) is a proposed hybrid idea that has inspired many state-of-the-art researchers. Elmahdy et al. [85] experimentally studied an innovative design using UHPC in mixed beams. The beam part is a GFRP hollow box. On top, a thin cover of UHPC is used to carry compressive stress.
At the bottom, steel or carbon fiber-reinforced polymer (SFRP/CFRP) sheets are used to withstand tensile stresses. These different materials are connected to each other with shear stud connectors and epoxy glue to provide adequate bonding between them. Research shows that by adding high-performance materials such as UHPC, the ability to mix beams is significantly improved.
Chen & El-Hacha studied another hybrid beam with the same GFRP hollow box section design under static bending loading. Research proves that UHPC is critical for achieving greater strength with lighter weight and smaller structural elements. Furthermore, Iskander et al. studied hybrid UHPC-GFRP hollow box sections with SFRP or CFRP plate bases to analyze the causes of failure under shear forces. Research shows that the failure is due in part to the fiber orientation design in the corner regions of this hybrid beam.
Many researchers have studied another hybrid idea, combining UHPC with regular concrete as a construction or repair material. As a building material, Hakeem and Azad & Hakeem studied the structural behavior of three different concepts of one-way simply braced precast floor units cast from ordinary concrete, UHPC made in different structural forms such as: (i) Elements with a layer of UHPC on the bottom tension plane, UHPC substratum and beam specimens of ordinary concrete; (ii) elements reinforced with prefabricated UHPC deformed bars. The concept of using UHPC sticks was introduced by Azad and Hakeem. UHPC steel bars were placed in the hybrid structural mold before pouring regular concrete. The UHPC rods were thermally cured at 900 C for 48 h to accelerate curing and strength development. Final hybrid specimen reinforced with two 50 × 50 mm UHPC rods; (iii) hybrid hollow unit with UHPC layers cast on the top and bottom surfaces.
Fatigue cracks have been observed in many orthotropic steel deck bridges around the world, for example: Severn Bridge in England, Sintal Bridge in Germany, Westgate Bridge in Australia, Leverkusen in Germany Moribashi. Many researchers have studied this fatigue problem to improve their performance.
One of the best-anticipated solutions to combat this fatigue is to use UHPC overlay as the bridge deck pavement instead of raw pavement. The fatigue response of UHPC cladding using an orthotropic steel deck was studied. Studies have shown that UHPC coverings can significantly reduce the magnitude of deck-side stresses. Furthermore, the transverse bending behavior of the steel-UHPC composite deck under bending moments was studied. Research has found that UHPC coverings have a considerable impact on the ultimate load-bearing capacity of decks.
Next-generation concrete (UHPC) offers astonishing quality levels never imagined before. After thoroughly reviewing the topic, the following conclusions were found:
1. It is clear that most researchers emphasize that the mechanical and environmental properties of UHPC exceed all expectations, creating the potential for wider applications in construction.
2. The mechanical properties of the new generation of concrete are much better than those of conventional concrete. These unparalleled values are a function of water-to-binder ratio, ultra-fine powder, optimized particle packaging, curing method, and microstructural enhancement.
3. Using this technology, structures can be built that are lighter, larger, or have longer spans than are usually designed. Its exceptional workability allows new concrete to be poured in irregular or very elongated shapes to create structures with an aesthetic appearance or an extraordinary finish.
4. However, the use of UHPC in construction is limited because it is not commercially feasible to replace conventional concrete in most applications for the following reasons:
a. An economic factor manifested by the high cost and lack of availability of some of its constituent materials. For example, steel fibers may cost more than other matrix materials combined.
b. Some technical aspects, such as limited design specifications and complex manufacturing and curing techniques.
c. Negative impact of cement production on the environment, as the amount of cement required for UHPC production is approximately twice that of traditional concrete.
5. One of the recommendations for promoting UHPC in construction is to explore alternative materials to replace UHPC’s expensive composite materials. It is recommended to use waste materials with gelling properties instead of Portland cement and silica fume, which have the following advantages:
a. Reduce concrete production costs,
b. Make concrete more environmentally friendly by reducing waste and gas emissions in cement production,
c. Improve the uniformity and density of concrete for better strength and durability.
