1. Durability properties
One of the main goals of UHPC concrete is to be as durable as a rock and to last for a long time without significant loss of quality. Typically, concrete structures can be set up where they are adherent to the immediate environment and may be exposed to unavoidable harsh conditions such as: water penetration, chemical attack, steel corrosion, alkali-silica reactions, freeze-thaw cycles, and carbonic acid change. Long-term exposure to such harsh conditions can cause concrete structures to deteriorate, resulting in increased structural maintenance costs. A key point in these durability issues is the permeability of the concrete matrix. If concrete is less permeable, it will be more durable. The types of constituent materials and new technologies used for UHPC concrete will enable the development of concrete with exceptional durability that can withstand harsh environments and have a long service life.
The main factors controlling permeability are the density microstructure and the porosity of the concrete matrix. Excluding coarse aggregate, adding fine and ultra-fine particles such as dune sand and silica fume, reducing the ratio of water to binder and dilute superplasticizer will work together to homogenize the mixture, thereby greatly reducing its pores.
Water absorption capacity is the permeability coefficient of UHPC concrete and can be considered a sign of its high durability. The decrease in the water absorption capacity of concrete means a decrease in the porosity of the concrete matrix. When the ratio of water to binder is reduced, these pores are reduced. Since the water-binder ratio of UHPC concrete is much lower than that of conventional concrete, the study by Dobias et al. showed that the water absorption coefficient of UHPC is 5 times lower than that of conventional concrete. When the pores are less than one-tenth of ordinary concrete, the matrix of UHPC concrete will be impermeable.
Many researchers have studied different environmental exposures such as freeze-thaw and weather behavior. Freeze-thawing occurs when water particles entering the concrete matrix freeze and expand outside the pores of the concrete. Many researchers have observed little deterioration and negligible quality loss after hundreds of freeze-thaw cycles to 800 cycles. Additionally, Hakeem, Azad, and Ahmad studied wet-dry and hot-cold cycles. They showed that UHPC maintained its strength under aggressive exposure conditions. In fact, the concrete substrate will be durable enough to last a long time without the need for coatings or paints.
2. Mechanical properties
Experimental studies conducted showed that UHPC has excellent mechanical properties as a building material. Compressive strength is the primary mechanical property of UHPC concrete and is critical to ensuring that the structure can withstand specified loads. It is also considered an indicator of other mechanical properties as well as high durability. In fact, high concrete strength is the result of two main principles: the packing of material particles and the ratio of water to binder. Effect of water-binder ratio on UHPC compressive strength. The content of silica fume can also increase compressive strength. The curing method also has a significant impact on compressive strength, with steam curing being superior to ordinary curing.
Steel fibers have no significant effect on the compressive strength, but on the other hand, can increase the tensile strength of UHPC concrete. For steel fibers, the tensile strength of UHPC is usually in the range of 15-20MPa, which is almost twice the tensile strength value of UHPC without steel fibers. This value is almost one-tenth to two-tenths of the compressive strength of UHPC concrete.
Regarding the value of compressive strength, some researchers say that the compressive strength is usually greater than 120 MPa, while others say that it starts from 150 MPa. In either case, it is much stronger than ordinary concrete. According to these researchers, the compressive strength and all mechanical properties of UHPC are far superior to those of ordinary concrete.
Furthermore, the flexural strength of UHPC is shown to be valued at up to 30 MPa, which is achieved in the high-temperature cured state or in the long-term normal state of 28 days. This value has been mentioned in many references. It has been mentioned as over 30 MPa, as high as 40 MPa, or in the range of 30-50 MPa. Since the results derived from the study referenced, the average 28-day flexural strength is 31 MPa, and the reference rule of thumb stating that the flexural strength of UHPC is more than five times that of ordinary concrete, the flexural strength of UHPC is shown in our table as Values up to 30 MPa.
The splitting tensile strength of UHPC is shown in the table as a maximum value of 20 MPa. This value has been mentioned in many references. For example, the reference states that "fiber-added UHPC matrices typically have tensile strengths in the range of 15-20MPa". Furthermore, the reference states that "mixtures have been reported to have split tensile strengths in the range of 8-15MPa". The tensile strength of UHPC can be regarded as one-tenth of its compressive strength, that is, if the compressive strength is 150MPa, it is 15 MPa tensile strength.
The tensile properties of UHPC are different from those of conventional concrete due to the tensile cracking capability of the cementitious composite matrix and the crack-bridging behavior of fiber reinforcement. Compared to fiber-reinforced conventional concrete, UHPC can exhibit significant, sustained post-cracking tensile capacity before crack localization, fiber pullout, and loss of tensile capacity.
Doo et al. pointed out that fiber characteristics such as fiber content, shape, aspect ratio, orientation, and distribution have a great influence on the tensile properties of fiber-reinforced ultra-high performance concrete. Increasing fiber content is the most convincing way to improve tensile properties, including tensile strength and breaking energy capacity. This means that the required tensile strength can be achieved by using a sufficient amount of fiber.
Habel et al. showed a schematic diagram of three different tensile behaviors that UHPC can exhibit: I) linear elastic behavior before cracking; II) strain hardening behavior after cracking and dispersed discrete cracking; and III) crack-specific softening during strain localization. Behavior.
