Ultra-High Performance Concrete: Mechanical Performance, Durability, Sustainability and Implementation Challenges

Outline

• Abstract
• 1 Introduction
• 2 Uhpc Composition
• 3 Mixture Design of Uhpc
• 4 Fresh Properties of Uhpc
• 5 Mechanical Properties
• 6 Durability Properties
• 7 Cost Estimation and Sustainability of Uhpc
• 8 Current Challenges for Implementation of Uhpc
• 9 Summary and Conclusions
• References
• Copyright Information

Conclusions

An extensive literature review was conducted in this study on the distinctive features of UHPC. The unique properties of UHPC have several advantages over normal-strength concrete (NSC) owing to its material ingredients and composition. The key factor in producing UHPC is to improve the micro and macro properties of its mixture constituents to ensure mechanical homogeneity and denser particle packing. UHPC yields high compressive strength (i.e. >150 MPa (22 ksi)) due to its improved internal micro- and macro-structure, leading to denser concrete. The application of thermal curing further densifies UHPC, which results in higher compressive strength properties. The typical heat treatment applied for UHPC is 90–400 °C (194–752 °F) for 2–6 days. The specimen size significantly affects the measured compressive strength of UHPC. Smaller size specimens can be used if the test machine capacity is limited. Furthermore, it was observed that the loading rate did not significantly affect the measured compressive strength of UHPC. The compressive stress–strain response of UHPC shows a linear elastic behavior up to 80–90 % of the maximum stress value.

UHPC exhibits high flexural strength properties (i.e. up to 48 MPa (7.0 ksi)) depending on its mixture design and curing regime. It was reported that horizontally cast beam specimens achieved nearly five times higher flexural strength compared to that of vertically cast beam specimens due to improved fiber orientation. Furthermore, the flexural strength of UHPC is dependent on the pouring method of concrete into molds. For instance, pouring concrete from one end of the mold increased the flexural strength compared to that of the same concrete poured from different places into the mold. This was mainly due to the strong fiber orientation (higher number of fibers crossing at particular sections) parallel to the flow direction. The flexural strength of UHPC increased linearly with increased fiber dosage. UHPC mixtures incorporating higher aspect ratio fibers had increased flexural capacity compared to that of those with lower aspect ratio fibers. The flexural strength of UHPC decreased as the specimen size increased.

UHPC exhibits higher bond strength to rebar and fibers owing to its dense micro- and macro-structures. UHPC elements typically require smaller reinforcement due to higher mechanical and durability properties. Smaller concrete cover in UHPC members further reduces the cross-sectional dimensions leading towards more economical construction. It was reported that the application of UHPC leads to reducing earthquake design loads due to a decrease in the overall structural weight. Furthermore, UHPC exhibited excellent performance against impact loading and is thus highly desirable in military structures where impact resistance due to blast loading is of concern. UHPC also demonstrated superior behavior against fatigue loading and showed no significant sign of failure after 106 load cycles.

UHPC exhibits high durability owing to a substantial decrease in the volume and size of pores. The application of pressure during the initial setting of UHPC specimens also reduced the overall porosity by removing entrapped air and additional water. UHPC shows very low water absorption due to its dense microstructure. The chloride diffusion coefficient of UHPC is significantly lower compared to that of NSC and HSC, leading to reduced corrosion risk. The reduced permeability and porosity of UHPC enables better resistance to freezing-thawing cycles. On the other hand, due to reduced porosity, UHPC structures are more vulnerable to fire and elevated temperatures due to obstruction in the release of vapor pressure, leading to physical damage. However, this issue can be mitigated by the use of polypropylene fibers.

The initial material cost of UHPC is higher than that of NSC due to the very high cement content and steel fiber addition. Globally accepted design provisions need to be developed in order to provide confidence to the design engineer in utilizing the high strength and other properties of UHPC. In short, it can be concluded that UHPC can be a sustainable material due to its improved mechanical and durability properties, ecological factors, economical benefits and its recycling ability in various applications.