FAQs about UHPC Topic
Is there a clear definition of ultra-high performance concrete?
Yes, it is, but there is not yet an internationally harmonized definition. Ultra-High Performance Concrete (UHPC), also known as Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) because of the general need to incorporate short-cut steel fibers or polymer fibers, is different from traditional high-strength concrete (HSC) and steel fiber concrete (SFRC), and is not the high reinforcement of the traditional meaning of "high-performance concrete (HPC)", but is a new generation of cement-based structural engineering materials with clear definitions and required characteristics are as follows.
● It is a cementitious composite material with an optimal grading of the constituent material particles;
●Water-cement ratio less than 0.25, containing a high proportion of micro-fine short steel fiber reinforcing materials;
●The compressive strength is not less than 150MPa; it has the toughness of the tensile state, and maintains the tensile strength of not less than 5MPa after cracking (France requires 7MPa);
●Internal disjointed pore structure with high resistance to gas and liquid immersion, and durability can be substantially improved compared with conventional concrete and high-performance concrete (HPC).
What the Features of UHPC material？
UHPC ultra-high performance concrete is made of ultra-fine active powder, cement, high-strength fibers, etc., through the optimal grading fomulation, high temperature thermal bonding and other specific processes to prepare a composite material, is currently the world's most strong and tough kind of concrete. UHPC's performance indexes are substantially better than ordinary concrete, high-performance concrete. It has a broad application in building structure, building decoration, bridge structure, bridge deck pavement, etc.
UHPC materials by improving the fineness and activity of the components, without the coarse aggregates, use of high-performance water reducing agent(usually polycarboxylate ether based superplasticizer) to significantly reduce the water-cement ratio, so that the internal defects of the material (pores and micro-cracks) to a minimum, in order to obtain high-strength and excellent durability, which ultra-high-performance concrete compressive strength of 100 to 200 MPa.
Is UHPC classfied of concrete or mortar?
concrete and mortar are classified by maximum aggregate particle size traditionally, a maximum size more than 5mm (or 4.75mm) means it contains coarse aggregate and is concrete, otherwise it is mortar. Some UHPC use coarse aggregate, some do not use , so UHPC can be both concrete and mortar.
The earliest UHPC (Denmark Densit ®) maximum aggregate particle size 16 mm. France R & D RPC (Reactive Powder Concrete, referred as one type of the UHPC), in order to improve the homogeneity of the material, do not use coarse aggregate, the use of fine aggregates with a maximum particle size of less than 0.6 mm. However, UHPC using coarse aggregate performs better in certain aspects, such as less shrinkage, better abrasion resistance, and the homogeneity of mixing is better. At present, the actual application of UHPC, the largest aggregate size is mostly in the 2mm ~ 8mm. should be based on the UHPC use, cost or special performance requirements, to determine the appropriate maximum aggregate particle size of UHPC.
Is ultra-high performance concrete stronger than steel?
NO. UHPC has no advantage Compared with steel strength, and its advantage is mainly compared with traditional reinforced concrete or steel fiber reinforced concrete, due to the low apparent density (bulk density), so that the specific strength (strength/mass ratio) and specific stiffness (stiffness/mass ratio) of the member can exceed the level of steel's.
●The ratio of tensile strength to compressive strength (T/C ratio) of concrete materials decreases almost linearly with the increase in compressive strength. the T/C ratio for C10 concrete is about 1/10, and for C80 it decreases to about 1/18. the UHPC matrix (without steel fibers) , is very brittle (due to the very low T/C ratio), not comparable to ductile materials such as steel.
● With the increase in strength and dosage of steel fibers, as well as the improvement of matrix strength, UHPC become toughness or high toughness materials, tensile strength increased substantially, but it is still an order of magnitude lower than the tensile strength of steel.
●UHPC relies on the strength of steel fibers and reinforcing bars-to achieve such high strength as a composite material or structure; therefore, it is not possible for UHPC to exceed the strength of steel.
What is the maximum strength of UHPC?
Currently, the highest compressive strength obtained in a research report is 810 MPa. Today, the strength commonly in the range of 1 5 0 MPa to 250 MPa. According to the application occasions, it is necessary to choose the appropriate fiber species and dosage to achieve the required tensile or flexural strength of UHPC; the configuration of reinforcing steel or pre-stressing technology to achieve the lightweight and high strength of the UHPC structure.
What are the application areas of ultra-high performance concrete?
UHPC has had many engineering applications Worldwide as follows:
● Bridges: a wide range of bridge structures for footbridges (the earliest application of UHPC structures in 1997), highway and railroad bridges, for increasing bridge spans or reducing bridge height/span ratios, for rapid bridge construction, and for improving bridge durability and longevity.
●Bridge decks: To solve the problem of rapid deterioration of bridge decks due to freeze-thaw and de-icing salts, to improve durability, and to enable rapid construction.
●Structural connection: Grouting connection of precast concrete bridge decks, wind power steel towers, etc., realizing high-strength structural connection.
Maintenance and reinforcement: beams and columns of bridges and buildings, light towers, road pavements, water-industrial washout structures, etc., for structural protection, functional recovery, or structural reinforcement, to improve structural load-bearing capacity, and to extend the service life of the structure.
●Housing construction: thin-walled balconies and staircases, hollowed-out curtain walls, etc., used for lightweight and beautiful structures, or structures with integrated load-bearing, functionality, and decoration.
Sewage facilities: Sewage pipes, sewage treatment plant facilities, reducing maintenance and repair costs, improving service life or replacing stainless steel to reduce construction costs.
●Street furniture: urban sculpture, street furniture, furniture, etc., beautiful shape, durable and long-lasting.
● Covers: High-speed rail cable trench covers (major application in China), reduced weight, high durability.
● Certain alternative cast iron and cast steel products: drilling pile drills, inspection manhole covers, rainwater grates, etc., reducing cost and improving service life.
●Explosion- and intrusion-resistant structures: military engineering, bank vaults, etc., improving security, etc.
What are the pros and cons of UHPC?
UHPC is suitable for the construction of "thin,slim, smart, light" concrete structures, changing the appearance of concrete structures "fatty beams and columns". Theoretically and from the current test results, in most of the harsh natural environment, UHPC structural life is expected to be HPC structural life of at least two times. In marine environments, it is entirely possible for UHPC structures to have a working life of more than 200 years.
In terms of specific strength and stiffness, steel reinforced UHPC (CRC, HRUHPC or R-UHPFRC) beams are able to reach the level of steel beams.
Cons：UHPC materials are expensive to prepare, the structural design and construction are complex, and the maturity of the current application technology development is still low. It is not easy to prepare UHPC with good and stable performance by industrial production
Now, the technical way of formulating UHPC and the use of materials showing diversified development, in addition to cement and silica fume, the application or research of mineral raw materials, including nano-silica, nano-calcium carbonate, nano carbon tubes, grinding or sorting of ultra-fine coal ash, ultra-fine mineral powder, ultra-fine cement, rice husk ash, metakaolin, quartz powder, glass powder, and so on, the influence of the performance of the UHPC material factors are diversified and complex, which increases the difficulty of the quality control of the production.
What are the bottlenecks in the engineering application of ultra-high performance concrete? Will it be widely used?
The bottlenecks are lack of design methods and standards, the lack of construction experience and the high cost of materials.
So far, France, Japan and South Korea have UHPC design guidelines that can be used as the basis for structural design, and the specifications or guidelines of some countries and international organizations (fib, ACI, etc.) are already in the drafting or perfecting stage.
In the UHPC material cost composition, steel fiber accounts for the largest proportion, reaching more than half of the total material cost of UHPC; matrix materials, including cementitious materials (cement, silica fume, etc.), water-reducing agents and aggregates belong to the general quality materials, although they need "carefully selected", the cost is high to a limited extent. Therefore, improve the efficiency of fiber reinforcement toughening, is the key to reduce the cost of UHPC materials. Now UHPC application scale is relatively small, is also the reason for its high cost. With the increase in the scale of application, UHPC raw materials, especially the large-scale industrial production of fibers and full market competition, the cost is expected to significantly decreased。
Compared to ordinary, high strength/high performance concrete, the complexity and higher cost of UHPC preparation and construction techniques dictate that UHPC will not be a universally applicable engineering material, but rather a useful complement to modern engineering materials, and is used to solve difficult problems encountered in engineering and to build better performing and more aesthetically pleasing engineering structures.
However, the value and potential of UHPC, which is now far from being fully explored and effectively utilized, is still facing a huge scope for development which will be reflected in AS FOLLOW:
● For innovative lightweight, high strength and toughness concrete structures; in some occasions, to reduce the overall construction cost of a structure or project;
● For building engineering structures with ultra-high durability and ultra-long working life, low maintenance and repair costs, and obtaining lower Life Cycle Cost than conventional structures;
● Application of UHPC to build structures with significant material savings, energy savings and emission reductions, which is conducive to sustainable development.
