Ultra High-Performance Concrete (UHPC)
Ultra High-Performance Concrete is a concrete which exceeds a compressive strength of at least 150 MPa. As consequence UHPC can compete technically and economically with steel in its load bearing capacity as well as in dimension and weight. In this way UHPC offers new markets like filled steel tubes, pillars, girder, bridges, driven pile, sheet pile wall, bullet proof walls, safes, etc.
UHPC is characterized by high cement contents (in the range of 700 to 900 kg/m3) and low water/cement ratios (w/c ≤ 0.20). Together with the use of reactive silica fume (10–20% of cement) and an optimized aggregate grading the resulting concrete has an extremely dense structure beside its high strength. Due to the high density the durability performance of this concrete is extraordinary. Weak points like fire resistance and its fragile fracture behavior can be eliminated with organic fibers and steel fibers. Besides strength optimization, the control of workability is a real challenge. A new generation of superplasticizers (PCE) enables the design of mixtures with nearly self-compacting properties.
The changes from 3 components concrete (cement, aggregates, water) to 5 components concrete (cement, aggregates, water, admixture, additives) enable new innovations in concrete technology. This technical progress opened the way to a new type of concrete: Ultra High-Performance Concrete UHPC is characterized by an extremely high compressive strength, which exceeds at least 150 MPa. In some laboratory tests about 500 MPa have already been exceeded under special production and curing conditions. UHPC used in practice today has in general a compressive strength of 150 – 220 MPa. Because of its extremely dense structure UHPC’s also improve the durability performance like low carbonation rates good protection of reinforcement steel high resistance against chemical attack high frost resistance.
The high load bearing capacities of UHPC will probably enable new markets for concrete. As an example, the comparison of the cross sections of girders indicates that UHPC constructions can be produced in a similar range of dimension and weight as steel girders. The production costs of UHPC seem to be high when calculated on the volume basis (m3). But if the cost calculation is done on a weight basis, as it is generally done for steel, it becomes obvious that UHPC can also economically compete with steel. Thus, possible applications and markets for UHPC are filled steel tubes, pillars, girder, bridges, driven pile, sheet pile wall, bullet proof walls, safes etc.
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Design of UHPC
The selection of cements with suitable quality is an essential part of successful mix optimization. But as UHPC is a multi-component system competences are necessary in the evaluation and selection of all materials used in UHPC. Know how must also be available in the design of concrete meeting the demanded properties with special focus on the workability and the intended mechanical performance.
The specific characteristics of UHPC composition commonly are:
? high cement content (600-700 kg/m3 or more)
? low water/cement ratio (below 0.25)
? addition of silica fume
? sand as aggregate
? steel fibers to improve ductility
? superplasticizers
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Cement:
Cement for UHPC should be of high strength the fineness and correspondingly the water demand should be low to allow sufficient workability with limited use of superplasticizers. many cases cements with a low C3A-content and low alkali level (Na O-equivalent ≤ 0.6 %) are recommended to be favorable. The effective interaction of cement and superplasticizers is one of the most decisive criteria when selecting appropriate materials for UHPC. For the evaluation of suitable cements, a simple screening test was developed. Within this screening test the cement paste flow is determined in combination with 3 different superplasticizers at given dosages. This superplasticizer is representing a wide range of admixtures. When a cement paste flow of 250 mm can be easily achieved or exceeded with each of the chosen superplasticizers the cement was found to be suitable for UHPC production.
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Aggregates:
The strength performance of UHPC strongly depends on the grain packing density (grading) particularly of material with grain size < 0.5 mm. Theoretically, UHPC can be produced with all kinds of maximum grain sizes. By increasing the grain size and adjusting the grading less binder is necessary as the void volume is reduced.
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Fibers:
Steel fibers are used in UHPC to improve the ductility, results taken from optimization procedure showed the strength improvement by fibers, especially the flexural strength increased significantly which allows the concrete to transmit tensile forces on a much higher level. Fiber content, shape and length are further parameters influencing the performance of UHPC. To increase the fire resistance of UHPC, polypropylene fibers can be added to the concrete.
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Silica Fume (micro silica):
Silica fume is a highly pozzolanic material with article diameter of 0.1 μm (100 times finer than cement). Due to the high finesses it contributes to strength physically by filling small voids between cement particles. The reaction with Ca (OH)2 remaining from Portland cement hydration results in the formation of additional CSH-phases giving a further strength increase (especially improving bond between aggregates and cement paste). Silica fume could increase the water and admixture demand of concrete. Especially when used as compacted powder, attention has to be paid on adequate dispersion during the mixing process.
Admixture:
Because of the high content of fine material in combination with very low w/c-ratio very effective superplasticizers are necessary to produce UHPC. The on-going development of PCE′s enabled solutions which were not possible with the traditional types of admixtures.
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UHPC is a product which can prove the innovative character of concrete in the competition with other construction materials. Because of its high strength it can easily compete with dimensions and weight of steel products. Moreover, the extremely dense structure of UHPC leads to a high resistance against all kinds of chemical and physical attacks and increases the durability.