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Ultra High Performance Concrete



Introduction


Within the past twelve years, High Performance steel Fiber Reinforced

Cementitious composites (HPFRCCs) which have compressive strength over 200

MPa has been developed by several researchers. These high performance concretes

allow remarkable flexural strength and very high ductility. Since UHPCs have

excellent impact resistance properties, they can be employed for;


  1. Military structures,

  2. Strategic structures against earthquake,

  3. Retrofitting of reinforced concrete structures.


They are also used for small or medium size

prefabricated elements. From the strength point of view, classification of high

strength/high performance concretes can be made as;


  • normal strength concrete up to grade 60 MPa,

  • high strength concrete, grades 60-90 MPa,

  • very high strength concrete, grades 90-130 MPa,

  • reactive powder concrete, grades 200-800 MPa,

  • high performance lightweght concrete, greater than 55 MPa.


Ultra high performance concrete are materials that can be defined by their high strength, high durability and high ductility. Unlike normal concrete, new generation superplasticizers are obtained by using fine aggregate, silica fume and steel fiber. The most important factor that enables them to be distinguished from high strength concrete is their ductile behavior. Thanks to this ductility feature, they have a very high energy absorption capacity compared to conventional concrete. Highly resistant silica sand and powder are used in order to achieve maximum fullness and grain optimization in the mixture. With the use of superplasticizers, the amount of water required for workability in concrete is minimized and gaps are reduced. The addition of steel fibers is necessary for micro level crack control and ductility. Finally, the silica fume is activated by applying heat treatment to the concrete produced in this way. After all these processes, an impermeable concrete is obtained by triangulating the aggregate-cement paste contact surface.


The first studies on ultra-high performance concrete have shown that concretes produced with silica fume, superplasticizer additive, fine aggregate and steel fiber can be heat treated at different temperatures and concretes with compressive strengths from 200 MPa to 800 MPa can be obtained. It is also noted that steel fiber reinforced concrete has much higher ductility, therefore energy absorption capacity, than lean ones.


Several experiment results found no reduction in strength of reactive powder concrete (RPC) subjected to 300 freeze-thaw cycles. In these experiments applied mercury porosimetry test to reactive powder concrete and observed that the porosity did not exceed 9%. Roux and colleagues, [4] found that RPC 200 (reactive powder concrete 200) has a 50 times lower diffusion coefficient and very low water absorption value than conventional concrete, thus making it a much denser and impermeable material.


In addition, silica sand with the largest particle size of 2 mm in order to ensure grain optimization in the study and to purify the mixture from large gaps, the water / binder ratio


Superplasticizer to increase compressive strength by minimizing and filling the gaps between cement grains and pozzolanic at high temperatures silica fume was used to take advantage of its properties.


What is Ultra High Performance Concrete ?


Ultra-High Performance Concrete (UHPC) is a cementitious, concrete material that has a minimum specified compressive strength of 17,000 pounds per square inch (120 MPa) with specified durability, tensile ductility and toughness requirements; fibers are generally included in the mixture to achieve specified requirements.


Strength

Compressive: 17,000 to 22,000 psi, (120 to 150 MPa)

Flexural: 2200 to 3600 psi, (15 to 25 MPa)

Modulus of Elasticity: 6500 to 7300 ksi, (45 to 50 GPa)

Durability

Freeze/thaw (after 300 cycles): 100%

Salt-scaling (loss of residue): < 0.013 lb/ft3, (< 60 g/m2)

Abrasion (relative volume loss index): 1.7

Oxygen permeability: < 10-19 ft2, (<10-20 m2)


Components/Ingredients of Ultra High Performance Concrete


Ultra-High Performance Concrete (UHPC), is also known as reactive powder concrete (RPC). The material is typically formulated by combining Portland cement, supplementary cementitious materials, reactive powders, limestone and or quartz flour, fine sand, high-range water reducers, and water. The material can be formulated to provide compressive strengths in excess of 29,000 pounds per square inch (psi) (200 MPa). The use of fine materials for the matrix also provides a dense, smooth surface valued for its aesthetics and ability to closely transfer form details to the hardened surface. When combined with metal, synthetic or organic fibers it can achieve flexural strengths up to 7,000 psi (48 MPa) or greater.


Fiber types often used in UHPC include high carbon steel, PVA, Glass, Carbon or a combination of these types or others. The ductile behavior of this material is a first for concrete, with the capacity to deform and support flexural and tensile loads, even after initial cracking. The high compressive and tensile properties of UHPC also facilitate a high bond strength allowing shorter length of rebar embedment in applications such as closure pours between precast elements.


Methods for achieving Ultra High Performance Concrete


The main factors determining the final mechanical performance parameters of UHPC concretes are:

  • The W/C ratio within the range of 0.16 - 0.23, depending on the consistency required.

  • The CEM I 52.5 type of cement with a low C3A content and Blaine value of about 4000 cm2 /g.

  • The amount of silica fume (SF), with dimensions of 0.1–2.5 µm and C/SF = 1:0. 25 .

  • The amount of glass powder (GP) or other filler, with dimensions of 0.8–4 µm and similar material parameters, C/GP = 1:0.25 .

  • The proportions of sand A (80–200 µm) and sand B (400–800 µm) at 30%/70%.

  • The type of superplasticizer used—polycarboxylate based, ideally at 0.5%–2.0%.

  • A sand-to-cement ratio of 1.1–1.4 .

  • The amount and type of steel fibers were obtained for L/S = 1.5%/0.5%. Experiments examined the influence of the type of steel fiber on the strength of the concrete; significant increases in the mechanical parameters of the concrete were achieved by using certain fibers

  • The mixing method, speed and duration presented the calculations for mixing time depending on the tool speed. The best result was obtained for the B sequence: cement and dry silica fume (1 min, 15–20 RPM), 80% water + 100% SP (3 min, 20–60 RPM), sand and quartz powder (4 min, 20–60 RPM), and 20% water (4 min 90 RPM).

  • The type and size of aggregate. Experiments showed that replacing fine sand with coarse aggregate of maximum size 8 mm had no effect on the compressive strength. It is reported that ultra-high-performance fiber-reinforced concrete (UHPFRC) including coarse aggregate with a maximum grain size ranging from 7 to 16 mm exhibited a slightly higher compressive strength of 178 MPa, compared to its counterpart without coarse aggregate (162 MPa).

Characteristics of Ultra High Performance Concrete


Compressive Strength

Compressive strength is an important property in the design of any concrete structure. The typical compressive strength of UHPC is in the range of 150 to 250 MPa. UHPC shows a linear elastic behavior until about 70% to 80% of the com-pressive strength. The failure of UHPC without fibers is of explosive nature. There is no descending branch in the stress-strain-diagram . The addition of fibers could im-prove the compressive strength of UHPC. A typical stress-strain curve for UHPC is shown in Figure . Due to the crack-bridging effect of the fibers the curve has a descending branch. The range of the possible descending depends on the fiber content and fiber orientation.


Tensile Strength

Depending on the content and type of the fibers, the tensile strength of UHPC is normally in the range of 7 to 15 MPa. Due to the crack bridging effect of fibers, the tensile behavior of UHPC becomes ductile. Graybeal proposed an idealized tensile stress-strain response of UHPC . This response is based on direct tension tests of two UHPCs with multiple fiber contents.


Figure 1 : Stress-Strain diagram of UHPC without Fibers


Figure 2: Stress-Strain Diagram of UHPC with Fibers



Figure 3 : Idealizad uniaxial tensil response of UHPC



Modulus of Elasticity

Due to the dense microstructure, the elastic modulus of UHPC is higher than that of NC and HPC when the same type and amount of aggregates are used. Normally, the value ranges from 40 to 70 GPa, depending on the composition of the mixture, amount and type of the aggregates and the curing regime.


Shrinkage

Three types of shrinkage may be present in UHPC: chemical shrinkage, autogenous shrinkage and dry shrinkage. All these three kinds of shrinkages are mainly affected by w/b and cement content. For UHPC, due to the very low w/b and high cement content, the autogenous shrinkage is predominant. In the early age, the high autogenous shrinkage leads to a high risk of micro-cracking if the UHPC element is restrained.

Several methods have been developed to offset the autogenous shrinkage of UHPC. One is to use an expansive additive or shrinkage reducing additive. They both have been proved to be very effective to reduce the autogenous shrinkage of UHPC. But the dosages of these additives have to been carefully determined.

Another solution is internal curing, which is defined by the American Concrete Institute (ACI) as "supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation". Lightweight aggregate is the most common material used as a water reservoir. However, the size of the lightweight aggregate makes it unsuitable to be used in UHPC. Researchers throughout the world are also investigating the possibility of using superabsorbent polymers (SAP) and RHA as internal curing agent. RHA is considered as a very promising material to act as internal curing agent in UHPC because of its porous structure and pozzolanic reactivity. The development of drying shrinkage of UHPC is similar as HPC. Due to the low w/b, high density of the matrix and the presence of fibers, the amount of drying shrinkage of UHPC is lower compared with that of HPC. For heat treated UHPC, dry shrinkage can be neglected after the end of heat treatment.


Creep


The creep of UHPC is generally less than that of NC and HPC. The creep of concrete can be expressed as a creep coefficient (=creep strain/initial strain) or specific creep (=creep strain/applied stress). The creep coefficient of UHPC ranges from 0.3 to 0.85, and the specific creep ranges from 5 to 47 μm/m/MPa. For comparison, the specific creep for NC is in the range of 35 to 140 μm/m/MPa.


Thermal Properties


The thermal expansion coefficients of UHPC measured by various researchers are in the range of 10 to 15 μm/m/K. In the French Interim Recommendations, 10.4–11.8 μm/m/K have been recorded as the general value. This value is in the same range as for normal concrete of about 11.0 μm/m/K.


Fire Resistance


Like all concrete, UHPC is non-combustible. However, due to the extreme dense microstructure, UHPC is more susceptible to explosive spalling during heating, which is the most dangerous damage mechanism for the structures built with this material. The spalling of UHPC is caused mainly by pore pressure due to the dehydration of hydrates and thermal stresses. A well-known solution to improve the fire resistance of UHPC is to include polypropylene fibers in the UHPC mixture. Although experience shows the effectiveness, efficiency mechanisms behind this are still under investigation. A widely accepted explanation is the interconnected pore system building-up at temperatures of about 160°C when polypropylene fibers melt. The interconnected pore system is then capable of distributing the developing vapor pressure with the volume gained by fiber melting, and finally releasing it.


Fatigue Behavior


Only a few studies were performed on the fatigue behavior of UHPC. This is due to the fact that this material still only has a few applications and research in this field is still in progress. The results of the studies showed that UHPC had a good resistance to fatigue. It can be said that, in contrast to other high strength materials, the high strength of UHPC does not leads to disadvantages with regard to the fatigue.


Impact Resistance


Thanks to the presence of fibers and the strong bond between the fibers and matrix, UHPC has a much higher impact resistance than NC or even normal fiber reinforced concrete (FRC). So UHPC has the potential to be used for military structures to resist the bomb blast or penetration. Bindiganvile compared the impact resistance of UHPC with that of conventional FRC. Under impact loading, UHPC was approximately twice stronger than conventional FRC and dissipated three to four times of energy. Lai studied the bullet penetration resistance of UHPC. The results showed that UHPC with coarse aggregates (Max. 16 mm) and more steel fibers had higher resistance to the bullet penetration.


Stress-Strain Behavior


The behavior can be divided into four phases. Phase I is the elastic phase, when the matrix stands the loads. Phase II is the multi-cracking phase, wherein multiple tightly-spaced cracks form in the UHPC matrix. The cracks occur when the stress in the matrix exceeds the tensile strength of the matrix. Phase III is the strain hardening phase. Individual cracks become wider in this phase, and no new cracks form. Lastly, Phase IV is the strain softening phase. The cracks begin to localize in this phase, and fibers begin to be pulled out from the matrix. After that, the material fails. It should be noted that the tensile stress-strain curve may not act exactly as illustrated in Figure , because the post- cracking behavior (phases II, III and IV) of UHPC largely depends on the content, type and orientation of the fibers. If very few fibers or even no fibers are used, there will not be phases II, III and IV.


Figure 4 : Idealizad uniaxial tensil response of UHPC

Figure 5 : Stress-Strain diagram of UHPC without Fibers



Advantages and Disadvantages of UHPC

ADVANTAGES

DISADVANTAGES

  • User Friendly

  • Scalable

  • Pourable

  • High Quality

  • Very High Strenght


  • An extended quality control

  • High Cost

  • Special Constituents

  • Should manufactured and placed very carefully

  • High transportation fees


Applications of UHPC


Because of the excellent performance of UHPC, its applications are increasing gradually in the recent years, especially in Europe, North America and Japan. The main applications of UHPC are bridges, buildings, structural strengthening and retrofitting, and some special applications. Some specific examples will be given in the following sections.


In Bridges


The advanced mechanical properties and durability of UHPC make it possible to reconsider the conventional design methods for many common bridge components. Many investigations have been conducted on the optimal designs with UHPC elements, resulting in the development and construction of the UHPC bridges all over the world. Ac-cording to the state of the art report on UHPC published by U.S. Federal Highway Administration in 2013, 55 bridges using UHPC have been built or are under construction in U.S. and Canada. There are 22 in Europe and 27 in Asia and Australia.



In Buildings


In addition to the applications on bridges, the field of building components, such as sunshades, cladding and roof components, was the leading domain of UHPC applications in the last decade. UHPC could be used to produce very slender, durable and aesthetic structures.



Picture :Fondation Louis Vuitton pour la Creationin,Paris France

Picture : The Haneda Airport


Picture :Fondation Louis Vuitton pour la Creationin,Paris France

Picture : Two Union Square Building , Seattle , USA


Picture :Pont du Diable Brigde , France

Picture :Sakata-Mirai Footbrigde ,Japan


Picture :Seonyudo Footbrigde ,South Korea


Picture :Fondation Louis Vuitton pour la Creationin,Paris France


Conclusion


At present, UHPC is one of the most modern technologies of the cementitious materials. By improving the homogeneity and packing density, UHPC with outstanding mechanical properties and durability could be prepared. It has an extremely dense micro-structure, and normally capillary pores do not exist in UHPC. The mechanical properties and durability of UHPC are both much better than HPC and NC. When using UHPC, attentions have to be paid to the autogenous shrinkage and fire resistance of this material. UHPC is a high-tech material following new technological rules regarding its composition, its production and the mechanical behavior, as well as regarding design and con-struction of structures.In addition, further scientific researches on UHPC and UHPC structures also must be continued. With all these efforts, UHPC may turn into a widespread ‘regular’ technology, thus resulting in more sustainable and durable infrastructures.



Thank you for your time and attention for the topic, see you at the next ones!


Have you checked my blog post called "Deep Talks About Earthquake" ?







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