ADVANCED CONCRETE TECHNOLOGY

ULTRA HIGH STRENGTH CONCRETE

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CONTENTS

1. Introduction 3

2. Composition of UHSC 3

3. Factors affecting the mechanical properties of UHSC 4

3.1. Effect of mineral admixtures 4

3.2. Effect of plasticizers 5

3.3. Effect of fibers 6

3.3.1. Compressive strength 6

3.3.2. Energy Absorption 7

3.3.3. Blast resistance characteristics 8

3.3.4. Workability 8

3.3.5. Disadvantages 8

3.4. Effect of curing method 8

3.5. Effect of temperature 10

4. Durability of UHSC

4.1. Resistance to chloride attack 11

4.2. Freeze-thaw resistance 11

4.3. Alkali-silica reaction 12

5. Applications of UHSC 13

6. Scope for future work 13

References

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1.Introduction:

According to Shah and Weiss (1998), UHSC is defined as a concrete mixture with

compressive strength greater than 150 MPa. It is a new generation concrete and it was

developed through microstructure enhancement techniques. This type of concrete was initially

developed in France and it was also called reactive powder concrete (RPC) because it contains a

larger quantity of silica fume. It is different from high-strength concrete not only because it does

not have coarse aggregates, but also the amount of powders and fibers used. A comparison of

normal-strength, high-strength, and ultra-high-strength concrete is given in the Table below.

Parameter Conventional High Strength Ultra-High Strength

Compressive Strength <50 MPa 50-100 MPa >150 MPa

Water-binder ratio 0.45-0.55 0.30-0.45 <0.24

Chemical Admixture Not Necessary

WRA/HRWRA

necessary HRWRA essential

Mineral Admixture Not Necessary

Fly

Ash/Slag/Rice

Husk Ash

Silica fume and fly ash

Composition of ultra-high-

strength concrete:Type of

aggregate

Gravel/crushed

stone Crushed stone Artificial aggregate

Maximum aggregate size Any Size 10 mm 5 mm

Fibres Not Necessary Beneficial Essential

Processing Conventional Conventional Heat and pressure

2.Composition of ultra-high-strength concrete:

The important characteristics of the ultra-high-strength concrete are:

(1) It has very low water–binder ratio. Since there is no polymer, UHSC can have very good

fluidity when compared with ordinary concrete.

(2) UHSC contains large quantity of silica fume. The incorporation of silica fumes in UHSC can

reach upto 25% by weight of cement.

(3) UHSC contain only fine sand or artificial sand. Since there is no coarse aggregates,

incorporation of large amounts of fibers into UHSC is very easy.

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(4) UHSC contain a high dosage of superplasticizers, so UHSC can have very good fluidity or

workability when compared with ordinary concrete. The high dosage is due to the high

percentage of silica fume and very low w/b ratio.

(5) In order to achieve a compressive strength greater than 150 Mpa, post-set heat treatment and

application of pressure before or during setting may be necessary.

3.Factors affecting the mechanical properties of UHSC:

3.1.Effect of mineral admixtures:

Halit Yazici [1] investigated the effect of high volume mineral admixtures (class C FA

and PS) and curing conditions on the mechanical properties of UHSC was investigated. In order

to obtain a compressive strength of 230MPa, the cement content requirement was as high as

1900 Kg/m3. This leads to high cost and this can be reduced by replacing part of cement with fly

ash and slag. Silica fume was used as binder and fly ash (FA) and slag (PS) of varying

percentage were used to cast the specimen and mechanical properties were studied.

Fig.1.A [1] Fig.1.B [1]

Fig. 1.A indicates the effect of FA (Class C fly ash were used) content on compressive

strength of steam cured (90°C) and standard cured specimens. It is evident from the graph that,

when FA content is increased up to 60% replacement level, there is increase in compressive

strength at all curing conditions. The increase in strength is due to hydration reaction between the

extra lime released from FA and SF. When the FA content exceeds 60%, the compressive

strength is reduced because of disintegration during steam curing.

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Fig. 1.B indicates the effect of PS content on compressive strength of steam cured (90°C)

and standard cured specimens. It is clearly understood from graph that, when PS content is

increased up to 60% replacement, there is increase in compressive strength observed at all curing

conditions. PS replacement above 60% resulted in large reduction in compressive strength for

standard curing. SF which were used as a binder resulted in secondary pozzolanic reaction and

thereby the compressive strength of all mixtures were increased. It can also be observed from

both the figure that, there is similar behavior between controlled mixture and FA or PS mixture.

This is due to presence of significant quantity of SF in the controlled mixture.

3.2.Effect of plasticizers:

In order to have very good workability at low water cement ratios (< 0.25), plasticizers

and superplasticizers are essential. Johann Plank & Christof Schroefl [2] investigated the

effectiveness of Polycarboxylate (PCE) superplasticizers in UHSC and PCE compatibility with

silica fume (SF) and the following observations was found in his research.

SF possess positive surface charge and it competes with the cement to adsorb the

negativley charged PCE molecules on its surface because the surface area of SF is greater than

the surface area of cement. Hence in order to achieve a highly flowable concrete there should be

effective dispersion of SF and not cement. It's because PCE molecules when dissolved in UHSC

mix recognizes SF surface for adsorption and this process in shown in figure

2.

Fig:2 [2]

His research also found that Methacrylate based Polycarboxylate disperses the cement

and Allylether based Polycarboxylate were more effective when SF is used. So a combination of

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Methacrylate based PCE & Allylether based PCE can be used to make UHSC. When different

types of mineral admixtures is used then the surface area of each mineral will vary accordingly

and in such cases combination of superplasticizers with different molecular structures can be

used.

3.3.Effect of fibers:

Conventional design uses high quantity of steel bars at various places in the concrete

which results in high cost and difficulty in placement of concrete. In order to improve these

properties fibers were introduced into the concrete. Fibers can be made of steel, glass and

organic polymers. Banthia et al. 1996 found that steel fiber reinforced concrete had better

energy absorbing and impact resistance capacity. Teng et al. 2004, Deng and Li 2007 also

concluded that incorporation of steel fibers improves the compressive strength, fracture

toughness and durability of concrete.

Yuh-Shiou Tai and Iau-Teh Wang [3] investigated mechanical properties of UHSC made with

different steel fiber fractions. The following results were found from his study.

Fig: 3.A [4] Fig:3.B [4]

3.3.1.Compressive strength

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From figure.3.A it can be found that addition of 1%, 2% and 3% steel fibers resulted in

compressive strength of 139 MPa, 157 MPa and 190 MPa respectively. When the compressive

strength reaches the maximum value, the mechanical behavior of UHSC has linear-elastic

behavior. The reason for the increase in strength can be explained as follows.

If there are no steel fiber or fraction of steel fibers are low, a large number of micro-

cracks is present in the ITZ and cracks extends and results in quick failure of the

specimen.

If higher qauntity of steel fibers are used and when the load is applied, because of the

bridging effect (refer to figure.3.B) of steel fibers, it provides high inertial resistance and

results in delayed failure and thereby increases the dynamic compressive strength.

3.3.2.Energy Absorption:

From figure 4 its evident that energy absorption of specimen with volume of steel fiber

fraction 3% is superior to that of all other specimen. It's clear that when the specimen is

subjected to dynamic loading, the dynamic energy absorption is directly proportional to the

compressive strength and volume of steel fiber fraction.

Fig:4 [3] Fig:5 [3]

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3.3.3.Blast resistance characteristics:

Na-Hyun Yi , Jang-Ho Jay Kim [3] investigated the blast-resistant capacities of UHSC

and RPC. Deflection, strain, and accelerometer measurements from the blast tests revealed that

UHSC and RPC specimens have higher blast-resistant capacities than normal strength concrete

(NSC) specimens [3]. So UHSC are suitable for structures that are susceptible to terrorist attacks

or accidental impacts. It was also found that only few cracks were present after the blast loading

test in UHSC & RPC specimen than NSC. So it needs less effort and cost to repair UHSC &

RPC members than NSC members.

3.3.4.Workability:

Amr S. El-Dieb [5] investigated the effect of including steel fibers with various fiber

volume fractions on the flowability characteristics of the concrete by the slump flow test. The

incorporation of steel fiber affected the slump value of the UHSC. Figure 5 shows the effect of

fibers volume fraction on the slump flow value. When steel fiber volume fraction exceeded

0.52%, there was reduction in flowability about 12%.It is evident that if slump value is needed to

be unaffected then the dosage of admixture should be adjusted.

3.3.5.Disadvantages:

If not compacted properly, steel fibers will tend to move to the top of the concrete

surface. When the concrete starts to harden, then the steel fibers will start to corrode and cause

cracking in the concrete. The cost of steel fiber is very high.

3.4.Effect of Curing method:

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The effect of different methods of curing for identifying the performance of the material,

the microstructure of UHSC was evaluated by Scanning Electron Microscope (SEM) by

Collepardi et al [6]. Figure 6 shows the fracture surface of different types of specimens.

Fig:6 [6]

In order to facilitate the observation of the cement matrix, specimens without steel fibres

were used in his research [6]. It is clear from the figure that the microstructure of autoclaved

specimen (Fig.6.C) is much more dense than that of the UHSC steam-cured at 90°C (Fig.6.B).

This appears be slightly less porous than the material cured at 20°C (Fig.6.A). When cured at

high temperature for prolonged period, it possibly causes extended pozzolanic reactions.

Shrinkage and swelling was found to be lower in steam cured specimen and autoclave

cured specimen when compared with the specimens which where cured at room temperature.

The change in dimensions of the autoclaved specimen was found to be significantly lower - than

the specimen cured at room temperature which can be found in Fig.2. This is due to the micro-

structural densification produced by the autoclave curing at 160°C (Fig.6.C).

Whereas, the shrinkage reduction of the steam-cured specimen with respect to the

specimen which was cured at room temperature was found to be less than 50%, which is clearly

supported by the SEM observation indicated above (Fig.7). Another reason is that the steam

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cured specimens at 90°C were less porous than the specimens cured at room temperature and this

justifies the difference in shrinkage and swelling and the same reason is applicable for

specimens cured at room temperature and that steam cured at 90°C.

Fig:7 [6]

3.5.Effect of temperature :

Taku Matsuda and Hirsohi Kawakami [7] conducted experimental studies on UHSC

subjected to high early temperature simulating the actual structure to examine the development

of mechanical properties. Their investigation revealed that when the concrete subjected to a

maximum temperature exceeds 45 to 60°C, the mechanical properties was found to be different

from the concrete cured at lower temperature. Figure 8 clearly indicates that there is

improvement in the compressive strength of the concrete when Tmax exceeds 45°C and this is

attributed because of low permeability of concrete due densified microstructure.

Fig:8 [7]

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Some studies have also found that at each temperature there is an optimum period of

curing that leads to a maximum compressive strength. Due to high temperature, the chemistry of

the hydration products are alerted. Thus the CSH is formed and converted to a crystalline product

called α-Calcium Silicate Hydrate which increases the concrete porosity thereby it reduces the

strength. Due to continued heating, α-Calcium Silicate is converted to tobermorite (C5S6H5) and

increases the strength.

4.Durability of UHSC:

4.1.Resistance to chloride attack:

Graybeal and Hartmann [8] conducted experimental investigations on concrete electrical

resistivity using Rapid chloride permeability test (RCPT) to evaluate the durability of concrete

against chloride attack. Figure 9 shows the total coulombs passed values and it indicates that

regardless of type of curing the chloride ion permeability is minimum. It can also be observed

that the permeability decreased significantly between 28 and 56 days for ambient air cured

specimens [8].

Fig:9 [8]

4.2.Freeze-thaw resistance:

Figure 10 provides the results from 300 cycles of freeze-thaw testing which was

investigated by Graybeal and Hartmann [8]. The results showed that steam, tempered steam and

delayed steam cured specimen retained the dynamic modulus value close to the original value.

On the other hand ambient air cured specimen dynamic modulus value increased with time [8].

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Fig:10 [8]

4.3.Alkali-silica reaction:

Figure 7 provides the test results of Alkali-Silica Reactivity Expansion value for different

type of curing methods ,which was investigated by Graybeal and Hartmann. In all the cases, the

14 day and 28 day expansion values are below limiting values. It is also evident that the delayed

steam cured specimen has less average expansion and this implies that the type of curing has

many impacts on the durability properties of the UHSC.

Fig.11 (Reference 8)

5.Applications of UHSC:

The impact resistance of UHSC is significantly improved when certain amount of steel

fibers are added. So UHSC is suitable for structures that are susceptible to terrorist attacks or

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accidental impacts. Experimental investigations also revealed that cross section of a beam and

column made with UHSC is similar to that of steel and so there is reduction in self-weight of the

member and also results in smaller cross sections. For example, in Singapore there is space

constraint and scarcity of land and if the columns and beams are made of UHSC then the use of

floor area can be increased.

UHSC with compressive strength of 200 MPa was first used to construct a pedestrian

bridge (Post tensioned precast bridge made with UHSC) in Sherbrook, Canada (refer cover page

photo 1). The thickness of the deck was only 35mm. It was also used to construct the canopies

of a MRT station in Shawnessy, Canada. The dimension of the canopy was 5.1m x 6.0m and

thickness was 20mm and it was supported by only one column. In China, it used to make

pavement cover plates for high speed trains (refer cover page photo 2). Since UHSC has very

low porosity it can also be used for nuclear waste storage structures and also in precast

structures.

6.Scope for future work:

1. Durability problems such as delayed ettringite formation, shrinkage, creep of UHSC can be

investigated.

2. Mechanical properties such as modulus of elasticity and split tensile strength of UHSC need to

be investigated.

3. Incorporation of other types fibers and their effects on mechanical properties of UHSC can be

investigated.

4. Since some type curing needs high temperature, it involves high energy cost, so efforts should

be made to reduce the energy costs.

5.Incorporation of fly ash and other SCM materials should be considered as partial replacement

for silica fume and cement thereby sustainability can be improved.

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References:

1) Halit Yazıcı. The effect of curing conditions on compressive strength of ultra high strength

concrete with high volume mineral admixtures. Building and Environment 42 (2007) 2083–2089

2) Johann Plank, Christof Schroefl, Mirko Gruber, Matthias Lesti and Roland Seiber.

Effectiveness of Polycarboxylate (PCE) superplasticizers in UHSC and PCE compatibility with

silica fume. Journal of Advanced Concrete Technology Vol.7, No.1, 5-12, February 2009.

3) Yuh-Shiou Tai and Iau-Teh. Elucidating the mechanical behavior of ultra-high-strength

concrete under repeated impact loading. Structural Engineering and Mechanics, Vol. 37, No. 1

(2011) 1-15.

4) Na-Hyun Yi , Jang-Ho Jay Kim, Tong-Seok Han , Yun-Gu Cho , Jang Hwa Lee . Blast-

resistant characteristics of ultra-high strength concrete and reactive powder concrete.

Construction and Building Materials 28 (2012) 694–707

5) Amr S. El-Dieb. Mechanical, durability and microstructural characteristics of ultra-high-

strength self-compacting concrete incorporating steel fibers. Materials and Design 30 (2009)

4286–4292.

6) S. Collepardi, L. Coppola, R. Troli, M. Collepardi. Mechanical Properties of Modified

Reactive Powder Concrete.

7) Taku Mastuda, Hiroshi Kawakami, Takao Koide and Takafumi Noguchi. Development of

mechanical properties of UHSC subjected to early curing. Journal of Advanced Concrete

Technology Vol.7, No.2, 183-193, June 2009.

8) Benjamin A. Graybeal, PE, PSI, Inc., McLean, VA Joseph L. Hartmann, PE, Federal

Highway Administration, McLean, VA. Strength and durability of ultra-high performance

concrete.

9) Mehta PK, Monteiro PJM. Concrete; microstructure, properties, and materials. McGraw-Hill;

2006.

10) Cheyrezy P. Composition of reactive powder concretes. Cem Concr Res 1995;25(7):1501–

21.

11) Yi NH, Kim SB, Kim JHJ, Cho YG. Behavior analysis of concrete structure under blast

loading: (II) blast loading response of ultra high strength concrete and reactive powder concrete

slabs (Korean). J Korean Soc Civil Eng 2009;29(5A):565–75.

12) Neville, A.M. “Properties of Concrete”, Fourth Edition, 1995.