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Mix Proportions and Properties Assessment of HPC and UHPC Using Low WaterBinder ratios

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High performance concrete (HPC) and ultra high performance concrete (UHPC) have been increasingly attracting industry’s attention worldwide. Reinforced concrete projects with particular requirements like high rise buildings, long span bridges, tunnel linings, offshore oil platforms, and nuclear power plants have employed such materials due to their superior properties compared to its normal strength counterparts, such as high compressive strength, modulus of elasticity, density, and resistance to chemical and physical deterioration. Durability, economy of longterm maintenance, economy of construction, opportunity for ergonomical and better aesthetical solutions by engineers and architects are among the benefits of utilising HPC and UHPC, but with the penalty of more stringent quality control requirements. European Union through the harmonized regulations of the EUROCODES do not cover neither HPC nor UHPC, thus creating tremendous competitive limitations for the European Construction Industry. The European code of practice for the design of structures for earthquake resistance (EUROCODE 8, EN1998Part 1 for buildings and EN1998Part 2 for bridges) limits the strength of concrete to C4050, while EUROCODE 2 1992Part 1 for the design of concrete buildings (without earthquake resistance requirements) allow concrete strength of C90105. Both EUROCODES 2 and 8 limit the grade of reinforcing steel to 600 MPa, while the commonly used grade is 500 MPa (e.g. Greece). Therefore, whenever such materials are to be utilized in an infrastructure project, the mechanical properties need to be verified by carrying out relevant experimental tests thus leading to unavoidable delays and indirect cost increases. On the other hand, it is unrealistic to assume that the structural behavior of buildings and bridges made of high and ultra high strength concrete can be understood simply by extrapolating the knowledge of current normal strength counterparts. The proposal to include HPC and UHPC in the EN version of EUROCODE 8 was put forward, but the drafting committee decided against it at the time due to the scarcity of information regarding the performance of structures using these materials under seismic loading. From previous experience it is evident that development of concrete technology mostly relies on empirical approaches 1. The presented experimental work is part of a broader experimental program conducted on 120 column specimens to assess their performance under uniaxial and repeated loading. The scope of this study was to define the mixture design method to be used in the production of HPC and UHPC using locally available materials.

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Mix Proportions and Properties Assessment of HPC and UHPC Using Low Water/Binder ratios

Dimirios Konstantindis Department of Civil Engineering

Alexander TEI of Thessaloniki

Thessaloniki, Greece

Georgios Sapidis Department of Civil Engineering

Alexander TEI of Thessaloniki

Thessaloniki, Greece

Angelos Patsios Department of Civil Engineering

Alexander TEI of Thessaloniki

Thessaloniki, Greece

Konstantinos Anagnostopoulos Department of Civil Engineering Alexander TEI of Thessaloniki Thessaloniki, Greece

Athanasios Valmis Department of Civil Engineering Alexander TEI of Thessaloniki Thessaloniki, Greece

Grigorios Grigoriou Department of Civil Engineering Alexander TEI of Thessaloniki Thessaloniki, Greece

Abstract—This paper presents the findings of a research program

aimed at developing a mixture design for high strength and ultra

high strength concrete using locally available materials Sixteen

different concrete mix designs were examined containing varying

water/binder and coarse/fine aggregate ratios Concrete mixes

with smaller size aggregates exhibited slightly higher strengths at

a given aggregate content level, while the reduction of

water/binder ratio with a simultaneous increment of

superplasticizer content resulted in a slightly higher strength

Keywords- High performance concrete, Ultra high performance

concrete, Mixture design

I INTRODUCTION

High performance concrete (HPC) and ultra high

performance concrete (UHPC) have been increasingly

attracting industry’s attention worldwide Reinforced concrete

projects with particular requirements like high rise buildings,

long span bridges, tunnel linings, offshore oil platforms, and

nuclear power plants have employed such materials due to their

superior properties compared to its normal strength

counterparts, such as high compressive strength, modulus of

elasticity, density, and resistance to chemical and physical

deterioration Durability, economy of long-term maintenance,

economy of construction, opportunity for ergonomical and

better aesthetical solutions by engineers and architects are

among the benefits of utilising HPC and UHPC, but with the

penalty of more stringent quality control requirements

European Union through the harmonized regulations of the

EUROCODES do not cover neither HPC nor UHPC, thus

creating tremendous competitive limitations for the European

Construction Industry The European code of practice for the

design of structures for earthquake resistance (EUROCODE 8,

EN1998-Part 1 for buildings and EN1998-Part 2 for bridges)

limits the strength of concrete to C40/50, while EUROCODE 2 1992-Part 1 for the design of concrete buildings (without earthquake resistance requirements) allow concrete strength of C90/105 Both EUROCODES 2 and 8 limit the grade of reinforcing steel to 600 MPa, while the commonly used grade

is 500 MPa (e.g Greece) Therefore, whenever such materials are to be utilized in an infrastructure project, the mechanical properties need to be verified by carrying out relevant experimental tests thus leading to unavoidable delays and indirect cost increases On the other hand, it is unrealistic to assume that the structural behavior of buildings and bridges made of high and ultra high strength concrete can be understood simply by extrapolating the knowledge of current normal strength counterparts The proposal to include HPC and UHPC in the EN version of EUROCODE 8 was put forward, but the drafting committee decided against it at the time due to the scarcity of information regarding the performance of structures using these materials under seismic loading

From previous experience it is evident that development of concrete technology mostly relies on empirical approaches [1] The presented experimental work is part of a broader experimental program conducted on 120 column specimens to assess their performance under uniaxial and repeated loading The scope of this study was to define the mixture design method to be used in the production of HPC and UHPC using locally available materials

II MATERIALS

Sixteen different mix proportions, with C1 to C16 notations were examined for producing HPC in the Concrete Laboratory

at the department of Civil Engineering of the Alexander Technological Educational Institute of Thessaloniki The constituent materials were cement, silica fume, natural crushed stone sand, natural crushed stone aggregate with a maximum

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size of 12.5 mm, tap water for mixing and curing and

superplasticizer The details of the mixing proportions are

given in Table I Portland cement type CEM I 52.5 N

according to EN-197-1 was supplied by TITAN cement

producer The amount of the cement content in the mix ranged

between 550 to 750 Kg/m3 Table II, shows the chemical

composition of the cement used It has a specific gravity of

3.15 and a blaine fineness of approximately 4.66 m2/Kg Silica

fume obtained from a ferro-chromite factory was used in all

mix proportions It has a specific gravity of 2.3 and a blaine

fineness of about 10000 m2/Kg Its’ chemical composition is

shown in Table III The amount of Silica fume added in the

mix ranged between 50 to 137.5 Kg/m3 In order to adjust

concrete workability a polycarboxylate ether based

superplasticizer provided by SIKA was selected as high range

water reducer Its’ properties are summarized in Table IV The

aggregates used to make HPC were brought from Mount

Olympus Two size ranges of natural coarse aggregates were

used, the first 12.5 to 6.3 mm and the second 6.3 to 4.75 mm

Natural fine course aggregate of 4.75 to 0.425 mm was also

used Table I, also summarizes the amount of water used in the

mix, which ranged between 0.20 to 0.28, along with the

water/binder ratio for each mix proportion, with the binder

including cement and Silica fume The amount of water

included in the superplasticiser was taken into account in the water content [2]

The mixing procedure was carried out very carefully in order to prevent agglomeration and also to promote uniform distribution of very fine particles First, all powders and aggregates were mixed for five minutes at low speed The mixing was continued for one more minute, while the required quantity of water was added, which already contained the superplasticiser After five minutes of stirring, the mixture became fluid The concrete mix produced was poured into steel molds and, 24 hours later the specimens were demoulded All specimens were cured in water immersion at 20o C until the day of the test

The assessment of unconfined compressive strength of the different concrete mix proportions was performed at 7 and 28 days of curing on cubic specimens (150mm x150mm x150mm) under a constant strain rate of 0.0043 mm/mm/sec For the same curing ages, splitting tensile strength tests were conducted following the instructions on cylindrical specimens with a height to diameter ratio of 300 mm/150 mm = 2 The tests were performed on four specimens and the average values were recorded An Instron servohydraulic (model 4500 KPX J4 Static Hydraulic Universal Testing System) compression testing machine was used for all tests

TABLE I M IX P ROPORTIONS AND N OTATION

Mix

Water/

Binder

ratio

Water (Kg/m3)

Cement (Kg/m3)

Silica Fume (Kg/m3)

Coarse aggregates (Kg/m3) 12.5> d > 6.3 (mm)

Coarse aggregates (Kg/m3) 6.3> d > 4.75 (mm)

Fine aggregates (Kg/m3) 4.75> d > 0.425 (mm)

Superplasticizer (Kg/m3)

TABLE II C HEMICAL C OMPOSITION OF T HE C EMENT SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O Ignition

loss

TABLE III C HEMICAL C OMPOSITION OF T HE S ILICA F UME SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O

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TABLE IV P ROPERTIES OF T HE S UPER P LASTISISER

Polycarboxylate ether

Recommended dosage*

(% by cement weight)

0.6 – 1.4

*by supplier

III TEST RESULTS AND DISCUSSION

Table V, summarizes the mechanical properties for all

concrete mix designs Mix C10, which contained only fine

aggregates of 0.425 to 4.75 mm size, resulted in the highest

compressive strength after 28 days equal to 120.2 MPa, as well

as splitting tensile strength after 7 and 28 days equal to 7.0

MPa and 8.2MPa The water/binder ratio in the mix was low

and equal to 0.20 However, C13 mix was the one that attained

the highest compressive strength of 104.5 MPa, as well as the

highest splitting tensile strength of 7.2 after 7 days

TABLE V M ECHANICAL P ROPERTIES OF HPC AND UHPC

Compressive

strength

(7 days)

Compressive strength (28 days)

Splitting tensile strength (7 days)

Splitting tensile strength (28 days)

In general, concrete mixing proportions with smaller size

aggregates exhibited slightly higher strengths at a given

aggregate content level This is evident by comparing C2 - C5

mixes with C6 - C9 The increase of the cement content beyond

550 Kg/m3 appeared to result in a reduction of strength for all

concrete mixes By comparing C2 with C3 - C5, this strength

reduction is more evident in the case of concretes incorporating

coarse aggregates than in the case of concretes containing finer

aggregates (compare C10 with C11-C13) Fig 1 depicts the

strength reduction due to the increase of cement content in

specimens with the fine aggregate’s and water/binder ratio of 0.25 in 7 and 28 days of testing In Fig 2 the same trend is depicted for specimens with water/binder ratios of 0.25 and 0.20

Figure 1 Effect of cement content on compressive strength

Figure 2 Effect of cement content on compressive strength for different

water/binder ratios

Fig 3, shows the relation between concrete compressive strength and water binder ratio As the water binder ratio reduces concrete compressive strength increases for both 7 and

28 days of testing The same trend was observed for splitting

80

85

90

95

100

105

110

115

Cement content (Kg/m 3 )

28 days

C7

C8 C7

C9

C6

C9

C8

85

90

95

100

105

110

115

120

125

Cement content (Kg/m 3 )

C6 w/b = 0.25 - 28 days w/b = 0.20 - 28 days

w/b = 0.25 - 7 days w/b = 0.20 - 7 days

C8

C9 C8

C9

C6

C7

C7

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tensile strength as shown in Fig 4 This is almost linear for

specimens tested after 7 days

Figure 3 Effect of w/b ratio on the compressive strength

Figure 4 Effect of w/binder ratio on splitting tensile strength

The addition of silica fume in the mix (see C14, C15 and

C16) in excess of 10 % by weight of cement resulted in the

reduction of both compressive and splitting tensile strength

The compressive strength development of all concrete

mixes after 7 days ranged from 81.4 to 91.4 % of that obtained

after 28 days Similarly the splitting tensile strength ranged

from 78.9 to 91 % of the one obtained at 28 days In addition, the splitting tensile strength values of all concrete specimens at

28 days of curing ranged from 6 to 7 % of the compressive strength values

Reduction of water/binder ratio with a simultaneous increment of superplasticizer content resulted in slightly higher strengths (Compare C1 with C2, C14 and C6-C9 with C10-C13)

Figure 5 Effect of cement content on splitting tensile strength

IV CONCLUSIONS

A comprehensive laboratory study was undertaken to investigate the effect of specific mix design parameters on the strength behavior of HPC and UHPC specimens Taking into account the data and results obtained in this study, the following conclusions can be drawn:

1 Increasing the silica fume content beyond 10 % by weight

of cement appeared to reduce compressive and splitting tensile strengths

2 Reduction of water/binder ratio with a simultaneous incremental increase of superplasticizer content resulted in

a slightly higher strengths

3 Concrete mixes with smaller size aggregates exhibited slightly higher strengths at a given aggregate content level

4 Increasing cement content beyond 550 Kg/m3 appeared to result in a reduction of strength for all concrete specimens The reduction of strength is more obvious in the case of concrete mixes incorporating coarse aggregates than in the case of concrete mixes containing finer aggregates

5 The early strength development (7 days) of all concrete mixes did not seem to be affected by any of the aforementioned parameters It ranged from 81.4 to 91.4 % and from 78.9 to 91 % of the one obtained at 28 days for

80

85

90

95

100

105

110

115

water/binder ratio

28 days

C2

C14 C2

C14 C1

5

5.5

6

6.5

7

7.5

8

water/binder ratio

28 days

C2

C14 C2

C14

C1

5 5.5

6 6.5

7 7.5

8

Cement content (Kg/m 3 )

28 days

C8

C9 C8

C9

C6

C7

C7

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compressive strength and splitting tensile strength,

respectively

6 Splitting tensile strength values of all concrete mixes at 28

days of curing ranged from 6 to 7 % of the compressive

strength values

ACKNOWLEDGMENT

This research has been co-financed by the European Social

Fund, European Union and Greek national funds through the

Operational Program "Education and Lifelong Learning" of the

National Strategic Reference Framework (NSRF) - Research Funding Program: ARISTEIA (Excellence) II The authors would also like to thank the companies TITAN Cement, Sidenor S.A and SIKA Hellas for providing materials

REFERENCES [1] P – C Aitcin, High Performance Concrete E & FN SPON, London and

NY, 1998

[2] C A Anagnostopoulos, “Effect of superplasticiser type on the properties of cement grouts”, in Advances in Cement Research, Vol 27 (5), 2015, pp 297-307

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