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.
Trang 1Mix 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
Trang 2size 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
Trang 3TABLE 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
Trang 4tensile 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
Trang 5compressive 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