1 STRENGTH DEVELOPMENT, FORMWORK REMOVAL AND TEMPERATURE CONTROLLED CASTING OF CONCRETE IN A HIGH-RISE BUILDING Ali Elmaskaya (1) Mehmet Ali Taşdemir (2) (1) Nida Construction & Turism Co, Inc., Istanbul (2) Istanbul Technical University, Istanbul Abstract The three meters deep foundation of the high-rise building studied, called Palladium Tower, was cast in three layers, one meter at a time. The maximum temperature of the mass concrete foundation at the core and the maximum allowable mean temperature of each foundation layer calculated in terms of core, bottom and edge temperatures were 55ºC and 53ºC, respectively. During hardening, the maximum allowable temperature difference between the internal average temperature and the surface temperature measured at the concrete cover depth was less than 20 o C for each layer. The average temperature differences between the layers were also limited to a maximum value of 20ºC. At early ages, a reliable basis for determining the proper formwork removal time was used to prevent cracking of reinforced concrete members. For this purpose, the curing parameters including the time, temperature, the method of placing concrete in the structure and the test specimens, especially cylinders cured on site, as well as the weather conditions were recorded. When formwork was removed, there was no excessive deflection or distortion and no evidence of cracking or other damage to the structural members were observed. Protection of the steel reinforcement against corrosion, due to chloride ion diffusion and/or carbonation, is the most important factor in achieving long service life of the structure. Thus, the concrete cover on the reinforcement was ensured to have enough thickness and strength. For this purpose, special repair techniques were applied to the locally segregated surfaces against corrosion of reinforcements.
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Palladium Tower Dayanım Gelişimi Kalıp Alma ve Beton Sıcaklığı Bildiri Mayıs 2015
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STRENGTH DEVELOPMENT, FORMWORK REMOVAL AND
TEMPERATURE CONTROLLED CASTING OF CONCRETE
IN A HIGH-RISE BUILDING
Ali Elmaskaya (1)
Mehmet Ali Taşdemir (2)
(1) Nida Construction & Turism Co, Inc., Istanbul
(2) Istanbul Technical University, Istanbul
Abstract
The three meters deep foundation of the high-rise building studied, called Palladium
Tower, was cast in three layers, one meter at a time. The maximum temperature of the mass
concrete foundation at the core and the maximum allowable mean temperature of each
foundation layer calculated in terms of core, bottom and edge temperatures were 55ºC and
53ºC, respectively. During hardening, the maximum allowable temperature difference
between the internal average temperature and the surface temperature measured at the
concrete cover depth was less than 20oC for each layer. The average temperature differences
between the layers were also limited to a maximum value of 20ºC. At early ages, a reliable
basis for determining the proper formwork removal time was used to prevent cracking of
reinforced concrete members. For this purpose, the curing parameters including the time,
temperature, the method of placing concrete in the structure and the test specimens, especially
cylinders cured on site, as well as the weather conditions were recorded. When formwork was
removed, there was no excessive deflection or distortion and no evidence of cracking or other
damage to the structural members were observed. Protection of the steel reinforcement against
corrosion, due to chloride ion diffusion and/or carbonation, is the most important factor in
achieving long service life of the structure. Thus, the concrete cover on the reinforcement was
ensured to have enough thickness and strength. For this purpose, special repair techniques
were applied to the locally segregated surfaces against corrosion of reinforcements.
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1. INTRODUCTION
In the design and construction of high rise buildings, high strength concrete is used more
often because element dimensions of these structures are larger compared to those built using
normal strength concrete. However, the heat of hydration of their mass concrete foundations
and the resulting temperature rise in the concrete can cause thermal cracking [1]. It is
commonly thought that mass concrete principles only apply to large dams, but they apply to
any large pours such as massive foundations, bridge piers, thick slabs, nuclear plants, and
some large scale structural columns and shear walls. Therefore, ACI 207 defines mass
concrete as “any volume of concrete with dimensions large enough to require that measures
be taken to cope with generation of heat from hydration of the cement and attendant volume
change to minimize cracking” [2].
The cracking of concrete at early ages can be prevented by controlling its volumetric
consistency during hardening, the time dependent deformations and their variations due to the
temperature differences between the interior and the exterior as well as between the layers, the
element sizes and the pouring order [3-8]. When dimensions are greater than 1m, the
temperature rise should be considered. Since the depth of foundation in Palladium Tower
investigated here is 3m, it meets the definition of mass concrete. The heavy reinforcement in
the foundations of this project does not mean that the concrete will not crack and certainly
will not prevent generation of heat [1]. Results from the literature suggest that the formation
of delayed ettringite is activated when concrete is subjected to elevated temperatures [9-15].
Thus, increased internal temperature due to casting of mass concrete must be considered. For
this purpose, a temperature monitoring method for the deep foundation in this project was
used to minimize the maximum concrete temperatures and temperature differentials, thus
preventing thermal cracking and damage through delayed ettringite formation. In addition to
temperature controlled casting of massive members, strength development of concrete,
formwork removal methods, and repair techniques in the local areas with surface defects were
employed.
2. ABOUT THE PROJECT
Palladium Tower was built in Ataşehir which is a district in the Asian part of Istanbul. Its
construction was completed in 2014. The total construction area in the project is 99.784 m2.
The tower is used as an A+ type office building. The tower consists of 4 basements, ground
floor, and 42 normal floors. The height of the tower is 180 m. The investor was Tahincioğlu
Group and main contractor was Nida Construction Co. In the foundations of the tower,
concrete class was C40/50. However, in the construction of columns, core and shear walls,
and slabs, concrete class of C50/60 was used.
3. EXPERIMENTAL WORK
The Palladium Tower Project required concrete designed for durability, unlike those used
in normal structures. In order not to cause cracks in concrete due to the early age temperature
differences between the interior and the exterior as well as between the layers, cements with
low heat of hydration, but with high strength were utilized.
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Binders and aggregates with low alkali and reactive silica contents were selected to prevent
undesired alkali-aggregate reactions which can appear after long years. The aggregates used
were continuously tested in batches to ensure that they are clean and have proper shape and
size distribution. In the selection of chemical admixtures, it was aimed to achieve the
consistency between cement and aggregate as well as the consistency of the fresh concrete
within its respective tolerances over time.
The main principle for durability was producing a concrete that was as impermeable and
crack free as much as possible. An impermeable concrete was achieved by using low
water/cement ratio and by keeping the properties of the binding material under control in
order to minimize the capillary pores. As a consequence of the achieved durability targets
through minimization of capillary pores, the concrete strength requirement was automatically
satisfied.
3.1 Cements
Cements used in this study were as follows; CEM I 42.5 R and CEM IV/B-P 32.5 N.
Densities of these cements were 3.16 g/cm3
and 2.86 g/cm3, respectively. Initial and final
setting times of CEM I 42.5 R were 166 and 210 minutes, respectively. These values were
188 and 230 minutes, respectively, for CEM IV/B-P 32.5 R. Additional tests on these
cements verified that their physical, chemical and mechanical properties satisfied the
requirements of TS EN 197 standard.
Heat of hydration of cements at 2, 7 and 28 days, determined according to TS EN 196-8 are
given in Table 1.
Table 1 Heat of hydrations of cements.
Cement
Heat of hydration, cal./g
2 days 7days 28 days
CEM I 42.5 R 71.9 86.1 92.5
CEM IV/B-P 32.5 R 48.0 61.2 66.4
3.2 Aggregates
Particle densities and water absorptions of aggregates used are shown in Table 2. An
aggregate sample containing a fraction of 10-14mm sizes taken from both coarse aggregates
was subjected to the abrasion test in accordance with TS EN 1097-2. After 500 revolutions,
the loss of abrasion was 20.3%, which corresponds to the Los Angeles Abrasion Category of
LA25.
Table 2. Particle densities and water absorptions of aggregates
Aggregate Density, g/cm3 Water absorption,%
Natural sand 2.58 1.6
Limestone fines (0-4mm) 2.74 1.1
Coarse aggregate No. I (5-12mm) 2.73 0.8
Coarse aggregate No. I (12-22mm) 2.75 0.6
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Petrographic analysis of limestone fines and coarse aggregates taken from the same quarry are
summarized below.
Rock contained similar proportions of cryptocrystalline/micritic and recrystallized/
microcrystalline calcite grains. Texture of the rock was homogeneous. Micritic calcite formed
the matrix of the rock. Grain size of the calcite was generally 0- 0.01 mm. Recrystallized
calcite was middle – large sized, generally 0.02 mm, with a homogenous grain size
distribution. Trace amount of opaque minerals was observed.
Table 3. Crushed Rock Mineralogical Constituents
Mineralogical Constituents (%)
Calcite (Primary, micritic) 45-60
Calcite (Secondary, recrystallized) 40-45
Opaque mineral + Iron oxide 1-1,5
From the ASR point of view, ASTM C 1260 states that if the expansion of mortar bar is
lower than %0.10 the aggregate is considered innocuous, if the range of expansion is
between 0.10% and 0.20%, the aggregate is considered potentially reactive and above %0.20
the aggregate is considered reactive. In natural sand, limestone fines and coarse aggregates
from the same quarry and in coarse aggregates, expansions recorded according to ASTM C
1260 were 0.04% and 0.08%, respectively. As a result, these aggregates can be considered
innocuous, because the expansion values measured according to ASTM C 1260 are less than
0.1%.
3.3 Main concrete mixtures used
In the composition of Mix 1 used for the foundation (i.e., C40/50), cements were as
follows; CEM I 42.5 R: 120 kg/m3 and CEM IV/B-P 32.5 N: 260 kg/m
3 and water /cement
ratio was reduced to 0.40. Densities of these cements were 3,16 and 2,86 g/cm3, respectively.
Thus, the mixture composition was: Total cement: Natural sand: Limestone fines: Coarse
aggregate No.I : Coarse aggregate No.II: Chemical admixture (in plant): Chemical admixture
(in construction site): Water = 1 : 1.251 : 1.017 : 1.880 : 1.271 : 0.019 : 0.004 : 0.40. For this
mixture, the target strength was as follows: ƒtg= ƒck + 1.48 ϭ = 40 + 1.48 x3.55 = 45.3 MPa,
here the 1.48 value corresponds to 93% confidence level in accordance with EN 206.
For cold weather conditions, in the composition of Mix 2 used for columns, walls and slabs
(i.e. C50/60); cements were as follows; CEM I 42.5: 300 kg/m3 and CEM IV/B-P 32.5 N: 120
kg/m3 and water /cement was reduced to 0.36. Thus, the mixture composition was: Total