Performance Confirmation Tests on C100 Concrete in Dubai, UAE

Transcription

1 Journal of Advanced Concrete Technology Vol. 5, No. 2, , June 27 / Copyright 27 Japan Concrete Institute 171 Technical report Performance Confirmation Tests on Concrete in Dubai, UAE Shusuke Kuroiwa 1, Yoshitaka Inoue 2, Kensuke Fujioka 3 and Adel William 4 Received 14 March 27, accepted 9 May 27 Abstract Numerous skyscrapers are being built using reinforced concrete construction in Dubai, UAE. Because the use of high-strength concrete is advantageous for skyscrapers from a number of aspects, concrete with a 1 N/mm 2 compressive strength was produced using materials locally available, and its properties while fresh and after hardening were investigated. Its placing performance in a mock-up column and in-situ strength development were also examined to investigate its applicability to actual construction. As a result, it was confirmed that the properties of fresh concrete are retained with little change over the required period, and that hardened concrete presents good mechanical and durability properties. Mock-up testing also revealed that the placeability of the concrete was sufficiently good, and the compressive strength and elastic modulus of cores drilled from the mock-up were found to be satisfactory for concrete of compressive strength of 1 N/mm Introduction 1 Senior Research Engineer, Building Engineering Research Institute, Technology Center, Taisei Corporation, Japan. shusuke.kuroiwa@sakura.taisei.co.jp 2 General Project Manager, International Building Construction Branch, International Division, Taisei Corporation, Japan. 3 Manager, International Building Construction Branch, International Division, Taisei Corporation, Japan. 4 Ready Mix Beton L.L.C., Dubai, UAE. Numerous reinforced concrete skyscrapers have been built in Dubai, UAE, where a construction boom is in full swing. Though the compressive strength of concrete used here is primarily 6 to 8 N/mm 2, the use of concrete with a higher strength will not only enable construction of higher buildings but also will bring about a number of advantages including reduction in building weight and increase in usable area due to reduction in column cross-sectional areas. On the other hand, concrete with 1 N/mm 2 compressive strength poses problems that need to be dealt with, such as vulnerability to explosive spalling under fire, blatantly large autogenous shrinkage, and the necessity to grasp the accurate elastic modulus and creep coefficient to deal with the large longitudinal deformation resulting from the relatively low elastic modulus compared with the high compressive strength. For this reason, the authors produced concrete with compressive strength of 1 N/mm 2 using materials available in Dubai and conducted tests to confirm its basic properties, which included the following: fresh properties, workability retention, initial and final setting times, cube compressive strength, modulus of elasticity, autogenous shrinkage, thermal expansion coefficient, creep coefficient, and fire resistance. In Dubai, demand for durability means high performance under harsh environmental conditions with high levels of chlorides and sulfates. It is well known that high-strength concrete that uses silica fume has good durability, and thus silica fume was used. We also carried out examinations of chloride permeability, water absorption and water permeability, which are the most commonly specified durability tests. In actual column members, high hydration heat due to increases in cement content also causes concern about temperature cracking at early ages and adverse effects on sound strength development at later ages. For example, in Japan the mixture proportions of high-strength concrete are decided based on the test results obtained by compression testing of cores taken from mock-up columns. For the above reason, placing tests were conducted in a reinforced concrete mock-up column having the full-scale cross-sectional area to confirm placeability, fillability, and internal temperature history subject to hydration heat. The compressive strength and elastic modulus of cores drilled from the column were also measured. However, since the placing tests were conducted at a time when the temperature was comparatively low, the properties of the concrete under high temperature conditions need to be examined in the future. 2. Outline of experiment 2.1 Test composition The test composition is shown in Table 1. This experiment consisted of two series: #1 tests to confirm the basic properties of fresh and hardened concrete with a compressive strength of 1 N/mm 2 ; and #2 placing performance tests while producing a mock-up reinforced concrete column with a cross-sectional area of the actual size and tests to confirm the strength properties after hardening. High-strength concrete having a dense microstructure is known to be prone to accumulate thermal stress and vapor pressure under fire, causing explosive spalling. The inclusion of fine organic fibers, such as polypropylene, is effective in preventing spalling under

2 172 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , 27 fire conditions. However, the amount of fibers necessary for measures against explosive spalling varies depending on the water-binder ratio and material type. Excessive fibers could adversely affect the fresh properties and compressive strength of the resulting concrete. For this reason, polypropylene fiber content was taken as the test parameter in series #1 to investigate its effects on the fresh properties, mechanical properties, such as compressive strength, and durability of concrete, as well as the spalling-inhibiting effect of the fibers. Series #2 using a mock-up column was carried out for a single mix designed to provide a compressive strength of 1 N/mm 2 with a polypropylene fiber content of.91 selected based on the results of series #1. Columns made of high-strength concrete with a high cement content undergo high temperature histories due to the high hydration heat of cement. Such a high temperature history can not only cause concern about temperature cracking but also can adversely affect strength development, possibly leading to large discrepancies between the strengths of in-situ concrete and control specimens compared with those of concrete of normal strength levels. It is therefore essential when dealing with concrete of this strength level to confirm the mechanical properties of concrete by testing cores drilled from a mock-up column specimen. 2.2 Materials, mixture proportions, and mixing Table 2 gives the concrete materials, all of which are normally available in Dubai. The cementitious materials were normal portland cement blended with fly ash and silica fume to reduce the viscosity and improve the pumpability of fresh concrete. The aggregates were a combination of crushed limestone with a maximum size of 1 mm, crushed and washed limestone sand, and natural dune sand. Crushed limestone and crushed limestone sand were adopted because these were expected to lead to concrete with higher dimensional stability with a low linear expansion coefficient, low drying shrinkage ratio, and high elastic modulus than concrete made using other types of aggregate. Consideration was also given to the pumpability and segregation resistance of concrete by selecting limestone with a lower density than that of gabbro, an available alternative, and setting the maximum aggregate size at 1 mm. A superplasticizer based on polycarboxylic ether polymers was used as a chemical admixture essential for producing concrete with a low water-binder ratio. The fibers were polypropylene fibers 12 mm in length and 18 microns in diameter to be used as a measure against explosive spalling under fire. Table 3 gives the mixture proportions of concrete. The water-binder ratio was.29 for all mixtures. In series #1 for confirming the basic properties of concrete, three types of mixtures were produced with %,.1% (.91 ), and.15% (1.37 ) polypropylene fibers by volume of concrete. In series #2 for confirming the placing performance while fresh and strength properties after hardening, a single type of mixture was produced with a polypropylene fiber content of.91 by volume of concrete. To attain the slightly large target slump flows of 65 and 75 mm with and without fibers, respectively, the sand-total aggregate ratio exceeded 5%. Series of Experiments Objective Concrete Material Cement Mineral admixture Fine Aggregate Coarse Aggregate Chemical admixture Fiber Table 1 Test composition. # 1 Tests to confirm basic properties Basic properties of fresh and hardened concrete with a compressive strength of 1 N/mm 2 1) without fiber 2) with.91 polypropylene fibers 3) with 1.37 polypropylene fibers # 2 Experiment using mock-up columns Placing performance tests while producing a mock-up reinforced concrete column with a cross-sectional area of the actual size and tests to confirm the strength properties after hardening 1) with.91 polypropylene fibers Table 2 Materials. Details Ordinary portland d= 3.15 g/cm 3, cement S= 32 m 2 /kg Fly ash d= 2.17 g/cm 3, S= 32 m 2 /kg Silica fume d= 2.2 g/cm 3, S= 21 m 2 /g Washed crashed sand d= 2.7 g/cm 3, (limestone) A= 1. % Natural dune sand d= 2.65 g/cm 3, A=.9 % d= 2.7 g/cm Crushed stone, A=.6 %, (limestone) Maximum size :1 mm superplasticizer Polycarboxylic ether polymers Polypropylene fiber () d :Specific gravity, S :Blaine fineness, A :Absorption Table 3 Mixture proportions. without fibers Length: 12 mm, Thickness: 18 microns with.91 with 1.37 air content, % Water, OPC, Fly ash, Silica fume, Fine Agg. (Washed crashed sand), Fine Agg. (Natural dune sand), Coarse Agg. (Crushed stone), Polypropylene fiber, Superplasticizer, ml Series of Experiments # 1 # 1 # 2 # 1

3 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , All concretes were mixed at a ready-mixed concrete plant using a pan-type mixer with a capacity of 3 m 3 to be used for actual construction. Three cubic meters, or two 1.5 m 3 batches, were produced for each mixture at this plant furnished with equipment that was sufficiently capable of producing high-strength concrete, including simultaneous treatment of multiple materials including silica fume, fly ash, and dune sand. Polypropylene fibers were charged into as-mixed concrete in agitating trucks and agitated at a high speed. 3. Tests to confirm basic properties 3.1 Test procedures The test items are listed in Table 4. (1) The test items for fresh concrete include concrete temperature, slump flow, time to 5 cm flow, air content, density, and initial and final setting times. For the first three items, measurements were done every 3 min from immediately after mixing to 12 min. (2) Compression tests were conducted using 15 mm cubes at 1, 3, 7, 28, 56, and 9 days in accordance with BS 1881, Part 116. (3) The elastic modulus was measured using cylindrical specimens 15 mm in diameter and 3 mm in height at an age of 56 days in accordance with ASTM C 469. (4) Autogenous shrinkage was measured for only the with.91 polypropylene fibers mixture. A strain transducer with a low apparent modulus of 4 N/mm 2 capable of measuring from immediately after placing was embedded in the center of each beam specimen measuring 1 by 1 by 4 mm to record autogenous shrinkage. In order to prevent molds from restraining autogenous shrinkage until demolding, a polytetrafluoroethylene (PTFE) sheet 1 mm in thickness was placed on the bottom, and polystyrene sheets 3 mm in thickness were placed on the bottom edges of both sides of each mold. Also, PTFE sheets.1 mm in thickness were placed on the inside of the sides, edges, and bottom of each mold, thereby preventing concrete from coming into contact with the mold. The top surfaces of the concrete were sealed with a PTFE sheet.1 mm in thickness immediately after placing to prevent moisture from escaping. The specimens were demolded on the following day and immediately sealed with.5 mm-thick aluminum foil adhesive tape to prevent drying. Note that the ambient temperature was kept constant at 2 C from an age of 4 days but uncontrolled until that time. The thermal strain was therefore calculated from the internal temperature of specimens based on the linear expansion coefficient described below, with which the measured strain was corrected to obtain the autogenous shrinkage. (5) The linear expansion coefficient was measured using specimens sealed with aluminum tape, which was used for measuring autogenous shrinkage to an age of 1 days. The specimens were subjected to three cycles of temperature rise and fall by stepwise changes of 15 C between 5 and 8 C. A single temperature step consisted of 3 min for changing the ambient temperature by 15 C followed by 4.5 hours for retaining the temperature. Note that the linear expansion coefficient was calculated from the temperature-strain relationship measured in the second and third cycles. (6) Compressive creep was measured only for with.91 polypropylene fibers concrete. Cylindrical specimens 1 mm in diameter and 2 mm in height were seal-cured until loading at 28 days. The load was 1/3 of the 28-day compressive strength of similarly seal-cured cylindrical specimens of the same size. When measuring the compressive strength, strain gauges were attached to these specimens to record the stress-strain relationship. The creep strain was measured using three each of air-dried specimens and specimens sealed with aluminum foil tape to prevent drying. Since the drying shrinkage strain ought to be subtracted to determine the creep strain of air-dried specimens, the shrinkage strain was separately measured using three unloaded air-dried specimens. The ambient temperature and relative humidity for creep testing were constant at 2 C and 6%, respectively. 1) Fresh properties and workability retention a. Concrete temperature: ASTM C 164 b. Workability by slump flow & T5: BS 1881 Part 12 c. Air content; BS 1881 Part 16 d. Fresh concrete density: BS 1881 Part 17 e. Initial and final setting times: ASTM C 43 Table 4 Test items. without fiber with.91 with ) Cube compressive strength: BS 1881 Part 116 3) Elastic modulus: ASTM C 469 4) Autogenous shrinkage - - 5) Thermal expansion coefficient - - 6) Compressive creep - - 7) Resistance to spalling in fire 8) Durability a. Rapid chloride permeability: ASTM C 122 b. Water absorption: BS 1881 Part 122 c. Water permeability: BS EN 1239 Part 8

4 174 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , 27 (7) Specimens for resistance to spalling in fire were cylinders 25 mm in diameter and 5 mm in height, each with a M22 through-threaded bolt longitudinally embedded in the center. After being seal-cured until the age of 3.5 months, specimens were subjected to heating testing, in which they were heated from all surfaces except the top, which was insulated through attachment of a ceramic blanket. Heat was applied for an hour in a refractory furnace following the heating curve specified in ISO 834. (8) Durability tests included rapid chloride ion penetration testing (ASTM C 122), which is often required in Dubai, permeability testing (BSEN 1239, Part 8), and water absorption testing (BS 1881, Part 122). Rapid chloride ion penetration testing was carried out as follows: Cut out disk specimens 95 mm in diameter and 51 mm in thickness from cubic specimens. Coat the curved surfaces of disk specimens with epoxy and subject them to pretreatment including drying. Place the specimens in chloride ion penetration cells and fill the anodic and cathodic cells of the voltage generator with a.3 N aqueous solution of NaOH and 3% aqueous solution of NaCl, respectively. Apply a current of 6 V to the specimens and determine the coulomb quantity (electric current integrated with time) after 6 hours. Permeability testing was carried out by applying water on one side of each specimen with a constant pressure and measuring the depth of water permeating into the specimen at the end of a specified period to evaluate the watertightness of concrete. Water absorption testing was carried out by drying specimens 75 mm in diameter and 75 mm in length for 72 hours at 15 C and immersing the cooled specimens in water at 2 C for 3 min to measure the water absorptivity. The test ages for these durability-related tests were all 28 days. 3.2 Test results (1) Properties of fresh concrete Table 5 and Fig. 1 give the results of the fresh concrete tests. These tests revealed that (a) the slump flow decreased and the time to 5 cm flow increased as the polypropylene fiber content increased; (b) a slump flow of 6 mm or more was retained up to 12 min even at concrete temperatures of 25 to 3 C provided the fiber content was not more than.91 ; and (c) it was difficult to obtain a slump flow of 6 mm or more with a fiber content as high as Also, the concretes showed good fresh properties without any indication of segregation, such as mortar outrunning ahead of the concrete body during slump flow testing, which is characteristic of concrete containing fibers. Setting times tended to increase as the polypropylene fiber content increased, due to the concomitant increases in the superplasticizer dosage to ensure the slump flow. Though such retardation can be improved by changing the superplasticizer type, care should be exercised when adopting jumping forms and slip forms. Test Concrete temperature (deg C) Slump flow (mm) Time to 5 mm flow T5 (s) Air content (%) Fresh concrete density ( ) Initial and final setting times (h:m) Slump flow, mm Table 5 Results of fresh concrete tests. Time without fiber with.91 with min min min min min min min min min min min min min min min min min min min Initial 9:1 13:4 19:2 Final 1:45 15:25 21: Time, minutes without 45 fiber kg/m (2) Compressive strength The compression test results are given in Table 6 and Fig. 2. The strength development appears to be affected by retarded setting up to around an age of 7 days, with the strength decreasing as the fiber content increases. The strength of with 1.37 polypropylene fibers concrete having the highest fiber content still tends to be slightly lower at 56 and 9 days. However, since all concretes nearly attain 12 N/mm 2 at 56 days, these are all proven to satisfy the requirements for a compressive strength of 1 N/mm T5, min Fig. 1 Slump flow, time to 5 cm flow T5 and change over time.

5 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , Compressive strength Compressive strength of cubes, N/mm Table 6 Compression test results. Age (days) without fiber with.91 with without fiber 3.91 kg/m 1.37 kg/m Age, days Fig. 2 Compressive strength development. (3) Elastic modulus The elastic modulus of cylinders 15 mm in diameter and 3 mm in height at 56 days is given in Table 7. According to the basic estimation equation specified in the Japanese Architectural Standard Specification for Reinforced Concrete Work (JASS 5) published by the Architectural Institute of Japan, the elastic modulus of concrete made using limestone aggregates with a compressive strength of 12 N/mm 2 and a density of 2.5 g/cm 3 is estimated to be (2.5/2.4) 2 (12/6) 1/3 = 55. kn/mm 2. Here, 1.2 is a coefficient to describe the effect of limestone aggregate. The results of the concretes under study can be roughly evaluated by the JASS 5 equation, though they are slightly high due to the use of crushed limestone and crushed limestone sand only. (4) Autogenous shrinkage Figure 3 shows the autogenous shrinkage of with.91 polypropylene fibers concrete, which reaches approximately 35 and 5 μ at 1 and 91 days, respectively. This can be regarded as being relatively Test Elastic modulus Table 7 Elastic modulus. Age (days) without fiber with.91 with small in view of the fact that the autogenous shrinkage of concrete made of ordinary portland cement with a design strength of 1 N/mm 2 in Japan can reach 6 μ at 91 days and tend to increase with increasing additions of silica fume, according to research of Miyazawa and others. The small autogenous shrinkage is presumably due to limestone aggregates with a high elastic modulus. According to the experience of the authors, even if concrete with autogenous shrinkage in the 5 μ level is used for reinforced concrete columns, cracks are hardly generated. Based on this experience, it was decided to check the shrinkage through a placing experiment, without using shrinkage reduction agents. Note that the strain resulting from temperature changes was corrected using the thermal expansion coefficient described below. (5) Thermal expansion coefficient Figure 4 shows the temperature-strain relationship measured using with.91 polypropylene fibers specimens. Whereas the thermal expansion coefficient of concrete is generally assumed to be / C, that of the concrete under study is slightly lower at / C due to the use of limestone aggregates with a low linear expansion coefficient. This low thermal expansion is an advantage because the thermal stress Autogenous shrinkage, x Age, days Fig. 3 Autogenous shrinkage of with.91. Strain, x Temperature, deg C Fig. 4 Temperature-strain relationship of with.91.

6 176 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , 27 resulting from heat of hydration can be reduced, and the deformation accompanying temperature change can also be reduced. (6) Compressive creep Figure 5 shows the stress-strain relationship measured during compression testing on cylindrical specimens 1 mm in diameter and 2 mm in height at 28 days, the age at which the creep testing began. The compressive strength, elastic modulus, Poisson s ratio, and density were 128 N/mm 2, 52,5 N/mm 2,.24, and 2,52, respectively. These results are similar to the above-mentioned 28-day compressive strength of 123 N/mm 2 and density of 2,51 by 15 mm cubes, and 56-day elastic modulus of 53,5 N/mm 2 by cylinders 15 mm in diameter and 3 mm height, proving that the effect of the shape of specimens is small in this strength class. Figure 6 shows the compressive creep of with.91 polypropylene fibers concrete, which was measured under a load corresponding to 1/3 of the compressive strength applied from an age of 28 days. The creep strain was calculated by subtracting the elastic strain under the load and the drying shrinkage strain measured using specimens with no load from the total load (creep strain = (total strain) (elastic strain under loading) (drying shrinkage strain)). The creep strain of air-dried specimens, which were simultaneously subjected to the load and drying, reached 5 μ at 365 days from the beginning of loading. On the other hand, the creep strain of seal-cured specimens, which were protected from drying by aluminum tape, was around 4 μ. The large difference shows that the effect of drying is more significant than a simple drying shrinkage strain. Such an effect should be considered in the design of columns for skyscrapers with large cross-sections, which are difficult to dry. (7) Resistance to explosive spalling in fire Figure 7 shows the results of 1-hour heating testing following the heating curve specified in ISO 834. The without fiber specimen having no polypropylene fibers began to undergo sporadic spalling 7 min after the beginning of heating. The spalling tentatively stopped at around 2 min, but the specimen eventually lost its original form due to large spalling that occurred 55 min after the beginning of heating. On the other hand, the with.91 polypropylene fibers and with 1.37 polypropylene fibers specimens caused no spalling. Their mass retention ratios after heating were 94.3% and 94.%, respectively, thus a similar level. This tests result can be considered to be appropriate since in Japan too, 1. fiber is usually used in concrete of design strength 1N/mm 2. (8) Durability testing Table 8 gives the durability test results. According to AASHTO, chloride ion penetrability levels of 1-1, 1-2, 2-4, and over 4 are rated as being very low, low, moderate, and high, respectively. The present test results therefore indicate very low chloride 2 ε H (μ) 1 σc,n/mm St rain, x ε V (μ) Fig. 5 Stress-strain relationship of with.91. Strain, x1-6 Strain, x air-dried specimens shrinkage creep strain shrinkage+creep total strain Time after curing, days sealed specimens creep strain total strain Time after curing, days Fig. 6 Compressive creep of with.91. penetrabilities. The results of water permeability testing in accordance with BSEN 1239, Part 8, and water absorption testing in accordance with BS 1881, Part 122, of the concrete under study also show sufficiently high durability, though the performance tends to slightly decrease as the fiber content increases. 4. Experiment using a mock-up column 4.1 Test procedures Placing tests of a mock-up concrete column were con-

7 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , Rapid chloride permeability Test without fiber with.91 with 1.37 Fig. 7 Results of 1-hour heating testing following heating curve specified in ISO 834. Table 8 Durability test results. Total charge passed (Coulombs) Chloride penetrability Age (days) 28 without fiber with.91 with Very Low Very Low Very Low Water permeability (mm) Water absorption (%) ducted using concrete containing.91 polypropylene fibers. Concrete was produced at a ready-mixed concrete plant, transported to the test yard in approximately 3 min, and placed in forms for a mock-up column. Figure 8 shows the outline of the mock-up with a cross section of 1, by 1, mm and a height of 2, mm. The column was reinforced with 16 longitudinal bars 4 mm in diameter and transverse bars 12 mm in diameter placed at 1 mm intervals, with the cover concrete depth being 6 mm. The formwork was made of 5-mm thick expanded polystyrene and 18-mm thick plywood, so as to minimize the temperature difference between the central and surface portions of the concrete. In consideration of the fact that boundaries between lifts of this type of concrete are prone to cracking due to the scarcity of bleeding, it was decided to stop placing at a mid-depth level and re-start following an interval of 3 min, to see if cracking would occur in this area after removing formwork. Though the fresh concrete had sufficient fluidity to ensure filling of all corners, consolidation was carried out using internal vibrators to treat the boundary between the lifts and remove air entrapped by fibers. Table 9 lists the test items. (1) The fresh concrete tests included concrete temperature, slump flow, time to 5 cm flow, and air content. These were measured immediately after producing concrete at the ready-mixed concrete plant, after arriving at the test yard, and at the time of re-starting placing in the upper half of the mock-up. (2) The internal temperature of the mock-up was measured using T-type thermocouples on three levels, each level consisting of three points: in the cross-sectional center, at 5 mm inward from a corner, and a middle point in between. (3) Tests on hardened concrete included compression tests on 15 mm cubic specimens and core tests on cores drilled from the mock-up column for compressive strength and elastic modulus. The cube tests were carried out in accordance with BS 1881, Part 116, at 7, 28, 56, and 9 days. Cores were drilled from inward (A) and outward (B) positions as shown in Fig. 8 at 28, 56, and 9 days, and five specimens from different longitudinal positions of each core were used for each test. The five specimens from each inward (A) position were used for measuring compressive strength in accordance with ASTM C 39. Those from each outward (B) position were used for measuring the elastic modulus in accordance with ASTM C469. Note that the compression test result of the A specimen on the same level at the same age was adopted to determine the stress corresponding to 4% of the ultimate load for each B specimen. Compression tests were also conducted on B specimens after measuring the elastic modulus to confirm the compressive strength. However, since the placing experiment was conducted at a season when temperature was comparatively low, the check of the performance in hot conditions was not performed. Moreover, when applying to super-high-rise construction, the conveyance to the height by a concrete pump is indispensable, but this too was not examined this time. These points need to be examined in the future. 4.2 Test results (1) Properties of fresh concrete The as-produced slump flow, time to 5 cm flow, air content, and concrete temperature were 65 mm, 1. s, 1.9%, and 26.8 C, respectively. The as-unloaded slump

8 178 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , 27 Fig. 8 Outline of mock-up with cross section of 1, by 1, mm and height of 2, mm. Table 9 Test items of experiment using mock-up column. 1) Fresh properties and workability retention 2) Internal temperature 3) Hardened concrete properties a. Concrete temperature: ASTM C 164 b. Workability by slump flow & T5: BS 1881 Part 12 c. Air content: BS 1881 Part 16 d. Fresh concrete density: BS 1881 Part 17 a. Compressive strength of cubes: BS 1881 part 116 b. Compressive strength of core: ASTM C 39 c. Modulus of elasticity of concrete in compression: ASTM C 469 flow, time to 5 cm flow, and air content at the test yard 5 min later were 645 mm, 1. s, and 2.1%, respectively, scarcely changing over the 5 min. The concrete retained sufficient placeability, filling into all corners without any problem. (2) Internal temperature history Figure 9 shows the internal temperature histories of the mock-up column. The temperature rose after an age of 24 hours, presumably due to the delayed setting resulting from a high superplasticizer dosage. Though the temperature in the center increased to a maximum of 7 C two days after placing, the difference between the temperatures in the center and 5 mm inward from the corner remained as low as 6 C, owing to the 5-mm thick expanded polystyrene insulation on the side surfaces. On the other hand, it took a week until the difference between the central temperature and the air temperature decreased to less than 25 C due to the temperature drop slowed by the expanded polystyrene insulation. Although a climbing formwork (jump form) is used to construct the cores of high-rise building in many cases, this suggests a necessity for meticulous care with respect to the time for removing the sheathing. (3) Hardened concrete Table 1 gives the hardened concrete test results. The compressive strengths of cubes were 112 and 115 N/mm 2 at 28 and 56 days, respectively, attaining a strength level of 1 N/mm 2. However, the strength of cores 15 mm in diameter and 3 mm in length taken from position A near the center ranged from 91.4 to 98.4 N/mm 2 and from 95.6 to 17 N/mm 2 at 28 and 56 days, respectively, thus lower than the cube strength. The compressive strength of cores taken from outward position B ranged from 91.4 to 99.4 N/mm 2 and from 93.4 to 13 N/mm 2 at 28 and 56 days, respectively, showing no marked difference from those of cores from inward position A, thereby proving the effect of insulation with expanded polystyrene for minimizing internal temperature differences. Nevertheless, slight differences are observed between the strengths from different longitudinal positions, with the strength of top cores being 4% lower than those of the others. The elastic modulus ranged from 46,4 to 48,9 N/mm 2 at 56 days. 5. Conclusions Numerous skyscrapers are being built using reinforced concrete construction in Dubai. Concrete with 1 N/mm 2 compressive strength made using materials lo- Recorded temperature, deg C air middle-center middle-25cm from the corner middle-5cm from the corner Age, days Fig. 9 Internal temperature histories of mock-up column.

9 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , Age (days) Table 1 Hardened concrete test results. Extracted cores Compressive strength of cubes No. Compressive strength of core A Elastic modulus of core B Compressive strength after elastic modulus of core B cally available was investigated in regard to its properties while fresh and after hardening, as well as its performance in a full-scale mock-up column to examine its applicability to actual construction. The findings obtained from these tests are summarized below. (1) High-strength concrete made using materials locally available, such as silica fume, superplasticizers, crushed stone, and crushed sand, was proven to meet the requirements for concrete with a compressive strength of 1 N/mm 2. The concrete ensured the retention of a slump flow of not less than 6 mm for 12 min after mixing even when containing.91 polypropylene fibers; achieved a compressive strength of 12 N/mm 2 at 56 days in accordance with BS 1881, Part 116; and reduced explosive spalling under heating conditions specified in ISO 834 by inclusion of.91 fibers, while spalling occurred without fibers. (2) However, the setting times of the concrete under study exceeded 1 hours due to the increase in the superplasticizer dosage to ensure slump flow after fiber addition. Though this can be improved by changing the superplasticizer type, care should be exercised when adopting jumping forms and slip forms. (3) The high-strength concrete under study made using crushed limestone and crushed limestone sand tended to show high size stability, such as an elastic modulus at 56 days exceeding 5 kn/mm 2 by testing in accordance with ASTM C 469; autogenous shrinkage as low as 5 to 6 μ at 91 days despite the combination of normal portland cement and silica fume with a low water-binder ratio of.29; and a thermal expansion coefficient as low as / C on average. (4) Whereas the creep strain of air-dried specimens reached 5 μ after loading for 24 days, that of sealed specimens remained as low as approximately 4 μ. Such a large difference suggests that the effect of drying is more significant than simple drying shrinkage, demanding care regarding drying, as columns for skyscrapers having large cross-sectional areas are slow in drying. (5) The durability performance testing included rapid chloride ion penetration testing in accordance with ASTM C 122, water permeability testing in accordance with BSEN 1239, Part 8, and water absorption testing in accordance with BS 1881, Part 122, all tests showing that the durability of the concrete under study is sufficiently high, though tending to decrease slightly as the fiber concrete increases. (6) The internal temperature of the mock-up column with a cross-section of 1, mm by 1, mm did not significantly increase until 24 hours after placing, presumably due to setting delayed by the high superplasticizer dosage. The temperature at the center reached a maximum of 7 C 2 days after placing, but the difference between the temperatures at the center and 5 mm inward from a corner remained as small as 6 C owing to the 5 mm-thick expanded polystyrene insulation. (7) The strength of cores 15 mm in diameter and 3 mm in length taken from near the center of the mock-up column made by placing the concrete under study ranged from 91.4 to 98.4 N/mm 2 and from 95.6 to 17 N/mm 2 at 28 and 56 days, respectively. These are slightly lower than the 56-day cube strength of 115 N/mm 2. However, no marked difference was observed between the cores from inward and outward positions, presumably due to the effect of insulation to minimize the temperature difference between cross-sectional positions. On the other hand, slight differences were observed between longitudinal positions of cores drilled from the mock-up, with the compressive strength of top specimens being lower than those of the other levels by approximately 4%. References Jinnai, H., Namiki, S., Kuroha, K., Kawabata, I. and Hara, T. (1999). Construction and design of high-rise building using 1MPa high strength concrete. Proceedings of the 5th International Symposium on the Utilization of High-Strength/High-Performance Concrete, 2, Jinnai, H., Kuroiwa, S., Watanabe, S., Namiki, S. and Hayakawa, M. (25). Development and construction record on high-strength concrete with the compressive strength exceeding 15MPa. Proceedings of Seventh International Symposium on the Utilization of High-Strength/High-Performance Concrete, SP228-65, Jinnai, H., Kuroiwa, S., Terauchi, R. and Abe, T. (27). Construction of high-rise building using 15 MPa concrete. Cement & Concrete, 723, (in Japanese) Kuroiwa, S., Kobayashi, Y. and Baba, S. (22). Fire performance of reinforced concrete columns using

10 18 S. Kuroiwa, Y. Inoue, K. Fujioka and A. William / Journal of Advanced Concrete Technology Vol. 5, No. 2, , 27 high-strength concrete with polypropylene fibers. Proceedings of 1st fib Congress, Session 11, Safety of Concrete Structures, Osaka, Japan, Kuroiwa, S., Jinnai, H., Kobayashi, Y., Kawabata, I., Nishikawa, Y., Kimura, Y., Abe, T. and Shimada, K. (22). Application of high-strength concrete with fire resistance improved by polypropylene fibers. AIJ J. Technol. Des. 16, (in Japanese) Miyazawa, S. and Tazawa, E. (25). Prediction Model for Autogenous Shrinkage of Concrete with Different Types of Cement. Proceedings of the Fourth International Seminar on Self-Desiccation and Its Importance on Concrete Technology, Walker, M. (22). Guide to the Construction of Reinforced Concrete in the Arabian Peninsula. CIRIA/Concrete Society.