Massive self-compacting concrete pumped in steel tube columns has been used more and more widely in super high-rise buildings and bridge engineering at present. The early-age expansion and shrinkage performance of its core mass concrete is an important index to ensure the stress state of triaxial compression and structural safety. However, no relevant reports have been found. In view of the actual building with the height of 265.15 meters, the early-age expansion and shrinkage tests of the massive self-compacting concrete pumped in full-scale columns with the height of 12.54 m and 12.24 m and diameter of 1.3 m and 1.6 m were carried out by means of strain gauges embedded in concrete-filled steel tubes (CFSTs). The early-age variation regularity of the vertical and horizontal expansion and shrinkage strains for the core concrete with the diameter of steel tube, development time, temperature, the pouring pressure, expansion stress, and so on is given. The calculation model of its early-age deformation strains is presented in this paper, which is in good agreement with the experimental results. It provides the basis of experimental and theoretical analyses for shrinkage compensation of massive self-compacting concrete pumped in steel tube columns.
As a composite component of steel and concrete, it must be ensured that its core concrete of concrete-filled steel tube (CFST) is in a complex stress state under three-dimensional compression and there is the rigid restraint effect for the core concrete formed in the steel tube, so as to improve the compressive strength and deformation capacity of the core concrete, promote the improvement of its plasticity and toughness, and prevent the brittle failure of the core concrete in steel tube. Therefore, the characteristics and changing regularity of the early-age expansion and shrinkage deformation for the core concrete are particularly important, which is the premise to ensure both works together. At present, the research on shrinkage and expansion properties of CFST at home and abroad is as follows.
Yang et al. [
For the massive self-compacting concrete pumped in steel tube column, the expansion and shrinkage of the core concrete affects the stress distribution of concrete-filled steel tubular members to a great extent, which can produce redistribution of internal stress in concrete-filled steel tubular members, including “redistribution of internal stress in the section and internal stress in the system.” For example, during the shrinkage process of the core concrete in CFST, the steel tube is forced to participate in the work alone, resulting in tensile stress in the core concrete and compressive stress in the steel tube, resulting in redistribution of the internal stress in the section, and the distribution values are large. Besides, when the core concrete shrinks, there is a gap between the core concrete and the steel tube, which not only destroys the joint work between the steel tube and the core concrete but also may cause corrosion on the inner surface of the steel tube during use. In addition, the excessive expansion of the core massive concrete in the steel tube will cause serious damage to the steel tube, thus making the structure in an unsafe state.
Based on the above, the early-age shrinkage and expansion properties for the massive self-compacting concrete pumped in steel columns have been studied in this paper.
The cementitious materials used for this investigation were Chinese standard P·I42.5R Portland cement (standard compressive strength higher than 42.5 MPa at the age of 28 days), Class I fly ash (fineness of 45
Mix proportions of massive self-compacting concrete pumped in steel tube columns.
Strength grade | Water/binder ratio | Water (kg/m3) | Cement (kg/m3) | Fly ash (kg/m3) | Silica fume (kg/m3) | Fine aggregate (kg/m3) | Coarse aggregate (kg/m3) | Superplasticizer (kg/m3) |
---|---|---|---|---|---|---|---|---|
C60 | 0.31 | 165 | 320 | 180 | 25 | 790 | 850 | 8.6 |
Major parameters of massive self-compacting concrete pumped in steel tube columns.
Mixture performance (1 hour after the concrete is discharged from the mixer) | Compressive strength/MPa | Dry shrinkage/×10−6 m/m | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Air contents | Slump | Slump flow | V-shaped funnel test | 3 d | 7 d | 28 d | 60 d | 3 d | 7 d | 14 d | 28 d | 60 d | 90 d | 180 d |
3.2% | 255 mm | 700 mm | 14 s | 39.5 | 54.8 | 78.9 | 86.7 | −79 | −176 | −237 | −305 | −378 | −412 | −431 |
The maximum outlet pressure of HBT105.21.280RS concrete pump with high reliability was designed to be 21 MPa, and the pressure reserve was 13.3%. The pumping rates were 35–45 m3/h. In order to facilitate the removal of gas in the steel tubes, it was necessary to open 4Ф20 mm vent holes on the wall of steel tubes by every 1.5–2 m along the length of the column. Test columns and their dimensions are shown in Figures
Test columns.
Design drawings of CFST columns used in engineering (unit: mm). (a) SYZ-1 test column. (b) SYZ-2 test column.
The vertical and horizontal expansion and shrinkage deformation of massive self-compacting concrete pumped in the core concrete of CFST were measured by the embedded strain gauges. The arrangement of the strain gauges of measurement points is shown in Figure
Arrangement drawings of test instruments for cross section of column (unit: mm). (a) Layout of deformation measurement points. (b) Layout of temperature measurement points.
BGK-4210 embedded strain gauges with a standard distance of 250 mm were adopted, which was suitable for strain measurement of massive concrete. A total of eight gauges for both CFST columns were needed. We ensured that the embedded strain gauges were geometrically aligned and placed horizontally. After the strain gauges and other devices were installed, it was necessary to calibrate them before the core concrete was poured. The early-age expansion and shrinkage deformation were collected regularly and continuously every day.
Wzp-pt100 armored platinum thermal resistance was used in the temperature test. In consideration of the symmetry of the test column, the temperature test section was taken as one-half of the diameter for testing. In addition to ensuring the thermal resistance required by the measuring point of the internal temperature of the core concrete, a temperature instrument was also set up to measure the temperature inside and outside the test shed. The specific layout of 1 to 6 measuring points is shown in Figure
BGK-4810 vibrating concrete pressure cells were used in the test of the pouring pressure and expansion stress of the core concrete. Four pressure cells were arranged for SYZ-1 column with the height of 940 mm, 4470 mm, 8650 mm, and 11890 mm, respectively; three pressure cells were arranged for SYZ-2 column with the height of 2280 mm, 6360 mm, and 10440 mm, respectively. Each height corresponded to 1∼4 measuring points, respectively. Figures
Arrangement photos of test instruments. (a) Deformation measurement of SYZ-1 column. (b) Deformation measurement of SYZ-2 column. (c) Temperature measurement. (d) Vibrating-wire pressure cell.
Test equipment. (a) Data collection instrument for vibrating-wire sensor. (b) Data collection computer. (c) Concrete pump truck. (d) Wire saw for cutting.
As shown in Figure
Test curves of temperature measurement of the core concrete. (a) SYZ-1 column. (b) SYZ-2 column.
As shown in Figure
It can be seen from Table
Maximum values of temperature at each measuring point of the core concrete.
Type of steel tube column | Measuring points | |||||
---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | |
SYZ-1 | 33.7°C | 32.6°C | 27.9°C | 22.3°C | 10.9°C | 11.5°C |
SYZ-2 | 39.1°C | 36.6°C | 29.0°C | 19.6°C |
Based on the above analysis, temperature parameters must be included in the early-age expansion and shrinkage model of massive self-compaction concrete pumped in steel tube column.
Figures
Test curves of pouring pressure and expansion stress of the core concrete for SYZ-1. (a) 1 hour. (b) 3 days. (c) 28 days.
Test curves of pouring pressure and expansion stress of the core concrete for SYZ-2. (a) 4 hours. (b) 3 days. (c) 28 days.
It can be seen from Figures
Maximum values of pouring pressure and expansion stress at each measuring point of the core concrete.
Type of steel tube column | Types of force | Measuring points | |||
---|---|---|---|---|---|
1 | 2 | 3 | 4 | ||
SYZ-1 | Pouring pressure | 0.025 MPa | 0.094 MPa | 0.197 MPa | 0.283 MPa |
Expansion stress | 0.407 MPa | 0.505 MPa | 0.530 MPa | 0.370 MPa | |
SYZ-2 | Pouring pressure | 0.208 MPa | 0.282 MPa | 0.370 MPa | — |
Expansion stress | 0.657 MPa | 0.712 MPa | 0.686 MPa | — |
From Figures
Figure
Comparisons between the calculation model and test curves. (a) SYZ-1 column. (b) SYZ-2 column.
It can be seen from Table
Maximum values of expansion and shrinkage strains of the core concrete.
Type of steel tube column | Expansion and shrinkage directions | Expansion strains | Shrinkage strains |
---|---|---|---|
SYZ-1 | Vertical | 30.295 | −414.518 |
23.792 | −402.060 | ||
Horizontal | 70.580 | −365.370 | |
87.019 | −335.686 | ||
SYZ-2 | Vertical | 6.824 | −323.532 |
8.777 | −302.180 | ||
Horizontal | 31.654 | −268.006 | |
38.513 | −269.299 |
For SYZ-1 column, the maximum values of the vertical and horizontal strains for early-age shrinkage of the core concrete in 28 days are −414.518
Through the analysis of the above experimental results, it can be concluded that, for SYZ-1 column, the difference values between vertical and horizontal shrinkage deformation are not significant in the first 6 days, but for SYZ-2 column, there was slightly greater difference for the above, and the difference values tend to increase with the change of time. For the CFST column of the same diameter size, the values of the vertical shrinkage strains are much greater than those of the horizontal shrinkage strains at the same time, which may be due to the influence of the self-weight of concrete and spatial effect on the vertical shrinkage of the core concrete. Therefore, the development trends of the horizontal and vertical shrinkage deformation show some differences. The values of the horizontal strains of temperature expansion are greater than those of its vertical ones at the same time, which is the main reason for the change caused by different spatial constraints. The values of the vertical and horizontal shrinkage strains of temperature expansion for small diameter CFST columns are greater than those of large diameter ones at the same time.
Based on the experimental measurement of hydration heat temperature, expansion pressure, expansion, and shrinkage strain of the core concrete and through theoretical analysis, the interaction mechanism of the above three aspects is as follows: at the initial stage of mass concrete pouring, a large amount of hydration heat is released from cement hydration reaction, which quickly accumulates in the concrete, making the concrete temperature rise and the expansion volume of concrete increase. Temperature stress and temperature difference stress will be generated, and their numerical sizes also involve many factors such as the plane size of the structure, structure thickness, constraints, characteristics of various composite materials of concrete, physical and mechanical properties, and construction technology; with the release of heat and the end of heat exchange with the environment, the temperature of concrete decreases, and the volume of concrete shrinks. When the shrinkage is constrained, tensile stress occurs. At this time, the newly poured concrete, no matter its tensile strength and force which are very small, is not enough to resist the tensile stress, so cracks appear in its concrete.
Based on the analysis for the experimental results in this paper and the existing shrinkage models of concrete at home and abroad [ The formula for the calculation model incorporates factors such as the limit shrinkage strain ( Although the tests show that the strength of the concrete itself does not affect the expansion and shrinkage deformation, the factors that affect the above deformation such as cement content, different water-to-binder ratios, and aggregate condition are closely related to the concrete strength in varying degrees. So, the influence of these factors can be reflected indirectly and comprehensively by introducing the compressive strength of concrete. Therefore, the formula of ultimate shrinkage strain ( Based on the analysis of shrinkage test of the core concrete pumped in steel tube in [ In recent years, high-performance concrete (HPC) has been widely used in the core concrete of CFST. The obvious difference between HPC and ordinary concrete is that its composition has the fifth active mineral materials such as fly ash, silica fume, and slag. Therefore, in this model, there is correction coefficient (
The formula is applicable to calculate expansion and shrinkage deformation of the core concrete in CFST. There is no reinforcement in the core concrete of steel tube. The time-development function of expansion and shrinkage is applicable for dimensions
The core concrete in CFST belongs to the massive concrete; that is, its section size is from 1000 mm to 1600 mm, and its time-development function is as follows:
The parameter values of time-development function for the early-age shrinkage and expansion.
Type of steel tube column | Expansion and shrinkage direction | Time-development function | Curve mark | |||
---|---|---|---|---|---|---|
Days of maximum expansion strains | ||||||
SYZ-1 | Vertical | −0.15 | 5.0 | 2.0 | 1.8 | Model in Figure |
Horizontal | −0.35 | 5.0 | 2.0 | 2.0 | Model in Figure | |
SYZ-2 | Vertical | −0.06 | 4.5 | 1.5 | 0.4 | Model in Figure |
Horizontal | −0.30 | 4.5 | 1.5 | 1.2 | Model in Figure |
It should be explained here that, before the day
The correction coefficient of volume-to-surface ratio of massive self-compacting concrete.
Type of steel tube column | The correction coefficient of volume-to-surface ratio | |
---|---|---|
Vertical direction | Horizontal direction | |
SYZ-1 | 0.70 | 0.55 |
SYZ-2 | 0.52 | 0.45 |
Influencing coefficient of substitution proportion of fly ash replacing cement [
Substitution proportion of fly ash replacing cement | 0 | 10∼20% | 20∼30% |
---|---|---|---|
1 | 0.95 | 0.90 |
Influencing coefficient of cement types on shrinkage [
Cement types | Ordinary or fast-hardening cement | Slow-hardening cement | Fast-hardening high-strength cement |
---|---|---|---|
1.0 | 0.7 | 1.15 |
The final shrinkage value (
Figure
Through shrinkage test, combined with hammering test of outer-wall of steel tube, inspection method of split damage test, and detection of cracks between the inner wall of steel tube and the core concrete by the electronic and visual crack observer, as shown in Figures
Detection of cracks between steel tube wall and the core concrete. (a) Detecting by electronic crack observer. (b) Detecting by visual crack observer.
Based on the experimental work and the analysis of the test results, the following conclusions can be drawn: The values of horizontal strains for early-age temperatures expansion are greater than those of the vertical ones. The two kinds of strains of the core concrete for small diameter CFST are greater than those of large diameter ones. The values of the vertical strains for early-age shrinkage are greater than those of the horizontal ones. The two kinds of strains of the core concrete for small diameter CFST are greater than those of large diameter ones. The shrinkage deformation of the core concrete in CFST develops rapidly in the early stage, and the values of its horizontal shrinkage are slightly less than those of the vertical ones at the same time. But the deformation rates show a decreasing trend with the increase of time. After 28 days, the shrinkage deformation curves in this paper become horizontal gradually. The change of section size and volume-to-surface ratio has a great influence on shrinkage deformation of the core concrete. With the increase of section size, the shrinkage deformation values of the core concrete decrease. The influence of volume-to-surface ratio of CFST should be reasonably considered in calculating shrinkage deformation of the core concrete. The calculation model for the early-age shrinkage and expansion is proposed for the massive self-compacting concrete pumped in steel tube column.
All data, models, or codes generated or used in this paper are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This study was funded by the Yuyou Talent Support Plan of the North China University of Technology (Grant no. 2018-39).