The effect of atmospheric pressure on performance of air entraining agent (AEA) and air entraining concrete was studied in Tibet and Hubei, China. The result shows that the reduced atmospheric pressure increased surface tension and lowered foaming property of solution. The AEA with excellent foaming ability and stability is preferred in low atmospheric pressure. The freeze-thaw deterioration process of hardened pastes is relatively faster under low atmospheric pressure. The effect of air pressure on concrete frost resistance performance is more sensitive than the mechanical property. The bigger pores and poor uniformity of internal pore size distribution led to the deterioration of concrete macroscopic properties. Therefore, the AEA varieties should be preferred, the dosage of AEA should be increased, and pore structure of pastes should be optimized to ensure the durability of concrete frost resistance for construction in low-pressure areas.
The altitude of China’s Tibet region is higher than 3000 m. The significant climatic environment of high-altitude area is low pressure, large temperature difference between day and night, dry and windy due to high and complex terrain, high-altitude atmospheric circulation, and solar and other strong radiation along with various other factors. In plateau climate conditions, air entraining concrete is used in hydropower, transportation, and civil engineering projects to resist the frequent freeze-thaw cycle to avoid freezing and other induced disease, thereby ensuring the safe operation of the building [
The AEA can introduce a large number of homogeneously distributed air micropores and, consequently, improve the mixing workability, antipermeability of concrete performance, and frost resistance [
Under different atmospheric pressures in Chinese Tibet (below 70.0 kPa) and Hubei (about 101.1 kPa), the effect of atmospheric pressure on the surface tension and air entraining performance of AEA is studied by AEA quality test. The effect of atmospheric pressure on macroscopic properties and micropore structure is studied by testing the air content of fresh concrete, the compressive strength of concrete, CT meso-map, and the mercury injection of freeze-thaw specimens. The aim of this study is to optimize the AEA varieties, improve the durability of concrete, improve the quality of engineering structure, and provide technical reference for prolonging the engineering life.
Moderate heat Portland cement PM 42.5 (Chinese standard GB175-2007), F class II level fly ash (Chinese standard DL/T 5055-2007), granite aggregates, and chemical admixtures are used in this study. Chemical compositions of cement and fly ash are shown in Table
Chemical composition of cement and fly ash by weight (%).
Compositions | CaO | SiO2 | Al2O3 | Fe2O3 | MgO | K2O | Na2O | SO3 | Loss |
---|---|---|---|---|---|---|---|---|---|
Cement | 62.05 | 20.71 | 4.44 | 5.12 | 4.12 | 0.28 | 0.11 | 2.23 | 0.9 |
Fly ash | 3.27 | 46.19 | 36.95 | 4.43 | 0.38 | 1.01 | 0.38 | 0.38 | 3.58 |
Physical properties of cement.
Specific area (m2·kg−1) | Fitness (%) | Density (g·cm−3) | Water requirement of normal consistency (%) | Stability | Setting time (minimum) | |
---|---|---|---|---|---|---|
Initial setting | Final setting | |||||
390 | 3.7 | 3.21 | 23.2 | Qualified | 185 | 270 |
Physical properties of fly ash.
Fitness (%) | Water demand ratio (%) | Density (g·cm−3) | Percentage of compressive strength |
|
---|---|---|---|---|
7 days | 28 days | |||
7.9 | 98 | 2.3 | 61.3 | 72.3 |
The chemical admixtures include superplasticizer and liquid AEA A and AEA B. The raw materials’ quality satisfied all the corresponding Chinese technical specifications. The maximum particle size of aggregates is found to be 40 mm. The slump of fresh concrete is found between 50 mm and 70 mm. The mix proportions of concrete are shown in Table
Mix proportions of concrete.
Water (kg·m−3) | Cement (kg·m−3) | Fly ash (kg·m−3) | Fine aggregate (kg·m−3) | Coarse aggregate (kg·m−3) | Superplasticizer (%) | AEA A (%) |
---|---|---|---|---|---|---|
124 | 174 | 74 | 675 | 1326 | 0.8 | 0∼0.08 |
The surface tension of the same batch of liquid AEA was tested by the platinum ring surface tension apparatus in three different places (Hubei Wuhan, 101.1 kPa, Tibet Shannan, 65.9 kPa, and Tibet Naqu, 57.2 kPa). The foaming capacity and foam stability of the AEA were tested by the shaking method. Shake 10 ml of 1% solution for 3 minutes, record the volume of foam, and observe the foam morphology from 0 minutes to 3 minutes.
The concrete is mixed and formed in Wuhan and Tibet indoor test labs, respectively. The condition of the mixing room is kept at temperature 20 ± 5°C and relative humidity 60% ± 5% according to the China Power Industry Standard DL 5150. The concrete air content is tested using a concrete air content measuring instrument from Sanyo, Japan. The concrete samples are tested for their compressive strength and frost resistance property using the specimens of 150 mm × 150 mm × 150 mm and 100 mm × 100 mm × 400 mm prisms, respectively.
The microdistribution of the cross section of the specimen before and after freeze-thaw cycles is measured by a CT tester. The internal paste pore structure of frozen-thaw cycle specimens is tested by the mercury injection method with the pore size determination range of 3 nm∼1000
The quality characteristics and effect of AEA are essential to ensure the frost resistance durability of concrete under low atmospheric pressure. The AEA is usually a surfactant which can significantly reduce the surface tension and interfacial energy of water. As a result, small and uniform, mostly under 200
In order to optimize the AEA suitable for high-altitude area, the surface tension and foam capacity of AEA under different air pressures are compared. As shown in Table
Surface tension and foaming property of AEA under different air pressures.
Sample | Atmospheric pressure (kPa) | Surface tension | Foaming ability | |||||
---|---|---|---|---|---|---|---|---|
Test value (mN·m−1) | Growth rate (%) | Foaming capacity (ml) | Foaming capacity at 3 min (ml) | Foam stability (%) | Defoaming time (min) | Bubble shape | ||
AEA A | 101.1 | 29.4 | 100 | Full | Full | 100 | ≥72 h | Small, much even |
65.9 | 32.9 | 112 | Full | Full | 100 | ≥48 h | Big, little, sparse | |
57.2 | 34.6 | 118 | 50 | 49 | 98 | ≥48 h | ||
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AEA B | 101.1 | 31.9 | 100 | 38 | 36 | 95 | ≥36 h | Moderate, even |
65.9 | 34.2 | 107 | 32 | 30 | 94 | ≥24 h | Big, little, sparse | |
57.2 | 36.3 | 114 | 29 | 26 | 90 | ≥24 h |
Surface tension histogram of AEA under different air pressures.
The relationship between dosage of AEA and air content of fresh concrete under different atmospheric pressures is shown in Figure
The relationship between dosage of AEA and concrete air content under different atmospheric pressures.
The relationship between atmospheric pressure and altitude is characterized by the following equation [
Table
Compressive strength of concrete formed under different air pressures.
Sample | Atmospheric pressure | Dosage of AEA (%) | Air content (%) | Compressive strength (MPa) | |||
---|---|---|---|---|---|---|---|
7 days | 28 days | 90 days | 180 days | ||||
WH | Tibet, 65.9 kPa | 0.04 | 3.6 | 15.9 | 27.2 | 34.3 | 38.9 |
XZ | Hubei, 101.1 kPa | 0.04 | 5.1 | 14.7 | 26.5 | 34.3 | 39.2 |
The concrete compressive strength generally decreases with the increasing air content value. The air content increases 1.3%, 2.9%, and 4.7% for the corresponding decreasing compressive strength of 4.7%, 14.3%, and 30.9%, respectively [
Table
Frost resistance of concrete formed under different air pressures.
Sample | Atmospheric pressure | AEA dosage (%) | Air content (%) | Frost resistance of 28 days | |||||
---|---|---|---|---|---|---|---|---|---|
Relative dynamic elasticity modulus (%) | Loss of mass (%) | ||||||||
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WH | Tibet, 65.9 kPa | 0.04 | 3.6 | 90 | 86 | 78 | 0.4 | 1.2 | 2.7 |
XZ | Hubei, 101.1 kPa | 0.04 | 5.1 | 93 | 90 | 84 | 0 | 0.3 | 1.0 |
CT scanning diagram of the concrete section after 150 freeze-thaw cycles. (a) Sample of XZ. (b) Sample of WH.
The meso- and microstructures of concrete reflect indirectly the macroscopic performance indexes. Aggregate, paste, the mesodistribution of pore structure, and the development of cracks after freeze-thaw damage can be shown by CT scanning [
Figure
Cumulative differential curve of mercury content in hardened paste before and after freeze-thaw cycles. (a) Forming in Hubei. (b) Forming in Tibet.
Pore structure of hardened paste.
Sample | Porosity (%) | Distribution of pore size (%) | <100 nm | ≥100 nm | |||||
---|---|---|---|---|---|---|---|---|---|
<5 nm | 5∼20 nm | 20∼50 nm | 50∼100 nm | 100∼200 nm | >200 nm | ||||
WH | 19.9 | 11.8 | 27.3 | 38.6 | 15.3 | 1.0 | 6.0 | 93.0 | 7.0 |
WH-D150 | 20.5 | 7.8 | 22.6 | 22.3 | 23.0 | 6.7 | 17.6 | 75.7 | 24.3 |
XZ | 17.3 | 9.2 | 25.4 | 31.8 | 10.6 | 5.8 | 17.2 | 77.0 | 23.0 |
XZ-D150 | 32.2 | 9.4 | 20.7 | 16.9 | 22.6 | 4.8 | 25.6 | 69.6 | 30.4 |
Pore size distribution of hardened paste.
The two groups of reference pastes (WH and XZ) under the different atmospheric pressures have the comparable, most probable pore size, but the peak of WH pastes is higher. The total porosity of hardened cement paste is 19.9% and 17.3% in Hubei and Tibet, respectively. However, the pore size of cement paste in Hubei is mainly concentrated <100 nm (accounting for 93%), most of them were less harmful and harmless pores, and the harmful pores are only 7.0%. The uniformity of pore size distribution is relatively poor in Tibet’s concrete, and the harmful pores larger than 100 nm are as high as 23.0%.
Most probable aperture in the hardened paste increases, and the peak in the curve shifts to the left after 150 times of freeze-thaw cycle. The total porosity of specimens is increased; especially, the pores above 50 nm in size. In general, freeze-thaw cycles lead to increased harmful pores in paste. On the other hand, the deterioration process of hardened paste in Tibet is faster and the final porosity is 32.2% (for <100 nm less harmful or harmless pores 69.6% and >100 nm harmful pores 30.4%), which is higher than 20.5% of Hubei.
Therefore, the freeze-thaw cycle process increases the porosity of the cement paste, especially, the increase of less harmful and harmful holes. The concrete formed at low pressure in Tibet is more vulnerable to freeze-thaw damage, and the sensitivity of air pressure to the frost resistance is more than the mechanical properties which are in agreement with the test results.
The adsorption of gas on the liquid surface and the solubility in liquid will decrease with the increase of atmospheric pressure. This will lead to the increase of the surface tension of AEA. The change of surface tension of AEA directly affects the size of pores. According to the Young–Laplace equation, the relationship between surface tension and bubble size is as follows:
According to (
Calculated results of bubble radius of AEA.
Location | Atmospheric pressure (kPa) | Surface tension |
Bubble radius |
||
---|---|---|---|---|---|
AEA A | AEA B | AEA A | AEA B | ||
Wuhan of Hubei | 100 | 29.4 | 31.9 | 58.8 | 63.8 |
Shannan of Tibet | 66 | 32.9 | 34.2 | 99.7 | 103.6 |
Naqu of Tibet | 57 | 34.6 | 36.3 | 121.4 | 127.4 |
It was shown [
Therefore, in winter, the frost resistance durability of concrete at high-altitude area can be guaranteed through optimize the variety of AEA, appropriately increase the amount of AEA and improve the internal pore structure of concrete.
When the ambient pressure is reduced from 101.1 kPa to 57.2 kPa, the surface tension of two kinds of AEAs is increased by 118% and 114% and the foaming ability is decreased. The AEA A has strong foaming ability and good foam stability, and it is more suitable for air entraining concrete in low-pressure environment. Air content of fresh concrete in the Tibet area is lower than that of in the Hubei area from 30% to 47%. When the dosage of AEA is more, the gap is more. When the difference of air content is 1.4%, the compressive strength of concrete is very close in two regions and the frost resistance of concrete affected by the pressure is more sensitive. The frost resistance of concrete formed at low air pressure is poor. As is shown in the CT diagram, there are obvious microcracks in the specimen except the edge damage, and some cracks connect into larger defects after freeze-thaw damage. The results of mercury injection show that the construction of cement paste becomes loose and the porosity and harmful pore ratio increase with the freeze-thaw process. The atmospheric pressure affects the air entraining effect of AEA solution which also affects the macroscopic properties of concrete and porous structure. In order to ensure the frost resistance of concrete in high-altitude area, the internal pore structure of concrete should be improved by optimizing variety and dosage of AEA.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
The work described in this paper was supported by the National Natural Science Foundation of China (nos. 51479011, 51779019, and 51778003) and the Central Non-Profit Scientific Research Fund for Institutes (no. CKSF2017052/CL)