Abstract
Low-field nuclear magnetic resonance (NMR) technology has the characteristics of nondestructive, rapid, and accurate. In the present paper, the mechanical properties and the size and distribution of pores of cement mortar during freeze-thaw cycles were studied by using the NMR technology for the first time. The change law of surface and quality, compressive strength, splitting tensile strength, and elastic modulus of cement mortar under 0, 25, 50, 75, and 100 freeze-thaw cycles were studied. And the changes of T2 spectra of cement mortar under different freeze-thaw environments were analyzed; the change rule between freeze-thaw cycles and the size of the pore within the cement mortar were also obtained. Moreover, the relationship between the mechanical properties and the pore structure of cement mortar was studied.
1. Introduction
In recent years, there are more and more concrete engineering developed in the cold regions [1, 2], and the need to accurately evaluate the stability of geotechnical engineering constructions and to prevent or control freeze-thaw hazards has become an urgent problem [3, 4]. Studies of the physical and mechanical properties of building material that undergo freeze-thaw cycles provide important data for the prevention of freeze-thaw hazards in cold regions. Low concrete temperature reduces the rate of the strength gain and extends the setting time [5, 6]. Freezing and thawing cycles deteriorate concrete because water freezes to ice and expands, and the volume change of water to ice causes high stress if there is no room for expansion. The ice then melts down water and increases the saturation level of the voids [7, 8]. Powers [9] originally proposed that hydraulic pressure (as water freezes, it expands and generates pressure on the pore wall) was the source of stress that causes damage. The following cycles of freezing and thawing in cold climates aggravate the resistance of the concrete [10, 11]. The increase of the volume of a fluid phase at freezing results in the occurrence of some effects: the crystallizing pressure of ice upon the walls of the pores and capillaries, the hydraulic pressure of the porous liquid, and the osmotic pressure caused by the freezing of water [12, 13]. Concrete internal damage caused by the frosting of internal moisture within concrete pores will lead to the generation and disintegration of microcracks. Studies of the pore structure of properties of cement mortar that undergo freeze-thaw cycles will provide important data for analyses of cement mortar engineering and the prevention of freeze-thaw hazards in cold regions.
Current detections of microscopic deterioration of solid materials are mainly conducted with traditional methods like the technique of CT scanning [14, 15], method of scanning electron microscope (SEM) [16], technique of digital imaging treatment [17], and acoustic emission [18, 19], yet these methods are not very satisfying. For example, results of CT scanning cannot reflect the features of microscopic structure [20, 21], and the experiment is costly; the method of electron microscope requires small specimen, and it can only be used in real time observation. As a new method for analysis and detection in physical tests, nuclear magnetic resonance (NMR) technology has the characteristics of nondestructive, rapid, and accurate [22], and it can be used to obtain parameters such as porosity, free fluid index, pore-size distribution, and T2 distribution of transverse relaxation time [23, 24]. It could be applied in experiments and detection researches on pore-size distribution, characteristics of the inner structure of concrete, and so on [25, 26]. In the present study, an extensive testing program is conducted to investigate the effect of concrete after freeze-thaw attack. The NMR technique was also applied to the measurement of concrete specimens. The mechanical property test is performed on the specimens after different freeze-thaw cycles. Freeze-thaw effects on the surface scaling, weight loss, water content, compressive strength, and elastic modulus are discussed and clarified. Several conclusions are then drawn based on the proposed study.
2. Experimental Methods
2.1. NMR Technology
Nuclear magnetic resonance (NMR) refers to the response of atomic nuclei to magnetic fields. Many nuclei have a magnetic moment, and they behave like a spinning bar magnet. These spinning magnetic nuclei can interact with externally applied magnetic fields, producing a measurable signal. The hydrogen proton (H+) is a particle with a positive charge, and proton spin is one of the important properties of the hydrogen proton. Hydrogen proton spin can produce a magnetic field, and magnetic axis directions of protons are random in no external magnetic field. The porosity of saturated cement mortar can be calculated by substituting the amplitudes of the detected signals into the relation determined by calibration samples. The porosity indicates the water content and volume in the cement mortar. If the specimens tested by the NMR are water saturated, the water volume is equal to the crack volume.
T2 is a time constant describing decay of the transverse component of magnetization. According to nuclear magnetic resonance theory, the transverse relaxation rate of nuclear magnetic resonance can be expressed as the following equation [24]:where is the relaxation time of fluid, is the transverse surface relaxation strength, is the pore surface area, is the pore volume, is the transverse surface relaxation rate, is the diffusion coefficient, is the gyromagnetic ratio, is the gradient of the magnetic field, is the echo time, and is the diffusion relaxation rate.
In this study, there is only one type of fluid (water) in the pores, and the volume relaxation is much slower than that of the area, so is neglected. When the magnetic field is even and adopted is short, the diffusion relaxation can also be ignored. Therefore, equation (1) can be simplified as
From equation (2), the rate of transverse relaxation depends on the surface-to-volume ratio of the pores. Thus, the T2 distribution reflects the pore size information: the smaller the T2 value is, the smaller the pore size is; the larger the T2 value is, the larger the pore size is.
2.2. Experimental Scheme
The cement mortar specimens were made of a mixture of water and cement according to the standard GB/T50082-2009. Ordinary Portland cement 42.5 R was used in this experiment, and the test report is shown in Table 1. The river sand is collected from the Xiangjiang river; the apparent density is 2640 kg/m3, bulk density is 1430 kg/m3, silt content is 0.8%, and fineness modulus is 2.91. The cement mortar mixture scheme is shown in Table 2.
The main experimental equipment included freeze-thaw cycle test machine, vacuum saturation device, NMR test system, and loading control system DCS-200, as shown in Figure 1. The NMR measurements were conducted using an AniMR-150 NMR system. The NMR test system was supplied by Niumag Corporation (Shanghai, China) with wide bore and vertical superconducting magnet. The magnetic field strength reaches 0.25 T, and the resonance frequency ranges from 8.5 to 12.8 MHz. The gradient coils can provide a maximum gradient strength of 0.15 T/m in X, Y, and Z directions, respectively. The required temperature of the experimental environment is 32°C to guarantee the system. Experimenting at such a temperature also ensures the testing system run at its best. The porosity, T2 distribution, and MR image can be obtained by using the Core analysis software and the MRI viewer software.
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The main steps of the test are as follows. (1) After casting molding and 24 hours of conservation, the cement mortar was placed in the standard curing box under the curing condition 20 ± 3°C and 95% RH. (2) The specimens were tested according to the standard experimental procedure. (3) The specimens were saturated by vacuum saturation device for 12 h and then placed in the freeze-thaw cycle test machine under the condition of freezing temperature of −20°C and thawing temperature of 20°C with reference to the weather condition. Each complete cycle of freeze-thaw lasted for 8 h, comprising 4 h for freezing and 4 h for thawing. (4) The specimens from the freeze-thaw cycle test machine were removed, water on the surface was wiped off, and the variations of appearance were recorded. (5) Porosity and T2 distribution were obtained by the NMR technology. (6) After recording the change rule of quality during NMR experiment, the next freeze-thaw test are carried out. The dimension of the additive cement mortar specimens was 70.7 mm × 70.7 mm × 70.7 mm, as shown in Figure 2.
3. Cement Mortar Damage Evolution during Freeze-Thaw Cycle
By observing the surface changes of the cement specimens during the freeze-thaw cycles as shown in Figure 3, there was no obvious change on the cement mortar after 75 cycles, but it had changed significantly after 100th cycles. There was a weak side on the surface that damaged after many freeze-thaw cycles, while the lower layer did not change much.
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Freeze-thaw causes the surface of cement mortar to change and fall off, and the quality change of cement mortar can be used as the evaluation index of the damage degree of the cement mortar specimen during the freeze-thaw cycle: the greater the mass loss, the greater the damage to cement mortar caused by the freeze-thaw cycle. Table 3 shows the quality of cement mortar after different freeze-thaw cycles. It shows that, as the number of freeze-thaw cycles increases, the quality of cement mortar is decreasing gradually, and it was reduced by about 8% after 100 freeze-thaw cycles.
Table 4 shows the changes law of mechanical parameters during the freeze-thaw cycle, and it shows that, as the number of freeze-thaw cycles increased, the compressive strength, tensile strength, and elastic modulus of cement mortar specimens have decreased.
4. NMR Characteristics of Cement Mortar during Freeze-Thaw Cycles
4.1. NMR T2 Spectrum Distribution
The changes of an NMR T2 spectrum could reflect the structural changes of pores within cement mortar. The sizes of pores in cement mortar specimens are in proportion to the fluid traverse relaxation time T2. When T2 is small, it means that the sizes of pores in cement mortar specimens are small; the location of peaks in the T2 spectrum is directly related to the size of pores, and the peak value in the T2 spectrum reflects the concentration degree of the distribution of pore sizes within cement mortar specimens. Therefore, the changes of T2 spectrum could qualitatively describe the structural changes of pores inside the cement mortar specimens. Figure 4 shows the T2 distribution of the cement mortar specimens after 25, 50, 75, and 100 freeze-thaw cycles.
It can be seen that the T2 spectrum distribution is mainly presented by 3 peak images: the first peak area is the largest, located near at 1.5 ms; the second one is smaller than the first one, located near at 75 ms; and the third peak is almost invisible, located near at 750 ms. The first spectrum peak in the T2 spectrum curve was considered as small pores, and the second and third spectrum peak were large pores. This indicates that the internal pores of cement mortar are mainly small pores, and the proportion of large pores and large pores is low. During the process of 100 freeze-thaw cycles, the T2 spectrum peak of cement mortar gradually increased, and the curve gradually expanded outward, which shows there are more internal pores produced inside the cement mortar after freeze-thaw cycles, and the small pores gradually become larger pores.
4.2. T2 Spectra Area Variation
Table 5 shows the variation of T2 spectrum area of cement mortar specimens after 100 freeze-thaw cycles. It can be seen that the total spectrum area increased with the increasing of freeze-thaw cycles. The proportion of the first spectrum peak area is more than 90%, which means the internal pores of cement mortar are mainly small pores. However, as there is an increase in the number of freeze-thaw cycles, the proportion of the second peak and the third peak is gradually increasing, while the first peak is decreasing, as shown in the ratio change rule of each spectrum peak in the T2 spectrum in Figure 5, which shows that the small pores inside the cement mortar are gradually getting larger, and the number of larger pores are increasing. Meanwhile, there are some new micropores constantly appearing, this change objectively reflects the microcosmic change and destruction mechanism of cement mortar during freeze-thaw cycles.
4.3. Relationship between NMR Characteristics and Its Mechanical Properties
The variation curves of compressive strength, splitting tensile strength, and magnetic resonance porosity of cement mortar during freeze-thaw cycles were shown in Figure 6; it can be seen that the nuclear magnetic porosity increased during freeze-thaw cycles, and there is a tendency to accelerate. While the tensile strength and compressive strength of the cement mortar decreased, the rate of decline is accelerating. It shows the porosity of cement mortar is negatively correlated with its tensile strength and compressive strength. The internal porosity of the cement mortar has been expanded after freezing, and its mechanical properties are gradually decreasing.
5. Conclusion
(1)When the cement layered while pouring module during freeze-thaw cycles, there was a weak side on the surface that damaged after many freeze-thaw cycles. As freeze-thaw cycles increased, the quality of cement mortar gradually decreased and the compressive strength, crack tensile strength, and elastic modulus of cement mortar also decreased.(2)The NMR technology had been applied to study the microstructure of cement mortar during the freeze-thaw cycle for the first time. The magnetic resonance T2 spectrum of cement mortar has three peaks, and the first crest ratio is over 90%, indicating that the internal pores of cement mortar are mainly microporous. With the increasing of freeze-thaw cycles, the microporosity inside cement mortar is gradually enlarged and new pores are constantly produced.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Authors’ Contributions
Taoying Liu carried out the figure preparation, analysis, manuscript preparation, and editing. Yunmin Wang and Keping Zhou planned and designed the research. Feng Gao contributed the conduct. Shenghua Xie performed the data collection.
Acknowledgments
This paper was funded by the National Key Research and Development Program of China (2018YFC0808404); the Key Research & Development Program of Hunan Province (2017GK2190); China Postdoctoral Science Foundation funded project (2018M643003); Anhui Postdoctoral Foundation Fund Project (2017B201); project was (2018JJ3676) supported by the Natural Science Foundation of Hunan Province; and open fund was provided by the State Key Laboratory of Safety and Health for Metal Mine (2016-JSKSSYS-02, 2018-JSKSSYS-02, and 2018-JSKSSYS-04). And we would like to express our thanks to Wenwu Tan for his help in this paper.