Study on Deformation Characteristics and Damage Model of NMK Concrete under Cold Environment

Abstract

To improve the ability of concrete structures to resist freeze-thaw damage in cold environments, explore the effect and mechanism of nano-metakaolin (NMK) on frost resistance of concrete. And make up for the deficiencies in the mechanical properties and deformation process of na-no-metakaolin concrete in freeze-thaw environments. Rapid freeze-thaw cycle experiment was car-ried out to detect the deterioration law of concrete. Physical and mechanical properties under freeze-thaw environment was measured. The modification mechanism of nano-metakaolin on con-crete frost resistance from micro and meso scales was analyzed. The effect of freeze-thaw damage on nano-metakaolin concrete was characterized. The influence law of stress strain is established, and the meso-statistical damage constitutive model of nano-metakaolin concrete under freeze-thaw action is established. The results show that: Compared with other nano-clays, adding 5% nano-metakaolin can effectively slow down concrete’s freeze-thaw cracking and crack propagation. After 125 freeze-thaw cycles, the surface crack width of concrete mixed with 5% nano-metakaolin is only 0.1mm. Without freeze-thaw cycles, the compressive strength of concrete mixed with 3% nano-metakaolin is the highest, which is 28.75% higher than that of ordinary concrete; after 125 freeze-thaw cycles, the loss rate of compressive strength of concrete mixed with 5% nano-metakaolin was 12.07%. After 125 freeze-thaw cycles, the peak strain is 0.45 times that of concrete without NMK, and the peak stress is 3 times that of concrete without NMK.

1. Introduction

Concrete is one of the most widely used engineering materials in civil engineering. With the extensive use of concrete structures, people pay more and more attention to the durability of concrete structures. Nowadays, the research on the durability of concrete structures has become a hot topic in civil engineering academia. In the northern cold environment, the concrete system in service will suffer freeze-thaw damage [1,2]. The service life of most concrete structures is only 20–30 years, far below the design life [3,4]. With the development of the social economy, many coastal offshore concrete structures, such as cross-sea bridges, and seaport dams, have put forward higher requirements for the durability of concrete materials. To resist the damage of harmful medium erosion and environmental factors on concrete structures, the development of high-durability concrete materials has become an inevitable trend in civil engineering, and related research needs to be carried out urgently.

In recent years, a great deal of attention has been drawn by nanotechnology given the potential use of nanoscale particles. Numerous studies have demonstrated the use of particles in nanoscale can significantly improve the specifications of cementitious base materials in a fresh and hardened state [2,5]. Considering theirtiny size and high specific surface area, nanomaterials make a better attachment with cement hydrates and improve permeability, durability, shrinkage, and strength of the concrete [6,7]. Nano-silica is one of the most commonly used nanomaterials across the world and is a desirable nanomaterial for incorporation in cementitious composites. Adding nanosilica to concrete increases it’s compressive, tensile, and flexure strength. It also reduces its setting time and water permeability and increases its resistance against the chemical attack. Therefore, they are used in many concrete structures [8,9,10,11,12]. However, the cost of nanoscale materials is an obstacle in its widespread application. On the other hand, their advantages over other admixtures are undeniable [13]. The recent decades, the use of nano-clay has been considered for lower production costs than nano-silica and their beneficial thermal properties [14,15,16,17,18,19]. Dejaeghere et al. [17] illustrated that most mixes containing 2.5% attapulgite nanoclay displayed a lower heat of hydration at 1 and 3 days than those containing 0.5%, indicating a reduced degree of cement hydration in the former. Hakam et al. found that the physical, mechanical and thermal properties of cement nanocomposites reinforced with calcined nanoclay (CNC) were enhanced due to the addition of CNC into the cement matrix and the optimum content of CNC was 1 wt% [18]. Ishida et al., determined that cement mortar mixtures with various replacement ratios, 0.5%, 1%, and 2% of nanoclay by weight of cement could improve the mechanical strengths of cement mortar; cement mortar reinforced with 2 wt.% nanoclay, where 11%, 5%, and 9% improvement in the compressive strength, flexural strength, and tensile strength, respectively [19].

At present, studies about the frost resistance of nanoclay (NC) modified concrete usually focused on the changes of the compressive strength, the mass and the dynamic modulus of elasticity [7,20,21]. Fan et al., revealed that concrete mixes containing 1–5% Nano-metakaolin clay (NMK) all had a higher value of relative dynamic modulus of elasticity (RDM) than the corresponding concrete mixes without NMK after 125 F-T cycles. In particular, 3% NKC led to the most enhanced concrete F-T durability, indicated by the highest RDM compared to the control [22]. Lomboy and Wang attested that with good air entrainment, SCCF concrete containing 1% attapulgite NC showed comparable F-T resistance to conventional pavement concrete after 300 F-T cycles [23]. NC can refine concrete microstructure, reduce porosity and the water absorption of concrete [24]. Low water absorption means that the probability of F-T damage is small. The existing literature shows that the use of NC generally improves concrete ant-frost. The failure mechanism of concrete under the common influence of freeze-thaw cycles is extremely complex and needs a comprehensive investigation. Only after a further investigation of the failure mechanism, the functions of the additives can be clearly understood [2]. Additionally, quantitative assessment of damage and the establishment of a constitutive model using amplitude as a parameter is a problem that requires further investigation [25]. Past traditional constitutive models often ignored the addition of NC in concrete. There is still a big gap for the reflection of critical state in the constitutive relation of concrete with NC.

NMK is made from raw ore, which has high pozzolanic activity, and its particles can fill the tiny pores in the cement matrix. This paper adopts the rapid freeze-thaw cycle test to check the Effect of different dosages of NMK on the frost resistance of concrete. At the micro-scale, the mechanism of NMK in concrete was analyzed by SEM. Based on the experimental study and damage mechanics, the freeze-thaw damage accumulation model of NMK concrete is proposed, providing a scientific practice and theoretical reference for developing new antifreeze concrete.

2. Materials and Methods

2.1. Raw Materials

Materials

The cement used in the test is Dalian Xiao YetianPO•42.5R ordinary Portland cement, and its chemical composition is shown in

Table 1. Chemical compositions of cement.

Chemical Composition	CaO	SiO2	Al2O3	Fe2O3	MgO	SO3	LOL
content/%	59.30	21.91	6.27	3.78	1.64	2.41	4.69

. Nano-clay is NMK, which is made by calcining nano-kaolin ore at a high temperature of 600–800 °C for 2 hand then grinding. The specific physical indicators and chemical composition are shown in

Table 2. Physical properties of NMK.

Mean Lamella Diameter/nm	Average Sheet Thickness/nm	Specific Surface Area/m2/g	Density/g/cm3
300–500	20–50	30	0.6

and

Table 3. Chemical compositions of NMK.

Chemical Composition	SiO2	CaO	Al2O3	Fe2O3	MgO	K2O	TiO2	Na2O
content/%	47.80	0.28	41.80	0.30	0.03	0.58	0.02	0.06

. The microstructures (SEM, TEM), EDS, and XRD patterns of NMK are given in Figure 1.

2.2. Preparation of Specimens

The concrete cube models of 100 × 100 × 400 mm were made to determine mass loss and elastic modulus in the rapid freeze-thaw cycle test process. Each group of three specimens has 12 specimens. The cube specimens of 100× 100 × 100 mm were made to measure the compressive strength of concrete and the development of internal cracks. There were 60 specimens in each group.

We studied concrete modified with 0, 1%, 3%, and 5%NMK by mass, and the water-binder ratio of concrete was 0.5. The specimen number and concrete mix ratio are shown in

Table 4. Mix proportion of concrete.

Specimen Number	Cement/kg/m3	NMKkg/m3	Water kg/m3	Sand kg/m3	Gravel kg/m3
NC0	455	0	228	700.8	1051.2
NC1	450.45	4.55	228	700.8	1051.2
NC3	441.35	13.65	228	700.8	1051.2
NC5	432.25	22.75	228	700.8	1051.2

. Before pouring NMK concrete specimens, the NMK is first ultrasonically dispersed in water for 15 min and then mixed with cement and aggregate evenly. The mixing process follows the relevant provisions of China’s current standard “Highway Engineering Cement and Cement Concrete Test Procedure” (JTG E30-2005). Immediately after the completion of the specimen into the standard maintenance room, maintenance to 24 h after demoulding. Then, the test was conducted after 28 days of supervision.

2.3. Test Methods and Processes

2.3.1. Rapid Freeze-Thaw Cycle Test

Following the relevant provisions of the ‘test method for long-term performance and durability of ordinary concrete(GBJ82-85), the freeze-thaw cycle test was carried out by the TDR-16 rapid freeze-thaw testing machine, and the dynamic modulus of concrete under the freeze-thaw cycle was measured by DT-12 dynamic modulus tester. The schematic diagram of the freeze-thaw cycle device and the temperature change curve are shown in Figure 2 and Figure 3, respectively.

2.3.2. Concrete Strength Measurement

After putting the concrete specimen into a freeze-thaw test box, take out one group every 25 cycles. The YAW-YAW2000A type 200t microcomputer-controlled electro-hydraulic servo pressure testing machine is used to test the compressive strength of concrete following the current standard Highway Engineering Cement and Cement Concrete Test Procedure (JTGE30-2005). The loading speed is controlled at 2400 ± 200 N/s.

2.3.3. Microstructure Analysis

SUPRA 55 SAPPHIRE field emission scanning electron microscope (SEM) was used to observe and analyze the microscopic morphological characteristics of the samples at different ages. When preparing the example, the piece is cured to the test age and broken, and the block of about 1 cm × 1 cm × 1 cm is taken and placed in anhydrous ethanol to stop hydration for 48 h. The test section of the sample is not subjected to any treatment.

3. Results and Discussion

3.1. Macroscopic Changes of NMK Concrete Surface

The surface erosion and failure of concrete specimens with different NMK content after 125 freeze-thaw cycles are shown in Figure 4. It can be seen that a large number of coarse aggregates are exposed on the surface of NC0 and NC1 specimens, there are many deep pits on the surface, and severe aggregate stripping occurs at the end. NC3 and NC5 specimen surface only a small amount of coarse aggregate exposed, surface pits less, no coarse aggregate stripping phenomenon; the erosion of NC5 specimen is more severe than that of NC3 specimen.

After 50, 75, and 100 freeze-thaw cycles, the surface cracks of NMK concrete are shown in Figure 5, and it shows that no obvious cracks appeared on the surface of NMK concrete before 50 freeze-thaw cycles. The concrete is in the induction period of freeze-thaw damage, and the micro-cracks in the matrix begin to appear but do not extend to the concrete surface. After 75 freeze-thaw cycles, macro cracks started to appear on the surface of NC0 and NC1 specimens and entered the accelerated period of freeze-thaw damage. The cracks gradually expanded and interconnected. There is no macroscopic crack on the surface of NC3 and NC5 samples, which is still in the induction period of freeze-thaw damage. When the number of freeze-thaw cycles reached 100, many visible cracks with wide width appeared on the surface of NC0 and NC1 specimens, accompanied by severe concrete spalling. At this time, the concrete has been in a stable growth period of freeze-thaw damage and is in a failure state. There are only a small amount of tiny cracks on the surface of NC3 and NC5 specimens, which have just entered the accelerated period of freeze-thaw damage to concrete. From the above analysis, it can be concluded that the appropriate amount of NMK can effectively hinder the occurrence, expansion, and connection of micro-cracks in the concrete matrix during the freeze-thaw cycle and delay the damage to the concrete structure. Visual inspections of the surface of different specimens are in good agreement with the results of Kalhori H, et al. [7].

The change of concrete surface crack width under different freeze-thaw processes is shown in Figure 6, it shows the NC0, and NC1 specimens showed obvious surface cracks after 75 freeze-thaw cycles. The NC3 and NC5 samples showed obvious cracks after 100 freeze-thaw cycles. After 75 freeze-thaw cycles, several obvious penetrating cracks appeared at the end of NC0 and NC1 specimens, and the average widths of cracks were about 0.1 mm and 0.3 mm, respectively. After 100 freeze-thaw cycles, the surface of ordinary concrete and NC1 specimens had an obvious cracking failure, and the center of the crack extended to both sides. There were only slight cracks on the surface of NC3 and NC5 specimens, and the average crack widths were 0.2 mm and 0.1 mm, respectively. However, the surface erosion of the NC5 specimen was more serious than that of the NC3 specimen, with a small amount of aggregate spalling and large surface holes.

3.2. Micro-Morphology Changes of NMK Concrete after Freeze-Thaw Damage

The microstructure of NMK concrete before and after freeze-thaw is shown in Figure 7. It can be seen from the diagram that before freeze-thaw cycles, the microstructure of the NMK concrete matrix is dense, the hydration products are filled between the matrix voids, and there are few cracks. After 30 freeze-thaw cycles, the internal components of concrete have different degrees of microcrack growth and expansion until the concrete cement slurry peeled off and the structure is crisp and loose. After 30 freeze-thaw cycles, apparent crack propagation occurred in NC0, and both CH crystal and C-S-H gel cracked. There were also apparent microcracks in NC1, and the inter-gel arrangement was loose. The microstructure of NC3 is still dense, and the number of pores is small; the microstructure of NC5 was more flexible than that of NC3, and the hydration products showed fine dispersion. From the above analysis, it can be concluded that an appropriate amount of NMK can effectively hinder the damage to concrete microstructure during freezing and thawing and slow down the initiation and propagation of microcracks.

3.3. Effect of Freeze-Thaw Action on Mechanical Properties of Concrete

After the concrete specimen is subjected to freeze-thaw cycles, cracks or holes will occur inside, which will lead to changes in its mechanical properties. Formula (1) is defined as the compressive strength loss rate of concrete specimen:

$$D_{fc}=(1-\frac{f_{cn}}{f_{c0}})\times 100\%$$

(1)

In the Formula, D_fc represents the loss rate of concrete compressive strength after freeze-thaw, unit: %; f_cn represents the compressive strength of concrete after n freeze-thaw cycles, unit: MPa; f_c0 represents the compressive strength of unfrozen concrete specimens, unit: MPa.

The relationship between the compressive strength of concrete specimens and NMK content during freeze-thaw cycles is shown in Figure 8. It can be seen that NMK particles can improve the compressive strength of concrete without freeze-thaw damage. The compressive strength of concrete specimens with 3% NMK is the largest, which is 28.75% higher than that of ordinary concrete. When adding 5% NMK, the compressive strength of concrete specimens decreases, mainly because the pozzolanic Effect of NMK can effectively promote the hydration reaction of cement in concrete, promote the formation of hydrated calcium silicate gel, and then improve the compressive strength of concrete. However, when the content of NMK increases, the agglomeration effect between particles reduces its dispersion in cement, which leads to a decrease in the compressive strength of concrete.

Under the early freeze-thaw cycles, the compressive strength of 1% NMK concrete is lower than that of ordinary concrete. The compressive strength of 3% and 5% NMK concrete did not change significantly. With the increase of freeze-thaw cycles, the compressive strength of concrete gradually decreases. Under the action of 100 and 125 freeze-thaw cycles, the compressive strength of concrete gradually increases with the increase of NMK content.

The variation of compressive strength of different NMK concrete with freeze-thaw cycles is shown in Figure 9. It can be seen that the compressive strength of concrete decreases after freeze-thaw cycles. The appropriate amount of NMK particles can effectively improve the compressive strength of concrete, and the compressive strength of concrete with 5% NMK is the best. After 125 freeze-thaw cycles, the compressive strength loss rate is the smallest; adding 5% NMK can reduce the damage and failure of concrete during freeze-thaw cycles. But, Shahrajabian et al. reported that 2% nano-clay in concrete can decrease the compressive strength reduction and weight loss of samples against freeze and thaw cycles [7]. Kalhori et al. revaled that the optimum content of nano-clay was 4 wt%, the use of high content of nano-clay decreases their efficiency [20].

3.4. Effect of Freeze-Thaw Action on Deformation Performance of NMK Concrete

3.4.1. Stress-Strain Curve

After the freeze-thaw cycle, the internal microstructure of concrete specimens will be damaged and deteriorate, which directly leads to the change of deformation behavior of materials. The stress-strain curves of NMK concrete under axial compression after different freeze-thaw cycles are shown in Figure 10. It can be seen from the diagram that the stress-strain curve of NMK concrete under the freeze-thaw cycle is roughly similar. Under different freeze-thaw cycles, with the increase of freeze-thaw cycles, the stress-strain curves of concrete specimens gradually flattened, and the peak point decreased and shifted to the right. This reflects that due to the freeze-thaw action, the internal damage of concrete is evolving and developing. The macroscopic performance is that the peak stress of the curve decreases gradually, the peak strain increases, and the elastic modulus decreases slowly. The stress-strain curves of different NMK concrete specimens are slightly different. The flat rate of ordinary concrete specimens is faster, and the flat rate of 5% NMK concrete specimens is the slowest. This is mainly due to the weak damage and strength of NMK concrete specimens in the freeze-thaw process. The brittle behavior of NMK in concrete is more potent than that of ordinary concrete, and it is prone to sudden brittle fracture during compression.

3.4.2. Shrinkage Cracking Characteristics of Mortar Plate

Different NMK concrete axial compression test results are shown in

Table 5. Compressive strength of the concrete with the addition of NMK under freeze–thaw cycles.

Freeze-Thaw Cycles		0	25	50	100	125
Peak strain/10−3	0	1.10	1.40	1.31	1.34	2.70
	1	1.40	1.10	1.40	1.55	1.81
	3	1.30	1.11	1.25	1.30	1.55
	5	1.20	0.99	1.20	1.15	1.20
Peak stress/MPa	0	32.93	36.42	29.97	18.06	10.02
	1	38.75	29.11	20.50	19.59	16.88
	3	41.10	39.02	32.33	23.57	19.14
	5	38.76	37.45	33.02	28.55	31.32

.

The relationship between the relative peak stress of concrete and the number of freeze-thaw cycles under different freeze-thaw cycles is shown in Figure 11. It can be seen from the figure that with the increase in the number of freeze-thaw cycles, the relative peak stress of concrete gradually decreases, indicating that the internal damage of concrete gradually increases and the pressure that can be progressively borne decreases. The peak stresses of NC0 and NC1 reduce considerably, indicating that the internal damage of concrete is extensive and cannot withstand enormous pressure. The decreasing trend of NC5 peak stress is slower than other concretes, meaning that the internal damage is light and the concrete matrix can withstand significant pressure.

The relationship between the relative peak strain of NMK concrete and the number of freeze-thaw cycles is shown in Figure 12. It can be seen from the diagram that the relative peak strain of concrete increases with the increase of freeze-thaw cycles, indicating that the ability of concrete to resist deformation under axial pressure gradually decreases with the increase to internal damage of concrete.

3.4.3. Initial Elastic Modulus and Peak Secant Modulus of Concrete

The slope of the compressive stress-strain curve (σ – ε) is its elastic modulus or deformation modulus, which is called the tangent elastic modulus when it is taken as the tangent slope dσ/dε; when σ/ε is taken as the secant elastic modulus.

When σ = f_c, ε = ε_c, the tangent elastic modulus is 0, and the peak secant modulus E_c is Formula (2):

$$E_c=\frac{f_c}{{\epsilon }_c}$$

(2)

when σ = 0, ε = 0, the tangent elastic modulus is equal to the tangent elastic modulus, and the initial elastic modulus E₀ is Formula (3):

$$E_0=\frac{f_{c1}}{{\epsilon }_{c1}}$$

(3)

The relationship between the initial elastic modulus, peak secant modulus, and freeze-thaw cycles of concrete under freeze-thaw action is shown in Figure 13. It can be seen from the figure that with the increase in freeze-thaw cycles, the initial elastic modulus and peak secant modulus of concrete show a decreasing trend. The initial elastic modulus and peak secant modulus of NC5 decrease slightly. NC0 and NC1 change significantly.

Li Yanlong et al. and Gong Xu et al. reveal that the peak stress decreases while the peak strain increases, the initial elastic modulus under compression decreases, and the stress–strain curve tends to be flat. Concrete after F–T damage increasingly exhibits more and more obvious brittle characteristics during the compression failure process. This is consistent with the findings of this study [25,26].

3.5. Freeze-Thaw Damage Deformation Model of NMK Concrete

3.5.1. Meso-Statistical Damage Deformation Model of Concrete

Many experimental studies [20,21] show that the probability distribution of uniaxial compressive strength of concrete conforms to Weibull distribution, and the fracture strength and corresponding fracture strain of concrete parallel rod model obey Weibull distribution. The degree distribution function is Formula (4):

$$F(\epsilon )=1-\exp \left[-{\left(\frac{\epsilon }{\eta }\right)}^m\right]$$

(4)

Damage variable D is expressed as Formula (5):

$$D(\epsilon )={\displaystyle {\int }_0^{\epsilon }f(x)}dx=1-\exp \left[-{\left(\frac{\epsilon }{\eta }\right)}^m\right]$$

(5)

Meso—statistical damage deformation model of concrete as Formula (6):

$$\sigma (\epsilon )=E\epsilon (1-D(\epsilon ))=E\epsilon \exp \left[-{\left(\frac{\epsilon }{\eta }\right)}^m\right]$$

(6)

After incorporating NMK into concrete, the mechanical properties of concrete will change, directly affecting its compression deformation relationship. Therefore, the influence coefficient k of NMK is introduced to obtain the meso-statistical damage deformation model of NMK concrete as Formula (7):

$$\sigma (\epsilon )=E\epsilon (1-D(\epsilon ))=E\epsilon \exp \left[-{\left(\frac{\epsilon }{k_2\eta }\right)}^{k_{1}m}\right]$$

(7)

In the Formula (7), k₁ and k₂ are the influence coefficients of NKC content on the deformation model parameters m and η, respectively.

The results of damage deformation equation parameters m and η of nano-metakaolin concrete under different freeze-thaw cycles are shown in

Table 6. Constitutive model parameters of NMK concrete under freeze-thaw cycles.

Freeze-Thaw Cycles/N	NC0	NC0	NC1	NC1	NC3	NC3	NC5	NC5
	m	η	m	η	m	η	m	η
0	4.10	0.71	2.99	0.69	4.64	0.72	7.03	0.76
25	2.60	0.69	2.86	0.69	7.27	0.76	3.72	0.98
50	2.80	0.69	1.20	0.86	2.35	0.69	3.67	0.70
100	1.24	0.84	1.09	0.93	1.34	0.80	3.51	0.69
125	0.51	3.76	0.93	1.08	0.91	1.11	5.97	0.74

. The relationship between the parameters m and η of the freeze-thaw damage deformation model of ordinary concrete and the number of freeze-thaw cycles N can be obtained by fitting the nlinfit function in Matlab software(R14, Yaojing, Dalian China), as shown in Formula (8).

$$\{\begin{array}{l}m=-0.03N+3.83\\ \eta =0.0196N+0.64\end{array}$$

(8)

In the Formula (8), N is the number of freeze-thaw cycles.

According to the data in Table 6, the relationship between NMK content and concrete deformation parameters is shown in Formula (9).

$$\{\begin{array}{l}{k}_1=\frac{0.51n+4.12}{4.12}\\ k_2=\frac{0.008n+0.71}{0.71}\end{array}$$

(9)

The meso-statistical damage deformation equation of NMK under uniaxial compression under freeze-thaw action can be obtained as Formula (10):

$$\sigma =E\epsilon \exp \left[-{\left(\frac{\epsilon }{\left(\frac{0.0008n+0.71}{0.71}\right)\left(0.019N+0.64\right)}\right)}^{\left(\frac{0.51n+4.12}{4.12}\right)\left(-0.03N+3.83\right)}\right]$$

(10)

In the Formula (10), n is NMK content; N is the number of freeze-thaw cycles.

3.5.2. Model Verification

In order to verify the applicability of the meso-statistical freeze-thaw damage deformation model of NMK concrete proposed in this paper, the stress-strain curves obtained are compared with the theoretical calculation results, as shown in Figure 14.

It can be seen from the figure that the meso-damage deformation model of NMK concrete during the freezing-thawing process is in good agreement with the experimental results, and the rise section of the theoretical curve is almost coincident with the experimental curve. This model can accurately reflect the change process of stress-strain relationship of nano-metakaolin concrete during freezing-thawing process.

4. Discussion

When the nano-metakaolin concrete begins to fail under the action of freeze-thaw cycles, small pieces with a particle size of 2–3 mm appear on the surface. With the increase in the number of freeze-thaw cycles, the amount of spalling on the concrete surface grows. And the size of the spalling addition from a few millimeters to a few centimeters. And the spalling progresses from the surface to the inside. With the addition of NMK, the microscopic pores inside the concrete are gradually filled [14,15]. In addition, NMK can promote cement’s hydration reaction, generate many hydration products, and progressively fill the internal pores of the concrete. In this way, the pore water in the concrete in the cold and humid environment is reduced. Under the freeze-thaw cycle, the damage to the concrete caused by the freezing expansion of the pore water is reduced [22]. The density of the concrete matrix increases with the addition of an appropriate amount of nanoclay. And then lead to the concrete strength of the substantial development. However, with insufficient or excessive NMK in concrete, the hydration reaction will be inadequate, and the strength of the concrete will be reduced. The optimization effect of nanoclay on the pore structure of concrete matrix [7] slows down the damage and degradation process of concrete mechanical properties under freeze-thaw cycles so that the peak stress and peak strain change slowly during the deformation process.

5. Conclusions

This paper mainly studies the effect and mechanism of NMK on frost resistance of concrete. The main conclusions are drawn as follows:

(1)

Cracks appeared on the surface of concrete mixed with 5% nano-metakaolin after 100 freeze-thaw cycles, the crack width was 0.1 mm, and the erosion of the concrete surface was weak.

(2)

Without freeze-thaw cycles, the compressive strength of concrete mixed with 3% nano-metakaolin is the highest, 28.75% higher than that of ordinary concrete; after 125 freeze-thaw cycles, the loss rate of compressive strength of concrete mixed with 5% na-no-metakaolin was 12.07%.

(3)

After 125 freeze-thaw cycles, the peak stress and peak strain of 5% nanoclay concrete changed little. The peak strain is 0.45 times that of ordinary concrete, and the peak stress is 3 times that of concrete without NMK.

(4)

The established nano-metakaolin concrete freeze-thaw meso-statistical damage constitutive model can better reflect the freeze-thaw damage process of concrete. 5% na-nometer metakaolin can improve the frost resistance of concrete.