Keywords

1 Introduction

Concrete is an anisotropic, heterogeneous, artificial material that is made of a cementitious binder, water, coarse aggregate, fine aggregate and possibly one or more admixtures to facilitate mixing, molding, pouring, and curing processes, through a complex series of physical changes and chemical reactions [1, 2]. The solid components of hardened concrete consist primarily of hardened cement paste, aggregates, and interfacial transition zone (ITZ) [3]. Presently, the hydrostatic pressure hypothesis [4] and the osmotic pressure hypothesis [5] had generally accepted as the mechanisms behind the F-T damage of concrete. There are two conditions for F-T failure of concrete: one is that the concrete saturated with water, and the other is the alternation of temperature. At present, most of the research on the frost resistance of concrete focuses on qualitative analysis [6,7,8,9], but there are few studies on the quantitative analysis of concrete related performance indicators under the action of F-T.

In this study, three kinds of concrete and mortar specimens with different water-binder ratios (0.35, 0.42 and 0.53) were prepared. Based on the rapid F-T cycle test, the mass loss rate, relative dynamic modulus of elasticity, change of mechanical properties and thickness of F-T damaged layer of concrete had measured. The internal damage and mechanical properties of specimens with different water-binder ratio under F-T action had studied systematically. The quantitative relationships between the relative dynamic elastic modulus of concrete and the relative dynamic elastic modulus of mortar, the thickness of F-T damage layer and the relative dynamic elastic modulus of concrete are established.

2 Experimental Details

2.1 Materials

P·II 52.5-grade Portland cement, Grade I fly ash, limestone graded gravel, river sand, and drinking water produced by Qinghai Qilian Mountain Cement Co., Ltd. were used in this study. The physical and mechanical properties of P·II 52.5 Portland cement are listed in Table 1, and the chemical compositions of the cement and fly ash are listed in Table 2. The limestone is produced by Jiaheng Magnesium Industry Co. Ltd. in Qinghai Province. The physical and mechanical properties are shown in Table 3, and the gradation curve of graded gravel is shown in Fig. 1. The river sand used was obtained from the Huangshui River in Xining. Its fineness modulus of 2.70 indicates that it is medium grain sand. The basic performance indicators shown in Table 4, the gradation curve of sand is shown in Fig. 2.

Table 1. Physical and mechanical properties of P·II 52.5 Portland cement.
Full size table
Table 2. Chemical composition of cementitious materials/%.
Full size table
Table 3. Basic performance index of limestone graded gravel.
Full size table
Table 4. Basic performance index of sand.
Full size table
Fig. 1.
figure 1

Grading curve of coarse aggregate.

Full size image
Fig. 2.
figure 2

Grading curve of sand.

Full size image

2.2 Sample Preparation

The concrete and mortar specimens with water–binder ratios of 0.35, 0.42 and 0.53 were used (Table 5).

Table 5. Mixture ratio of concrete specimen.
Full size table

2.2.1 Concrete Specimens

Firstly, the cement, fly ash, aggregate and sand was mixed into the horizontal ready-mix concrete for 1 min, after which the water and admixture were mixed for 3–4 min. Finally, the mixture was poured into 100 × 100 × 100 mm molds and placed on a shaking table for vibration and compaction, then placed in an environment of 20 ± 2 ℃ and a relative humidity of >50%, and sealed and hardened whilst wrapped in a plastic film. After curing for 24 h, the mold was removed and then placed in the standard curing chamber with a temperature 20 ± 2 ℃ and a humidity of  ≥95% for 28 days.

2.2.2 Mortar Specimens

By using the method of “wet screen mortar”, the concrete mixture with the corresponding proportion was poured into the square screen with the aperture of 4.75 mm. It was put on the shaking screen machine in order to get rid of the aggregate whose particle size is more than 5mm, and then the mortar with corresponding proportion can be prepared. The mortar was poured into a cubic mold of 70.7 mm, and placed on the shaking table for compaction. The mortar specimens were placed in the same environment as the concrete specimens to harden, after whom the molds were removed, and the samples were left to harden in the same curing chamber as the concrete samples. The serial numbers of mortar specimens prepared from CFL35, CFL42 and CFL53 concrete are MF35, MF42 and MF53.

2.3 Sample Testing and Analysis

2.3.1 Compressive Strength Test of Concrete and Mortar Cube Specimen

The compressive strength of concrete and mortar cube specimens was measured by microcomputer controlled electro-hydraulic servo pressure tester (YAW4306) in accordance with the Standard for Test Methods of Mechanical Properties of Ordinary Concrete (GB/T 50081-2019). The calculation result is accurate to 0.01 MPa.

2.3.2 Rapid F–T Cycle Test

The rapid F-T test machine (NJW-HDK-9) is used, and the F-T cycle method and test method refer to the relevant methods of the Standard for Long-term Performance and Durability Test Methods of Ordinary Concrete (GB/T 50082-2009) [11]. The concrete prism specimens were put into the rapid F-T testing machine, and the mass loss rate and relative dynamic elastic modulus (Fig. 3) were measured after every 25 cycles of F-T (NM-4B). When the mass loss reaches 5% of the initial mass or the relative dynamic modulus decreases to 60% of the initial value, the experiment is finished. The mass loss rate and relative dynamic modulus of elasticity of the specimen are calculated according to Eqs. (1) and (2):

$$ \Delta W_{n} = \left[ {(W_{0} - W_{n} )/W_{0} } \right] \times 100 $$
(1)

where Δ\(W_{n}\) is the mass loss rate of specimens after F-T cycles, \(W_{n}\) is the mass of the specimen after several F-T cycles and \(W_{0}\) is the Initial mass of the specimen before the F-T cycle test.

$$ RDEM = (E_{t} /E_{0} ) \times 100 = (V_{t} /V_{0} )^{2} \times 100 = (T_{0} /T_{t} )^{2} \times 100 $$
(2)

where \(RDEM\) is the relative dynamic elastic modulus of concrete and wet screen mortar, \(E_{0} ,V_{0} ,T_{0}\) are the initial dynamic modulus, ultrasonic wave velocity and ultrasonic acoustic time respectively. \(E_{t} ,V_{t} ,T_{t}\) are the dynamic modulus, ultrasonic wave velocity and ultrasonic sound time at the end of every 25 F-T cycles respectively.

Fig. 3.
figure 3

Schematic diagram of ultrasonic detection (a) and arrangement of measuring points (b).

Full size image

At the same time, at the end of every 25 F-T cycles, the thickness of F-T damage layer of concrete prismatic specimens was measured according to technical specification for detecting concrete defects by ultrasonic method (CECS21-2000) [12,13,14]. The test method is shown in Fig. 4. Firstly, the “Time-Interval” diagram of corroded concrete is drawn by using the acoustic time value and the corresponding distance value of each measuring point (Fig. 5). The thickness of concrete damage layer is calculated by the ultrasonic time value data and the inflection point of the curve in the “Time-Interval” diagram of each specimen, and then the regression linear equations of damaged and undamaged concrete l and t are obtained by regression analysis method. Finally, the thickness of the damaged layer of the concrete specimen is calculated according to Eqs. (3) and (4):

$$ l_{0} = \left( {a_{1} b_{2} - a_{2} b_{1} } \right)/\left( {b_{2} - b_{1} } \right) $$
(3)
$$ h_{f} = (l_{0} /2) \cdot \left[ {(b_{2} - b_{1} )/(b_{2} + b_{1} )} \right]^{1/2} $$
(4)

where \(h_{f}\) is thickness of F-T damaged layer of concrete, \(a_{1} ,b_{1} ,a_{2} ,b_{2}\) is the regression coefficient of the line being the intercept and slope of the damaged and undamaged concrete line in Fig. 5.

Fig. 4.
figure 4

Schematic diagram of thickness detection of concrete damage layer.

Full size image
Fig. 5.
figure 5

Time-distance diagram of damage layer detection.

Full size image

3 Results and Discussion

3.1 Surface Spalling of Concrete Under F-T

Figure 6 shows the surface spalling of concrete specimens with different water-binder ratios under F-T. It can be seen from the figure that as the number of F-T cycles increases, the peeling off the concrete specimen surface becomes more and more serious. When the specimen is damaged, the degree of F-T damage increases with the increase of water-binder ratio. In the early stage of F-T, the surface of the specimen was partially rough, a small amount of cement mortar peeled off the attached coarse aggregate, and the damage was unobvious. With the number of F-T cycles increased, the amount of cement mortar spalling also increased, and pits appeared on part of the surface. When the concrete is about to fail, the surface of the specimen shows serious peeling, the coarse aggregate is exposed, and the corners of the local corners are dropped.

Fig. 6.
figure 6

Destruction mode of CFL35 (a, b, c), CFL42 (d, e, f) and CFL53 (g, h, i) under F-T.

Full size image

3.2 Variation of Mass and Relative Dynamic Elastic Modulus of Specimens Under F-T

Figure 7 shows the mass loss and relative dynamic elastic modulus change curves of concrete and mortar specimens with different water-binder ratios under F-T. It can be seen that with the increase of F-T times, the mass and relative dynamic elastic modulus of the specimen decrease to some extent. After 250, 75 and 50 cycles of F-T cycles, the relative dynamic modulus of elasticity of CFL35, CFL42 and CFL53 decreased to less than 60% (49.84%, 54.28% and 56.01%), the mass loss rate was 1.135%, 0.456% and 0.523% respectively. After 250, 75 and 50 cycles of F-T cycles of MF35, MF42 and MF53, the relative dynamic elastic modulus decreased to less than 60% (52.9%, 58.18%, 59.24%), the mass loss rate was 0.836%, 0.184% and 0.417% respectively. The basic reason for this phenomenon lays in the difference of pore structure parameters in concrete. With the increase of water-binder ratio, the internal structure of concrete also tends to be loose, resulting in a decline in its frost resistance.

Fig. 7.
figure 7

Mass loss rate (a) and relative dynamic modulus of elasticity (b) of specimens under F-T.

Full size image

At the same time, it is found that the relative dynamic elastic modulus of concrete is lower than that of mortar under the same F-T cycles. Through the regression analysis of the relative dynamic elastic modulus of concrete and the relative dynamic elastic modulus of mortar under F-T (Fig. 8), it is found that there is a significant linear relationship (Eq. (5)) between them:

$$ RDEM_{,c} = 1.033RDEM_{,m} - 7.224 $$
(5)

where \(RDEM_{,c}\) is the relative dynamic elastic modulus of concrete, \(RDEM_{,m}\) is the relative dynamic elastic modulus of mortar. In this fitting relationship, the number of samples is 15, and it can be seen from the figure that \(R = 0.9798\) > \(R_{0.001} (13) = 0.760\). This shows that the relative dynamic elastic modulus of concrete and the relative dynamic elastic modulus of mortar in the F-T are more than 99.9% significant. It is preliminarily shown that one of the factors of F-T damage of concrete is the damage of mortar.

Fig. 8.
figure 8

Relation between relative dynamic modulus of concrete and relative dynamic modulus of mortar.

Full size image

3.3 Comparative Analysis of Basic Mechanical Indexes of Specimens Before and Aater F-T

Figure 9 shows the change rule of the basic mechanical indexes of concrete and mortar specimens with different water-binder ratios before and after F-T. It can be seen that the mechanical properties of the concrete and mortar specimens have different degrees of loss under F-T. When the concrete and mortar specimens with water-binder ratios of 0.35, 0.42 and 0.53 have undergone 250, 75 and 50 F-T cycles in sequence, they reached the condition of F-T failure. It is found from Fig. 9a that the compressive strength loss rates of CFL35, CFL42 and CFL53 specimens are 61.24%, 26.93% and 37.50% respectively after F-T failure. The compressive strength loss rates of corresponding mortar specimens (MF35, MF42 and MF53) were 53.11%, 21.27% and 29.02% respectively. It is found from Fig. 9b that the splitting loss rates of CFL35, CFL42 and CFL53 specimens are 41.54%, 51.0% and 66.02% respectively, and the splitting loss rates of corresponding mortar specimens (MF35, MF42 and MF53) are 40.81%, 24.32% and 57.92% respectively. It is found from Fig. 9c that the loss rates of flexural strength of CFL35, CFL42 and CFL53 specimens are 73.12%, 76.15% and 83.71% respectively. It can be seen from the above data that the loss of mechanical properties of the mortar is less than that of the concrete specimens. The reason for this phenomenon is that the larger the porosity of the inner structure and the looser the structure, the worse the frost resistance of the specimen with large water-binder ratio. Compared with the concrete specimen, the mortar specimen is more homogeneous, which leads to a lower loss rate of strength.

Fig. 9.
figure 9

Variation of compressive strength (a), splitting tensile strength (b) and flexural strength (c) of concrete and mortar specimens before and after F-T.

Full size image

3.4 Variation Law of Thickness of F-T Damaged Layer of Concrete Under F-T

Figure 10 is the change curve of thickness of F-T damaged layer of concrete with different water-binder ratios under the action of F-T. The results show that the thickness of damage layer increases with the increase of F-T cycles, and the growth rate increases with the increase of water-binder ratio. When the specimen reached the failure condition, the thickness of the F-T damage layer from high to low was CFL35 (47.52 mm), CFL53 (43.59 mm) and CFL42 (31.49 mm). This is due to the expansion of the pores caused by the water expansion in the pore structure of concrete under the action of F-T, which leads to the increase of the thickness of the damaged layer. When the specimen reaches the condition of F-T damage, the thickness of the F-T damage layer is between 31–48 mm, which indicates that the specimen is in the state of thoroughly frozen.

Fig. 10.
figure 10

Thickness variation of F-T damage layer of concrete specimen under F-T.

Full size image
Fig. 11.
figure 11

Relationship between the thickness of F-T damaged layer and the relative dynamic elastic modulus of concrete.

Full size image

Through further analysis of the thickness of F-T damage layer of concrete, it is found that the variation law is basically consistent with the relative dynamic elastic modulus of concrete. Regression analysis of the two (Fig. 11) revealed the following nonlinear relationship (Eq. (6)):

$$ h_{f} = - 0.0079(RDEM_{,c} )^{2} + 0.5306(RDEM_{,c} ) + 36.456 $$
(6)

where \(h_{f}\) is the thickness of F-T damaged layer of concrete, \(RDEM_{,c}\) is the relative dynamic elastic modulus of concrete. In this fitting relationship, the number of samples is 19, and it can be seen from the figure that \(R = 0.860\) > \(R_{0.001} (17) = 0.693\). The results show that the prediction model of the thickness of the F-T damage layer of concrete and relative dynamic elastic modulus of concrete is more than 99.9% significant. It shows that as the relative dynamic elastic modulus of concrete decreases, the thickness of the F-T damage layer is also increasing. At the same time, it can be seen that the F-T damage of concrete is a process from the outside to the inside, which not only occurs inside the concrete but also occurs on the surface of the concrete. Through the above analysis, it is found that one of the reasons for the F-T damage of concrete is caused by partial F-T damage of the mortar.

4 Conclusions

This study systematically explored the changes of relevant indexes of concrete and mortar specimens with different water-binder ratios under F-T. Based on the experimental results, the following conclusions can be drawn:

  1. 1.

    With the increase of F-T times, the quality and relative dynamic elastic modulus of the specimens both decreased to varying degrees. This is caused by the difference in the parameters of the pore structure inside the concrete. Through the regression analysis of the relative dynamic elastic modulus of concrete and the relative dynamic elastic modulus of mortar, it is found that there is a very significant linear relationship between them. It is preliminarily shown that one of the factors leading to concrete F-T damage is mortar damage.

  2. 2.

    After the concrete and mortar specimens experienced F-T effects, their mechanical properties showed varying degrees of loss. The loss rate of strength of mortar is relatively low. This is caused by the relatively homogeneous interior of the mortar specimen.

  3. 3.

    When the concrete reaches F-T failure, the degree of exfoliation on the surface of the test piece increases with the increase of the water-binder ratio, and the concrete is basically in a state of being frozen through. Through regression analysis of the thickness of the concrete F-T damage layer and the relative dynamic elastic modulus of concrete, it is found that the two have a very significant non-linear relationship. It is further confirmed that one of the causes of concrete F-T damage is caused by mortar damage. At the same time, it shows that the F-T damage of concrete is a process from surface to interior.