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Influence of Aggregate Wettability with Different Lithology Aggregates on Concrete Drying Shrinkage
The correlation of the wettability of different lithology aggregates and the drying shrinkage of concrete materials is studied, and some influential factors such as wettability and wetting angle are analyzed. A mercury porosimeter is used to measure the porosities of different lithology aggregates accurately, and the pore size ranges that significantly affect the drying shrinkage of different lithology aggregate concretes are confirmed. The pore distribution curve of the different coarse aggregates is also measured through a statistical method, and the contact angle of different coarse aggregates and concrete is calculated according to the linear fitting relationship. Research shows that concrete strength is determined by aggregate strength. Aggregate wettability is not directly correlated with concrete strength, but wettability significantly affects concrete drying shrinkage. In all types’ pores, the greatest impacts on wettability are capillary pores and gel pores, especially for the pores of the size locating 2.5–50 nm and 50–100 nm two ranges.
Concrete drying shrinkage is important in crack and deformation generation. For hydraulic structures, such as dams and hydropower stations, cracks caused by drying shrinkage are the main causes of leakage . Previous study shows that concrete drying shrinkage does not only refers to traditional theoretical phenomena, such as moisture loss, cementitious materials shrinkage, and aggregate limitation. Aggregate constitutes approximately 70%–90% of concrete volume. Moreover, aggregate strength directly determines concrete strength; thus, under capillary tension theory, aggregate water absorption is the main factor that affects concrete strength and drying shrinkage [2–5]. Numerous studies have been conducted on lithology aggregates and the effect of aggregates on concrete strength and drying shrinkage [6–8]; reliable physical models have been established according to different theories . Further research has shown that aggregate wettability is closely correlated to concrete drying shrinkage; according to the capillary bundle model based on capillary tension theory, the complex effect of internal capillary strength on aggregate absorbance has been initially solved. Based on previous research, the porosities of different lithology aggregates are measured accurately in this study using a mercury porosimeter, the pore sizes that significantly affect the drying shrinkage of different lithology aggregate concretes are verified, and the effects of different lithology aggregates on concrete drying shrinkage are summarized [10, 11].
Cement P.O 42.5, superplasticizer (FDN), quartzite, marble, basalt, sandstone, and waste concrete were obtained from Wanzhou District, China. Samples from four rocks and waste concrete were collected, some cylindrical specimens with 50 mm diameter and 100 mm height were made, and wettability of the specimens was tested. Rocks and waste concrete were crushed and processed into coarse aggregates. Table 1 shows the performance parameters of the aggregate (tested in accordance with the standard of “Test Methods of Aggregate for Highway Engineering (JTG E42—2005)”). The aggregates were processed into quartzite concrete (QC), marble concrete (MC), basalt concrete (BC), sandstone concrete (SC), and recycled aggregate concrete (RAC). The mix design of the cylindrical specimens is shown in Table 2. The curing is in accordance with “Standard for Test Method of Mechanical Properties on Ordinary Concrete (GB/T 50081-2002).”
2.2. Principles and Methods
The Washburn equation was essential to calculate the contact angle and create the curve ( is water rising height in cm over time ), but measuring height “” accurately was difficult because of irregular internal pore distribution. Thus, the curve was converted to the relationship between and , and the effective radius was introduced according to capillary tension theory; thus, concrete contact angle can be calculated according to linear fitting relationship and the following formulas.
The specimens were dried to a constant weight and placed in a self-made contact angle testing equipment . Wettability was analyzed through the Washburn method; thus, the relationship between the wettability of aggregates and that of aggregate concrete was determined, along with the relationship between the strength of aggregates and that of aggregate concrete. The same grain sizes for QC, MC, BC, SC, and RAC were considered. The aggregate porosity of these concretes could be measured using an AutoPore 9500 mercury porosimeter; the of QC, MC, BC, SC, and RAC were measured similarly. The 28 d strength and drying shrinkage value of the different aggregates were tested. Finally, the pore distribution curve of the different coarse aggregates and coarse aggregate concretes was summarized according to (1)–(3), and the contact angle was calculated:where is the weight in g of the water sunk into RAC over time , is the porosity of RAC to the decimal meter, is the water density, is the th sectional area of the cylindrical capillary tube, is liquid surface tension in dyne/cm, is the viscosity of the liquid in Pa·s, is the effective capillary radius in cm, and is contact angle in °. Slope is computed with (2) and is used to obtain contact angle in (3):
The curve is almost a straight line according to these formulas.
Experimental methods of the mechanical properties testing are in accordance with “Standard for Test Method of Mechanical Properties on Ordinary Concrete (GB/T 50081-2002)”; experimental methods of the drying shrinkage test are in accordance with “Standard for Test Method of Mechanical Properties on Ordinary Concrete (GB/T 50081-2002).”
3. Results and Discussion
3.1. Strength of Different Coarse Aggregate Concretes
The C30 natural concrete mix proportion is used as reference. Table 2 shows the mix design and 28 d compressive strength of the concrete.
In Table 1, “NA” is natural aggregates. As shown in Table 1, the order of the compressive strength of the concrete from the strongest to the weakest is QC, BC, MC, SC, and RAC. According to Table 2, the strength of the different lithology aggregates of concrete in descending order is QC, BC, MC, RAC, and SC. Therefore, the concrete strength is determined by the aggregate strength. The water absorption of different lithology aggregates from the largest to the smallest is RAC, SC, BC, MC, and QC, which illustrates that the aggregate strength is closely correlated with water absorption, and both are inversely proportional.
3.2. Distribution Value and Curve of Different Coarse Aggregates
Pores with a size of 3–100 nm significantly affect concrete drying shrinkage according to the physical model established in previous research . Table 3 shows the pore distribution ratios of different lithology aggregates, which are tested using a mercury porosimeter and are summarized via a statistical method. The curve is shown in Figure 1.
As shown in Table 3 and Figure 1, the distribution ratio of the pore size 3–100 nm of the different coarse aggregates in descending order is RAC, SC, BC, MC, and QC. Water absorption has the same descending order; therefore, pore size distribution is the main factor that affects water absorption, given the 3–100 nm pore size.
3.3. Distribution Value and Curve of Different Coarse Aggregate Concretes
Open-pore volume of the specimen whose radius is between and is expressed as ; thus the pores size distribution function (in volume) can be shown in the following equation:
In formula , and are constant, so , where is applied pressure, Pa, is radius of pore, nm, is surface tension of mercury, N·m−1, the surface tension of mercury is 0.48 N·m−1 at 20°C, is the wetting angle of mercury solid surface, °, and the wetting angle of mercury-cement material is 125°. Therefore
Because of the pore volume what is measured directly with the instrument is the volume whose radius is greater than , which can be expressed as , where is total open-pore volume, and is open-pore volume whose radius is smaller than in specimen. If pushed mercury-content curve depicted the function of versus , then the curve slope can be measured by the experiment, so the above formula can be written as
The right-hand side value in (6) is known or can be measured, and the derivative in (6) can be solved by graphic differentiation to obtain . Finally, the pore size distribution curve can be obtained by drawing point corresponding to the value.
For convenience, directly measured data can be plotted in the cumulative pore volume variation diagram of versus (as shown in Figure 2), and the corresponding values are attached to the -axle. Thus, taking several intervals according to need in the figure and finding the increment of in each corresponding interval, the pore size distribution table can be listed , as shown in Table 4. The curve is illustrated in Figure 2.
As shown in Table 4 and Figure 2, for the different lithology aggregate concretes, the 3–100 nm pore distribution ratio of RAC is the largest. The distribution ratios of SC, QC, and BC are equal, and MC has the lowest ratio. According to Tables 2 and 4, pore porosity and concrete strength are closely correlated, and both are usually inversely proportional.
3.4. Wettability Curve of Different Coarse Aggregate Concretes and Contact Angle of the Curve
Figure 3 shows that the different lithology aggregates and concrete moisture absorption performance can be divided into two stages; moisture content is large at the prophase, moisture content and time have a rough logarithmic relation, and moisture and time are inevitably correlated. The curve flattens in the later period. This phenomenon is mainly caused by the large pore, and connected capillary bundles form in the previous hydration process stage of concrete, during which both water absorption and water loss are large. Along with the hydration process of concrete, both pore and capillary bundle shrink gradually, and concrete drying shrinkage stabilizes.
Table 5 shows the aggregates in descending order according to the contact angle: MC, BC, RAC, QC, and SC. When the moisture absorption ratios of the aggregates and the curve are combined, the aggregate contact angle and moisture absorption become inversely proportional, which are influenced by shape, and the RAC ingredient can be disregarded. The different coarse aggregate concretes are arranged in descending order based on the contact angle as follows: MC, BC, QC, and SC. Concrete contact angle is directly determined by the aggregate contact angle.
3.5. Drying Shrinkage Characteristics of Different Lithology Aggregates and Concretes
The drying shrinkage value of prepared concrete at different stages is tested on the basis of the drying shrinkage characteristics of different lithology aggregate concretes, as indicated by the curve in Figure 4.
Figure 4 shows the drying shrinkage from the largest to the smallest as RAC, SC, BC, QC, and MC; thus, the 3–100 nm pore distribution and wettability are roughly directly proportional to concrete drying shrinkage.
As can been seen from Figures 2 and 4 and Table 4, in all types’ pores, the greatest impacts on wettability are capillary pores and gel pores, especially for pores with sizes ranging from 2.5 nm to 50 nm and from 50 nm to 100 nm.
The main physical factors that affect concrete drying shrinkage are wettability, contact angle, aggregate water absorption, and pore distribution. The following conclusions are drawn.
Concrete strength is determined by aggregate strength, and both types of strength are directly proportional. Aggregate wettability is determined by contact angle and the 3–100 nm pore size distribution; thus, surface water absorption is also influenced.
Aggregate contact angle and surface water absorption are inversely proportional, and concrete drying shrinkage is significantly affected by contact angle, wettability, and water absorption rate. Thus, the wettability and water absorption rate of different lithology aggregates are roughly directly proportional to concrete drying shrinkage.
Concrete drying shrinkage shows obvious periodic regularity, which is closely related to the concrete hydration process.
The ingredients and characteristics of recycled concrete materials are complex and difficult to compare with other different lithology aggregates to obtain the regularity. However, this study shows that the pore porosity of recycled aggregates determines drying shrinkage, which is consistent with general regularity.
Conflict of Interests
The authors declare that they do not have any commercial or associative interest that represents a conflict of interests in connection with the work submitted.
This project was supported by the Project of National Natural Science Foundation of China (51202304), the Natural Science Foundation Project of CQ CSTC (cstc2012jjA50005), Project funded by China Postdoctoral Science Foundation (2014M552320), and Scientific, Technological Talents’ Special Funds of Wanzhou District and Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1401016).
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