Abstract
Pore structure plays an important role in the drying shrinkage of recycled aggregate concrete (RAC). High-precision mercury intrusion and water evaporation were utilized to study the pore structure of RAC, which has a different replacement rate of recycled concrete aggregate (RCA), and to analyze its influence on drying shrinkage. Finally, a fractal-dimension calculation model was established based on the principles of mercury intrusion and fractal-geometry theory. Calculations were performed to study the pore-structure fractal dimension of RAC. Results show the following. (1) With the increase in RCA content, the drying shrinkage values increase gradually. (2) Pores with the greatest impact on concrete shrinkage are those whose sizes ranging from 2.5 nm to 50 nm and from 50 nm to 100 nm. In the above two ranges, the proportions of RAC are greater than those of RC0 (natural aggregate concrete, NAC), which is the main reason the shrinkage values of RAC are greater than those of NAC. (3) The pore structure of RAC has good fractal feature, and the addition of RCA increases the complexity of the pore surface of concrete.
1. Introduction
Shrinkage deformation of concrete material has attracted extensive studies here and abroad with regard to the complexity of the shrinkage-deformation mechanism and heterogeneity of nature of concrete material. Although corresponding theoretical models have been built, capillary tension theory still has a more extensive application range [1, 2].
Recycled aggregate concrete (RAC) mainly refers to new concrete prepared by recycled concrete aggregate (RCA), which is made from construction waste, to replace or partially replace natural aggregate. Hasaba and Kawamure [3], Katz [4], Debieb and Kenai [5], Courard et al. [6], Domingo-Cabo et al. [7], Evangelista and Brito [8], and other experts have carried out numerous experimental studies on drying shrinkage. The same research results showed that, under the same test condition, RAC has greater shrinkage deformation than natural aggregate concrete (NAC). Coupling with the physical and mechanical property defects of RCA, RAC is rarely applied to the project instance. Thus, determining how to reduce the shrinkage and improve the crack resistance of RAC materials has become an important concern. Based on capillary tension theory, porosity and pore characteristics have great influence on the uneven shrinkage of RAC. Therefore, studying the pore structure and characteristics is particularly important in the research on RAC shrinkage.
2. Experimental
2.1. Materials and Mix Proportion
Cement: P.S 32.5. Construction waste (CW): generated from a demolished building in Chongqing City, China. Natural coarse aggregate (NCA): commercial gravel. Sand: commercial mountain sand. Table 1 shows the performance parameters of the aggregates.
C30 natural mix proportion is utilized as reference. Table 2 shows the mix design. Considering the guaranteed rate of 0.9 after standard curing of 28 days, the compressive strength of the concrete is shown in Figure 1.
2.2. Principle and Methods
2.2.1. Test of Drying Shrinkage and Mechanical Properties
Drying shrinkage test of RAC is conducted according to the GB/T 50082-2009 standard for test methods of long-term performance and durability of ordinary concrete. Specimen size is 100 mm × 100 mm × 515 mm. Mechanical properties of concrete are tested according to the GB/T 50081-2002 standard for test method of mechanical properties on ordinary concrete.
2.2.2. Test of Pores
Water evaporation and mercury intrusion are the two methods utilized to characterize the pore structure in this study.
Water evaporation can be utilized to determine the capillary pores and stoma whose aperture is more than 30 nm (referred to herein as macroporous, otherwise referred to as capillary pores) [9]. With 90% relative humidity, the water loss rate of saturated RAC specimens can be expressed as the macropore porosity of RAC [10]. In the case of 105°C, the water loss rate of saturated RAC specimens can be expressed as the total porosity of RAC. Following this calculation, the difference between the above water loss rates can be expressed as capillary porosity.
The volume porosity of RAC can be calculated using where is the volume porosity of RAC, %; is the saturated quality of the RAC sample, g; is the sample quality after losing the water, g; is the density of RAC, gcm−3; is the density of water, gcm−3; is the volume of water, cm3; and is the volume of concrete, cm3.
Specimens after standard curing were crushed into 3 mm to 5 mm particles, rinsed, and then dried in gradual rising temperature. Next, the pore structure was tested by AUTOPORE 9500 automatic pressure mercury analyzer. Pore size range measured utilizing the instrument was 0.003 μm to 1000 μm.
Under atmospheric pressure, mercury can only enter open pores whose radius is greater than 7 μm because of the resistance of surface tension. External force must be applied to overcome the surface tension and make the nonwetting liquid mercury enter a narrow capillary force [11], as shown in Figure 2.
3. Results and Discussion
3.1. Drying Shrinkage of RAC
The tests results are shown in Figure 3.
Figure 3 shows that, with the increase in RCA content, drying shrinkage values tend to increase gradually.
3.2. Pore Structure and Its Influence on Drying Shrinkage
Concrete is a porous aggregate, and its pores can be divided into gel pores, capillary pores, and stomata (macropores). Among them, the pore size of stomata is the largest at 1 mm to 0.01 mm. However, stomata have little influence on concrete shrinkage. Capillary pores and gel pores have the greatest impact on concrete shrinkage, especially for those with sizes between 2.5 nm and 50 nm. Pores with sizes between 50 mm and 100 nm also have some influence on concrete shrinkage. Therefore, the proportion of pores with the above two ranges can reflect the shrinkage ability of the concrete material [12–15].
Table 3 shows the effect of different pores on the properties of hardened cement paste.
3.2.1. Water Evaporation Method
The impact of recycled aggregate on the porosity of RAC in water evaporation is shown in Figure 4 [12].
Figure 4 shows that, with the increase in recycled aggregate replacement rate, the total porosity of RAC increases significantly. Compared with NAC, the increase in porosity of the capillary pores of RAC is evident.
3.2.2. Mercury Intrusion Method
For cement-based materials, drying shrinkage mainly comes from the drying shrinkage of cement stone. Hence, in the mercury test in this study, specimens were taken from a cement mortar section of RAC. The cumulative curve of pore volume test in mercury intrusion is shown in Figure 5.
Figure 5 and the test data show the sample RC0. The injection of mercury into the sample starts when the pore size is 45.352 μm. The intrusion volume of mercury per gram of samples is 0.0036 mL/g when the mercury pressure is 1.994 psia. No mercury is reinjected into the sample when the pressure is 29992.57 psia. The pore-size range of RC0 is 3.015 μm to 45.352 μm. Median radius is calculated according to the volume method; when intermediate interpolation is applied, the median radius is 34.2 nm.
For sample RC30, mercury starts injecting into the sample when the pore size is 45.253 μm. The intrusion volume of mercury per gram of samples is 0.003 mL/g when the mercury pressure is 1.998 psia. No mercury is reinjected into the sample when the pressure is 29992.48 psia. The pore-size range of RC30 is 3.015 μm to 45.252 μm. The median radius is calculated according to the volume method; when intermediate interpolation is applied, the median radius is 31 nm.
For sample RC100, the injection of mercury into the sample starts when the pore size is 45.352 μm. The intrusion volume of mercury per gram of samples is 0.0022 mL/g when the mercury pressure is 1.998 psia. No mercury is reinjected into sample when the pressure is 29992.5 psia. The pore-size range of RC100 is 3.015 μm to 45.352 μm. The median radius is calculated according to the volume method; when intermediate interpolation is applied, the median radius is 34 nm.
The Washburn equation shown in (2) is an important theoretical formula of mercury intrusion: where is the applied pressure, Pa; is the radius of pore, nm; is the surface tension of mercury, Nm−1, and the surface tension of mercury is 0.48 Nm−1 at 20°C; and is the wetting angle of mercury to solid surface, °, and the wetting angle of mercury to cement material is 125°.
In this experiment, the unit of pressure is psia and 1 MPa = 145 psia. Equation (3) can be obtained after the unit conversion:
According to the formula , the open-pore volume of specimen is expressed as , and the open-pore volume of specimen with radius between and is expressed as . Thus, the pore-size distribution function (in volume) can be shown as
In the formula , and are constant. Hence, . Therefore,
Owing to the pore volume, what is measured directly with the instrument is the volume whose radius is greater than the , which can be expressed as , where is the total open-pore volume and is the open-pore volume whose radius is smaller than in the specimen. If the pushed mercury-content curve is depicted in the function of versus , the curve slope can be measured by the experiment. Hence, 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 (shown in Figure 6) and the corresponding values attached to the -axle. Thus, obtaining several intervals according to the 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.
(a) RC0
(b) RC30
(c) R100
Figure 6 and Table 4 show that as far as concrete mortar is concerned, the proportion of pores with sizes ranging from 3 nm to 100 nm is as follows: RC100 accounts for 95.3%, and RC30 accounts for 94.9%. Both are greater than that of RC0 (NAC) 87.9%, which is consistent with the curve tendency in Figure 4, the main reason the shrinkage values of RAC is greater than natural concrete (as shown in Figure 3). However, pores greater than 10 μm are smaller than those of natural concrete, which indicates that the addition of RCA mainly increases the content of capillary pores.
The proportions of various types of pores are different in water evaporation and in mercury intrusion, mainly because the different specimens were obtained in two ways.
3.3. Pore-Fractal Law
Pore geometry and particle geometry of concrete show the fractal behavior over the range of atom scale to grain size [16]. The introduction of fractal-dimension concept makes the quantitative description of concrete pore structure possible.
According to the fractal definition, the power-law relationship meets between the number of objects with the same scale and their measurement linear scales: where is the number of scale feature bodies accommodated in fractal objects; is the pore radius, nm; is the fractal dimension of fractal objects; and , where is the proportional factor.
When a cylinder with the size of height and cross-sectional radius is utilized to measure the pore volume of concrete, . Therefore,
When (3) is substituted into (8), we have
Both sides of (9) are obtained logarithms; thus, we have
With both and as constants, exhibits linear change along with , and is the slope of the line. Thus,
Therefore, the pore-fractal dimension can be determined by the straight-line slope . As long as the linear relationship between and exists, the pore-fractal dimension possesses the fractal feature of pore distribution.
Pressure and pore volume of concrete specimens can be obtained through mercury experiments. The corresponding and are then calculated, the curve of versus is drawn, and their linear fit is performed (as shown in Figure 7). Table 5 shows the fitting parameters and pore-fractal dimension of the sample.
(a) RC0
(b) RC30
(c) RC100
Figure 7 and Table 5 show that linear-fitting correlation coefficients of the three curves are above 99%, which indicates that the pore distribution of RAC meets the fractal feature. The fractal dimension of pore volume characteristics can be calculated according to the curve slope . The pore-fractal dimension of RC0 is 1.605, RC30 is 1.626, and RC100 is 1.627. According to the fractal theory, if the pore-fractal dimension is higher, surface pores are more complex, which indicates that the complexity of the pore surfaces is . Therefore, the addition of RCA increases the complexity of the pore surface of concrete.
4. Conclusions
(1)With increase in RCA content, the drying shrinkage values increase gradually.(2)The pore types with the greatest impact on concrete shrinkage are capillary and gel pores, especially for pores with sizes ranging from 2.5 nm to 50 nm and from 50 nm to 100 nm. In the above two ranges, the proportions of RAC are greater than those of RC0 (NAC), which is the main reason the shrinkage values of RAC are greater than NAC. (3)Through mercury intrusion analysis, the median radius of RC0 is 34.2 nm, the median radius of RC30 is 31 nm, and the median radius of RC100 is 34 nm.(4)The pore structure of RCA has good fractal feature, and the addition of RCA increases the complexity of the pore surface of concrete.
Acknowledgments
This project was supported by the Project of National Natural Science Foundation of China (51202304), Natural Science Foundation Project of CQ CSTC (cstc2012jjA50005), and Project of Science and Technology Committee of Wanzhou District of China (WZ011R003).