How To Study The Permeability Of Pervious Concrete

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1 Laboratory and Analytical Study of Permeability and Strength Properties of Pervious Concrete B. Huang, J. Cao, X. Chen and X. Shu Dept. of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37996; PH (865) ; FAX (865) ; Abstract This paper presents a study in which the effects of aggregate gradations on the permeability and mechanical properties of pervious concrete were investigated. Pervious concrete with three aggregate gradations was characterized through laboratory tests. Air void distributions were evaluated through conventional procedures and image analysis. Theoretical and laboratory methods were employed to evaluate the permeability properties of the concrete mixtures. The mechanical properties of the concrete mixtures were characterized through compressive and split tensile strength tests. The results from this study indicated that aggregate gradations have significant influence on both strength and permeability properties of pervious concrete in this study. The theoretical analysis method employed in this paper can be potentially used in the mixture design of pervious concrete. Introduction Pervious concrete has been increasingly used to reduce the amount of runoff water and improve the water quality near pavements and parking lots. The open pore structure that allows high rates of water transmission is the key characteristic of pervious concrete. This pavement technology creates more efficient land use by eliminating the need for retention ponds, swales, and other storm-water management devices. The rapid drainage of water through interconnected voids can minimize wet weather spray, improve visibility, minimize glare and etc. (Paul, 2004) Numerous researches have been reported in the literature on topics of pervious concrete and its applications in pavement. The effects of aggregate gradation, amount, and size on pervious concrete s mechanical properties and void contents were widely studied. Effective void contents in pervious concrete typically range from 15 to 35 percent. Mix design for pervious concrete in cold weather climates was developed and reported (NRMCA, 2004). The desired void content can be achieved by modifying the level of compacting effort or by adjusting the aggregate gradation, proportions and properties. Single-sized aggregate could provide concrete with high porosity but lower strength. Inclusion of sand and latex can improve the workability of Portland cement pervious concrete (Kevern, et al, 2005). The effect of water quality on properties of pervious concrete was reported (Park and Tia, 2004). The pore structure of pervious concrete was evaluated by image analysis (Neithalath and Weiss, 2003). Above a certain level, increasing cement content results in a reduced concrete porosity with insignificant influence on concrete strength. The addition of sand and latex significantly improved the workability and resulted in higher strength, appropriate permeability, and freeze-thaw 1

2 resistance. Chemical admixtures can improve the workability significantly, obtain the higher strength at relative lower cement contents and results in relative higher porosity (Kevern, et al, 2005, Yang and Jiang, 2003). Permeability is an important parameter of pervious concrete since the material is designed to perform as drainage layer in pavement structures. As the concrete in this study are of large air void, Darcy s Law is no longer valid even for a very low flow speed. In this study, a permeability measurement devise, a permeameter developed by Huang et al (1999) as shown in Figure 1, was used to measure the permeability of pervious concrete. The details to analyze the hydraulic conductivity under non-laminar flow conditions can be found in Huang et al. (1999.) Figure 1. Water Permeability Testing Apparatus (Huang et al., 1999) The objective of the present study was to study the effects of aggregate structures on the permeability and mechanical characterization of pervious concrete through lab and analytical methods. Laboratory prepared pervious concrete specimens with three aggregate gradations were characterized for their volumetric properties, compressive and split tensile strength, and permeability. Materials Type I cement employed in this experiment was obtained from a local supplier. Three gradations of gravel sieved were obtained from a local supplier: 4.75 mm (No. 4), 9.5 mm (3/8 inch) and 12.5 mm (1/2 inch). The properties of coarse aggregate were measured according to ASTM standard and listed in Table 1. Aggregate size (inch) Table 1. Physical Properties of Coarse Aggregate unit weight, kg/m 3 bulk specific gravity absorption, % 1/ / No incompact void content, % 2

3 Aggregates of three gradations were used to make pervious concrete. No fine aggregates or chemical admixture were added. The proportion of cement, coarse aggregate and water remained same (cement: coarse aggregate: water = 1:4.5:0.35). Mixtures were mixed by concrete mixer for 10 min and poured into PVC mold. The specimens were compacted by manual vibration, demolded at next day and cured in 100% humidity curing room at room temperature. Compressive Strength Test Compressive strength was measured after 7-day curing. Compressive strength testing was conducted on INSTRON loading frame on the three cylinder specimens with mm mm (6 inch by 12 inch). The test protocol was follow by ASTM C39. Indirect Tensile Strength Test (IDT) The indirect tensile strength (IDT) was used to determine the tensile strength of the concrete. The specimens for IDT were cylindrical: diameter 152mm (6 in) height 76mm (3 in). Three specimens were measured. MTS load frame was employed to load until failure at a 50.8 mm/min (2 inch/min) deformation rate. The vertical load was continuously recorded and indirect tensile strength was computed as follows: S T 2 Pult = π t D (1) Where S T = Tensile strength, MPa P ult =Peak load, N t =Thickness of the specimen, mm D =Diameter of the specimen, mm Permeability The falling head permeability test was performed in the permeameter developed by Huang as illustrated in Figure 1. Two pressure transducers installed at the top and bottom of the specimen give accurate reading of the hydraulic head difference during the test. Automatic data acquisition makes continuous reading possible during falling head test so that the test can be conducted even at very high flow rate, such as in pervious concrete. The specimen is placed in an aluminum cell. Between the cell and the specimen is an anti-scratch rubber membrane that is clamped tightly at both end of the cylindrical cell. A vacuum is applied between the membrane and the cell to facilitate the installation of the specimen. During the test, a confining pressure of up to kpa (15 psi) is applied on the membrane to prevent short-circuiting from the specimen s side. The top reservoir tube is with a diameter of 57mm (2.25 in) and a length of 900mm (3 ft). A vacuum is applied on the top of the reservoir tube before the test to saturate the specimen. 3

4 The specimen is with a diameter of 152mm (6 in) and a height of 89mm (3.5 in). Each specimen was tested three times and three specimens were measured for each gradation. Air Voids Measurement by CoreLok TM Air void content is another important property for pervious concrete. In order to obtain the air void content, it is necessary to know the bulk volume of the compacted concrete. Since the pervious concrete has high interconnected air voids, it is not suitable to use the submerged weight measurement to obtain the bulk volume. Geometrical measurement of the specimen dimension will not reflect the surface texture (for different sized aggregates). A vacuum package sealing device, CoreLok TM, commonly used to measure the specific gravity for asphalt mixtures, was used to obtain the bulk specific gravity for the pervious concrete specimens (Cooley, et al, 2002). Air Voids Analyses through Digital Image Processing Image processing technology was employed to analyze void content in concrete. Different phases in the concrete will exhibit different color, reflecting in different RGB values. The histogram can be generated by counting the RGB values based on 8 bit images. The intensity range can be defined based on image grayscale intensity distribution. Two ways can be employed to determine the intensity range: first, the edge of difference phase can be determined based on histogram, then visually determining whether the computerized edge match the real edge of different phases; second, the distribution can be analyzed by fitting of Gaussians. As a preliminary research, the intensity range for void was visually determined. Figure 2. Flow Chart of Image Analysis Program One cylinder specimen (152.2 mm mm) was cut with a water-cooled diamond saw every one inch length. The cross section of specimen with ½ inch aggregate was pictured directly with digital camera; hence the void on the cutting surface with the 4

5 other size aggregates was filled with flour and smoothed, then pictured by camera. A rectangular portion was selected from the cross section picture for image analysis. The flow chart for the image analysis was showed in Figure 2. The program was completed with MATLAB. Results and Discussion Compressive Strength. The only difference among the three mixtures is the aggregate size. Table 2 shows the compressive strength at seven days of pervious concrete with different aggregate size. As indicated, the compressive strength increased as aggregate size increased. The trend is different from the research results as published (Kevern et al, 2005). This paper only showed the preliminary results. The repeated experiment will be carried out in the next step. One possible reason for the result was that there is more cement paste between the coarse aggregate in pervious concrete with larger size aggregate. The pervious concrete always failed at the cement paste bond between the coarse aggregates. Considering the aspect ratio of aggregate, there would be more surface for smaller size aggregate at unit weight or unit volume. Therefore, the strength of pervious concrete with large size aggregate would be higher. Table 2. Compressive Strength at 7-Day Aggregate Size in Mixtures 1/2 3/8 No.4 Compressive Strength at 7-Day (MPa) ± ± ± Permeability. Test data are plotted and processed to obtain the pseudo-coefficient of permeability K and the shape factor m presented in Figures 3 and 4. The test results shown in Table 3 indicate that hydraulic conductivity varies from different concrete with different size aggregate. The pseudo-coefficient of permeability (K ) gives a good benchmark to compare the hydraulic conductivity of different size concrete. K increased as aggregate size increased, that means the hydraulic conductivity increased as aggregate size increased. Table 3. Permeability of pervious concrete Aggregate size in the mixture 1/2 3/8 No.4 K (mm/s) m R

6 Concr et e Per meability Hydr aul i c Head( mm) y = x x R 2 = y = x x R 2 = y = x x R 2 = D=13 D=9. 5 D= Ti me( Sec) Figure 3. Hydraulic head vs. time in falling head test Di schar ge Vel oci t y v( mm/ s) Concrete Permeabi l i t y y = x R 2 = y = x R 2 = y = x R 2 = D=13 D=9. 5 D= Hydr aul i c Gr adi ant ( i ) Figure 4. Discharge Velocity vs. Hydraulic Gradient Indirect Tensile Strength. Table 4 presents the indirect tensile strength at seven days of pervious concrete with different aggregate size. As indicated, the indirect tensile strength at seven days increased as aggregate size increased. One of the reasons for the result is that the air void increased as aggregate size increased. Another reason is more cement paste between the coarse aggregate in pervious concrete with larger size aggregate. Table 4. IDT Results of Pervious Concrete at 7 Days Aggregate Size in Mixtures 1/2 3/8 No.4 Indirect Tensile Strength (MPa)

7 Void content measuredby CoreLok TM. The steps involved in sealing and analyzing the samples are as follows: (1) measure the oven dried pervious concrete specimen; (2) Place the sample in the bag; (3) Place the bag containing the sample inside the Corelok chamber; (4) Close the chamber door. The vacuum pump will start automatically and evacuate the chamber; (5) In approximately two minutes, the chamber door will open automatically. The sample is completely sealed, ready for water displacement analysis. (6) Perform water displacement analysis using the existing AASHTO or ASTM standards for specific gravity calculations. Correct the results for the bag density and the displaced bag volume as suggested by ASTM method D1188. Table 5 lists the air void measurement results. It was clear that the majority of air voids was interconnected for the pervious concrete considered in this study. Table 5. Air Voids of Pervious Concrete measured by CoreLok Size Total Void Interconnected Unconnected Void (%) Void (%) (%) 1/2'' /8'' No Void content by Image analysis. Figures 5 and 6 showed the image analysis results: (a) shows the original image for different aggregates; (b) the grayscale image was obtained by MATLAB internal command; (c) showed the histogram of the grayscale image; (d) showed the edge between two phases. As shown in the figures, the histogram in Figure 5 and 6 are different. The void corresponded to the first peak in Figure 5, the last peak in Figure 6, which is due to the flour filler. The main reason to use the flour filler is that the intensity gradually changed on the slope of the void which made the difficulty to recognize the intensity range of the void, aggregate or cement. Compared the histograms, the slope corresponding to the void filled with flour is sharper than the other one, which increased the precision of the analysis results. Table 6 presents the calculated air voids of pervious concrete through image analysis, which is quite consistent with the results obtained through CoreLok TM measurement as presented in Table 3. Figure 7 shows the void content in pervious concrete along the specimen length. As showed, the slopes for all three mixtures are near zero, which means that void content is irrelevant with the position and there is no bleeding and segregation during preparation of the samples. The scatter of the void content results may be attributed to (a) uniform of the concrete and (b) the cutting (some coarse aggregate will loose due to the weak bond, as seen in image.) Compared the void content values in Tables 3, 5 and Figure 4, pervious concrete with ½ inch aggregate was more compact than the other two, which may have been resulted by different image analysis method. Pervious concrete with ½ inch aggregate was more compact than that with 3/8 inch aggregate. 7

8 Table 6. Void Content of Pervious Concrete Obtained by Image Analysis Aggregate Size in Mixture Void Content (%) ½ 22.8 ± / ± 3.48 No ± 4.78 (a) (b) (c) (d) Figure 5. Image Analysis Result of Pervious Concrete with ½ inch Aggregate (a) original image without filler; (b) grayscale image derived by program; (c) histogram obtained by counting the pixels at same intensity (first peak corresponding to the void); (d) edges between solid phase and void 8

9 (a) (b) (c) (d) Figure 6. Image Analysis Result of Pervious Concrete with ¼ inch Aggregate (a) original image with flour filler; (b) grayscale image derived by program; (c) histogram obtained by counting the pixels at same intensity (last peak corresponding to the void); (d) edges between solid phase and void Void fraction on the cross section 0.4 3/8" pervious concrete 1/4" pervious concrete 0.35 y = x y = 4E-05x y = -5E-05x /2" pervious concrete Distence from the end (mm) Figure 7. Air Void Content along the Length of the Cylinder Samples 9

10 Summary and Conclusion A preliminary study has been conducted to study the effect of aggregate gradations on the strength and permeability characteristics of pervious concrete. Based on the limited laboratory and analytical results, the following can be concluded. 1. The compressive strength and indirect tensile strength of pervious concrete with larger size aggregate was larger, which can be attributed to the smaller air void and the more cement paste between aggregates if considering the aspect ratio. 2. For pervious concrete, due to high air void, Darcy s Law is no longer valid. Therefore, proper procedures should be used to characteristic the permeability of pervious concrete. The values of pseudo-coefficient of permeability K increased as aggregate size increased, which means the hydraulic conductivity increased as aggregate size increased. 3. The results obtained by two different methods, image analysis and CoreLok density measurement, are consistent. The void content in the pervious concrete is irrelevant with position. Considering the different surface preparation method, the concrete with smaller aggregate should be more compact. Acknowledgement The authors would like to express their thanks to Mr. N. Randy Rainwater, Mr. Chun-Yip Chan and Mr. Stephen Horne, who carefully prepared specimens and helped conduct concrete testing. Reference: Cooley, L.A., Prowell, B.D., Hainin, M.R., Buchanan, M.S. and Harrington, J. (2002). Bulk specific gravity round-robin using the Corelok Vacuum Sealing Device, NCAT report 02-11, Kevern, J., Wang, K., Suleiman, M.T. and Schaefer, V.(2005). Mix Design Development for Pervious Concrete in Cold Weather Climates, Proceedings of 2005 Mid-Continent Transportation Research Symposium, Ames, Iowa Huang, B., Mohammad, L.N., Raghavendra, A. and Abadie, C. (1999). Fundamentals of permeability in Asphalt Mixutres, Journal of the Association of Asphalt Paving Technologists, 68, National Ready Mixed Concrete Association (2004) Freeze-thaw Resistance of Pervious Concrete, 10

11 Neithalath, N., Marolf, A., Weiss, J., Olek, J. (2005). "Modeling the Influence of Pore Structure on the Acoustic Absorption of Enhanced Porosity Concrete", JCI Journal of Advanced Concrete Technology, 3 (1), Park, S. and Tia, M. (2004). An experimental study on the water-purification properties of porous concrete, Cement and Concrete Research, 34, Tennis, P.D., Leming M.L. and Akers, D.J. (2004). Pervious Concrete Pavements, Portland cement Association Yang, J. and Jiang, G.(2003). Experimental study on properties of pervious concrete pavement materials, Cement and Concrete Research, 33,