The Influence of Porosity & Aspect Ratio on the Compressive Behavior of Pervious Concrete. Alexander Hango

Transcription

1 The Influence of Porosity & Aspect Ratio on the Compressive Behavior of Pervious Concrete by Alexander Hango 1

2 Clarkson University The Influence of Porosity & Aspect Ratio on the Compressive Behavior of Pervious Concrete A Thesis Proposal by Alexander Hango Department of Civil Engineering Mentor: Prof. Narayanan Neithalath March 11,

3 I) Abstract The goal of this thesis project is to investigate the influence that porosity and aspect ratio of test specimens has on the mechanical properties of pervious concrete. The ability to quantify these influences, particularly that of aspect ratio can help better understand pervious concrete, as well as its testing procedures. Many studies have been done on compressive strength and other mechanical properties of pervious concrete. Using this predisposed information this research can be done with expected results while examining how aspect ratio can alter the peak compressive strength, the pre and post-peak stress-strain behavior, and any trends can easily be observed. The results of this research can be used to help establish a more defined testing procedure of pervious concrete, as well as act as a baseline for comparison of testing specimens in the future. II) Introduction Pervious concrete is a unique and effective solution to address important environmental issues and support sustainable construction and development of infrastructure. The use of pervious concrete has and continues to grow because of its environmentally friendly advantages over traditional concrete in certain applications. Pervious concrete, which is typically made up of limestone aggregate, Type 1 Ordinary Portland Cement, and water is relatively porous therefore it contains more voids in the structure leading to higher permeability compared to conventional concrete, but the structural strength of it is compromised [1,2]. Since cement paste in pervious concrete is very thin to bond coarse aggregate together, porous concrete tends to fail at the interface between the aggregates and results in the low compressive strength [2]. The main benefit of pervious concrete is that it is porous and can allow water to permeate through it. This can reduce storm water run-off of paved areas and allow for some ground-water recharge. This can eliminate the need for retention ponds, storm water treatment, and lessen the impact of new construction on the surrounding environment. Typically pervious concrete has been used for low volume applications such as parking lots and sidewalks [1]. In recent history, there have been studies done which characterize the mechanical properties of pervious concrete and examine the effects of changing aggregate size and porosity on these properties [1]. Using this previous research as a baseline, this project intends to characterize the mechanical properties of pervious concrete while examining the effect of changing the aspect ratio of the specimens tested, as well as the porosity of the specimens. Generally, in an experiment set, all the specimens tested are done so in a standard mold size and the data collected is consistent to the aspect ratio of the mold. This study intends to look at and identify a relationship between aspect ratio of the concrete specimen created and the mechanical properties. By altering the aspect ratio of the specimens it can be determined if aspect ratio does in fact effect the mechanical property data collected in the lab. Specifically, compression and UPV tests will be done; the stress-strain curves for each mixture proportion will be looked at and compared to identify relationships and differences in strength, Elastic Modulus, and post-rupture behavior of each specimen. The results of this experiment will provide valuable information about the testing procedures of pervious concrete, and how or if the aspect ratio of a sample can altar laboratory results. 3

4 III) Background & Purpose Traditionally in the determination of the compressive strength of concrete there are two shapes of test specimens; cubes and cylinders [11]. Studies have been done with nominal concrete that involved creating several different aspect ratios of specimens, cube and cylinder alike. This study was able to produce relationship and conversion factors between non-standard specimen shapes and standard cube strength. Additionally, it was found that the higher the strength of the concrete, the less aspect ratio influenced the specimen s compressive strength. The major goal of this thesis work is to perform a similar study for pervious concrete with cylindrical cylinders varying in aspect ratios of 1.5, 2 and 2.5. Mixture Proportioning In order to work with and test pervious concrete, it is crucial to understand the basic mechanical properties of pervious concrete, and know what can be expected in all phases of the testing process. One of the main challenges of this project is the mixture proportioning. Generally the aggregates used are limestone in the range of: (i) #8 (retained on 2.36 mm sieve), (ii) #4 (retained on 4.75 mm sieve), (iii) 3/8 (retained on 9.5 mm sieve), and (iv) 1/2 (retained on 12.5 mm sieve) [1]. When proportioning pervious concrete mixtures, the goal is to obtain a target void content that will allow for the percolation of water [3]. To do this correctly there are several parameters that need to be accounted for. Some of these parameters include moisture content of the aggregate being used, the water to cement ratio, and the relative densities of the material being used [4]. It has been found in previous studies involving mixture proportioning that the design porosities were always lower than the fresh porosities obtained when batching. This was attributed to the reduced compaction effort in the ASTM C1688 procedure that effectively results in an increased void volume in the mixture [1]. To correct for this, one common method is to increase the amount of cement paste in the mixture [1]. Although higher cement paste content increases the cost of the mixture, it has also proven to increase the compressive strength. Because of the degree of accuracy required in mixture proportioning of concrete, it is important to measure these quantities before mixing. Batching of Concrete Specimens As previously mentioned, pervious concrete is porous allowing it to permeate water through pores in its structure. Pervious concrete has a porosity that typically ranges from 15 to 30% [1,2]. In order to ensure the mixture proportioning is correct, ASTM has developed a standard method to test for fresh porosity of specimens as produced. The ASTM C1688 Standard Test Method for Density and Void Content of Freshly Mixed Pervious Concrete establishes a set methodology which allows for the fresh density to be calculated using mass and volumetric relationships [4]. Using a standard measure, a proctor hammer, and the volume and mass of the measure, the fresh density of the pervious concrete can be calculated after mixing is complete. This fresh density can then be compared to the theoretical density of concrete, and the porosity can be found. 4

5 Testing of Concrete Specimens Common tests to perform on pervious concrete specimens are: a compression test, a UPV test, and a permeability test. This study will focus mostly on the UPV and compression testing to characterize the mechanical properties of the specimens. A UPV or ultra-sonic pulse velocity test method employs the principle of measuring the travel velocity of ultrasonic pulses through a material. The wave velocity is calculated using the time taken by the pulse to travel the measured distance between the transmitter and the receiver [5]. This can then be used to find the Dynamic Modulus of the specimen. The compression testing is done in a load cell operating in displacement controlled mode. This can provide load vs. displacement data. This can then be converted to stress-strain curves and additional data can be extrapolated. The preparation of the pervious concrete specimens is difficult and needs to be carried out with a high degree of precision. Due to the porous and irregular surface of the faces, platento-platen displacements are recorded to determine the strains since strain gauges usually can t be mounted on the specimen surface [1]. In order to ensure that no eccentricity of loading occurs, time must be taken to smooth both surfaces of the specimen, and ensure that the surfaces are parallel. It is suggested that neoprene pads be used on each end of the specimen to help reduce the potential for eccentricity. Analysis Due to the number of studies and tests done on pervious concrete, there have been some distinct and repeatable trends and patterns occurring in the stress-strain curve relationships that should occur and be observed in this study. First, increasing porosity indicates decreasing peak compressive strength of the specimen [1,4]. Second, the pre-peak response of the specimen (portion of stress-strain curve prior to the peak) decreases with a smaller aggregate size being used. This is likely due to the smaller aggregate size having a more uniform pore structure [1,6]. Also, increasing the cement-paste content will increase the peak compressive strength of the specimen. Specimens with higher cement-paste content will also exhibit greater strain levels at the ultimate stress than a low paste-content specimen [1,6,10]. These relationships provide a large amount of information prior to doing any testing. The results of this study, such as expected strength, the shape of the stress-strain curve, and the effects of changing porosity, can easily be compared to previous studies. It will be easy to tell if the data being collected is accurate, and representative of the corresponding sample. As stated earlier, the main goal of this study is to obtain similar data and use it to compare a parameter that hasn t been used before, aspect ratio. Due to the numerous sources of potential error and subjectivity in preparing and testing pervious concrete, six specimens will be made per mix in order to obtain at least 3 good samples per mix. This study, with good data, has the potential to reveal valuable data on the testing methods of pervious concrete, and how the results are or are not affected by the aspect ratio of the specimen tested. 5

6 IV) Research Methodology In order to complete this project, several stages of work must occur. In order to observe any data, the specimens that are to be tested need to be created. The methodology of this process can be broken down into several stages. Preliminary Work One of the first steps in the process is to gather the necessary supplies. This research will occur in a laboratory containing all the appropriate equipment for mixing, curing, and testing of concrete specimens. The materials needed to batch the twelve mixtures are Type 1 Ordinary Portland Cement, limestone aggregate, and an admixture. The bulk materials will be supplied by a local quarry. The admixture to be used will be a rheology modifying admixture to improve the workability of the mixtures at a dosage of 680 g/100 kg of cement as recommended by the manufacturer [1]. The twelve mixes will all use #4 (retained on 4.75 mm sieve) aggregate that will need to be sieved before use. Additionally, before a mix can be made, several aggregate, cement, and volumetric parameters need to be addressed in a pre-made mixture proportioning spreadsheet. This spreadsheet encompasses many material properties and allows for the design of a mix based on the inputs of desired porosity, aggregate size, and the volume of the mix desired. Several material properties including: relative density of oven dry (OD) aggregates, absorption of aggregates, packing density of aggregates, and the relative density of cement were measured in a previous study done at Clarkson University [4]. Unless the results from these mixture proportions become undesirable, the current values for the previously stated parameters will be used. Before any mix, the moisture content of the sample will be measured by preparing C for twelve hours. By doing this, the moisture content of the aggregate can be determined. As stated earlier, this research will consist of twelve mixtures. Six of the mixes will be done at 19% desired porosity and the other six at 27% desired porosity. These porosities were selected due to information available from previous studies done with 19% and 27% porosities [1]. In most compression testing of concrete, both nominal and pervious, an aspect ratio (length to width) of 2 is commonly used [9]. Commonly 3 x6 and 4 x8 cylinders are used for compression testing [1,7,8]. To determine the effect that aspect ratio has on the mechanical properties of pervious concrete, aspect ratios of 1.5, 2, and 2.5 will be used. Each aspect ratio will have 4 different mixes. The mixes to be performed in this lab are as shown below: 6

7 Table 1- Mixes Description Aspect Ratio Dimensions Porosity Agg. Size 4"x6" 19% # "x6" 27% #4 4"x8" 19% #4 2 4"x8" 27% #4 4"x10" 19% # "x10" 27% #4 3"x4.5" 19% # "x4.5" 27% #4 3"x6" 19% #4 2 3"x6" 27% #4 3"x7.5" 19% # "x7.5" 27% #4 For each mix, six cylinders of that size will be made for compression and UPV testing, as well as two cylinders at 3.75 x8 for permeability testing. The procedures for these tests will be described later. Mixing The mixing procedure will be done in accordance with ASTM C192, in order to ensure a quality mix [4]. Once the mixing has been done, a quick check that can be done is to visually examine the mix for wetness, consistency, and workability. ASTM also has a procedure C1688 to determine the fresh density of mixes by mass. This procedure will be used to determine if the fresh porosities achieved are close to the desired porosities. A sample of fresh pervious concrete should be placed in a standard measure and compacted at twenty blows in 2 lifts with a standard Proctor hammer. The fresh density of the pervious concrete can be calculated based on the mass of the concrete in the standard measure [3,6]. This fresh density can then be compared to the theoretical density, and a fresh porosity can be calculated [1]. From previous studies, it has been determined that a compactive effort of 5 drops of a Proctor hammer per layer for 2 layers in the cylindrical molds was sufficient to bring the hardened state porosities close to the design porosity (and the fresh porosity) [1]. This compactive effort will be used unless changes are deemed necessary during the ASTM C1688 procedure. Curing The industry standard for curing of concrete is 28 days [1,9]. Since this study is being done with the comparison of specimens being done all relative to each other, the test date can and was arbitrarily selected. A test period of two weeks will be used for this study. All samples will be removed from the molds after 24 hours and immediately placed in the curing room for a two week period. This will give a consistent test period and the curing process should be equal for all specimens. 7

8 Testing All of the concrete mixes will be tested for permeability, compression strength, and dynamic modulus using a UPV test. As in a previous study done at Clarkson University, the compression test will be done on a 490 kn closed-loop universal testing machine [1]. In previous studies, two strain rates; 10 με/s and 100 με/s have been used for testing. It has been shown that there is a small increase in peak stress and the slope of the ascending stress strain curve, and a decrease in the strain corresponding to peak stress, when the strain rate is increased. [6]. Because this difference is minor, and all samples will be tested at the same strain rate, a rate of 100 με/s will be used. Since strain gauges can t be used due to the irregular surface of pervious concrete, a platen-to-platen method will be used to measure strain. Volumetric porosity of the pervious concrete specimens will be measured by submerging a 3.75 x8 cylinder in water for 24 hrs to fully saturate the specimen [1]. The specimens sides will then be sealed with latex and the cylinder will be secured and sealed at the base to a steel plate. Water can then be added to the top of the cylinder and the mass of water required to fill the cylinder can then be converted into a volume. This test will show the actual porosity of the cured specimens [1,9]. The third test that will be conducted is a UPV test. A UPV test quickly determines the dynamic modulus of the specimen. Each specimen will be tested three times, and the average value will be taken. Analysis Using the data from the compression test, a stress-strain curve can be created and compared. These curves can provide adequate means to compare the compressive behavior of pervious concretes proportioned for different porosities and hopefully provide a comparison of the influence of aspect ratio on the mechanical properties. It has been found that the stress strain relationships are influenced by the testing conditions, material properties, as well as specimen preparation [1,6,12]. A method for analyzing stress-strain curves, which takes into account experimental errors, has been developed in multiple studies. It has been noticed that in a compression test of a pervious concrete specimen, there is a slight curvature at the beginning of the stress strain curve. Theoretically, at up to 30% of the peak stress, the stress strain curve should behave linearly [1,6,12]. After that point, it should begin to behave non-linearly. To account for this experimental testing error, which has yet to be attributed to anything specific, a method has been developed to correct this curve. First, a value corresponding to 30% of the peak stress can be found. Next, a tangent to that point is drawn down to the x-axis, and that tangent becomes the linear portion of the stress strain curve [1,12]. This method helps to examine and look at data that has not been influenced by error. From the stress-strain curves, toughness can also be extrapolated. Toughness can be determined by measuring the area underneath the stress-strain curve and its energy of mechanical deformation per unit volume prior to fracture. The stress-strain curves that these are done on usually go only to a strain value of three times the peak stress. Generally this point is the beginning of the plateau of the stress-strain curve for pervious concrete, or the point where the specimen can only strain but not bear additional weight [1,12]. The toughness parameter can be summed by using a numerical area calculation method. 8

9 Preliminary Data At this point in the project, a few of the mixes have been made, and a complete set of tests have been done. The first mix that was batched was a standard 4 x8 cylinder with both 19% and 27% porosity using #4 aggregate. Shown below in Figure 1 and Figure 2 are two curves from the data-set for a 4 x8 cylinder with 19% porosity and #4 aggregate. The linear pre-peak portions of the curves were corrected using the previously stated method. These figures show a peak stress of approximately 11 MPa and 15 MPa respectively. The shape of the curve matches curves that have been created in previous studies; however this set of data shows a rather low peak stress for the samples [1]. An expected peak stress for a mix as such is MPa [1]. However, these curves give the correct shape and behavior of a pervious concrete specimen. These curves can be compared to curves of 19% porosity at different aspect ratios, and the location of peak stress, the stress-strain response, and the toughness can all be determined. Figure 1: Figure 2: 9

10 V) Timeline Table 2: Thesis Timeline & Goals Month December 2010 January 2011 February 2011 March 2011 April 2011 Summer 2011 September 2011 October 2011 November 2011 December 2011 Goal Determine focused topic and begin literature review Meet with advisor, work out specifics of thesis; what tests will be run, number of mixes to make, type of mixes to make. Make 4-6 of the 12 mixes, continue literature review, and begin testing of first set of mixes. Build molds for non standard test specimens. Complete 1 st draft of proposal by March 4 th. Have 6 of the 12 mixes done before spring break (March 12 th ). Continue to test as needed. Examine Preliminary results. Complete all 12 mixes by April 8 th. Continue testing specimens. Rewrite proposal as needed. Begin to do in-depth data analysis of test results. Start to write paper for Prof. Neithalath. Make sure all lab work is done by the end of the school year. Work on paper as needed. Keep in contact with Prof. Neithalath. Continue data analysis. Finish data analysis. Continue writing paper. Keep in contact with Prof. Neithalath. Finish paper for Prof. Neithalath. Expand this paper to use as thesis report. Work on thesis paper. Complete thesis paper by the 1 st week in November. Have it edited by Prof. Neithalath and perhaps other professors. Present thesis. Have thesis requirements done by the end of this semester!! 10

11 References [1] Deo, O., Neithalath, N. (2010) Compressive Behavior of Pervious Concrete Mixtures Proportioned for Desired Porosities. [2] Lian, C., and Zhuge, Y., Optimum mix design of enhanced permeable concrete An experimental investigation, Construction and Building Materials, Vol. 24, No. 12, 2010, pp [3] ASTM C1688 / C1688M - 10a (2008) Standard Test Method for Density and Void Content of Freshly Mixed Pervious Concrete. Section on Safety Precautions, Manual of Aggregate and Concrete Testing, Annual Book of ASTM Standards, Vol [4] NRMCA Engineering Division. (2009) Pervious Concrete: Guideline to Mixture Proportioning. National Ready Mixed Concrete Association [5] Hall, B. and John, V. (1998) Non-destructive testing. Springer-Verlag. [6] Deo, O., and Neithalath, N., Compressive behavior of pervious concretes and a quantification of the influence of random pore structure features, Material Science and Engineering A, Vol. 528, No.1, 2010, pp [7] Meininger, R.C., No-Fines Pervious Concrete For Paving, Concrete International, Vol. 10, 1988, pp [8] Malhotra, V.M., No-Fines Concrete Its Properties and Applications, ACI Journal, Title No , [9] Yang, J., and Jiang, G., Experimental study on properties of pervious concrete pavement materials, Cement and Concrete Research, Vol. 33, No. 1, 2003, pp [10] Chindaprasirt, P., Hatanaka, S., Chareerat, T., Mishima, N., and Yuasa, Y., Cement paste characteristics and porous concrete properties, Construction and Building Materials, Vol. 22, No.1, 2008, pp [11] Mansur, M. A. and Islam M. M. Interpretation of Concrete Strength for Nonstandard Specimens Journal of Materials in Civil Engineering, Vol. 14, No. 2, April 1, [12] Puri, Sunil (2003) Assessing the Development of Localized Damage in Concrete Under Compressive Loading Using Acoustic Emissions Purdue University. 11