Mechanical Properties of Concrete Containing Recycled Asphalt Pavement and Class C Fly Ash

Transcription

1 2015 World of Coal Ash (WOCA) Conference in Nasvhille, TN - May 5-7, Mechanical Properties of Concrete Containing Recycled Asphalt Pavement and Class C Fly Ash Pranshoo Solanki 1, Bharat Dash 2 1 Illinois State University, Department of Technology, Campus Box 5100, Normal, Illinois 61790; 2 Illinois State University, Department of Technology, Campus Box 5100, Normal, Illinois KEYWORDS: Recycled asphalt pavement (RAP), class C fly ash (CFA), Portland cement concrete. ABSTRACT The objective of this study was to evaluate the influence of recycled asphalt pavement (RAP) as an aggregate replacement and class C fly ash (CFA) as Portland cement replacement on compressive and tensile strength of concrete. A total of 28 concrete mixes containing different replacement level of RAP and CFA were designed in addition to one control mix. A total of two types of RAP namely, chips and screenings were used for replacing coarse and fine aggregates, respectively. Using the mix designs, cylindrical specimens of concrete were prepared, cured in water tank, and tested for compressive and tensile strength after 28 days. The properties tested include the compressive and tensile strength values of concrete. Experimental results showed aggregate substitution with RAP was found to decrease both compressive and tensile strength of concrete. Further, this reduction was dependent on the percentage of aggregate replacement with RAP screenings, percentage of cement replacement with fly ash, and type of aggregate being replaced (coarse or fine). This decrease in strength could be attributed to the weak bonding between the RAP particles and the cement-fly ash matrix. INTRODUCTION Reduction of natural resources and aggregate quarries for the road and other construction purposes is a severe problem to procure materials. The concrete contains no less than 75% by volume of aggregate materials which may be locally available but in some places it may be economical to substitute those natural aggregates by more cheaply and abundantly available materials, Okafor (2010) 9. Therefore it is very essential to recycle or reuse the material. Each year as much as 100 million tons of hot mix asphalt (HMA) are reclaimed during road resurfacing and widening projects. About 80 million tons (80%) are reused as recycled asphalt pavement (RAP) (Solanki et al., 2013) 8. Even though RAP has been used in the U.S. for over 25 years, with increased environmental awareness and focus on recycling, its enhanced use in construction has become a topic of national importance.

2 Consequently, the aim of the proposed study was to evaluate the influence of RAP as an aggregate replacement and class C fly ash (CFA) as Portland cement replacement on compressive and tensile strength of concrete. In this research study, a total of 28 concrete mixes containing different replacement level of recycled asphalt pavement and fly ash were designed in addition to one control mix. A total of two types of RAP millings namely, chips (particles ranging in size from 1 in. to 0.19 in) and screenings (particles ranging in size from 0.19 in. to less than mm) were used for replacing coarse and fine aggregates, respectively. Using the mix designs, cylindrical specimens of concrete were prepared and cured in water tank for 28 days. Then, specimens were tested for compressive and indirect tensile strength in accordance with ASTM test procedures. OVERVIEW OF PREVIOUS STUDIES Class C fly ashes are as effective as Portland cement in developing 28-day strength (Cook, 1981) 10. Therefore, CFA is highly beneficial in the production of high-strength concrete. Several studies were also conducted from CFA for partially replacing Portland cement in concrete. A study by Naik and Ramme (1990) 5 have substantiated that super plasticized CFA concrete with low water-to-cementitious materials ratio can be proportioned to meet the very early-age strength as well as other requirements for precast/prestressed concrete products. The maximum cement replacement with the fly ash was reported to be 30% for such high-early strength concrete application. Another study by Naik et al. (1994) 4 developed mixture proportions for paving roadway concrete using large amounts of fly ash. These mixtures were composed of 50% Class C fly ash and 40% Class F fly ash as a replacement of Portland cement. The results indicated that high volumes of class C and class F fly ashes could be used to produce high-quality concrete pavements with tremendous performance. Naik et al. (1994) 4 also indicated that concrete containing large volumes of CFA can be proportioned to meet strength and workability requirements for construction applications. Delwar et al. (1997) 7 investigated use of RAP as an aggregate replacement in PCC. It was found that increasing the RAP aggregate in the concrete mixture reduces its compressive strength and tensile strength; however, it increases its ductility and elastic behavior. Delwar et al. (1997) 7 suggested the feasibility of RAP aggregate usage in driveways, sidewalks, pipes, barriers and gutters. Further it was concluded that, concrete made with RAP can be potentially mixed using conventional equipment and methods. Naik and Chun (2003) 3 studied the effects of incorporating high volumes of CFA on the properties of concrete. Their results further indicated that economical self-consolidating concrete with 28-day strengths up to 9,000 psi can be made using high-volumes of fly ash. Such concretes can be used for a wide range of applications from cast-in-place to precast concrete construction.

3 Recent research studies examined the feasibility of incorporating RAP in Portland cement concrete (Huang et al. 2005, 2006) 11, 14. Huang et al. (2006) 11 used two gradations of RAP (coarse and fine) materials to replace the fresh aggregate from a control concrete mixture. The study results indicated that concrete samples made with only coarse RAP resulted in strength reduction and improved toughness. Furthermore, Huang et al. (2006) 11 attributed reductions in the strength to the fact that asphalt film around the aggregate particle were much softer than the concrete matrix and aggregates. Additionally, the slump test results of concrete showed that coarse or fine RAP containing concrete slump values were lower than that of control concrete. Huang et al. (2005, 2006) 11, 14 suggested that reduction in slump values could be attributed to the asphalt coating around both coarse and fine RAP so that the aggregates will absorb less percentage of water. Okafor (2010) 9 study focused on the replacement of aggregates with RAP. The physical properties of RAP aggregates were compared with similar concretes made with natural aggregate. The results indicated that the strength of concrete made from RAP is dependent on the bond strength of the asphalt-mortar. His study also indicated that when natural aggregate was compared to RAP, it was found that RAP has lower specific gravity and lower water absorption. In a recent study, Ibrahim et al. (2014) 1 studied the self-consolidating concrete mixtures preparation which included different proportions of RAP, CFA, sand, virgin aggregates and water. The second task was the laboratory testing which included fresh properties, mechanical properties and durability. Properties of fresh concrete were investigated in this study which included flowability, deformability, filling capacity and resistance to segregation. Ibrahim et al. (2014) 1 test results indicated that when increased the RAP content from 0 to 25% and 50% for it caused the spilt tensile strength of all mixtures to decrease suggestively. Furthermore the results obtained in the study recommended not replacing the coarse aggregate in self-consolidating concrete with more than 25 percent RAP. These samples indicated towards the most recent advances of high performance concrete in the contemporary industry. The American Coal Ash Association (ACAA, 2006) 13 has testified that almost 26,720 metric tons (29,450 tons) of ash were used as asphalt mineral filler in A solution to a combined RAP and fly ash disposal problem would be to use RAP as an aggregate replacement and CFA as Portland cement replacement in concrete, which has not received much attention. MATERIALS AND TEST PROCEDURES Collection of Materials The RAP used in this study was collected from McLean County Asphalt Inc. located in Bloomington, Illinois. A total of two types of size fractions namely, RAP chips (particles ranging in size from 1 in. to 0.19 in) and RAP screenings (particles ranging in size from 0.19 in. to less than mm) were used for replacing coarse and fine aggregates,

4 respectively. Figure 1 shows two different size fractions of RAP. Both ordinary Portland cement and class C fly ash (CFA) were collected in cooperation with Lafarge Cement North America office located in Chicago, Illinois. The CFA was produced in a coal-fired electric utility plant and the source was Pleasant Prairie. The properties of CFA and cement are presented in Table 1. The differences between the chemical composition and physical properties among CFA and cement are clearly evident from Table 1. Virgin aggregates were collected from Prairie Material, a local ready-mix concrete plant located in Normal, Illinois. (a) RAP Chips (b) RAP Screenings Figure 1: Recycled Asphalt particles used in the experimental study. Table 1: Chemical properties of ordinary Portland cement and CFA used in this study. Percentage by Chemical compound/property weight (%) OPC CFA Silicon dioxide (SiO2) a Aluminum oxide (Al2O3) a Ferric oxide (Fe2O3) a Calcium oxide (CaO) a Magnesium oxide (MgO) a Sulfur trioxide (SO3) a Alkali content (Na2O+K2O) a Free lime b Loss on ignition (LOI) c CKD: cement kiln dust; OPC: ordinary Portland cement; a X-ray fluorescence analysis; b ASTM C 114 alternative method B; c ASTM C 114

5 MIX DESIGN A total of 28 concrete mixtures containing different replacement level of RAP and CFA were designed in this study. Additionally, one control mixture was prepared by mixing Portland cement, fine aggregates and coarse aggregates in the ratios of 1:2:2 and water-to-cement ratio of 0.5. Table 2: Design of Concrete Mix Proportions Mixture# Cementitious Material Portland Fly Cement Ash (%) (%) Coarse Aggregates Virgin Aggregates (%) RAP Chips (%) Fine Aggregates Virgin Aggregates (%) Control Mixes prepared by replacing coarse aggregate with RAP screenings Mixes prepared by replacing fine aggregate with RAP screenings RAP Screenings (%)

6 Table 2 shows proportion of all concrete mixtures which were considered in this study. Fine and coarse aggregates were replaced with recycled asphalt screening and chips, respectively, with replacement level of 0%, 10%, 20%, 30% and 40% by weight. On the other hand, Portland cement concrete was replaced with 0%, 10%, 20% and 40% CFA by weight as shown in Table 2. The water-to-cementitious material (Portland cement and fly ash) ratio for all mixtures were fixed at 0.5. SPECIMEN PREPARATION AND LABORATORY TESTING All mixtures were prepared by adding required amount of dry ingredients in a Hobart mixer. After preparation of mixtures, the workability of mixtures was evaluated by conducting slump test in accordance with ASTM C 143. Then, cylindrical specimens of concrete were prepared in accordance with ASTM C 192. Cylindrical specimens were cured in a water tank, as shown in Figure 2 (a), with the water being renewed on a monthly basis. After 28 days of curing (Figure 2 (b)), these cylinders were tested for compressive strength and indirect tensile strength in accordance with ASTM C 39 and ASTM C 496 test methods, respectively. Both compressive strength and indirect tensile strength tests were conducted in a compression testing machine. (a) (b) Figure 2 (a) Cylindrical Samples under Curing Process (b) After Curing PRESENTATION AND DISCUSSION OF RESULTS Effect of Replacement of Fine Aggregate with RAP Screenings Compressive Strength The individual results of the 28-day unconfined compressive strength tests of concrete containing RAP are graphically presented in Figure 3. All the specimens tested in this study generally showed a reduction in the UCS values with an increase in the amount of RAP. For example, UCS values of cylindrical specimens containing 10%, 20% and 40% RAP with 20% CFA decreased respectively by 9%, 8% and 25%, as compared to a control specimen containing no RAP. The decrease in the UCS values can be partially explained by weak bonding between the asphalt of RAP particles and the cement-fly ash matrix. Additionally, it is important to note that RAP has lower water absorption. Since part of the aggregates is replaced by RAP particles, overall strength and density reduction is expected. It was noted that the density of

7 cylindrical specimens containing, 10%, 20% and 40% RAP screenings reduced to 138.3, and lb/ft 3, respectively. Further, it was noted that the amount of reduction in strength was dependent on the percentage of cement replacement with fly ash. It is also evident from Figure 3 that increase in amount of CFA in specimens without any RAP increased UCS values of cylindrical specimens. However, no clear trend was found between amount of CFA and UCS values in specimens containing RAP. Unconfined Compressive Strength, UCS (psi) % Fly Ash 10% Fly Ash 20% Fly Ash 40% Fly Ash % RAP 10% RAP 20% RAP 40% RAP Amount of RAP Screenings Figure 3: 28-day compressive strength values of concrete containing different percentage of RAP screenings and fly ash. Indirect Tensile Strength - The indirect tensile strength values of specimens containing different percentages of CFA and RAP screenings are presented in Figure 4. All specimens tested showed decrease in the indirect tensile strength values with an increase in the percentage of RAP screenings. For example, indirect tensile strength values of cylindrical specimens containing 10%, 20% and 40% RAP with 20% CFA decreased respectively by 33%, 50% and 45%, as compared to a control specimen containing no RAP. However, no clear trend was observed between amount of CFA and indirect tensile strength values for specimens containing RAP screenings. Similar trend of tensile strength results were reported by Ibrahim et al. (2014).

8 1400 Incirect Tensile Strength (psi) % Fly Ash 10% Fly Ash 20% Fly Ash 40% Fly Ash % RAP 10% RAP 20% RAP 40% RAP Amount of RAP Screenings Figure 4: 28-day tensile strength values of concrete containing different percentage of RAP screenings and fly ash A photographic view of broken cylindrical specimens containing different percentage of RAP screenings is presented in Figure 5. As indicated in Figures 5 (a) through 5(f), decrease in percentage of RAP in concrete showed failure pattern with wider cracks. (a) (b) (c) (d) (e) (f) Figure 5: Photographic view of cylindrical specimens containing (a) 10% RAP, (b) 20% RAP, (c) 40% RAP after compressive strength testing, (d) 10% RAP, (e) 20% RAP, and (f) 40% RAP after indirect tensile strength testing. Effect of Replacement of Coarse Aggregate with RAP Chips Compressive Strength The individual results of the 28-day unconfined compressive strength tests of concrete specimens containing different percentage of coarse aggregate and RAP chips are graphically presented in Figure 6. As noted with RAP screening, specimens containing RAP chips showed a reduction in the UCS values with an increase in the amount of RAP. For example, UCS values of cylindrical specimens

9 containing 10%, 20% and 40% RAP with 20% CFA decreased respectively by 52%, 25% and 27%, as compared to a control specimen containing no RAP. The failure in compression is probably originated by the seeming weak bond between the asphaltmortar and aggregate. Also, decrease in the compressive strength values might be due to a poor bonding between the cement paste and the RAP particles. Further, UCS results of specimens containing RAP screening and chips were compared from Figures 3 and 6, respectively. A replacement level of 10% resulted in higher UCS values for specimens containing RAP screenings as compared to corresponding specimens containing RAP chips. However, no clear trend was found for replacement level greater than 10% Unconfined Compressive Strength, UCS (psi) % Fly Ash 10% Fly Ash 20% Fly Ash 40% Fly Ash % RAP 10% RAP 20% RAP 40% RAP Amount of RAP Chips Figure 6: 28-day compressive strength values of concrete containing different percentage of RAP chips and fly ash Indirect Tensile Strength - The indirect tensile strength values of specimens containing different percentage of CFA and RAP chips are presented in Figure 7. It was noted that the amount of reduction in indirect tensile strength was dependent on the amount of RAP as well as CFA. For example, indirect tensile strength values of cylindrical specimens containing 10%, 20% and 40% RAP and 20% CFA decreased by 24%, 23% and 36%, respectively, as compared to a specimen containing no RAP and no CFA. No clear trend was observed between tensile strength values of RAP screenings (Figure 4) and RAP chips (Figure 7) containing concrete samples.

10 1400 Indirect Tensile Strength (psi) % Fly Ash 10% Fly Ash 20% Fly Ash 40% Fly Ash % RAP 10% RAP 20% RAP 40% RAP Figure 7: 28-day tensile strength values of concrete containing different percentage of RAP chips and fly ash. CONCLUSIONS This study was conducted to evaluate the feasibility of utilizing RAP as an aggregate replacement and CFA as Portland cement replacement in concrete. Aggregate substitution with RAP was found to decrease both compressive and tensile strength of concrete. The reduction in strength values was dependent on the percentage of aggregate replacement with RAP, percentage of cement replacement with CFA, and type of aggregate being replaced (coarse or fine). The decrease in the UCS values could be partially attributed to weak bonding between the asphalt of RAP particles and the cement-fly ash matrix. Further, increase in the amount of CFA in specimens without any RAP increased both compressive and tensile strength values of concrete. Concrete mixes containing RAP and CFA could be used for low-strength construction such as driveways, sidewalks, gutters and patching. ACKNOWLEDGMENTS AND DISCLAIMER Amount of RAP Chips Financial support for this study was provided through University Research Grant, College of Applied Science and Technology at Illinois State University. The material collection assistance provided by John Drew and Mike Dearing at Prairie Material located in Bloomington, Illinois, is gratefully acknowledged. The trend, conclusions and recommendations reported in this paper reflect the behavior of concrete containing aggregates, sand RAP and class C fly ash used in this study. This cannot necessarily be extrapolated to other aggregates, sand, class C fly ash and RAP.

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