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Rheological and Mechanical Behavior of Concrete Mixtures with Recycled Concrete Aggregates

Adam M. Knaack1, S.M.ASCE and Yahya C. Kurama2, M.ASCE

1Univ. of Notre Dame, Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall of Engineering, Notre Dame, IN, 46556; email: aknaack@nd.edu 2Univ. of Notre Dame, Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall of Engineering, Notre Dame, IN 46556; email: ykurama@nd.edu ABSTRACT This paper investigates the behavior of concrete mixtures that use recycled concrete aggregates (RCA) as replacement for virgin coarse natural aggregates. The direct weight, equivalent mortar, and direct volume replacement methods utilizing varying amounts of replacement are compared based on concrete workability, compressive strength, and elastic modulus. A total of 144 mixtures with 16 different RCA sources are used in the experimental program. It is found that the direct volume and equivalent mortar replacement methods result in the best and worst workability of fresh concrete, respectively. Regardless of the replacement method, the compressive strength of RCA concrete is moderately reduced as compared with the strength of concrete with virgin aggregates. There is a greater loss in the elastic modulus as the amount of RCA is increased. The results suggest that the material properties that most affect the mechanical behavior of RCA concrete are the aggregate water absorption and the deleterious content. INTRODUCTION This paper focuses on the use of recycled concrete aggregates (RCA) as replacement for virgin coarse natural aggregates in normal strength concrete mixtures. About half of the construction and demolition waste in the U.S. consists of old concrete rubble. At the same time, the mining, processing and transportation of natural aggregates (e.g., crushed stone, gravel) for the construction and maintenance of our civil infrastructure consume large amounts of energy and adversely affect the ecology of forested areas and riverbeds. By recycling demolished concrete as replacement for virgin coarse aggregates in new construction, it may be possible to substantially reduce the demand for new aggregates.

Most recycled concrete aggregates readily pass the requirements for coarse aggregates in structural concrete (ASTM 2009a); however, the use of recycled concrete as aggregate in U.S. construction has been largely limited to non-structural applications such as sidewalks and sub-base for roadways (FHWA 2008). Furthermore, almost all of the existing research on RCA concrete was conducted outside the U.S., limiting the applicability of the findings domestically because of variations in materials and quality control. To advance the use of RCA in structural reinforced concrete applications, it is necessary to determine the effects of RCA on the rheological and mechanical properties of concrete. With this objective in mind,

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this paper provides an experimental evaluation of three aggregate replacement methods: (1) direct weight replacement; (2) equivalent mortar replacement; and (3) direct volume replacement utilizing varying amounts of RCA determine concrete workability, compressive strength, and elastic modulus. Sixteen different RCA sources from the Midwestern U.S. are included in the study to determine the variability of RCA properties. BACKGROUND

Previous research on the material properties of RCA and the mechanical behavior of RCA concrete dates back to the late 1970s, with the majority being completed outside the U.S. (Buck 1977; Corinaldesi 2010; Dhir et al. 1999; Etxeberria et al. 2007; Fathifazl et al. 2009; Hansen 1986; Nishibayashi et al. 1984; Obla et al. 2007; Topcu and Sengel 2004; Yamato 1998). In a state-of-the-art paper, Hansen (1986) reported that mortar left adhered to the original natural aggregate in RCA was a cause for decreased specific gravity, increased water absorption, and increased L.A. abrasion loss. The most notable of these differences was the absorption, which was found to range between approximately 3.6% and 8.7%, as compared to the absorption of between 0.2% and 4.0% for natural aggregates (Kosmatka et al. 2002). As a result of the increased aggregate water absorption, RCA concrete tends to be less workable and have greater water demand (ACI 555 2001), which can be resolved by using water-reducing admixtures and fly-ash. Hansen (1986) recommended presoaking the aggregate and using RCA with less than 7% absorption. Similarly, Obla et al. (2007) suggested using 5% more mixing water for RCA concrete to produce equally workable material as natural aggregate (NA) concrete. Generally, RCA concrete has been shown to have decreased compressive strength and lower modulus of elasticity, depending on the amount of aggregate replacement and the quality of the RCA. While Yamato (1998) found strength losses as much as 45% at full aggregate replacement, the majority of the previous research found smaller losses ranging from 10% to 20%. In comparison, the effect of RCA on the elastic modulus of concrete is generally greater, with losses ranging from 10% to 33% at full replacement (ACI 555 2001). EXPERIMENTAL PROGRAM Aggregates. As listed in Table 1, one type of natural fine aggregate (FA), two types of virgin coarse natural aggregates (NA), and 16 sources of recycled concrete aggregates (RCA) were used in the experimental program. INDOT (2012) No. 23 “concrete sand” was used as the natural fine aggregate (FA). The fineness modulus of the fine aggregate was determined as 2.54 according to ASTM C 136 (2009a). For virgin coarse natural aggregate, moraine pea gravel (NA-PG) and crushed limestone (NA-CL) from a local ready-mix concrete plant were used. These fine and coarse natural aggregates are typical in ready mix-concrete construction.

To provide a regional geographical representation of the variability in material quality and properties, the 16 RCA sources used in this research were located across

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Illinois, Michigan, Indiana, and Ohio. Out of these sources, 15 were construction and demolition recycling plants. Since these facilities take debris from many sources (e.g., sidewalks, pavements, buildings, bridges), the properties of the original concrete are not known. Furthermore, the RCA from the recycling plants had gradations typical for road sub-base use in each respective state (including significant fine fraction), and also contained varying amounts of deleterious substances (e.g., brick, asphalt, wood). The other source of RCA was a 0.4-scale precast concrete shear wall laboratory specimen previously tested to failure under reversed-cyclic lateral loading (Smith et al. 2011). After the testing of the wall, the precast panels were crushed at a mobile recycling facility, and then sieved and combined into two different gradations (RCA-ND and RCA-ND1 in Table 1). The reason for crushing a test specimen into RCA was the availability of detailed information on the mix design as well as the fresh and hardened properties of the source concrete. In addition, since the RCA from the test specimen was not mixed with other material (i.e., it contained one concrete mix and little deleterious substances), it served as a benchmark for the RCA from the other 15 sources.

Table 1 shows the type/source location, specific gravity [bulk dry, saturated

surface dry (SSD), and apparent], water absorption (!!" for NA, !!"# for RCA), residual mortar content (RM), deleterious material content (!!"#), gradation, and LA abrasion loss (ASTM 2009a) of the aggregates (with the RCAs listed in order of increasing absorption). Since significant variations in aggregate size can produce different fresh and hardened concrete properties (e.g., workability, strength)

Table 1. Natural and recycled concrete aggregate properties

Aggregate ID

Type/ Source Location

Specific Gravity Water Absorption, !!" or !!"# (%Weight)

Res. Mortar Content,

RM (%Weight)

Deleterious Mat. Cont.,

!!"# (%Weight)

Gradation LA

Abrasion Loss

Bulk Dry SSD App.

FA Concrete Sand 2.59 2.63 2.69 1.39 - - INDOT #23 - NA-PG Pea Gravel 2.47 2.55 2.70 3.48 - - ASTM #8 23.2 NA-CL Limestone 2.71 2.73 2.76 0.74 - - INDOT #8 21.0

RCA-RR 1. Mishawaka, IN 2.31 2.41 2.56 4.33 26.1 22.1 INDOT #8 35.6 RCA-R 2. Chicago, IL 2.27 2.37 2.53 4.59 23.6 10.2 INDOT #8 31.3 RCA-W 3. Indianapolis, IN 2.26 2.36 2.52 4.70 18.8 3.27 INDOT #8 32.3 RCA-P 4. Chicago, IL 2.33 2.45 2.64 5.07 34.9 5.70 INDOT #8 32.7

RCA-GT 5. South Bend, IN 2.29 2.41 2.61 5.30 37.4 4.84 INDOT #8 35.4 RCA-L 6. Chicago, IL 2.34 2.46 2.67 5.31 36.9 4.00 INDOT #8 30.9 RCA-A 7. Cleves, OH 2.32 2.44 2.64 5.31 37.6 1.99 INDOT #8 35.2

RCA-EL 8. Cincinnati, OH 2.31 2.43 2.65 5.55 32.4 5.12 INDOT #8 35.2 RCA-EG 9. Elk Grove, IL 2.33 2.46 2.68 5.68 22.2 4.45 INDOT #8 30.5 RCA-HP 10. Highland Park, MI 2.28 2.41 2.62 5.69 42.1 1.91 INDOT #8 36.4 RCA-ND 11. Notre Dame, IN 2.28 2.41 2.63 5.69 60.5 0.00 ASTM #8 29.8

RCA-ND1 11. Notre Dame, IN 2.30 2.43 2.66 5.85 63.6 <1.00 INDOT #8 38.0 RCA-S 12. South Bend, IN 2.18 2.32 2.52 6.06 30.4 5.68 INDOT #8 37.8 RCA-T 13. Taylor, MI 2.24 2.38 2.61 6.29 46.5 3.55 INDOT #8 38.9

RCA-SB 14. South Bend, IN 2.13 2.28 2.49 6.73 51.5 0.00 ASTM #7 36.7 RCA-E 15. Elkhart, IN 2.14 2.32 2.61 8.44 35.1 1.25 INDOT #8 39.6 RCA-G 16. Goshen, IN 2.1 2.29 2.59 8.94 34.3 2.03 INDOT #8 40.5

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(Kosmatka et al. 2002), it was necessary to use aggregates with a consistent gradation. The three target gradations used were ASTM C33 (2009a) No. 8, ASTM C33 No. 7, and INDOT (2012) No. 8 [corresponding to nominal maximum aggregate sizes of 9.5 mm (3/8 in.), 12.5 mm (1/2 in.), and 19.0 mm (3/4 in.), respectively]. To achieve these gradations, each coarse aggregate was sieved into individual size fractions and then recombined as necessary. The details of the specific gravity, absorption, residual mortar content, and deleterious content testing are described in Knaack and Kurama (2011). Lastly, the LA abrasion loss, which measures the change in aggregate mass due to mechanical degradation through tumbling and falling, was determined according to ASTM C 131 (2009a).

NA Concrete Mix Designs. As listed in Table 2, three target virgin natural aggregate concrete (NAC) mix designs formed the basis for the RCA concrete mixes in this research. The original NAC mix (Smith et al. 2011) for the laboratory wall test specimen (i.e., the source for RCA-ND and RCA-ND1) served as one of the target mixes. This mix used NA-PG as the coarse aggregate and was designed with a water-to-cement (w/c) ratio of 0.44 for a target 28-day strength of 41.4 MPa (6.0 ksi), slump of 12.7±2.54 cm (5±1 in.), and air content of 5±1.5%. From previous data (Smith et al. 2011), the average measured 28-day strength of the concrete was 44.1 MPa (6.4 ksi). Because the original mix was designed for precast concrete construction (which requires high early concrete strength for removal of formwork), ASTM C 150 (2009) Type III portland cement was used. Keeping all of the volumetric mix proportions the same, a second target mix design using NA-CL with Type I cement was also determined. For both the NA-PG and NA-CL target designs, Sika® AEA-14 and Sikament® 686 were included as air-entraining agent (AEA) and high range water reducer (HRWR), respectively.

The third target mix (NA-PG-Wet) contained NA-PG coarse aggregate, ASTM C 150 Type III portland cement, Sika® AEA-14 air entraining agent, and no water reducer. The mix was designed with a w/c ratio of 0.45 for a target 28-day strength of 31.0 MPa (4.5 ksi) and slump of approximately 17.8 cm (7 in.). While the w/c ratio of this mix and the other two target mixes were nearly equal, the volumetric ratio of fresh mortar to coarse material for the third mix was greater (2.23 compared to 1.38), resulting in a much “wetter” mix. RCA Concrete Mix Designs. Three different aggregate replacement methods are explored in this paper as the direct weight replacement method (DWR), equivalent mortar replacement (EMR), and direct volume replacement method (DVR). A detailed comparison of these replacement methods is given by Knaack and Kurama (2011). By replacing the NA in the mix designs above, a total of 144 RCA mixes were made. The aggregate replacement ratio, ! was used to quantify the amount of

Table 2. Dry weight proportions of NAC target mix designs Name Water

(lb/yd3) Cement (lb/yd3)

NA (lb/yd3)

FA (lb/yd3)

HRWR (fl oz/yd3)

AEA (fl oz/yd3)

NA-PG Target 253 574 1733 1140 48.5 8.1 NA-CL Target 253 574 1916 1140 48.5 8.1

NA-PG-Wet Target 324 726 1249 1308 - 21.8

Note: 1 lb/yd3 = 0.5933 kg/m3; 1 fl oz/yd3 = 38.67 mL/m3

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NA replaced by RCA as given by Equations 1, 2, and 3 for the DWR, EMR, and DVR methods, respectively.

! = 1−!!"!"#/!!"

!"# (1) ! = 1− !!"!"#/!!"!"# (2) ! = 1− !!"!"#/!!"!"# (3)

where, !!"!"# =  weight of NA in DWR mix; !!"

!"# =  weight of NA in NAC mix; !!"!"# =  volume of NA in EMR mix; !!"!"# =  volume of NA in DVR mix; and !!"!"# = volume of NA in NAC mix. Sample RCA mix designs utilizing the DWR, EMR, and DVR methods are shown in Table 3, with the footnotes at the bottom of the table listing all of the remaining RCA mixes studied.

For the DVR and DWR methods, the water reducer was batched

proportionally to the amount of cement (per HRWR manufacturer’s specifications) so as to maintain a consistent composition for the fresh mortar. For the EMR mixes, this approach resulted in a decrease in water reducer (since the amount of cement decreases as ! increases in EMR mixes). To explore this effect, the maximum amount of water reducer recommended by the manufacturer was used in the EMR mixes based on the NA-CL target while the water reducer was varied proportionally to the amount of cement in the EMR mixes based on the NA-PG target.

Aggregate Preparation. Following dry sieving, the recycled and virgin natural coarse aggregates were washed with water over a No. 8 (2.36 mm) sieve to further

Table 3. Dry weight proportions of sample RCA-ND mix designs (with NA-PG target) Name !

(%) Water (kg/m3)

Cement (kg/m3)

NA (kg/m3)

RCA (kg/m3)

FA (kg/m3)

HRWR (mL/m3)

AEA (mL/m3)

DWR1,2

21 150 341 812 216 657 1876 313 41 150 341 607 421 640 1876 313 61 150 341 401 627 623 1876 313 81 150 341 195 833 606 1876 313

100 150 341 - 1028 590 1876 313

EMR3,4,5

4 142 322 987 110 640 1775 298 9 132 301 940 235 597 1659 279

14 122 276 887 380 549 1520 255 20 109 248 824 549 492 1365 228

DVR1,6,7,8

21 150 341 815 203 676 1876 313 41 150 341 606 404 676 1876 313 61 150 341 400 600 676 1876 313 81 150 341 198 787 676 1876 313

100 150 341 - 971 676 1876 313 Note: 1 lb/yd3 = 0.5933 kg/m3; 1 fl oz/yd3 = 38.67 mL/m3

1Similar mixes also made at  ! = 10, 30, 50, 70, and 90%. 2Mixes repeated with RCA-SB at  ! = 10, 20, 30, 40, 50, 60, 80, and 100%. 3Mixes repeated with RCA-SB at  ! = 5, 10, 15, and 20%. 4Using NA-CL target, similar mixes made with RCA-G at  ! = 20, 40%, and with RCA-RR at  ! = 20, 40, 60%. 5Using NA-PG-Wet target, similar mixes made with RCA-ND at  ! = 10, 20, 30, 36, 40, 50%. 6Mixes also repeated with RCA-SB at  ! = 10, 20, 40, 50, 60, 80, 100%. 7Using NA-CL target, similar mixes made with RCA-RR, RCA-R, RCA-W, RCA-P, RCA-GT, RCA-L, RCA-A, RCA-EL, RCA-EG, RCA-HP, RCA-S, RCA-T, RCA-E, RCA-G, RCA-ND1 at  ! = 20, 40, 60, 80, 100%. 8Using NA-PG-Wet target, similar mixes made with RCA-ND at  ! = 10, 20, 30, 36, 40, 50, 60, 70, 80, 90, 100%.

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remove excess fines. All coarse and fine aggregates were then dried in an oven at 110 °C (230 °F) for at least 24 hours. To ensure that the aggregates did not absorb mix water (in turn pulling water from the cement paste and leading to decreased workability and cement hydration), the material was removed from the oven, batched according to the dry weights in Tables 2 and 3, blended (as necessary) according to the gradations in Table 1, and then soaked in water for a period of 18 to 24 hours. After soaking, the excess water was decanted from the aggregates, which were then weighed to determine the amounts of absorbed and residual water. Using the absorption values from Table 1, the amount of residual water beyond the saturated surface dry (SSD) condition of the aggregates was subtracted from the required mix water for each concrete batch. Mixing. Following aggregate preparation, each concrete batch was mixed in a rotating drum mixer [with a wet material capacity of 0.02 m3 (0.80 ft3)] according to ASTM C 192 (ASTM 2009b). The fresh concrete was immediately removed from the mixer and placed in a non-absorbent metal pan for slump testing and cylinder molding. In removing the fresh concrete from the mixer, some mortar adhered to the inside of the drum. This mortar loss was accounted for by adding an estimated amount of mortar left inside the drum to the original mix design. To determine this estimated amount, three batches each were made for the NA-PG and NA-CL target mixes. The average amount of mortar loss for each set of three batches was determined by subtracting the total material removed from the mixer from the total weight of the material placed in the mixer. The amounts of fine aggregate, water, cementitious material, and liquid admixtures were adjusted proportionally to account for this loss in all subsequent mixes. Concrete Slump. To determine the workability of each mix, a variation of the ASTM C 143 (2009a) slump test was conducted. The ASTM test requires a slump cone with a concrete volume of 5663 cm3 (0.20 ft3). However, because of the relatively small amount of RCA available from each source, the volume of each batch was limited to 3398 cm3 (0.12 ft3) to make four 7.6 x 15.2 cm (3 x 6 in.) cylinders. To allow for slump measurement, the dimensions of the ASTM slump cone were scaled by half, thus reducing the required concrete volume to 721 cm3 (0.03 ft3). The correlation between the full slump cone and the scaled “mini slump” cone is described in Knaack and Kurama (2011) and is given by Equation 4 as:

!"##  !"#$% = 2.13 ∗ !"#"  !"#$% (4)

Curing. Following slump testing, four 7.6 x 15.2 cm (3 x 6 in.) cylinders were made from each batch. Three cylinders were used to determine the compressive strength and stiffness of the concrete, while the fourth cylinder was left unbroken for future reference and analysis. The cylinders were made using plastic disposable molds, which were capped and stored at room conditions for approximately one day. The day after casting, the cylinders were removed from the molds and were cured for 27 days in water at room temperature, bringing the total curing duration to 28 days. In accordance with ASTM C 511 (2009a), high-calcium hydrated lime (calcium

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hydroxide) was added to the curing water to prevent the leaching of lime from the concrete. Testing. After curing, the specimens were hand dried using paper towels and tested in a moist state according to ASTM C 39 (2009a). Each cylinder was capped using unbonded steel caps with rubber bearing pads to help maintain an even load distribution during testing. A servo-controlled hydraulic universal testing machine was used to apply a constant stress rate of 241 kPa/s (35 psi/s) and the concrete axial strain was measured using an Epsilon© 3542RA rock averaging extensometer with 5.1 cm gauge length. So as to not damage the extensometer, each test was briefly paused (for less than 10 s) to remove the sensor at a stress of 20.7 to 37.9 MPa (2.5 to 5.5 ksi), depending on the expected strength of the concrete, after which the loading continued at the same stress rate until specimen failure. The compressive strength was determined from the peak stress, !!" reached in each test, and the secant modulus, !!"# was determined from the slope between two points on the measured stress-strain curve, first point at a strain of !!! = 0.00005 and the second point second at a stress of !!! = 0.40!!". RESULTS

The results of the slump, compressive strength, and secant modulus of elasticity for the mixes shown in Tables 2 and 3 are discussed below. Slump. The DVR method should not result in a significant decrease in workability provided that similar aggregate gradations are used and the absorption of the aggregates is accounted for. Conversely, since the DWR and EMR methods decrease the fresh mortar to coarse material ratio, the workability of these mixes is expected to decrease. Figs. 1(a), 1(b), and 1(c) show the mini slump results from the DWR, EMR and DVR mix design methods. It can be seen that the DVR method provides similar workability to the target NA mix designs (cases with !=0%). In comparison, for large !, the DWR method tends to result in smaller slump values than the target mix design. The EMR mixes perform the worst and become unworkable at small amounts of replacement, with the slump reaching zero between !=14% and !=60%, depending upon the RCA source and the target mix design used.

(a)

(b)

(c)

Figure 1. RCA concrete mini slump: (a) DWR

mixes; (b) EMR mixes; and (c) DVR mixes

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As stated previously, for the EMR method, the proportion of HRWR to cement was held constant in the NA-PG mixes, but was increased in the NA-CL mixes to the maximum recommended by the HRWR manufacturer. The NA-PG-Wet mixes did not have any HRWR. While the increased HRWR in the NA-CL mixes and the increased fresh mortar in the NA-PG-Wet mixes resulted in increased slump, the DVR and DWR methods still resulted in significantly more workable concrete than the EMR method. Note that the mixes using RCA with higher residual mortar content (RM, see Table 1) tended to have less workability, because the EMR method dictates less fresh mortar for higher RM in the mix. It is concluded that in order to achieve high levels of replacement in EMR mixes, the target mix should have a relatively high fresh mortar to coarse material ratio. Further, the RCA should have low RM so that only a small reduction in fresh mortar is needed to achieve equivalent volumes of total mortar in the RCA and target mix designs. Compressive Strength. Fig. 2 shows the 144 average (from a minimum of 3 samples for each batch) 28-day compressive strength, !!", results for the RCA mixes designed based on the NA-PG, NA-PG-Wet, and NA-CL target mixes. The !!" data is based on a total of 602 cylinders designed using the DVR, DWR, and EMR methods. A significant distinction cannot be made between the DWR, DVR, and EMR mixes; and thus the replacement design method does not seem to be an important factor for strength. Depending on the RCA source, it is possible to produce concrete with a comparable strength as the target NA concrete. On average, the mixes using the NA-CL target mix show a decrease in strength with increased R, whereas the mixes using the NA-PG and NA-PG-Wet target mix tend to have a slight strength increase.

It is interesting to note that RCA-ND and RCA-ND1 did not result in the strongest RCA concrete even though these materials were obtained from a known source with known properties and little or no deleterious inclusions. The three aggregates that consistently produced the weakest concrete are: 1) RCA-RR, which had the highest deleterious material content, !!"# (see Table 1); and 2, 3) RCA-G and RCA-E, which had the highest absorption, !!"#. It is also important to point out that there is a significant amount of variability in the compressive strength results based on the source of the RCA material.

(a)

(b)

(c)

Figure 2. RCA concrete compressive strength: (a)

all target mixes; (b) NA-PG and NA-PG Wet target

mix; and (c) NA-CL target mix

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Modulus of Elasticity. Fig. 3 shows 111 average secant modulus, !!"# results (from a minimum of 3 samples for each case) for the RCA mixes designed based on the NA-PG, NA-PG-Wet, and NA-CL target mixes. Note that the number of data for !!"# is smaller than the number of data for !!" in Fig. 2 because an extensometer was not available for some of the tests. In general, the !!"# findings are consistent with those for !!" that the mix design method does not seem to have a large effect on the results. For mixes based on the NA-PG and NA-PG-Wet target mixes, there is no discernable change in !!"# as ! is increased; whereas, for mixes based on the NA-CL target, a significant decrease in !!"# is observed (the decrease is more than 30% at !=100%). One possible explanation for the above trends is the effect of aggregate water absorption on the elastic modulus of concrete. As shown in Table 1, the absorption of NA-PG (!!"=3.48%) is closer to the absorption of the RCA ( !!"# =4.33% to 8.94%) whereas NA-CL has a much lower absorption (!!" =0.74%). Since according to ASTM C 192 (2009a), all aggregates are added to the mixer in a fully saturated condition, the water stored in the aggregate pore space can migrate into the surrounding cement paste during hydration, thus resulting in a higher water-to-cement ratio (i.e., weaker cement paste) in the interfacial zone adjacent to the aggregate as compared to the bulk paste away from the aggregate. This interfacial transition zone (ITZ) surrounding the aggregate has long been considered the “weak link” (Maso 1996), and therefore it is possible that aggregates with higher absorption (i.e., increased porosity) can add to that weakness by locally increasing the w/c ratio.

Similar to the concrete strength in Fig. 2, RCA with larger absorption and deleterious content tend to result in smaller secant modulus in Fig. 3. However, there is much less variability in the secant modulus data than in the strength data. For example, for the DVR mixes based on the NA-CL target mix at !=100%, the coefficient of variation in !!"# is only 5.4% as compared with 9.8% for !!". One explanation for this difference may be that !!" is a failure property (which is a relatively unstable event for concrete) and therefore inherently experiences a significant amount of variability even between concrete cylinders of the same batch, whereas !!"# is a linear-elastic property and is measured during a stable portion of the stress-strain behavior.

(a)

(b)

(c)

Figure 3. RCA concrete elastic modulus: (a) all

target mixes; (b) NA-PG and NA-PG Wet Target

mix; and (c) NA-CL Target mix

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SUMMARY AND CONCLUSIONS This paper investigates the rheological and mechanical properties of concrete mixtures that use recycled concrete aggregates (RCA) as replacement for coarse virgin natural aggregates. Three mix design methods utilizing direct volume replacement (DVR), direct weight replacement (DWR), and equivalent mortar replacement (EMR) are compared based on concrete workability, compressive strength, and elastic modulus. The important conclusions from the study are given below. In interpreting these conclusions, it should be noted that the results are limited to the materials and mix designs tested.

1) The workability of fresh concrete is affected by the aggregate replacement method. The DVR method produced concrete with similar slump as the target natural aggregate concrete mixes. While the DWR mixes had somewhat decreased slump, the concrete was still easily placed and finished in the cylinder molds. In comparison, the decreased fresh mortar in the EMR mixes resulted in significantly reduced slump such that replacement ratios greater than R=60% were not possible even with the maximum amount of water reducer.

2) The compressive strength, !!" and elastic modulus, !!"# of concrete do not seem to be significantly affected by the mix design method.

3) Generally, RCA with larger water absorption, !!"# and deleterious material content, !!"# tend to result in smaller !!" and !!"#. The use of RCA has a greater effect on !!"# than on !!". However, there is much less variability in !!"# than in !!".

ACKNOWLEDGMENTS The authors thank Danny Atkinson of Concrete Recycling Center and Mark Zeltwanger of American Mobile Aggregate Crushing for their help in acquiring some of the RCA. Additional materials were provided by ACT Recycling, Aggregate Industries, Buzzi Unicem, Evans Landscaping, Great Lakes Aggregates, Green Tech Transfer and Recycling, Lindahl Brothers, INC., Reliable Asphalt Corporation, R & R Excavating, Sika Corporation, Transit Mix South Bend, Vulcan Materials Company, and Walsh Construction Company. The authors acknowledge Dave Schelling of the LaPorte district INDOT office for his help in conducting the LA Abrasion testing. Any opinions, findings, conclusions, and/or recommendations in the paper are those of the authors and do not necessarily represent the views of the individuals or organizations above.

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