i

Aggregate Source Effects on RCC Green PropertiesJordan Ouellet, Ph.D. Student, University of Illinois, Urbana-Champaign, Illinois, USA*Gail M. Scott, M.S. Student, University of Illinois, Urbana-Champaign, Illinois, USAJeffery R. Roesler, Professor, University of Illinois, Urbana-Champaign, Illinois, USA

*Corresponding author: jordano2@illinois.edu

KEYWORDS: Roller-Compacted Concrete, Aggregates, Green Properties, Volumetrics

Conflict of Interest: None

Word Count: 4,250 + 2,500 (10 figures) = 6,750

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1. ABSTRACT

Interest in roller-compacted concrete (RCC) pavement has been increasing because of its low initial cost, construction efficiency, and ability to open to traffic early. Current RCC mixture design methods do not directly consider the aggregate source or optimal cement content. An experimental testing plan was developed to batch RCC mixtures with several aggregate sources, gradations, paste contents, and water-cement (w/c) ratios. Each aggregate blend was compacted in order to determine the intergranular volume of voids. The volume of cement paste was varied to underfill, equifill, and overfill the compacted aggregate voids. Green properties (green strength, green modulus, and softening modulus) were measured on the lab compacted specimens to assess the fresh RCC capacity, stability, and plasticity. Green and hardened properties were then related to the RCC mixture volumetric parameters. Green properties were sensitive to aggregate type and gradation, ratio of voids filled by paste, w/c ratio, and total paste content, which all affected the mixes adhesion and shear resistance. In most cases, high cement contents did not improve green or hardened properties. RCC mixtures containing crushed aggregates achieved maximum green strength and stability with underfilled voids, while RCC with rounded aggregates required overfilling the voids. Workability (Vebe time) measurements were not sensitive unless voids were overfilled and not highly sensitive to w/c ratio.

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2. INTRODUCTION

Multiple laboratory studies have demonstrated that RCC strengths for pavements can be successfully achieved with a wide variety of mixture constituents and proportions [1, 2]. However, this does not guarantee that a particular RCC mixture will provide the necessary fresh properties for mixing, transporting, and constructing the pavement at this lower cement content. The aggregate source and physical properties are critical components to RCC pavement compactability, stability, roll-down, and edge support during construction [1, 3, 4]. Existing fresh property tests such as the Vebe time are not adequate to quantify the mixture’s fresh state for all relevant RCC pavement construction goals. For example, improper consideration of paste content with respect to the aggregate structure can lead to excessive roll-down, poor edge support, inadequate compaction, or poor surface characteristics [2]. RCC mixture proportioning and fresh properties can also affect the pavement opening time after construction [5, 6]. Extrusion and direct shear parameters have been shown to be positive indicators of zero slump cementitious materials [7, 8]. Additional tests are needed to assess the compacted RCC fresh capacity, stability and plasticity, referred herein as “green” properties. RCC in its fresh state behaves closer to asphalt concrete or cohesive granular soil [5] with adhesive forces and internal friction between the particles dominating [9].

Since aggregates make up approximately 85% of the RCC mixture volume, they are a significant factor in mixture proportioning and field construction performance. In existing RCC mixture designs, aggregates in acceptable gradation limits are proportioned based on meeting bulk RCC properties such as wet density and compressive strength. However, the aggregate type, shape, and texture and its influence on the required volume of paste and properties needed during construction are not directly integrated into the mixture design process.

The two most common RCC mix design approaches are the concrete consistency method (USACE procedure [10]) and the soil compaction method [11], popularized as the ACI procedure [12] or the modified Proctor density-testing procedure [13]. The gyratory compactor has also been used in multiple studies to evaluate RCC compactability [1, 2, 14, 15], but there has not been a direct connection between RCC mixture volumetrics and field construction performance other than acceptable Vebe times. Several other theoretical approaches for RCC mixture design methods with limited application include the Solid Suspension Model [16] and the Optimal Paste Volume Method [17]. The volumetric mix-design approach proposed by Marchand, et al. [17] selects the paste volume based on filling the aggregate void structure. This was motivated by an earlier study linking aggregate void structure with RCC compaction performance [18]. Gagné [19] later correlated the Vebe time to the degree to which the voids are overfilled with paste, but did not study other void filling states such as underfilled or equifilled.

A laboratory investigation was performed to characterize the RCC’s freshy compacted capacity, stability, and plasticity, called green properties, using different aggregate sources and gradations, as well as different paste volumes and water-cement ratios. The mixture design constituents and proportions allows for measuring RCC green properties in the underfilled, equifilled, and overfilled states.

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3. RESEARCH SIGNIFICANCE

RCC pavement mixtures use a variety of local aggregate sources, particle size distributions, and qualities. The most widely accepted RCC mixture design approaches do not directly integrate the aggregate sources and properties with the required cement and water content. Therefore, a mechanistic RCC mixture design approach that selects aggregate and cement proportions to achieve desired fresh properties and the design strength can significantly improve the RCC mixture design process, field construction performance, and minimize the initial cost.

4. RCC MIXTURE DESIGN AND TESTING

The goal of this laboratory study was to identify the key RCC mixture volumetrics that impact fresh properties for several aggregate sources and gradations. The different RCC volumetric constituents are defined in Figure 1. The intergranular voids represent the voids produced from efficient aggregate packing for a given compaction method and energy. When cement paste is added to blended aggregates, these intergranular voids may be underfilled, equifilled, or overfilled. Extragranular voids are generated when RCC compaction is inadequate or following aggregate unpacking by overfilling the intergranular voids. The ratio at which the intergranular voids are filled with paste is defined as VFP. When the VFP is greater than 100%, the aggregates must unpack to accommodate the additional paste volume, while VFP less than 100% means voids still exist after adequate compaction.

Figure 1 – RCC phase diagram for constituents with aggregate-paste relationships

Aggregate Pores

Extragranular Voids

Intergranular Voids

Aggregates

Minimal Volume

Compact

Pores

Agg.

Pores

Agg.

Pores

Agg.

Pores

Agg.

Inter. Voids

Inter. Voids

CompactedAgg. Blend

OverfilledVFP >100%

EquifilledVFP =100%

UnderfilledVFP <100%

Cement

Water

Unpacking

WaterCement

Water

Cement

4

To achieve the study goal, eight different mixtures were created to vary the RCC mixture volumetrics as detailed in Table 1. Several fine, intermediate and coarse aggregate sources were acquired to obtain different aggregate angularities, i.e., crushed and naturally round particles. A combination of both round and crushed angularities (hybrid) produced another aggregate structure. A dense gradation was selected for most of the RCC mixtures with the intergranular void content a result of the aggregate angularity and particle packing efficiency. One mixture was batched with a gap-graded aggregate combination to accommodate more total paste volume. In addition to these 8 distinct mixtures, the w/c and cement content were also varied to determine their sensitivity on the RCC’s green properties.

Table 1 - Summary of RCC Mixture Design CasesMixCase

Aggregate Source

Gradation Type Target w/c Cement Content

% weight Oven Dry Aggregate. Admixture

A Hybrid1 Dense-Graded 0.28, 0.41, 0.44, 0.47, 0.55 13.0 None

B Hybrid1 Dense-Graded 0.50 5.5, 7.0, 8.5, 10.0, 11.8, 13.6, 15.5 NoneBB Hybrid1 Dense-Graded 0.35 9.0, 10.8, 12.8, 13.8, 18.8 NoneC Hybrid1 Dense-Graded 0.35 8.5, 10.8, 13.2, 15.4, 16.8, 19.4, 19.9 RMA2

D Crushed Dense-Graded 0.20 12.5, 13.5, 15.1, 20.5 NoneDD Crushed Dense-Graded 0.35 5.5, 8.5, 12.6, 17.4 NoneE Round Dense-Graded 0.35 6.2, 8.0, 9.5, 10.1, 12.8, 17.0 NoneF Hybrid1 Gap-Graded 0.35 11.0, 25.5 None

1Coarse and intermediate crushed particles with natural (round) sand 2RMA: Rheology-Modifying Admixture

Aggregate Source, Gradation and Void Content

Crushed and naturally round aggregate sources were used to produce five final aggregate blends presented in Table 2. The crushed aggregates were dolomite including the manufactured sand (fines). The natural blend incorporated coarse and intermediate round river gravel with a natural (round) sand. The hybrid mixture contained coarse and intermediate crushed dolomite with natural sand. The aggregate size distribution of the mixes is presented in Table 2. The coarse and intermediate particle sizes for the dense-graded mixtures followed the power-0.45 curve [19, 20]. The fine gradation was selected to be below power-0.45 for workability purposes [2]. The gap-graded mixture had similar coarse and fine gradations to the dense graded mixtures but with the intermediate size removed. The aggregate blends were compacted with a vibratory hammer at 5.5% moisture content as in ASTM C1435 [21] and then the intergranular voids were estimated. As seen in Eq. (1), the dry density obtained in the lab was compared to the theoretical maximum (solid) density of the blend to estimate the compacted aggregate void content. As expected, different intergranular void contents were obtained for the dense-graded and open-graded mixtures as shown in Table 2. The voids varied slightly for dense-graded mixtures, depending on the aggregate shape and angularity and gradation variation. All aggregates were batched and mixed at stock moisture condition.

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IntergranularVoid Ratio (% )=(1−OD Packed Density ( g

cm3 )OD Maximum Solid Blend Density ( g

cm3 ))×100 (1)

Table 2 – RCC aggregate blends and proportions

Sieve Testing Program Mix CaseA&B BB&C D&DD E F

Opening (mm)

Dense-Graded Hybrid

Dense-Graded Hybrid

Dense-Graded Crushed

Dense-Graded Natural

Gap-GradedHybrid

25.400 100.0 100.0 100.0 100.0 100.019.000 94.8 95.3 94.9 89.8 86.012.700 83.8 82.2 81.4 81.2 49.49.500 79.0 76.0 75.7 76.8 32.64.750 53.8 53.4 62.0 51.0 17.62.400 40.0 41.3 41.0 38.8 13.42.000 - 39.4 35.4 37.2 12.91.200 32.6 31.6 22.7 32.2 10.90.600 24.5 18.2 12.5 20.3 7.90.300 9.1 5.0 7.6 5.5 4.40.150 2.0 2.3 5.7 1.9 3.30.075 1.3 1.6 4.0 1.2 2.5

Fraction Weight PercentageCoarse 25.0 33.3 33.6 18.0 90.0

Intermediate 30.0 25.4 10.0 42.3 0.0Fines 45.0 41.3 56.4 39.7 10.0

Volumetric Intergranular Void PercentageVoids 18.4 18.4 19.1 13.9 26.6

Volume of Paste and Cement Content

To evaluate the effect of the aggregate type and gradation on RCC fresh properties, the volume of paste was varied for each mixture case in Table 1. Case A was defined as the RCC control mixture for this study with cement content of 13.0% by mass of the dry aggregates. The optimal moisture content (OMC) of 5.5% was determined per ASTM C1435. This OMC was then used as the reference w/c ratio of 0.35 for subsequent RCC mixture cases in the aggregate stock condition. The percentage of intergranular voids filled by paste (VFP) was computed in the stock water condition as shown in Eq. (2). The sensitivity of RCC fresh and hardened properties were assessed by varying the w/c ratio for the hybrid and crushed mixture cases in Table 1 at a fixed paste volume. The cement content was selected to vary the amount of paste in the intergranular voids from underfilling (VFP<100%) to overfilling as shown in Table 1.

Voids Filled by Paste (VFP ,% )=( Volumetric PasteContent ∈Stock water condition (% )Intragranular Void Ratio ( %) )× 100(2)

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7

RCC Specimen Preparation and Tests

A pan mixer was used for mixing the proportioned RCC constituents. Immediately after mixing, a 13.5 kg RCC sample was subjected to a consistency evaluation using the Vebe apparatus in conformance with ASTM C1170, Procedure A [22]. The wet density of each compacted RCC sample was also measured for yield and the air content was subsequently calculated based on the mixture volumetrics.

RCC specimens were compacted with a mechanical hammer into 4 in. x 8 in. (102 mm x 203 mm) cylindrical molds using a modified version of ASTM C1435 [21], as recommended by [2, 3, 23]. Samples were compacted in 3 uniform lifts for 20 sec/lift or until a ring of paste was formed around the tamper plate (98 mm diameter). The maximum compaction energy deployed per sample with the procedure was estimated at 20 MJ/m3 [23], whereas the Modified Proctor test only imparts 2.7 MJ/m3 to each sample per ASTM D1557 [24].

The compacted RCC samples for compressive strength testing were covered for the first 24 hours and then demolded and placed into a moist curing room until final testing. The 7-day compression testing was performed on triplicate samples per ASTM C39 [25]. Load-deflection tests were performed on unconfined, triplicate specimens (102 mm x 203 mm) in the fresh compacted or “green” state with a 10 kN load frame and deformation rate of 2 mm/minute (see Figure 2). Neoprene caps were used to distribute the force evenly on the sample surface during testing. Green properties, defined here as green strength, green modulus, and softening modulus, are calculated from the load-deformation response of each sample and described the RCC mechanical performance in the fresh state. These green properties are unconfined indicators of the RCC’s capacity, stability, and plasticity in the fresh state, respectively.

(a) (b)Figure 2 – Example of stress-strain curve of unconfined RCC sample in fresh state for specimens with

(a) lower and (b) higher green properties

Green Strength

Softening Modulus

Softening Modulus

Green Strength

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5. EXPERIMENTAL RESULTS AND DISCUSSION

Green Strength

Green strength is defined as the unconfined compressive strength of freshly compacted RCC, in undrained testing conditions. The green strength of RCC is expected to behave like a cohesive granular material. As seen in Figure 3a, the magnitude of RCC green strength is comparable to a very stiff soil and it is sensitive to cement content. The peak green strength was achieved between 10% to 15% cement content as seen in Figure 3a, depending on the particular aggregate type, aggregate gradation, paste content, w/c ratio and presence of rheology modifying admixture. The highest green strength of all RCC mixtures is 270 kPa for Case D with crushed aggregates and w/c of 0.20. For Case D, the green strength also decreased 70% for a 6% increase in cement. Although the aggregates were different between the natural (E) and crushed (DD) mixtures, the peak green strength was approximately the same at 0.15 MPa. For the hybrid aggregate combination, the cement content required to reach peak green strength increased by 5% (B vs. C) with the addition of a rheology-modifying admixture (RMA).A reduction in the w/c from 0.35 to 0.20 for crushed aggregate combination (D vs. DD) increased the peak green strength from 0.16 to 0.27 MPa. Alternatively, the hybrid mixtures (B vs. BB) did not significantly impact the peak green strength when changing w/c from 0.35 to 0.50, but changed the cement content at which the peak green strength occurred.By plotting VFP instead of cement content, the behavior of aggregate blends on RCC green strength can be ascertained. As shown in Figure 3b, the peak green strength for a given aggregate source maximized at a similar VFP. Crushed aggregate mixtures (D, DD) had peak green strength with underfilled aggregate voids (VFP 72-80%), hybrid mixtures (B, BB) had peak green strength closer to equifilled voids (VFP 90-94%), and peak green strength occurred for natural aggregate mixture (E) with overfilled voids (VFP 130%). The RMA case C tested on the hybrid reference mixture (BB) reduced the peak green strength by 40% at a VFP of 96%.

5% 7% 9% 11% 13% 15% 17% 19% 21%0.00

0.05

0.10

0.15

0.20

0.25

0.30

Cement Content (% wt. OD Aggregates)

Gre

en S

tren

gth,

MPa

40% 60% 80% 100% 120% 140% 160% 180%0.00

0.05

0.10

0.15

0.20

0.25

0.30

VFP at Stock Moisture, (% total vol.)

Gre

en S

tren

gth,

MPa

(a) (b)Figure 3 – RCC green strength as a function of (a) cement content and (b) percent voids filled with paste

(VFP)Both aggregate shear resistance and paste adhesion contribute to RCC green strength by providing and maintaining interlocking respectively [26]. The crushed aggregate mixtures (D,

9

DD) had a lower green strength when overfilled as compared to the hybrid mixtures with natural sand (B, BB, C). Overfilling voids in crushed mixtures is reducing interparticle friction by aggregate unpacking while severe underfilling resulted in insufficient paste to stabilize the interlocked particles. For natural aggregate mixtures, underfilling exposed the lower shear resistance of the smooth, round particles. In general, the green strength of RCC with overfilled voids is controlled by the paste adhesion while aggregate shear strength controls underfilled RCC systems. The gap-graded structure (F) in Figure 3 was not able to develop any green strength given the high intergranular void content (26.6% vs. 18.4%) that prevented sufficient interparticle friction when underfilled. When the gap-graded mix (F) was overfilled, the larger void structure prevented proper adhesion by reducing surface tension between the particles [9].

Green Modulus and Softening Modulus

To further explore the RCC fresh behavior to the mixture proportions, green strength alone was not sufficient as it only represents the peak capacity in the fresh, unconfined state. The green modulus represents the pre-peak stress-strain response of RCC in its fresh state, measured as a chord modulus. The chord is defined as the stress/strain between 50% and 90% of the peak strength as seen in Figure 2. A low green modulus indicates higher deformation under the same stress level, which may indicate instability and high roll-down in the field. Whereas, a high (negative) post-peak softening modulus represents a low plasticity with rapid stress reduction as the RCC material shears, dilates, and collapses, which may indicate unstable and collapsing edges in the field. The softening modulus was defined in Figure 2 as the stress/strain between 90% and 50% of the post-peak strength. Plasticity in the field is important for proper pavement edge compaction given its challenge and potential limitation of quality RCC construction [27].Figure 4a shows the green modulus with respect to VFP. Similar to green strength behavior, crushed mixtures (D, DD) had the greatest green modulus when underfilled (VFP 80%), hybrid mixtures (B, BB, C) peaked when close to equifilled voids (VFP 95%), and natural mixture (E) had largest green modulus when overfilled voids (VFP 130%). The peak green modulus of the crushed mixture decreased by more than 50% when the w/c increased from 0.20 to 0.35. The rheology-modifying admixture (C vs. BB) reduced the peak green modulus by 40%. The gap graded mixture (F) did not achieve sufficient stability to even support its own weight. The greater the green modulus of the RCC mixture, the higher its stability in the fresh state.Figure 4b shows the green softening modulus with respect to VFP. A less negative softening modulus means higher plasticity of the RCC mixture. For all RCC mixtures, a higher VFP or total paste content lead to a less negative softening modulus or greater plasticity. Modifying the rheology of the hybrid reference mixture (BB vs. C) significantly increased the plasticity, especially when the aggregate voids were underfilled (more adhesion). Crushed mixtures (D, DD) led to a rapid mixture collapse when VFP was less than 90%. Rounded mixture (E) maintained highest softening modulus in the overfilled state. Overall, RCC mixtures had lower green softening moduli (higher plasticity) with increased paste content and higher plastic deformations in the overfilled state with low interparticle friction.

10

40% 60% 80% 100% 120% 140% 160% 180%0

5

10

15

20

25

VFP at Stock Moisture, (% total vol.)

Gre

en M

odul

us, M

Pa

40% 60% 80% 100% 120% 140% 160% 180%

-60

-50

-40

-30

-20

-10

0

VFP at Stock Moisture, (% total vol.)

Softe

ning

Mod

ulus

, MPa

(a) (b)Figure 4 - (a) Green modulus and (b) softening modulus as a function of VFP for RCC tested in

compression in the fresh, unconfined state.

Workability and Air Content

RCC mixtures must have the proper workability in the field to compact and consolidate the RCC pavement [28] without significant roll-down or too much compaction energy expended. The most accepted standard to assess RCC consolidation potential is the Vebe test [22]. A consolidation time in the lab ranging between 20 to 75 seconds corresponds empirically to an acceptable RCC mixture in the field [29]. A lower Vebe time implies reaching the maximum density for less compactive effort, where the paste starts to be flow out of the surface faster. The Vebe test is known to be a highly subjective and prone to large variability [30]. Within a mix design approach, a satisfactory Vebe time can be obtained for many different material configurations and does not imply the minimum possible paste content is reached.

Figure 5a demonstrates that the aggregate type and gradation influences the Vebe time for a given amount of paste. In order to achieve a Vebe time of 60 seconds, the RCC natural aggregate mix (E) required 19% paste and the crushed aggregate mix (DD) required 22.5%. The hybrid aggregate mixes (B, BB) consolidated faster than the crushed mixtures (D, DD). The shape and angularity of the particles influenced the consolidation performance with the RCC mixtures with rounded particles (B, BB, E) requiring less paste for the same Vebe time. The addition of the RMA (Case C) decreased the consolidation performance as measured by Vebe test because it increases the viscosity of the paste. An alternative presentation of the Vebe times for the various RCC mixtures as a function of VFP are shown in Figure 5b. Underfilling the aggregate voids for any RCC mixture did not produce a ring of paste even at 120 seconds for any of the aggregate combinations. Therefore, the Vebe is only able to quantify the consistency of RCC mixtures in the overfilled state. For a fixed Vebe time of 60 seconds, most aggregate structures required 108% to 120% VFP. The natural aggregate mixture required 136% VFP to reach the same Vebe time. The Vebe is useful as a field quality control tool to check consistency of a given RCC mixture from batch to batch. Mix design methods using Vebe may force designers to increase the paste content to reach requirements, whereas reducing the intergranular voids may be more efficient to reach the overfilled state.

11

15% 20% 25% 30% 35% 40%0

20

40

60

80

100

120

Paste Content at Stock Moisture (% total vol.)

Vebe

Tim

e, s (

ASTM

C11

70 A

)

80% 90% 100%110%120%130%140%150%160%170%180%0

20

40

60

80

100

120

VFP at Stock Moisture, (% total vol.)

Vebe

Tim

e, s (

ASTM

C11

70 A

)

(a) (b)Figure 5 - Vebe Results up to 120 sec with respect to (a) cement content and (b) VFP

For each RCC mixture, the gravimetric air content was backcalculated from density measurements at a fixed compactive energy in stock moisture condition. Gravimetric air content is calculated based on the measured wet density with known mixture proportions and physical properties. Slightly negative values can be theoretically obtained in stock condition when moisture content of the aggregates differs from the estimated value due to absorption rate of free water. Reporting volumetric proportions and air content at stock moisture condition best represents the fresh tested state, compared to oven-dry (OD) or saturated-surface-dry (SSD) being a theoretical condition. As the VFP increased in Figure 6, the air content decreased. Hybrid (B, BB) mixtures started approaching zero air content at a VFP around 100%. All mixtures reached an air content below 5.0% when equifilled, except for the use of an admixture (C) and for low w/c crushed mixture (D). The use of an admixture (C vs. BB) led to higher air content, most likely due to the modified paste rheology that entrained more air than reduced compactability [31]. The reduced w/c ratio for the crushed mixture (D vs. DD) likely reduced the compactability and consequently the density.

40% 60% 80% 100% 120% 140% 160% 180%-1%

1%

3%

5%

7%

9%

11%

13%

15%

VFP at Stock Moisture, (% total vol.)

Air C

onte

nt a

t Sto

ck M

oist

ure,

% v

ol. T

ot.

Figure 6 - Gravimetric air content with respect to VFP

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Compressive Strength

The strength is the main hardened property used to design RCC pavements [32] and a minimum compressive strength of 24 MPa and 31 MPa is typically specified at 7 and 28 days, respectively. As shown in Figure 7a, all of the RCC mixtures meet the 7-day strength requirements with greater than 12% cement and meet the 28-day strength at 7-days with 13% cement content except for the use of an RMA Case C. When the compressive strength is plotted versus VFP (see Figure 7b), a trend with aggregate type is seen for the various RCC mixtures. The maximum compressive strength is achieved in the overfilled state where lower air content is reached. Crushed and hybrid mixtures are much stronger than the natural aggregate mixtures in the underfilled state, where high interparticle friction is higher. The compressive strength plateaus at different VFP in the overfilled state. More paste cannot increase the strength to compensate for the unpacking of the interlocked aggregate structure. For maximum compressive strength, RCC mixtures should have at least VFP of 100% with the aggregate source gradation controlling the magnitude of the strength. The typical 28-day minimum compressive strength is reached for all mixtures at 7-days, even when underfilling the voids with the exception of the natural mixture (E) that only met the 7-day specification when equifilled. Thus, overfilling the voids to reach peak strength is not required for obtaining recommended minimal compressive strengths.

5% 10% 15% 20% 25% 30%0

10

20

30

40

50

60

70

Cement Content (% wt. OD Aggregates)

7-d

UCS,

MPa

40% 60% 80% 100% 120% 140% 160% 180%0

10

20

30

40

50

60

70

VFP at Stock Moisture, (% total vol.)

7-d

UCS,

MPa

(a) (b)Figure 7 - 7-day compressive strength with respect to (a) cement content and (b) VFP

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6. CONCLUSIONS

Common RCC mixture design methods are experience based and extensive trial and error is necessary to reach optimal constituent selection and proportion. Furthermore, existing methods do not directly consider the effect of aggregate type, gradation, and cement content on the desired fresh properties for construction but primarily link performance to consistency (Vebe) and compressive strength. In this study, rounded, crushed, and a combination of the two (hybrid) aggregates were used to cast RCC specimens. For each aggregate combination, the paste content was varied to underfill, equifill, and overfill the compacted aggregate void structure (VFP). The water-cement ratio was also varied and a rheology-modifying admixture (RMA) was employed. The testing objective was to determine how mixture volumetrics impacted fresh and hardened properties.

Experimental program demonstrated that RCC green properties (capacity, stability, plasticity) can be measured easily in the lab on unconfined, freshly compacted RCC specimens. Green properties can be related to field performance in terms of roll-down, edge support and early traffic capacity. Results showed that RCC green and hardened performance is highly influenced by volumetric properties for different aggregate sources that could be considered within a mechanistic-oriented mix design approach. The aggregate type and the intergranular void content of the compacted aggregate structure affected the green and hardened properties primarily as a function of the VFP. The VFP state (i.e., underfilled, equifilled or overfilled) influenced the contribution of adhesion and interparticle friction, the two mechanisms governing workability, green properties, and strength. The optimal VFP state to achieve maximum green strength, green modulus, and softening modulus depended on the aggregate type.

RCC mixtures with rounded aggregate produced the best results (peak green strength, green modulus, and softening modulus) when overfilled. Crushed aggregates in RCC produced peak green strength and modulus when the aggregate void structure was underfilled. The hybrid mixture had peak green strength and modulus near the equifilled state. Generally, an increase in paste content above a certain VFP for any aggregate source did not further improve the green or hardened properties. Compressive strengths peaked for all mixtures in the overfilled state, however, additional cement after that did not change the strength. Acceptable 7-day compressive strengths were reached in the underfilled or equifilled state for all mixtures. Satisfactory Vebe times were reached for all RCC mixtures only in the overfilled state.

7. ACKNOWLEDGEMENTS

The authors would like to thank the Roller-Compacted Concrete Pavement Council for partial support of this study, Yixuan Wen and Dr. Sushobhan Sen for assistance, and finally, Open Road Paving Company, Prairie Materials, and DOW Chemicals for donation of materials.

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