Typical strength of asphalt mixtures compacted by gyratory compactor

Dina A. Rasool; Miami M. Hilal; Mohammed Y. Fattah

doi:10.1515/jmbm-2022-0023

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Typical strength of asphalt mixtures compacted by gyratory compactor

Dina A. Rasool , Miami M. Hilal und Mohammed Y. Fattah

Veröffentlicht/Copyright: 30. Mai 2022

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Aus der Zeitschrift Journal of the Mechanical Behavior of Materials Band 31 Heft 1

Abstract

Design of asphalt mixes and quality testing is influenced by the laboratory compaction procedure. Laboratory specimens must be manufactured in a way that suitably resembles field compaction for a performance test to give reliable mechanical properties. The internal structure of the mixture, which is referred to in this article as the spread of aggregate and air voids, provides the basis for the simulation. Gyratory compaction uses a kneading effort to produce cylindrical specimens. The goal of the present article is to determine the required strength for asphalt mix compaction of 40–50 per surface. This look was achieved with three distinct types of filler material and two kinds of sand. The asphalt mixture’s compressibility was tested. By adding the cumulative energy expended during gyratory specimen compaction to the compression data, the force applied to the sample during gyrations may be calculated. The relation between the number of gyrations and forces demonstrates pressure resistance. The measured fore force vs number of gyrations for combinations containing limestone, cement, and fly ash as fill materials was presented. The force required to compact the six bituminous materials was found to be influenced by the filler content. As the number of gyrations increases, its compaction properties change until it achieves a steady state. Except for the asphalt–cement mixture, compaction strength in mixes containing river sand requires less strength than compaction strength in mixes containing crushed sand.

Keywords: asphalt; gyratory compactor; HMA; load; strength

1 Introduction

One of the most common procedures used in constructing highways is the compaction of asphalt mixes. Compaction reduces mix air gaps, allows aggregates to interlocking and lowering the thickness of the (hot mix asphalt (HMA)) matting according to the specification thickness [1,2]. Aggregate grading, type, shape, roughness, asphalt content, and grading are all elements that affect mix compaction. Gyratory compaction is one technique for determining mix compactability. The current approach to the asphalt design mix is to use the gyratory compactor to create cylindrical HMA samples (superpave gyratory compactor (SGC)). The compaction resistance of a mix can be in expressed terms of measurement of height, the number gyrations is used to determine its workability and compactability. In the lab, difficult-to-compact mixtures should behave the same way in the field. For the contractor to obtain acceptable density, these mixes would demand greater compaction effort and time. Workability is determined by the ease with which mixed components could be blended by construction equipment. Workability suffers dramatically as the asphalt modifiers begin to elevate their viscosity at lower temperatures. Under stress, the mix’s stability as well as resistance to densification are indicators of compactability. Densification occurs when the air voids decrease substantially towards zero and the density of the mix reaches its maximum limit. In laydown operations, compaction of the pavement is a critical responsibility. Premature failure can happen if the job is not done correctly. Stiff mixtures have a hard time compacting, as a result of breaking and deboning at the interface. Mixes that have compaction issues in the lab are more likely to have compaction issues on the job. The SGC applies force to specimen during compaction to apply vertical pressure and gyration angle. Height loss caused by such forces can be used to determine a mix’s compactability. There are two techniques for employing gyratory compactors for assessing mixed compactability that have been documented in the literature. The first is to analyze the shear stress of the mix undergoing compaction using traditional static equilibrium modeling as investigated by many researchers, e.g., [3–7]. Using this method, shear strength was used to study the stability of mixes underneath load as resistance to distress, including permanent deformation. Second strategy is to build compaction indices using SGC data [8–14]. According to common understanding, a good mix should be easy to compact during the construction phase and endure deformation adequately during the transportation loading cycle. As a result, at both these stages, compaction quality must be evaluated. Studying the compaction curve revealed the compaction properties of regular sample molding. Fattah et al. [15] employed the SGC to construct force indices to evaluate the workability and compactability of mixtures during the normal mixture design phase and before laydown operations.

2 Work in progress

Asphalt binder (40–50) penetration grade, the aggregate of various gradations, and three kinds of mineral fillers: limestone, fly ash, and Portland cement were used. The asphalt binders were sourced from the Al Daurah oilfield in Baghdad. The crushed quartz utilized in the laboratory study came from the Al-Nibaie quarry in Iraq, which is located north of Baghdad. Routine tests, including sieving, shape, specific gravity, flat and elongated, toughness (Los Angeles abrasion), and soundness, were used to assess the physical qualities. These tests follow the following specifications: ASTM C127 (ASTM 2015a), ASTM C131 (ASTM 2014a), ASTM C88 (ASTM 2018), ASTM D4791 (ASTM2010), ASTM C128 (ASTM2015b), ASTMD2419 (ASTM 2014b), and ASTM C142 (ASTM 2017). Six combinations of these materials were produced and piled in the presence of a load cell, with the force required to compact each mix being calculated. The gradation, bitumen grade, and content of the mixes, as well as aggregate size, temperatures, and compaction type, all influence the mix’s compaction. These considerations will be taken into account while determining compaction indices in this investigation. Physical parameters of coarse aggregates are shown in Tables 1–3. Cement, limestone dust, and fly ash have different physical characteristics, as shown in Tables 4–6, respectively.

Table 1

Physical characteristics of coarse aggregate

Test	ASTM designation	Specification	Results
Bulk specific gravity	C-127	—	2.624
Apparent specific gravity	C-127	—	2.680
Water absorption (%)	C-127	—	0.48
Percent wear (Los Angeles abrasion)	C-535	Max. 30%	20.5
Soundness loss by sodium sulfate solution (%)	C-88	Max. 12%	4.1%
Flat and elongation
(1) Flat	D-4791	Max 10%	1%
(2) Elongation	D-4791	Max 10%	4%

Table 2

Physical characteristics of fine aggregate crushed sand

Test	ASTM designation	Specification	Results
Bulk specific gravity	C-128	—	2.639
Apparent specific gravity	C-128	—	2.699
Water absorption (%)	C-128	—	0.720
Sand equivalent (%)	D-2419	Min 45%	72%

Table 3

Physical characteristics of fine aggregate river sand

Test	ASTM designation	Specification	Results
Bulk specific gravity	C-128	—	2.660
Apparent specific gravity	C-128	—	2.782
Water absorption (%)	C-128	—	0.720

Table 4

Physical characteristics of Portland cement

Properties	Result
No. 200 sieve (percentage passing) (0.075 mm)	97%
Specific gravity	3.12

Table 5

Physical characteristics of limestone dust

Properties	Result
No. 200 sieve (percentage passing) (0.075 mm)	96%
Specific gravity	2.92
Plasticity index	N.P

Table 6

Physical characteristics of fly ash

Properties	Result
No. 200 sieve (percentage passing) (0.075 mm)	96%
Specific gravity	2.5

3 Choosing the best asphalt content

The ideal asphalt binder content could be determined by assessing the performance of each sample blend. As a result, the test blends must have a range of asphalt content across each optimum asphalt value. As a result, determining the optimal asphalt composition is the first step of sample preparation. The asphalt content of the trial blend is then calculated based on this estimate. The samples are compressed using a Marshall hammer to apply the apparatus number of blows to every face based on the projected traffic load. After the samples have cooled, remove them from the mold. The thickness and bulk specific gravity of the specimens were measured after they had reached room temperature. For mixtures containing (mid-value of prescribed limits) 7% Portland cement (by weight of the total aggregate) and five different bitumen contents of 40–50 grade from 4 to 6% (by weight of the total mix) with increased periods (0.5) percent, a series of test results for Marshall stability, flow, and density-void analysis are carried out. For each mix variable, three specimens are made and tested. Figure 1 shows the results of Marshall test determined using the best asphalt content. It is discovered that it makes up 5% of the total mixture.

Figure 1

Results of Marshall test. (a) stability vs. asphalt content, (b) unit weight vs. asphalt content, (c) air voids vs. asphalt content, (d) flow vs. asphalt content, (e) VMA (void mineral aggregate) vs. asphalt content, (f) VFA (void filler aggregate) vs. asphalt content.

4 Modification of gyratory compactor

During compaction, the gyratory compactor actuators provide forces to the specimen in the case of applying vertical pressure and gyration angle. The mix’s response to these factors can be tracked and used to assess mix stability [15]. To attain this goal, two basic approaches have been used in the past. The first method depends on examining compaction curve properties, such as slope, and linking them to mix stability as suggested by Mallick [8] and Bahia et al. [10]. The essential point to note about this method is the fact that it focuses on the compaction curve’s average slope. However, Fattah et al. [16] point out that the compaction curve is divided into two halves. The first part has a high rate of change in air voids, which is linked to densification during roller building at high temperatures, but the second half has a lower rate of change in air voids and the aggregate structure is subjected to severe shear stresses. Performance of the HMA in service has been linked to the second part’s compaction characteristics at ambient temperatures [16,17]. The second way for employing a gyratory compactor is to develop experimental instruments and methodologies for determining shear stress throughout compaction and relate it to stability. During gyratory test machine compaction, McRea presented an equation for calculating the shear stress in HMA [3,18]. The formulation of the HMA’s equilibrium analysis and compacted mold was used to develop this equation. The same equation was employed to demonstrate that the estimated shear stress is sensitive to the variations in binder type [12]. Mohammed et al. [19] calculated the shear stress and compacted energy in asphalt mixtures using a Finland gyratory compactor and installed a device on the Superpave gyratory compactor to calculate the forces at the specimens are when the bottom is compacted [4]. Dessouky et al. [6] demonstrated that, while various gyratory compactors were employed to measure shear stress in the past, they all relied on different variants of McRae’s equation. A gyratory compactor from Superpave with such a gyratory load cell mold was used to test the mechanical properties of the material. The components listed in the following sections make up the device. On top of the sample, the load cell allows for the modification of forces to be measured during gyration. Resistance effort (w) as a function of gyration count was calculated using mix’s response to applied forces in the SGC, as illustrated in Figure 2.

Figure 2

Designation of the load cell mold [15].

The load cell can be used in compression applications with limited space. The loading button is somewhat convex for accurate weight distribution. Threaded mounting holes on the bottom surface allow it to be attached to its base. The load cell is constructed of 17-4 PH steel material that has been sealed against temperature for practical application. Figure 3 depicts the part of load cell portion of the load cell and its components.

Figure 3

Sensor network that operates wirelessly.

Figure 4 depicts the WSGx-1 presented in a compact enclosure (75.6 × 59 mm × 29 mm), it uses two AA batteries, and can be installed directly on a sensor. It is powered by USB bus, so there will be no extra charge components required to manage remote devices from a PC.

Figure 4

During gyration, measuring units are prepared in the SGC mold [15].

5 Discussion of findings

Compression tests were performed in the laboratory to use a Superpave gyratory compactor. Specific compressive strength values were computed. At a temperature of 165°C, compressive measurements of asphalt mixes and asphalt mixtures were made. Figures 5–7 show the measured fore force vs the number of gyrations for combinations containing limestone, cement, and fly ash, respectively. The strength required for compaction varies depending on variables, such as the mixture of asphalt with limestone and river sand, which requires a strength increase of 5.95 percent, while for the mixture of asphalt with limestone and crushed sand, it requires a strength increase of 71.5 percent, and the cement-river mixture, which requires a strength increase of 71.5 percent. Sand river requires an increase of 8.69%, while cement mixture with crushed sand requires an increase of 19.35%, and a mixture of fly ash and river sand requires an increase in force to raise the strength. The plate covering the specimen rotates at a predetermined gyration angle as well as frequency and provides kneading pressure, which is represented by forces generated by the pressure and plates angle. After each gyration, the specimen is compacted, and the height difference is recorded. Volume changes and compaction rates can be calculated using the height change (volume change rate).

Figure 5

The measured force on the sample obtained from the modified gyratory apparatus using limestone as filler.

Figure 6

The measured force on the sample obtained from the modified gyratory apparatus using cement as filler.

Figure 7

The measured force on the sample obtained from the modified gyratory apparatus using fly ash as filler.

The combinations using crushed sand and limestone as a filler have the highest strength, indicating that the mix is stiff, as shown in Figures 5–7. The role of aggregate orientation in granular material shear strength and stiffness has been extensively researched. As a result, as the gyrations occur, the measured force varies.

6 Conclusion

The goal of the article was to determine the required strength for compaction of asphalt mixtures with 40–50 per surface layer. Three different types of mineral filler, as well as two different types of sand, were evaluated. A link between gyratory and measured forces was used to research the compressibility of an asphalt mix. Following conclusions were drawn:

When the six asphalt mixtures are compacted, it is discovered that the strength necessary to compact them differs depending on the filler material. The mixes using crushed sand and limestone as a filler have the highest strength, indicating that the mixes are stiffer.
As the number of gyrations rises, the compaction strength increases until it stabilizes.
Except for asphalt–cement combinations, the strength of compaction in mixtures using river sand requires less strength than mixtures containing crushed sand.

Funding information: The authors state no funding involved.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: Authors state no conflict of interest.

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Received: 2022-02-05

Revised: 2022-03-12

Accepted: 2022-03-24

Published Online: 2022-05-30

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/jmbm-2022-0023

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asphalt; gyratory compactor; HMA; load; strength

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