A methodology of implementing target mixing ratio for asphalt mixture

Abstract

In order to implement the objective mix design of hot mix asphalt adequately during the construction process, the significance of objective mixing ratio was elaborated, and the variability of materials and its control method were analyzed. An engineering example is used to illustrate the debugging process of asphalt mixture batching & mixing plant and the dynamic quality management methods. The results show that the set of methods can not only implement the objective mixing ratio of hot mix asphalt adequately, but also control production during the construction process effectively.

Keywords

asphalt mixture / target mixing ratio / mix design implementation / batching & mixing plant debugging

Introduction

There are mainly three mix design methods of asphalt mixture: (1) the standard method (also known as “Marshall test method”). The asphalt mixing ratio is based on Marshall standard compaction test, and the technical indicators include voids, Marshall stability, flow value, residual stability and asphalt saturation, etc.; (2) Superpave method. The method employs gyratory compaction molding specimen. First, for the same asphalt content, the optimum one is selected from three different gradations by weight and volume analysis. Then four different asphalt content and the best gradation of aggregate are used for manufacturing mixture specimens. This method is similar to the asphalt mixture Marshall, which determines the optimum asphalt content by volume index and strength test. The Surperpave method is very strict in the requirement of the aggregate shape, and it is limited to the aggregate gradation area [1–3]; (3) the comprehensive design method. The method is to supplement and overcome the shortcomings of the Marshall test method, which considers various possible failure forms of asphalt pavement and the pavement performance of the asphalt mixture. It includes two categories: the first category is when a mineral aggregate gradation is selected, according to the asphalt mixture field performance, the optimum asphalt content is determined; the second method is like the Strategic Highway Research Program (SHRP), coarse, medium and fine three gradations are selected. The gradation types are finally determined according to field performance adapted to different climate zone [4].

The optimum mixing ratio of asphalt mixture represents the good quality of the pavement. The realization of the target mixing ratio of asphalt mixture is the key to ensure the construction quality of the asphalt concrete pavement. Due to the complexity of the material variation, construction environment and other conditions, the realization of the target mixing ratio has a lot of difficulties. The analysis of quality control in the construction process has lots of complex influencing factors. Using appropriate technology management in mixing ratio and implementation methods, the quality of asphalt mixture can be improved, early distress can be avoided, the pavement service life can be prolonged and pavement ride quality can be improved.

Material variablity control

The materials are the main components of pavement, where the properties of various materials will have direct impacts on the performance of asphalt mixture, and material properties in many aspects exist variability [5]. Although there have been many researches in asphalt material area [6–10], how to control the variability of the material is still not well understood, where it is the key to achieve target mixing ratio.

Control the variability of aggregate

Aggregate is a part of the asphalt mixture structure. Aggregate plays the structure role in the asphalt mixture [11,12]. And it is the main source of the mechanical properties of asphalt mixture, so it is an important factor to affect the quality of asphalt pavement.

(1)‚ Variability in sieving results of single particle size aggregates

The variation in sieving results of single particle size aggregate will directly affect aggregate gradation. The wrong ways of material packing, mixing in station, and sieving will lead to material sieving results mutate. In practical engineering projects, "vertical placing in horizontal pile" principle should be followed. A wall should be set up between the adjacent cold material piles. The height of the baffle plate between the cold silos should be strictly controlled. The forklift should fetch the material from the bottom to the top and cannot not fetch only from the surface of the aggregate. The sieving results and the deviation of the average value should meet the requirements of Table 1.

When the qualified rate is too low, the material should be treated to meet the requirements.

(2)‚ Moisture content of aggregates

If aggregate is too wet, the excess moisture will be counted into aggregate weight. The uneven distribution in humidity will make cold material metering out of control. Also it may cause excessive heating and deterioration of asphalt in the mixed material, which seriously affects asphalt mixture quality. Therefore, the aggregate drying should be strictly ensured in feeding and material storage.

(3)‚ Variations in aggregate morphology

The morphological characteristics of aggregate including particle shapes, angularity and texture are critical to the structure strength and stability of asphalt mixture, and the ideal aggregates should have edges and corners, which is close to the cube. While the flat and elongated particles are prone to break along the axe direction [13–19]. In the construction of asphalt pavement, the content of the aggregates with needle shape should be less than 15%, and some construction projects have stricter standards than the construction technology, less than 10%.

Control the variability in mineral fillers

Filler fineness and dosage are important indexes to reflect the performance of asphalt mixture. Technical specification for construction of asphalt pavement regulates that, for filler size less than 0.075 mm, 75%–100% satisfy the requirements. The broad definition of this indicator and un-standard processing process mineral fillers will cause index fluctuations in asphalt pavement construction. In the asphalt mixture production process, filler dosage is not stable. Different amount of filler will result in different properties of asphalt mixture [20–22]. At the same time, the excessive use of recycled powder will not be able to make the asphalt fully dispersed, and great decrease the adhesion, result in the rapid decreasing in mixture performance and early damage in asphalt pavements.

Control the variability of asphalt

Asphalt plays an important role in high temperature stability and low temperature crack resistance of asphalt mixture. In the actual production, the quality of asphalt needs to be real-time monitored. The commonly used evaluation indexes are: penetration grade, softening point, and ductility. If an index is unusually large, the production should be stopped to find the reasons. For example, if the penetration grade decreases greatly, it may result in aging during storage and transportation. At the same time, asphalt of different locations needs to be detected, including asphalt plant, asphalt in the field. If something abnormal occurs, the previous process should be carefully analyzed.

Mixing station equipment requirements

The accuracy of the cold material supply system, mixed material sieving system and electronic weighing system need to be paid great attention on. Only when cold silo feed ratio and feed speed, vibration of sieving system, corresponding sieve size, sieve placement, and highly accurate electronic scales are compatible with each other, the mineral aggregate gradation and asphalt aggregate ratio meet the production requirements.

The common failures are sieve breakage, the hot material storehouse diaphragm damage, silo door opening and closing out of control, not tightly closed, bitumen pipeline impeded and so on. In the production process, these processes should always be checked. If necessary, the sieving or re calibration measurement can be changed. In addition, in order to realize the dynamic quality management in production, mixing station equipment should also have the disc-to-disc print function.

Mixing station debugging

Determination of hot silo mesh size

In order to avoid the overflow in each hot aggregate silo when machine is fully loaded. The volume and target mixing ratio should be determined reasonably in the hot material storehouse, where the specific steps are as follows: (1) mixed material are filled in until the hot material storehouse is full; (2) empty the hot aggregate silo one by one, weigh the hot material storehouse of quality, and then calculate the hot aggregate silo volume ratio; (3) according to thermal silo volume ratio and the target gradation curve, calculate the thermal silo suitable mesh size.

Determination of speed of cold silo

In the transport of cold material, the main cold silo discharge opening and the lower belt speed need to be focused on. Under normal circumstances, the opening of the material in the production process is a constant value, while the cold feed rate adjusting is controlled by the speed of the belt. The discharge ratio of the hot material storehouse directly affects the mixture gradation, and cold silo discharging ratio will affect the grading of the hot material storehouse the storehouse. So in the actual production, the feeding proportion of the material and cold silo speed cannot arbitrarily change.

First of all, the relationship between the cold silo and the feeding material quality needs to be determined. Three speeds are set normally. When the feeding becomes stable, the feeding material quality at different speed per unit time needs to be calculated using a stopwatch. Usually it takes 10–15 minutes. The statistical curve is drawn (generally approximate straight line), as shown in Fig. 1.

The feeding material mixing ratio of each cold silo can be determined based on the sieving results and the designed gradation curve (Table 2). The synthetic gradation curve of cold silo is shown in Fig. 2. If the production efficiency is determined, the cold silo material weight can be determined. Compared with Fig. 1, the cold silo belt rotation frequency can be determined, as shown in Table 3.

Determine the hot silo discharge ratio

After determining the cold silo frequency, the mixing procedure can be conducted. After the system becomes stable, the hot mixture will be sieved in each silo. Finally, according to the target gradation curve, the feeding ratio of each hot material storehouse is fitted, as shown in Table 4. The hot silo synthetic gradation curve is shown in Fig. 3.

Control of import and export balance

Cold silo synthetic gradation and the hot silo synthetic gradation are not in full compliance with the target gradation. It may result in the inconsistency between import and export of the material, and with the increasing of time, the difference will gradually increase. At the same time, due to feed gradation instability, it may ultimately lead to overflow phenomenon of material. The solutions are as follows:

(1)‚ Strictly set the hot silo size in accordance with the methods described in Section 4.1;

(2)‚ Try to make the cold silo of synthetic gradation consistent with the hot silo gradation;

(3)‚ Check if the material source is stable or not. If the variability results of the sieving exceed the requirements, it should be poured off;

(4)‚ Check if the cold silo opening is blocked and the belt is loose, and the mixing station metering device is accurate or not;

(5) ‚When the emergence of overflow trend, to fine tune the cold silo speed, in order to achieve the balance of import.

Production mixing ratio validation

First, three asphalt contents including optimum asphalt content and the optimum asphalt content of±0.3% are determined by the Marshall test. The optimum asphalt content is determined. Then, the mixing machine tests the trial mixing, paves on the test section, and uses the core samples drilled from the road for Marshall test. The standard production mixing ratio is then determined. The standard mixing ratio should be used as the standard basis for production control and quality inspection.

Dynamic quality management

In order to ensure the realization of the obtained mixing ratio maintaining for a long time, the production process of asphalt mixture needs to be conducted using dynamic quality management. The principle of mathematical statistics analysis is employed to analyze the mix design data and timely monitoring of production stability. The disc-to-disc print data includes the amounts of asphalt, mineral fillers and the amount of heat storage silo for every mixing. Fig. 4 shows the change of asphalt content in one section of Daguang Highway in China. The coefficient of variation can be calculated by the distribution of the scattering. The amount of material can be timely analyzed and controlled.

Conclusions

In this paper, the importance of the target mix design is evaluated, and the method to achieve the target mix design is systematically explained as follows:

(1)‚ The control of the variability of materials (including the set of variations in the material properties, variation of asphalt aggregate gradation, filler variation, material approach and storage project management) is explained;

(2)‚ The variability of control equipment (including the supply system, the screening system and the electronic weighing system, etc.) is explained;

(3)‚ In order to make full use of storage of hot silo and realize the material import and export balance, a reasonable set of hot silo sieve needs to be set;

(4)‚ The discharge ratio of the hot material storehouse directly affects the mixture gradation, and cold silo discharging ratio will affect the grading of the hot material storehouse the storehouse. Thus the mixing speed of cold silo needs to be determined. According to the target gradation curve, the feeding ratio of each hot material storehouse is fitted and a reasonable mixing proportion of the material in hot silo needs to be determined;

(5)‚ The production mixing ratio is verified based on the Marshall Test, by which the optimum asphalt content is determined. The production process of asphalt mixture needs to be conducted by using dynamic quality management

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