Proportion Design Method and Water Stability of Cement-Emulsified Asphalt Cold Recycling Mixtures

Nowadays, a large amount of work has been done in the field of emulsified asphalt cold regeneration. Three different proportions of emulsified asphalt cold recycling mixtures were designed by mixing different ratios of new aggregates and RAP, the effect of cement content and RAP content on the mixtures’s water stability was studied. The results show that the addition of cement can improve the mixtures’s water stability, but there is an optimum content; the mixtures’s water stability reduces with the increase of RAP content.


Introduction
In china, the total number of highways was 4,773,500 kilometers by the end of 2017. Asphalt road occupies 76% in high-grade road construction, so there will be a large amount of recycled asphalt mixture in the road pavement renovation, renovation and conservation every year. If calculate by 10% of highway pavement need repair, there will be 2.2 million tons recycled asphalt mixture every year. It is easy to be predicted that more and more recycled asphalt mixture will be generated in the future. The recycled asphalt mixture was treated as waste in some engineering projects, which is a waste of resources. In addition, they will occupy a lot of land, and pollute the environment. Asphalt cold recycling can be a good solution, which can make the old pavement materials reused and save 40%-50% of the total investment [1,2,3] .In recent years, the cold regeneration technique of recycled waste road materials by foam asphalt has been famous for its remarkable advantages in energy saving, environmental protection and economy. Nowadays, a large amount of work has been done in the field of emulsified asphalt cold regeneration [4,5] .
The effect of emulsified asphalt cold regeneration is influenced by many factors, such as the type and amount of emulsified asphalt, the gradation of the mixture, the dosage of the new aggregate, and the dosage of the cement. Based on the different proportions of RAP and new aggregate expected to cooperate, three kind of emulsified asphalt cold recycled mixture were designed. The optimum water consumption of the mixture was determined by the compaction test. By designing the proportion of emulsified asphalt cold recycled mixture combined with the water stability experiment, for this mixture the best proportion are determined. The results of this study can provide some reference for the design of practical emulsified asphalt cold regeneration project.  Each of the three gradations meets the requirements of the medium-grain grading of <Technical Specification for Road Asphalt Pavement Regeneration> (JTG F41-2008). Fig.1. is cold recycling mixtures gradation curves which compared with the standard grading.

The amount of cement and emulsified asphalt
Cement is only used as a stabilizer and it is not considered as active filler, regardless of its effect on the mixture grading. In order to investigate the effect of cement content change on the stability of mixed material water, this study set five kinds of cement content, which are 0%,1%,2%,3% and 4% of the RAP and new mixture

The amount of cement and emulsified asphalt
Mix and mix well and need a moderate amount of water. If the water is too little, the emulsified asphalt is difficult to disperse, and the lubrication between the aggregate particles is not enough. It is difficult to compaction the mixture and the cement cannot be fully hydrated. If there is too much water, the pressure of moving water will increase, and the mixture will be hard to compaction. At the same time, emulsified asphalt and cement slurry loss. Hydrate crystal development is not sufficient, and health time is long, and material strength is low. Therefore, it is necessary to determine the optimum mixing water consumption through experiments. The amount of emulsified asphalt is 4.2% of the RAP and new mixture total quality. The optimized amount of water (OWC) used and the maximum dry density were determined by the compaction test, and the mixing water consumption is calculated. The impaction test was referring to the method in <Highway Engineering stable inorganic binder materials testing procedures> (JTG E 51-2009). The RAP and new aggregates used in this experiment were air-dried, which could be considered as 0% water content. Therefore, the added amount of water is equal to the difference value between the OWC of the mixture and the water content of the asphalt emulsion. Table 4 shows the total amount of mixed materials and the maximum dry density obtained by the compaction test, and calculates the actual amount of water needed in the experiment.  It can be seen from Fig. 2. that the dry splitting strength and wet splitting strength of the mixture 3 are increased with the increase of cement content. When the cement content is less than 3%, the dry splitting strength and wet splitting strength of A and B increase with the increase of cement content. Table 5. is the water stability of different cement content mixtures. As shown in table 5, for certain types of mixtures, the water stability increases with the increase of cement content. When comparing 1% cement content and 0% cement content, the increase of water stability of the three mixed materials was 20.3%, 20.8% and 22.6% respectively. Therefore, the addition of cement to the water stability of the mixture is very obvious.

Table 5. Water stability of different cement content mixtures
After the hydration of the cement in cold cycling mixture, some of the hydration products C4AH13 react with gypsum: C4AH13+ 3C-S-H2+14H=AFt+CH. The needle bar AFt will be filled space within the mix and form a network structure that increased mixture strength.
Others hydration products C-S-H coagulation forms a fibrous structure that further increased mixture strength [7] . Hydration products develop to the surrounding space of the cement particles, criss-cross, and gradually fill the mix of all the capillary space. Thereby it forms a uniform, dense, pore closed the whole structure, and enhances the overall strength of the mixture of water and anti-damaging. On the other hand, the crystal structure of cement hydration products can be formed and developed in the water phase of asphalt emulsion. The cement hydration products and the released asphalt after emulsion breaking are intertwined, and the mixture is tightly integrated into a whole. Under the combined action of these two aspects, the splitting strength and water stability of the mixture are improved with the increase of cement content. However, cement content is not the higher the better [8] . Because the increase of cement dosage will lead to the increase of the dry shrinkage of the mixture, resulting in a severe shrinkage crack to reduce the strength. This is the reason that when the cement content is 4%, the strength of Mixture 2 and Mixture 3 is slightly lower than that of 3% cement. Therefore, the optimal cement dosage for this test was 3%. When the cement content was selected as 3%, the dry and wet splitting strength values of the three mixed materials were compared, and the results were shown in Fig. 3.

The effect of RAP dosage on the water stability of mixture
The RAP dosage of mixture 1, mixture 2 and mixture 3 was 25%, 50% and 75% respectively. As shown in Fig.  3, with the increase of RAP proportion in the mixture, the dry and wet splitting strength of the mixture decreased gradually. The dry and wet splitting strength of the three kinds of mixtures was 0.967, 0.955 and 0.804 respectively. But 25%RAP content is only 1.2% higher than the 50% RAP content that is 16.3 percent larger than the 75% RAP mixture. This is due to the fact that as the amount of RAP increases, the content of stone debris in the mixture will be reduced. Thus, the adhesion of aggregates and emulsified asphalt is weakened, and the dry, wet splitting strength and water stability of the mixture are reduced. However, due to the cementation effect of cement, the amount of RAP content increased from 25% to 50%, the reduction of the mixture splitting strength and water stability were minor. The cementation of cement is not sufficient to compensate the strength of the loss due to the attenuation of aggregates and emulsified asphalt, when the amount of RAP in the mixture is further increased. The results show that the mixture drying, wet splitting strength and water stability are greatly reduced.
In the actual cold regeneration construction, the lower the new aggregate amount in the mixture, the

Conclusions
By adding new aggregates of certain amount and granule, it can be combined with recycled asphalt pavement materials (RAP) to meet the design requirements. The design of the emulsion asphalt cold regenerated mixture gradation meets <Technical specification for road asphalt pavement regeneration>(JTG F41-2008). The addition of cement can significantly improve the splitting strength and water stability of emulsified asphalt cold regeneration mixture. However, the high cement content will cause serious shrinkage crack in the mixture, which will reduce the strength of the mixture.
With the increase of RAP dosage, the splitting strength and water stability of the mixture were reduced. Compared with the mixture of 25% RAP content, the mixture of 50% RAP, mixture of 25% RAP has a small reduction in splitting strength and water stability. Compared with the mixture of 25% RAP content, the mixture of 75% RAP, mixture of 75% RAP has a big reduction in splitting strength and water stability.
The best mixture ratio of this study is the Mixture 2 (50%RAP+25%1#new mixture+25%2# new mixture), and the cement content is 3%, and the emulsified asphalt content is 4.2%, and the water content is 5.15%. Best of proportion of mixture of 15℃ dry splitting strength is 0.554 MPa, and wet splitting strength is 0.529 MPa, and dry wet splitting strength ratio 0.955. All these meet the requirements of the design of the emulsified asphalt cold regeneration mixture.