Keywords

1 Introduction

In road maintenance or reconstruction, a large number of waste cement stabilized macadam base materials will be produced by milling or digging. Reasonable utilization of waste cement stabilized macadam base material can not only alleviate the difficulty of stone shortage in engineering construction, but also benefit the ecological civilization construction and the realization of double carbon goal in China. Lu et al. [1] found that adding a certain amount of fly ash into the recycled cement mixture can improve the strength and frost resistance of the recycled cement mixture, and adding a small amount of emulsified asphalt into the recycled old cement stabilized macadam base material can improve the frost resistance and flexibility of the recycled cement mixture. Wang et al. [2, 3] added emulsified asphalt to recycled waste cement stabilized crushed stone, and found that it can improve the dry shrinkage performance of recycled cement stabilized crushed stone mixture, enhance the flexibility of the mixture, and suppress the generation of cracks in the base layer. The research results of Lu et al. [4, 5] showed that the strength and crack resistance of recycled cement stabilized macadam mixture can be improved by adding fiber materials.

Redispersible latex powder is a kind of high molecular polymer powder, which can be redispersed when exposed to water, and forms organic mucosa on the surface of solid materials such as crushed stone to enhance the integrity of the structure. At the same time, the redispersible latex powder has the advantages of dissolving in water and being easy to mix evenly. In China, there are related studies on adding latex powder to cement mortar [6,7,8]. Adding latex powder to cement mortar can improve the bonding strength and flexural performance of cement mortar. At present, there is no research on its application in recycled cement stabilized macadam base. This paper studied the influence of latex powder on the road performance of recycled cement stabilized macadam, especially the effect of improving its crack resistance.

2 Properties of Recycled Materials for Waste Base Course

2.1 Morphology of Recycled Aggregate

Recycled aggregate is produced by milling cement stabilized macadam base during highway overhaul, as shown in Fig. 1. Most of the surface of recycled aggregate is covered with cement mortar, and the cross sections of coarse aggregate become more (full of edges and corners) and the surface is rough. The cement mortar particles in the recycled materials of milling base course are small and the strength is low. The particle size of recycled aggregate used in the test ranges from 0 to 26.5 mm, and 78% of the aggregate particles are larger than 4.75 mm.

Fig. 1.
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Recycled aggregate

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2.2 Technical Index of Recycled Aggregate

According to “Test Methods of Aggregate for Highway Engineering” (JTG E42–2005), the technical indexes of recycled aggregate and natural aggregate are shown in Table 1. The water absorption and crushing value of recycled aggregate are obviously higher than those of natural aggregate.

Table 1. Technical indexes of aggregate
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3 Experimental Design

3.1 Raw Materials

According to the gradation of recycled materials, natural aggregate is still needed in the test, which can be divided into 1 # (20 ~ 30 mm), 2 # (10 ~ 20 mm), 3 # (5 ~ 10 mm) and 4 # (0 ~ 5 mm) according to the particle size. P.O42.5 cement was used, and its technical indexes are shown in Table 2.

Table 2. Technical indexes of cement
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The 8020 redispersible latex powder produced by Anhui Wanwei Company was used in this study. The technical parameters of latex powder are shown in Table 3. Redispersible latex powder is a kind of high molecular polymer powder, and its appearance is milky white powder.

Table 3. Technical indexes of latex powder
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3.2 Mix Proportion

According to “Technical Guidelines for Construction of Highway Roadbases” (JTG/T F20–2015), C-B-1 was selected as the gradation type, which can be used for expressways and first-class highways. The designed gradation of recycled cement stabilized mixture is shown in Table 4, and the content of recycled aggregate is 44%. The compaction test shows that the optimal moisture content of recycled cement stabilized crushed stone is 5.6%, and the maximum dry density is 2.260 g/cm3.

Table 4. Grading of cement stabilized macadam mixture
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3.3 Performance Test

Strength test

The size of the compressive strength specimen is Ф100 mm × 100 mm cylindrical specimen. The bending tensile strength specimen is 100 mm × 100 mm × 400 mm mid beam specimen. The bending tensile strength test piece was formed by static pressure method, and it was conducted on a universal testing machine after 28 days of curing.

Freeze-Thaw performance test

The test method referred to section T0858–2009 in JTG E51–2009. We made same cylinder specimens as compressive strength test with 6 specimens in each group. After curing for 7 days, three specimens were tested for direct compressive strength, and the other three specimens were tested for compressive strength after freeze-thaw cycle. Freeze-thaw temperature setting: -18℃ cryogenic box for 16 h, 20 ℃ water tank for 8 h, freeze-thaw cycle once.

Dry shrinkage performance test

The test was based on section T0854–2009 of the JTG E51–2009. Same beam specimens as flexural-tensile test (3 specimens in each group) were made. To assess the dry shrinkage properties of cement stabilized macadam material at the initial stage of construction, the test began after being cured for 1 day in the standard curing room. A dial indicator was inserted into a specially designed mold, as shown in Fig. 2. The specimen's weight was measured daily to determine the water content loss, while both water content loss and deformation shrinkage were continuously recorded each day.

Fig. 2.
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Specimen of dry shrinkage test

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4 Result Discussions

4.1 Effect of Latex Powder on Compressive Strength of Mixture

The unconfined compressive strength of cement stabilized macadam for 7 days is shown in Fig. 3(a). The compressive strength of cement stabilized macadam with recycled materials is slightly higher than that of natural aggregate. From the physical point of view, the reason is that the angularity of recycled aggregate increases and the surface is rougher, which leads to the increase of internal friction. In terms of microstructure, many micro-cracks on the surface will inhale new cement particles, which makes the hydration of the contact area more sufficient and the interface structure more compact.

After adding latex powder, the compressive strength of cement stabilized macadam decreased slightly. The reason is that after the emulsion is cured, the elastic modulus of the film formed is small, which can not play a rigid supporting role when the whole test block is compressed, so that its compressive strength is reduced.

4.2 Effect of Latex Powder on Flexural Tensile Strength of Mixture

Natural aggregate, recycled aggregate, recycled aggregate + latex powder (8%) were mixed with cement stabilized macadam mixture. After 28 days of curing in standard curing room, the flexural and tensile strength of different mixture specimens was measured. The test results are shown in Fig. 3(b).

The flexural-tensile strength of recycled aggregate cement stabilized macadam is 5.9% higher than that of natural aggregate cement stabilized macadam, and the flexural-tensile strength of recycled aggregate + latex powder mixture is 9.8% higher than that of natural aggregate mixture. The reasons for adding latex powder to improve the flexural strength of recycled cement stabilized macadam are as follows: The latex powder emulsion plays a filling role and improves the internal structure of hardened cement paste. In addition, the film with high adhesive force formed by emulsion dehydration connects the hydrated products of cement more effectively, and the two interweave each other to form a firmer and more flexible three-dimensional network connection structure, which improves the flexural strength of the sample.

Fig. 3.
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Strength test results

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4.3 Effect of Latex Powder on Frost Resistance of Mixture

When cement stabilized macadam base is applied in a seasonal frozen area, its bearing capacity decreases after several freezing and thawing seasons. As a result, it may not be sufficient to support the vehicle load transmitted by the pavement, leading to cracking and other forms of damage. In order to assess the frost resistance of cement stabilized macadam, a freeze-thaw cycle test was conducted to evaluate the impact of latex powder. The test was conducted in accordance with the method specified in section T0858–2009 of the JTG E51–2009. The compressive strength loss (BDR) of the mixture was calculated using formula (1) from this section, and the results are presented in Table 5.

$$BDR={R}_{DC}/{R}_{C}\times 100$$
(1)

In formula (1): BDR is the compressive strength loss of the specimen after freeze-thaw cycle, %; RDC is the compressive strength of the specimen after freeze-thaw cycle, MPa; RC is the compressive strength of the contrast specimen, MPa.

Table 5. Compressive strength loss of specimens after freeze-thaw cycles
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The BDR value of cement stabilized crushed stone increased with the addition of latex powder, and its frost resistance improved. The cement stabilized macadam recycled base material has a few voids inside. When the base material passes through the freeze-thaw cycle, the water in the voids inside the base material will crush the void wall of the mixture because of the frost heaving action, thus reducing the strength of the mixture. Adding latex powder will fill some voids in the mixture, and the void wall has certain elasticity, which will have less freeze-thaw damage. The effect of frost resistance improvement will be more obvious after multiple freeze-thaw cycles.

4.4 Effect of Latex Powder on Dry Shrinkage Performance of Mixture

Four groups (natural aggregate, natural aggregate + latex powder, recycled aggregate, recycled aggregate + latex powder) of mid beam specimens (3 specimens in each group) were made, to compare the effects of latex powder on dry shrinkage performance of natural aggregate and recycled aggregate mixture. According to T0854–2009 in JTG E51–2009, the total dry shrinkage coefficient of the specimen can be calculated by formula (2) ~ ((6). The changes of total water loss rate, total dry shrinkage strain and total dry shrinkage coefficient with time are shown in Fig. 4, Fig. 5 and Fig. 6 respectively.

$${\omega }_{i}=\left({m}_{i}-{m}_{i+1}\right)/{m}_{p}$$
(2)
$${\delta }_{i}=\left({\sum }_{j=1}^{4}{X}_{i,j}-{\sum }_{j=1}^{4}{X}_{i+1,j}\right)/2$$
(3)
$${\varepsilon }_{i}={\delta }_{i}/l$$
(4)
$${\alpha }_{di}={\varepsilon }_{i}/{\omega }_{i}$$
(5)
$${\alpha }_{d}=\sum {\varepsilon }_{i}/\sum {\omega }_{i}$$
(6)

In formula (2) ~ (6): \({\omega }_{i}\) is the i-th water loss rate, %; \({\delta }_{i}\) is the i-th dry shrinkage of observation, mm; \({\varepsilon }_{i}\) is the i-th dry shrinkage strain, %; \({\alpha }_{di}\) is the i-th shrinkage factor, %; \({m}_{i}\) is the weighing mass of the i-th standard specimen, g; \({X}_{i,j}\) is the reading of the j-th dial indicator in the i-th test, mm; \(l\) is the length of standard specimen, mm; \({m}_{p}\) is the constant weight of standard specimen after drying, g.

The dry shrinkage strain and dry shrinkage coefficient of cement stabilized macadam mixture without latex powder are bigger than those of the mixture with latex powder. After adding latex powder, the dry shrinkage coefficient of natural aggregate mixture decreased by 53.8% in 7 days and 55.8% in 14 days. The dry shrinkage coefficient of recycled aggregate mixture decreased by 57.4% in 7 days and 28.5% in 14 days. Adding latex powder will significantly reduce the dry shrinkage coefficient of cement stabilized macadam, and improve the dry shrinkage performance of the material.

Fig. 4.
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Total water loss rate

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Fig. 5.
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Total drying shrinkage strain

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Fig. 6.
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Total dry shrinkage coefficient

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In order to analyze the mechanism of improving dry shrinkage performance of cement mixture with latex powder, the cement mortar part of recycled mixture specimen and recycled mixture with latex powder was taken respectively, and the two samples were observed by electron microscope. SEM test pictures are shown in Fig. 7 and Fig. 8.

The mixture samples without latex powder have more macropores and channels under microscopic conditions. In the sample of latex powder mixture (Fig. 7b), it can be observed that the polymer film formed after the reaction of latex powder covers the hydration products of cement, and the mortar formed by latex powder and cement binds the mineral particles, which reduces the voids between the particles.

Fig. 7.
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SEM pictures of mucilage part of mixture (1000 times)

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In the 3000 times SEM picture of the latex powder mixture sample, the network-like connection structure can be observed, which is the network-like connection structure formed by cement hydrate, latex powder polymer, etc. (Fig. 8b). This network structure has certain flexibility due to the addition of latex powder. The flexible latex powder cement skeleton connection structure (elastic skeleton) improves the flexural strength of cement macadam mixture. At the same time, the deformable joint structure can absorb the shrinkage deformation of the material, thus reducing the shrinkage deformation of the specimen and the dry shrinkage strain.

The volume shrinkage of cement stabilized macadam materials will be caused by capillary action, adsorption, intermolecular force, interlayer water between mineral crystals or gels and carbonization shrinkage due to the decrease of water in cement stabilized macadam mixture. Based on the analysis of dry shrinkage test results and micro-electron microscope test results of cement-stabilized macadam with latex powder, there are two main reasons why latex powder can improve the dry shrinkage performance of cement-stabilized macadam mixture: (1) The latex powder polymer formed after adding latex powder fills some pore channels, and the latex film formed by latex particles coagulates on the surface of particles, which also hinders the migration of water and makes it difficult to evaporate and lose water. (2) In the latex powder mixture, the latex powder polymer particles form a network-like connection structure. This network-like connection structure (elastic skeleton) with deformation ability can absorb the shrinkage deformation caused by water loss in the material and reduce the volume shrinkage of the specimen. Therefore, the test data show that the dry shrinkage strain of the specimen is obviously reduced. Comparing the dry shrinkage test water loss rate and dry shrinkage strain data, the network structure of latex powder polymer particles is the main factor.

Fig. 8.
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SEM pictures of mucilage part of mixture (3000 times)

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5 Conclusions

  1. (1)

    The compressive strength and flexural strength of the cement stabilized macadam with recycled aggregate are higher compared to those with natural aggregate. After the addition of latex powder, the flexural and tensile strength of the cement stabilized macadam mixture increased, while the compressive strength decreased slightly. The result of freeze-thaw test shows that the BDR value of cement stabilized macadam with latex powder increased, and the frost resistance of mixture increased.

  2. (2)

    Adding latex powder can reduce the shrinkage strain and shrinkage coefficient of cement stabilized macadam, and significantly improve its crack resistance. After adding latex powder, the dry shrinkage coefficient of natural aggregate mixture decreased by 53.8% in 7 days and 55.8% in 14 days, and the drying shrinkage coefficient of recycled aggregate mixture decreased by 57.4% in 7 days and 28.5% in 14 days.

  3. (3)

    There are two main reasons why latex powder can improve the dry shrinkage performance of cement mixture: First, the formed latex powder polymer fills part of pore channels, and the latex film formed by latex particles coagulates on the surface of particles, which also hinders the migration of water and makes it difficult for water to evaporate and lose. The second is the network connection structure formed by latex powder in the mixture. This network connection structure (elastic skeleton) with deformation ability can absorb the shrinkage deformation caused by water loss in the material and reduce the volume shrinkage of the specimen, so the test data show that the dry shrinkage strain of the specimen is obviously reduced. Comparing the dry shrinkage test water loss rate and dry shrinkage strain data, the network connection structure is the main factor.