Zhang Liqiang
Tianjin Qirui Municipal Landscape Engineering Co., Ltd
Abstract: Because the cement stabilized gravel base has the advantages of high overall strength, good impermeability and water stability, wide source of raw materials, low cost, etc., it has become the most common form of pavement base in the mainland. However, due to the fact that the problem of reflection cracks caused by the cracking of the cement-stabilized gravel base has not been effectively solved, the problem of dry shrinkage cracks of the cement-stabilized gravel base has become the most important factor restricting the service life of the road. In this paper, the changes in flexural and compressive properties of cement mortar were analyzed by adding 0, 1%, 2% and 3% rubber powder to cement mortar. 0, 1%, 2% and 3% rubber powder were added to the cement-stabilized gravel mixture, and the unconfined compressive strength and dry shrinkage properties were tested. The results show that with the increase of rubber powder content, the strength of cement mortar increases first and then decreases, and the strength is the highest when the rubber powder content is 1%. The unconfined compressive strength of the cement-stabilized gravel mixture showed a downward trend, and the dry shrinkage coefficient also decreased, indicating that the rubber powder could improve the dry shrinkage performance of the cement-stabilized gravel mixture while reducing its strength. Therefore, combined with the influence of rubber powder on the dry shrinkage performance and unconfined compressive strength of cement stabilized gravel mixture, the cracking problem of cement stabilized gravel base can be improved by adding rubber powder, but the rubber powder should be mixed with low dosage, and the recommended dosage is about 1%, and the specific dosage needs to be determined according to field tests.
Keywords: rubber powder; cement-stabilized crushed stone mixture; dry shrinkage performance; intensity change; dosage;
About author:ZHANG Liqiang (1982-), male, from Chifeng, Inner Mongolia, engineer, research direction: municipal highway construction technology research.
1 Introduction
As of the end of 2021, the total length of highways in mainland China was 5,280,700 kilometers, of which 169,100 kilometers were expressways, ranking first in the world. Because the semi-rigid base has the characteristics of "strong base and thin surface" [1], it can effectively reduce the thickness of the pavement and reduce the cost of road construction, so it is used by most roads in the mainland. At the same time, the cement-stabilized gravel base has the advantages of high initial strength, rapid plate formation, high overall strength, good impermeability and water stability, wide source of raw materials, and low cost, and has become the most widely used semi-rigid base form in the construction of mainland grade highways [2]. However, the cement-stabilized gravel base is more sensitive to temperature and humidity changes, and after the temperature and humidity changes, it is easy to form temperature shrinkage cracks and dry shrinkage cracks, and the cracks induce crack reflection to cause cracking of the surface layer under the further action of the dry shrinkage and temperature shrinkage stress of the base layer, resulting in early damage to the road, so that most roads can not reach the designed service life. Therefore, improving the cracking of the grassroots is an urgent problem to be solved. In order to solve the problem of cracks in the grassroots layer, scholars at home and abroad mainly conduct research on optimizing gradation, incorporating admixtures, and pre-cracking. In terms of gradation optimization, Pan Zhaoping [3,4,5,6] analyzed the effect of cement dose on the crack resistance of the hydraulically stabilized base layer around the addition of cement dose, and concluded that the amount of cement dosage directly affects the dry shrinkage performance of the hydraulically stabilized base layer, the higher the cement dose, the greater the dry shrinkage and temperature shrinkage, and when the cement content is less than 3%, the dry shrinkage coefficient is more sensitive to the change of cement dose, and the dry shrinkage coefficient and temperature shrinkage coefficient both increase with the increase of cement dose. In terms of the incorporation of admixtures, Siripun K et al. [7] demonstrated in laboratory experiments that the incorporation of fibers in cement-stabilized gravel can reduce the occurrence of cracks. Fu Chunmei et al. [8] proved that the compressive strength of cement-stabilized gravel increased first and then decreased with the increase of fiber content, and there was an optimal content of fiber. Scholars at home and abroad have also conducted a lot of research on the use of admixtures such as water reducers, swelling agents, anti-cracking agents and emulsified asphalt. In terms of pre-cracking, some foreign researchers were the first to find that artificial micro-crack nets can better release shrinkage stress and avoid the occurrence of wide cracks. Focusing on the application of rubber powder in roads, the study of Xue Gang et al. [9] showed that the fatigue life of rubber concrete was higher than that of plain concrete, and the fatigue life of concrete was higher with the increase of rubber powder content. Wang Haipeng et al. [10,11,12] incorporated rubber powder into cement stabilized pellets, and studied the mechanical properties and shrinkage properties of the mixture.
At present, there are few studies on the crack resistance of rubber additives on cement stabilized gravel substrates. Therefore, this paper mainly focuses on the effects of different rubber content on the crack resistance of cement stabilized gravel substrate, so as to provide reference for related research and application.
2. Raw materials and mix ratio
2.1 Raw Materials
Rubber powder cement stabilized gravel is composed of rubber powder, aggregate and cement, the quality of raw materials is the key to ensure the quality of the project, and it is necessary to test the performance of raw materials used in accordance with the requirements of the Technical Rules for Highway Pavement Foundation Construction (JTG/T F20-2015).
2.1.1 Rubber powder
The rubber powder used in the test is made from waste tires by crushing, and is divided into four types according to the number of rubber powders: rubber particles (10 mesh ~ 30 mesh), rubber powder (30 mesh ~ 60 mesh), fine rubber powder (60 mesh ~ 120 mesh) and fine rubber powder (more than 120 mesh). 40 mesh rubber powder was used in this test, and the main performance indicators are shown in Table 1.
Table 1 Detection indicators of rubber powder Download the original image
2.1.2 Cement
The test uses ordinary Portland cement with a strength grade of 42.5, and the main detection indicators are shown in Table 2.
Table 2 Cement detection indicators Download the original image
2.1.3 Aggregates
The aggregates used in the test are divided into 0~5mm, 5mm~10mm, 10mm~20mm, 20mm~30mm four grades, of which stone chips are produced in Jinan, Shandong, and coarse aggregates are produced in Tai'an, Shandong, in accordance with the "Highway Engineering Aggregate Test Regulations" (JTG E42-2005) The main technical indicators of raw materials are tested, and the test results are shown in Table 3 and Table 4.
Table 3 Grading of aggregates Download the original image
Table 4 Test results Download the original image
2.2 Mix design
In accordance with the grading range specified in the current "Technical Rules for the Construction of Highway Pavement Base Layer" (JTG/T F20-2015), the grading range of cement stabilized gravel meets the requirements of C-A-3, and the specific grading is shown in Table 5.
Table 5 Synthetic gradation of cement-stabilized gravel base mixture Download the original drawing
3. Analysis of the influence of rubber powder content on the strength of rubber sand
According to the requirements of the "Cement Mortar Strength Test Method (ISO)" (GB/T17671-2021), 450g of cement, 1350g of standard sand and 225g of water are mixed with 40mm×40mm×160mm cement prism specimens according to C:S:W=1:3:0.5, and the health conditions specified in the specification are carried out, and the flexural strength and compressive strength tests are carried out after reaching the required curing age.
In order to study the influence of rubber powder on the strength of cement mortar specimens at different curing ages, according to the rubber powder content of 0, 1%, 2% and 3% (mass fraction), the mixing method adopts the dry mixing method, that is, after adding cement to the mixing pot, rubber powder is added, stirred with a mixer together with the glue sand, formed on the vibrating table, and the flexural and compressive strength tests of the cement mortar specimens are carried out respectively for 3D, 7D and 14D, and the test results are shown in Figure 1 and Figure 2.
Fig.1 Variation of compressive strength with curing time under different dosages of rubber powder Download the original figure
Fig.2. Variation of flexural strength with curing time under different dosages of rubber powder Download the original picture
It can be seen from Fig. 1 and Fig. 2 that the compressive and flexural strength of the cement mortar specimen increase slightly after adding 1% rubber powder, and the compressive and flexural strength of the cement mortar specimen increase with the increase of curing age. However, when the content of rubber powder increases to 2% and 3%, the compressive and flexural strength of cement mortar specimens are weakened. The test results show that the addition of low amount of rubber powder to cement mortar specimens will increase its compressive and flexural strength, but when the amount of rubber powder is too large, it will have an adverse effect on its compressive and flexural strength.
4. Analysis of the influence of rubber powder on the mechanical properties of cement stabilized gravel
In accordance with the test method specified in the "Test Regulations for Inorganic Binder Stabilized Materials for Highway Engineering" (JTG E51-2009), the maximum dry density and the best moisture content of cement stabilized gravel are determined; Five cement doses (3%, 4%, 5%, 6%, 7%) were set to measure their 7-day unconfined compressive strength, and the test results are shown in Table 6. In accordance with the provisions of the "Technical Rules for the Construction of Highway Pavement Base Layer" (JTG/T F20-2015), the optimal cement dosage is determined to be 5%.
Table 6 7-day unconfined compressive strength of cement-stabilized gravel with different cement dosages Download the original figure
4.1 Analysis of unconfined compressive strength changes
The optimal cement dosage of 5% was used, and 0, 1%, 2%, 3% and 4% rubber powder were mixed with respectively. In accordance with the provisions of the "Test Regulations for Inorganic Binder Stabilized Materials for Highway Engineering" (JTG E51-2009), the forming and health of cement stabilized gravel specimens are carried out, and the unconfined compressive strength test is carried out after the specimens reach the age stage, and the test results are shown in Table 7 and Figure 3.
Table 7 Test results of unconfined compressive strength Download the original figure
Fig.3. Change of unconfined compressive strength under different rubber powder content Download the original image
From the analysis of Table 7 and Figure 3, it can be seen that under the standard curing conditions of 7d and 28d, with the increase of rubber powder content, the unconfined compressive strength of cement stabilized gravel mixture gradually decreases, that is, the strength of cement stabilized gravel mixture will be reduced after adding rubber powder. This is mainly due to the fact that the material composition properties of rubber powder and cement stabilized gravel mixture are different, rubber powder does not react with cement hydration, rubber powder particles exist independently inside the material, under the action of external load, when the stress is transferred from the hydraulic stabilized material to the rubber powder, because the deformation capacity of the rubber powder is relatively large, the deformation of the recycled mixture is small, and the deformation coordination ability of the two is different, so it is easy to destroy the material caused by excessive strain, resulting in microcracks, and the greater the amount of rubber powder. The more obvious the interface defects between cement stone and rubber powder, the looser the inside of the material, the lower the density, and the more obvious the strength attenuation.
4.2 Dry shrinkage
The optimal amount of 5% cement was used, mixed with 0, 1%, 2% and 3% rubber powder respectively, and the molding and health maintenance of the specimen were carried out in accordance with the provisions of the "Test Regulations for Stable Materials of Inorganic Binders in Highway Engineering" (JTG E51-2009), and the dry shrinkage coefficients of 7d, 14d, 28d, 35d and 42d were detected respectively, and the influence of rubber powder on the dry shrinkage performance of cement stabilized gravel mixture was analyzed, and the test results are shown in Table 8 and Figure 4.
Table 8 Detection results of dry shrinkage coefficient of cement-stabilized gravel Download the original figure
Fig.4 Changes in different curing times under different rubber powder dosages Download the original image
It can be seen from Table 8 and Figure 4 that after the addition of rubber powder to the cement stabilized gravel mixture, the dry shrinkage coefficient of the cement stabilized gravel mixture decreases continuously with the increase of rubber powder content. This is due to the large elasticity of rubber powder, which reduces the capillary tension caused by water evaporation to a certain extent, as well as the adsorption of water and intermolecular forces, thereby reducing the dry shrinkage coefficient of cement-stabilized gravel mixture. The results show that the addition of rubber powder can effectively slow down the occurrence of shrinkage cracks in the cement-stabilized gravel base.
5 Conclusion
By adding rubber powder to the cement mortar and cement stabilized gravel mixture, the compressive strength, flexural strength, unconfined compressive strength and dry shrinkage tests at different ages are carried out respectively, and the following conclusions can be obtained:
(1) After adding rubber powder, the strength of cement mortar is the largest when the rubber powder content is 1%, and then its compressive strength and flexural strength will decrease with the increase of rubber powder content.
(2) After adding rubber powder, the strength of the cement stabilized gravel mixture will decrease with the increase of rubber powder content, that is, the rubber powder will reduce the strength of cement stabilized gravel.
(3) After adding rubber powder, the dry shrinkage coefficient of cement stabilized gravel will decrease with the increase of rubber powder content, that is, rubber powder will effectively slow down the occurrence of dry shrinkage cracks in cement stabilized gravel base.
(4) Considering the influence of rubber powder on the strength and dry shrinkage performance of cement stabilized gravel base, the dosage of rubber powder in cement stabilized gravel mixture should not be too high, generally about 1%, and the specific dosage needs to be determined according to field tests.
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