In China’s road construction, rural road construction has great differences from urban road construction. Comparing from the road construction conditions, urban construction is more convenient, which is not only reflected in construction technology, but also in material transportation and equipment application. Compared with rural road construction, urban road construction has higher requirements and standards, mainly because the traffic flow in cities is much larger than in rural areas, while the implementation of China’s rural is an environmentally friendly ecological construction route, so the air around rural roads has higher quality requirements, and thus also put forward higher requirements for construction technicians.
1 cement concrete pavement overview
The so-called cement concrete pavement generally refers to the construction of the pavement surface layer using higher strength concrete, and the block of load is spread through the nesting action between the concrete blocks so as to form the pavement. It has a structure consisting of concrete blocks, bedding layer, kerbs and embedded joints. The process of paving cement concrete pavement is relatively simple and easy to repair. Cement concrete block is generally hexagonal, thickness is more than 10cm, generally in the low grade highway, geological poor transition section often used to cement concrete pavement.
2 Selection of cement concrete construction materials
2.1 Selection of cement
To ensure that the incoming cement has a laboratory list and product certification, the quality of the selected cement should be guaranteed to meet the requirements. The flexural strength of rural roads is generally in the range of 4.0-4.5MPa, and the cement used is 32.5 grade. After the cement comes in, it needs to be neatly stacked. During the storage of cement, because of the poor construction conditions in rural areas, special attention needs to be paid to moisture and water resistance. Cement stored for more than 3 months needs to be sampled in time for testing to ensure its availability, and if the cement appears caked with moisture, it is not allowed to be used. It is important to ensure that the stability and strength of the cement at all ages are in line with the design requirements; when paving manually, it is important to ensure that the cement has an initial setting time of more than 3 hours and also a final setting time of more than 6 hours. If conditions do not allow, medium and coarse sand can be used, and through the ratio test, fine sand can be used, but make sure its content of clay impurities is below 3%.
2.2 Selection of sand
The selected sand should ensure to meet the designed fineness modulus and gradation, and more than 2.3 medium and coarse sand, the selected sand has a low impurity content, clean and dry; if the 0.075mm sieve size through the quality percentage to be less than 5%.
2.3 Selection of crushed stone
The strength of the selected crushed stone must be above grade 3, with a hard texture, and the maximum particle size is generally less than 31.5cm; the best crushed stone particle shape is generally square hexagonal, and the flaky particles and needle-like particles must be below 15%. In the crushed stone, for 0.6mm particles below the need to do plasticity index test and liquid limit test, the liquid limit is required to not more than 28%, plasticity index below 9.
2.4 The choice of water
The water selected is generally the drinking water that can be used directly, to ensure clean and free of impurities.
3 Pre-construction preparatory work
3.1 Choose the right mixing site
Suitable mixing site is to ensure the smooth construction of highway projects is an important guarantee, first of all, to ensure that there is sufficient water and power supply; secondly, to ensure that there is sufficient mixing site space to avoid the problem of insufficient stacking space in the construction process; again to ensure that there is sufficient cement warehouse and other infrastructure to build space; finally, to ensure that there is a good mixer drainage channels.
3.2 Preparation of construction machinery
When carrying out construction, a wide variety of apparatus and machinery will be used in the construction of cement concrete for rural roads, so a series of equipment such as flat vibrators, steel formwork and forced mixers need to be prepared and checked in advance.
3.3 Concrete material trial preparation
Before the cement concrete pavement construction, the concrete proportioning scheme and other experimental work need to be done in advance according to the design of the pavement construction. In the selection of ingredients, two sizes of coarse aggregates need to be prepared and matched according to the performance of different materials, in addition, the compressive and flexural experiments can be carried out according to different coarse aggregates, through a certain weight of the age of the application of pressure stage, and finally the best performance of the mix ratio will be selected and used.
3.4 Inspection and maintenance of the base layer
Further inspection and maintenance of the pavement subgrade is required before carrying out construction. Strictly check and test all aspects of the width, strength and height of the pavement, the construction of cement concrete pavement needs to reach certain standards before it can be carried out, and the construction unit must promptly return to repair the places that do not meet the standards, and also need to pay special attention to the wetness of the surface of the base layer to prevent moisture loss from appearing in the bottom layer of the concrete, which will have some impact on the pavement.
4 Construction process of cement concrete for rural road projects
4.1 Placing construction
According to the design requirements, the center line and edge line of the pavement are placed; the expansion joints of the pavement panel, the parting plate and the position of the structure are checked; the elevation can be checked by the level point set up in the construction of the base layer during the construction process.
4.2 Formwork support
It is necessary to sweep the leveling layer or deal with the floating soil or debris above the grass-roots level, and then carry out the support of the template. The height of the formwork should be consistent with the thickness of the road surface. Make sure that the formwork has close and smooth splicing and needs to be firmly supported, and the formwork is fixed by drilling into the subgrade by steel brazing to avoid displacement phenomenon when leveling, paving and vibrating, and sealing the gap with the ground by mortar.
4.3 Installation of formwork
Before laying the concrete, the formwork first needs to be installed. Inspection and correction work needs to be carried out regularly before the formwork is used to eliminate the formwork that has been deformed or bent in time, and in addition, the concrete should be kept at the same height as the formwork. In the installation of the formwork, it is necessary to place the formwork on the adjusted base layer or well-done base layer according to the prior placement position, followed by drilling the brazier into the base layer from the outside and subsequently fixing the formwork, to effectively control the distance between the brazier, in the process of concrete paving and vibrating, the formwork is always in a fixed state, and the plane position and elevation of the standard fixed formwork. After laying the concrete pavement, you need to pull out the brazier, this time to ensure that the formwork is in a solid, upright state, and at the same time can be tightly combined with the joints, in addition to not allowing the front and back of the misalignment or uneven height. To use the mix to fill the vacant space below the formwork, to avoid leakage or sinking phenomenon in the concrete pounding process.
4.4 Concrete paving
The mixture delivered to the pouring site is poured directly into the road trench with the side forms installed, and then filled by manual means. In the paving process, the mix needs to be broken up by tools such as large iron palladium, and then the mix will be palladed by shovel, using the scraper to pave, using the square shovel to spread the mix near the edges of the formwork and other edges by means of the buckling shovel method and inserting the pounding several times until the mortar is pounded out to prevent any hollow honeycomb situation. During the paving process, ensure that the loose concrete is about 10% higher than the design height of the top surface of the formwork. During the construction operation, if the paving construction exceeds 30 minutes of interruption, the transverse construction joints need to be set.
4.5 Vibrating concrete
In general, the thickness of rural roads should be controlled within the range of more than 18cm and less than 22cm. For concrete slabs with a thickness of 22cm or less, it is necessary to use an insertion-type vibrating bar to conduct sequential vibrating against the corners, followed by using a flat vibrator with a power of 2.2kW or more to conduct longitudinal and transverse full vibrating, while ensuring an overlap of 10-20cm during vibrating, followed by a Vibrating beam to complete the pounding and dragging flat, in the pounding process to prevent the location of steel deformation. When pounding, it is also necessary to level by manual means and check the formwork to correct loosening, sinking and deformation in time. When leveling the concrete mix, a concrete mix with finer gravel needs to be used to fill the slab surface, and pure cement mortar is not allowed to be used in filling the leveling. If the pavement panel is constructed in two widths, there is a road arch design in the road surface, it needs to be leveled through the pavement forming plate. In the leveling to maintain a clean and tidy formwork top surface, the joints of the plate surface smooth and flat.
4.6 Finishing of concrete
After the vibrating is completed, the slurry is repeatedly rolled along the longitudinal direction by means of rollers. The surface is smoothed with a large wooden trowel until the surface is free of water, and then calendered with an iron trowel for 2 to 4 times. No dry cement or cement paste powder is allowed to be added to the surface when finishing is carried out. During the finishing process, a 3m straightedge is used to check the flatness. After finishing the plastering, embossing is done with an embossing machine along the cross slope direction to ensure the safety of traffic and people. It is necessary to set a 2% to 3% transverse slope of the concrete pavement to provide convenience for drainage when the surface is closed.
When demoulding, the formwork support and brazier should be removed first, and then a flat-headed tool should be inserted gently between the formwork and the concrete and slowly pried outward, which will cause damage to the edge of the concrete slab if the force is too strong.
4.8 Maintenance construction
After the completion of the brushing work, it is necessary to carry out the curing operation. In general, the water sprinkling wet curing method is used. In having sufficient water for nourishment or rainy days, wet straw mat, wet sack or straw bag can be sprinkled with water to cover the nourishment, in the process of sprinkling maintenance, the concrete surface is not allowed to pour water directly, the way of surrounding water nourishment should not be used. When the concrete strength reaches 40% of the design bending strength after the pedestrian can pass, to reach more than 80% of the design strength, you can remove the grass mat or wet sack, open traffic.
To sum up, in the construction of cement concrete pavement of rural roads, construction units and technical personnel must strictly follow the relevant standards and regulations to ensure the standardization and standardization of construction, through strict control of construction quality, so that the overall project quality is guaranteed, so that the construction performance of the road is constantly improved, so that the phenomenon of poor rural roads in general is gradually improved.