Cement concrete pavements are widely used and have played a great role in the national economic construction and sustainable development. In use, it is also found that a considerable part of the pavement has different degrees of cracking, broken plate, subsidence, misplaced platform and other structural diseases , especially transverse cracks (regular and irregular) in the pavement, which not only bring great difficulties to maintenance and repair, but also seriously affect the function and life of the pavement, and become a technical difficulty that restricts the development of cement concrete pavement. In this paper, combined with the actual yueqing mountain old company line highway reconstruction project, focusing on cement concrete pavement transverse random cracking hazards, causes and prevention measures to be explored, in order to benefit the future road construction.
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Transverse Random Cracks
Cement concrete pavement transverse cracks have regular cracks and irregular cracks. Regular transverse cracks, refers to the direction perpendicular to the direction of travel, and the basic parallel to the shrinkage or expansion joints, through the full thickness of the concrete pavement panel cracks; irregular cracks, refers to the crack direction is not perpendicular to the direction of travel, not parallel to the shrinkage or expansion joints and not in the shrinkage or expansion joints position, but in the middle of the concrete pavement slab or close to the shrinkage or expansion joints position of the slab cracks, such cracks we call it transverse irregular cracks or transverse random cracks.
Transverse random cracks on the cement concrete pavement structure is extremely harmful, resulting in direct infiltration of rainwater, under the action of traffic load, the internal formation of high-pressure water, continuous infiltration, scouring, erosion, so that the concrete slab under the plate dehollow, cracks plus giant, and finally caused the concrete plate overall fracture and loss of bearing capacity and use of function, endangering the safety of traffic.
Transverse Random Cracks To Explore The Cause
The causes of transverse random cracks in cement concrete pavement are many, and according to the relevant information at home and abroad, it is roughly considered to be related to concrete dry shrinkage (plastic shrinkage), cold shrinkage (cooling shrinkage), construction quality, boundary impact and other factors.
1. Cracks caused by dry shrinkage of concrete
Cement concrete road panels in the hardening process, due to the gradual evaporation of water, panel volume contraction generated by dry shrinkage, its linear shrinkage index of 1.0 ~ 2.0 × 10-4, that is, 0.1 ~ 0.2mm per meter shrinkage; and temperature changes generated by the linear expansion and contraction index of 1.0 × 10-5, that is, the temperature per 1.0 ℃ rise and fall, 0.01mm per meter deformation. The former in the cement concrete panel hardening process the former occurs in the hardening process of cement concrete panels, and the latter occurs in the whole process of hardening and after hardening. When the sudden change in temperature, such as summer night rain, winter day and night temperature difference, coupled with the impact of wind, concrete surface moisture evaporation accelerated, especially when the temperature rise and fall of more than 10 ℃, panel hardening, dry shrinkage and temperature changes in the superposition of expansion and contraction, concrete panels such as not timely demoulding, regeneration, cutting joints, resulting in cracks broken plate is inevitable.
2. Cold shrinkage (cooling shrinkage) cracks
Cement concrete has thermal expansion and contraction properties. Thermal expansion belongs to the compression deformation, while cold shrinkage belongs to the tensile deformation, tensile deformation is very easy to cause concrete structure cracking. Concrete tensile strength is only 1/8 to 1/7 of the compressive strength, about 4.3 to 5.0 mpa, when the external concrete slab subjected to tensile stress once exceeded the ultimate strength of concrete at that time, the slab will produce transverse random cracks.
3. Construction quality influence
Poor quality of grass-roots construction
A road project because the road through the mountainous areas, poor construction conditions, grass-roots (water-stabilized layer) construction all with manual paving, resulting in poor surface flatness of the grass-roots level (height difference ± 3 ~ 5cm), increasing the friction between the grass-roots level and the panel, affecting the free expansion of the panel, in the low-temperature season is easy to cause the panel to produce transverse irregularity random cracks; in order to grab the schedule, the other half of the road is in the summer construction, the grass-roots surface is excessively dry in order to rush the construction schedule, the other half of the pavement is constructed in the summer, the surface of the grass-roots level is overly dry, absorbing part of the moisture of the surface concrete, making the bottom concrete water loss serious, shrinkage inconsistent, strength reduced, resulting in panel cracking; the most serious cracking lot, due to uneven settlement of the roadbed, coupled with the aforementioned impact, resulting in uneven strength of the grass-roots level, so that the concrete panel in the location of low strength transverse random cracks.
Uneven thickness of concrete panel
This leads to the road panel to produce transverse random cracks in more locations, the reason for this is mainly the grass-roots elevation control is not strict so that the concrete panel thickness is not uniform. According to the site investigation, the thickest concrete panel 26 ~ 27cm, the thinnest only 21 ~ 22cm, designed for 25cm. Too thin or thick and thin junction of stress concentration and the formation of weak sections, in the concrete shrinkage is difficult to withstand the tensile stress and lead to random cracking; in addition, the depth of the shrinkage joints cut with the seam cutter is generally only 4.5 ~ 5.5cm, and the location of the shrinkage joints during construction may is very thick at the plate, because the strength of the concrete panel cross-section weakened enough, cracking will not occur in the location of the cut plate joints, but in the location of the weak strength of the concrete plate, thus forming random cracks.
Untimely seam cutting
Concrete transverse shrinkage is very sensitive to the early strength of concrete, in the actual construction if due to machine failure or operator cutting time mastering inaccurate (not timely) or cutting depth is not enough, will cause the concentration of stress in the concrete, the weakness in the concrete often form transverse random penetration cracks.
Concrete construction ratio
During construction, the aggregates and sand are stacked in the open air, and their water content distribution is very uneven, that is to say, there is an obvious error between the water content of the material and the measured water content of the test during actual construction. Thus, the adjustment of the construction mix ratio according to the measured water content may not be accurate to the original theoretical mix ratio, and the resulting construction water-cement ratio compared with the design water-cement ratio also has certain errors. In addition, another factor affecting the accuracy of the construction adjustment ratio is the temperature, the same construction slump of cement concrete, high temperature than the low temperature construction is much more difficult.
Concrete panel maintenance influence
Concrete maintenance is extremely important for its early strength growth and prevent shrinkage cracks. The actual construction site personnel are basically natural maintenance, so that the surface of the concrete occurred in a number of irregular small cracks.
Premature passage of vehicles
Due to insufficient concrete strength and prematurely open traffic, the resulting load stress leads to transverse random cracks in the road panel.
4. Boundary causes
The influence of the first cast slab on the later cast slab
When the temperature changes more than a certain amplitude, the concrete slab on both sides of the fractured shrinkage joint shrinks, so that the concrete slab of the “After-cast slab” Which has not been cut is subjected to large tensile stress (the concrete strength is still low at this time), when the tensile stress is greater than the initial tensile strength of concrete, it will be in the “After-cast slab when the tensile stress is greater than the initial tensile strength of concrete, transverse random cracks will occur at the location corresponding to the “First cast slab” Shrinkage joints, and such cracks also exist on other concrete pavements. The longer the interval between “First cast slab” And “After cast slab”, the more likely to cause “After cast slab” Cracking.
The effect of setting ties for longitudinal shrinkage joints
The purpose of setting tie bar for longitudinal shrinkage of concrete panel is to prevent the gap from increasing due to displacement of slab, but at the same time, it also forces the horizontal expansion and contraction of “After-cast slab” And “First-cast slab” To be consistent. Originally, the expansion and contraction of the slab can be adjusted at the shrinkage or expansion joints before the construction of the “First cast slab” And the “Later cast slab”, but after the construction of the “Later cast slab”, its expansion and contraction through the longitudinal ties must synchronized with the “First cast slab”. However, the initial strength of the concrete of the “Post-cast slab” (before 50 hours) is much lower than that of the “Pre-cast slab”, if combined with the uneven thickness of the concrete slab or improper concrete mix ratio, which leads to uneven distribution of concrete strength, then the weakness of the concrete in the “Post-cast slab” Will be reduced. (1) if the concrete slab thickness is not uniform or the concrete mix ratio is not appropriate, resulting in uneven distribution of concrete strength, it is easy to form transverse random cracks in the weak section of the “After-cast slab”.
(3) if the grass-roots stabilization layer has cracked, the grass-roots level on both sides of the crack shrinks when the temperature drops, due to the role of friction, the same will also drive the above initial concrete panel cracking.
Prevention And Control Measures
- The quality of each raw material of concrete pavement must be qualified.
- Control concrete pavement cracks must ensure the quality of roadbed filling. Roadbed compaction quality requirements to achieve the specified compaction value and bending value, to ensure that the roadbed has sufficient, uniform strength.
- Carefully control the elevation and levelness of the grass-roots level. Establish a slot inspection system to ensure uniform thickness and strength of the base layer; control the construction elevation of the top layer of the base.
- Strictly control the concrete ratio. Conduct repeated tests according to the water content of aggregate and sand measured on site, and convert the theoretical mix ratio into construction mix ratio as the basis of concrete construction mix material for accurate batching.
- Try to avoid the construction of high temperature season.
- Prevent the generation of concrete dry shrinkage and cold shrinkage cracks. ① choose the appropriate variety of cement; ② increase the amount of fly ash, the use of high-efficiency water reducing agent.
- Timely cut joints. After demoulding, cut joints should be organized in time, usually controlled at 200-300℃/h or when the compressive strength of concrete is 8-10mpa. Reasonable and timely cutting joints can partially release the shrinkage stress and direct the stress concentration to the design cutting section, thus forming regular cracks.
- Set transverse seam transfer bar. Transverse shrinkage transfer bar can permanently improve the load transfer capacity of pavement shrinkage joints, improve the force state of the grass-roots level and road panel, and eliminate the random cracks caused by temperature changes.
- Boundary impact control. (1) try to cut joints in time for the panel of “Post-poured plate”; (2) try to shorten the construction interval between “First-poured plate” And “Post-poured plate”; (3) after the grass-roots level is finished, pour cement concrete pavement in time to prevent the grass-roots level from being exposed. (3) after the grass-roots level is cured, pour the cement concrete pavement in time to prevent the grass-roots level from dry shrinkage cracks due to long exposure time.
Conclusion
From the engineering point of view, it is inevitable that the cement concrete pavement produces transverse random cracks, and the reasons for its formation are also many, but as long as we seriously grasp the key and measures, the concrete pavement transverse random cracks, this persistent problem can be effectively controlled.
