During the construction of cement concrete pavement, if the demolition of the mold too early, due to the lack of early strength of the concrete, it is easy to damage the newly built pavement during the demolition of the mold, resulting in off-edge, off-angle, adhesion and other phenomena, affecting the durability and strength of the concrete pavement; if the demolition of the mold too late, it will lengthen the turnover cycle of the formwork, reduce the frequency of the use of the formwork, thus affecting the construction period. At the same time, if the demolition of the formwork is not timely, it will also cause the delay of the roadway slit, which will cause irregular shrinkage cracks in the low temperature or large temperature changes, resulting in structural damage to the concrete roadway slab. Therefore, how to reasonably choose the concrete roadway construction of demolition of the formwork and timing of cutting joints, to avoid hidden engineering quality problems, is particularly important. In this paper, we take the construction of a concrete pavement as an example and use field experiments to study the timing of concrete pavement demolition and cutting joints.
Project Overview
The cement concrete pavement thickness is 28cm, the slab size is 4.5m×5m, using Huaxin No. 425 ordinary Portland cement produced by Xinyang Cement Plant, gravel concrete, mixing plant centralized mixing, Vibe consistency between 25 ~ 30s, is super dry hard concrete. In the construction season from the end of September to the end of November, the rainfall is less, and the temperature drops significantly after the middle and late October, with a temperature difference of 8-10℃ and an average temperature of 10-15℃.
Experimental research
Timing study of mold removal
The earliest demolition time is controlled mainly to avoid all kinds of damage to the concrete course during demolition, such as falling edges, corners, and sticking, and the later the demolition, the higher this safety factor will be. For the temperature of the project area, in mid to late October, it should be removed 60 hours after the concrete is formed. If the joints are cut after demolition, due to the large temperature difference between day and night, the concrete road surface may have shrinkage cracks, so that the concrete slab irregular pull crack, causing very serious structural damage, in order to avoid this risk, it is necessary to cut joints without demolition of the formwork. However, it is difficult for the cutting machine to cut to the slab edge, so the slab edge is usually left 20-30cm close to the slab edge, and the slab is cut after the demolition of the mold. If the temperature difference between the construction environment is small, the practice is still okay, but if the ambient temperature difference is large, the lower part of the roadway panel at the false joints cutting place is easy to appear local fracture, at this time, the uncut slab edge parts due to stress concentration will appear slab edge corner fracture damage. Obviously, it is undesirable to remove the formwork too late to cause structural damage to the roadway surface.
- Comparison test on the timing of mold removal
To scientifically determine the best time to demolish the mold, in 10.25 ~ 10.26, the average day and night temperature of 14.5 ℃, respectively, in the concrete molding after 12h, 16h, 20h, 24h, mold demolition comparison test can be seen, with the postponement of demolition time, demolition caused by concrete road surface damage gradually decreases to 24h, to get good quality concrete products, no apparent This is 60% shorter than the 60h required by the specification, indicating that early demolition is completely feasible, and greatly improves the use of the formwork efficiency and saves turnaround time.
- Demolition formwork construction requirements
(1). demoulding time should not be earlier than 24h, if the subsequent construction conditions allow, can try to delay the demoulding time.
(2). demolition of the mold, should first clear the mortar around the template, stones and other debris, parallel demolition and removal, not hard smash, hard pry, brutal construction.
(3). set up tie-rod joints in the template, should be straightened before demolding, and clean the eyelets.
(4). demolition of the concrete slab should be promptly painted on the side walls of the asphalt protective coating.
(5). In order to ensure early demolition of concrete strength, concrete pouring time should be controlled, generally from the early morning pouring, to the temperature is getting higher when the completion of the warehouse.
(6). for a small amount of off-edge, off-angle parts, must be processed in a timely manner after demolition of the mold.
Research on the timing of cutting joints
Concrete roadway panels should be cut in time. If the slit is cut too early, the concrete strength will be low, and the edges and corners of the slit will be easily damaged by the blade of the slitting machine, resulting in the edges and corners falling off; if the slit is cut too late, the concrete strength will be too high, which will make it difficult for the slitting machine to cut the slit and ensure the straightness, and the blade will wear out more, resulting in waste, and more seriously, the concrete slab will shrink and crack. So the scientific choice of cutting time is very important, too early and too late are not conducive to concrete construction. According to the specification: gravel concrete appropriate in the compressive strength of 9MPa when cutting seam.
- test study
On October 18, samples were taken on site and 6 groups of simultaneous specimens were prepared in the testing room. 18h, 24h and 48h concrete specimens were selected for compressive test with the average strengths of 8.3MPa, 10.3MPa and 14.4MPa respectively. 18h, 24h and 48h concrete specimens were selected for compressive test due to the use of super dry hard concrete. The strength of the specimen is higher than that of ordinary concrete at the same age.
When the specimen is prepared for 18-24h, the strength can meet the requirement of slitting, but if the curing time is too long, the concrete strength will increase greatly, which is not good for slitting construction and is prone to hidden danger. 24h for slitting, the false seam is straight, uniform, natural fracture and good stress condition; 48h for slitting, the concrete strength will increase, the blade of slitting machine will not run smoothly, and the operator will lose strength suddenly at the end of the slab, which will make the machine easy to be slightly damaged. Wobble, but instead of edge off phenomenon, and the lower part breaks too early, and a certain amount of deviation from the false seam, the force conditions are reduced.
The comparative analysis with the mold removal time found that the best mold removal time and the best seam cutting time are both about 24h, which can be used for full section seam cutting immediately after mold removal, the two work well together, avoiding the contradiction between mold removal and seam cutting in timing, which is good for construction.
- Cutting seam construction requirements
(1). The cutting time of the concrete roadway slab should be 18h, not too late.
(2). Independent warehouse concrete by steel formwork constraints, reducing the risk of irregular broken slab, cutting joints can be delayed to about 24h, can be cut immediately after the demolition of the mold.
(3). The cutting joint construction should be completed in the whole section (transverse) at one time to prevent the phenomenon of corner drop.
(4). The cement slurry in the body of the seam should be cleaned immediately after the cutting and covered with geotextile for early recovery.
Conclusion
(1). Through the field comparison test, it was determined that the demolition time of cement concrete pavement construction can be advanced to 24h, which is 60% shorter than the specification requirement, and the construction efficiency is improved while guaranteeing the project quality.
(2). Through the compressive strength test of specimens of different ages, it is confirmed that the 9MPa compressive strength of the airport super dry hard concrete can be reached in 18~24h.
(3). The contradiction between the timing of dismantling the concrete pavement and the timing of cutting joints is solved during the construction of the concrete pavement, which effectively avoids the hidden danger that the concrete pavement slab is prone to irregular fracture if the joints are cut too late.
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