In actual construction, skilled technicians can determine the initial setting and final setting of concrete based on their experience and the appearance of unhardened concrete (such as the traces after the disappearance of air bubbles on the concrete surface). Differences in management level, quality problems related to setting time occur from time to time

1.2 Reasons

The main reason is that the amount of admixtures (especially the components that retard setting) in the concrete is excessive, and the workability of the concrete mixture is poor when it is put into the mold. One of the following two situations can cause excessive amount of admixtures: 1. The precipitation-prone admixture is not stirred evenly when it is added, and the concentration at the bottom of the container is relatively large. The addition of an admixture with a larger concentration at the bottom will cause the excessive amount of the admixture; ② When the admixture is added, the measuring instrument malfunctions or the excessive amount is caused by man-made reasons. The performance of poor workability: the concrete mixture has greater fluidity, but its cohesion and water retention are poor.

1.3 Hazards

The cement mortar component in the concrete that guarantees the strength of the concrete is lost from the gap of the formwork, and the strength of the solid concrete is greatly reduced; after the formwork is removed, the components have exposed ribs, holes, and honeycomb defects (there are many lost mortars on the floor that must have such defects).

2.1 Symptoms

After the concrete is poured into the mold, the initial setting and final setting are soon followed by the shrinkage of the concrete during the initial setting and final setting process, resulting in irregular plastic shrinkage cracks. Such cracks often occur in late spring and early summer (the temperature rises suddenly), mainly distributed on the cast-in-place slab, and can be found a few hours after the concrete is poured; the cracks on the slab surface are shown in Figure 3, and the slab after the slab surface is impounded The bottom cracks are shown in Figure 4; the number of cracks on the board surface is more than the board bottom; the board surface crack width is larger than the board bottom, and the maximum crack width can sometimes reach 2mm; , and some cracks do not penetrate the plate section.

2.2 Reasons

The concrete mixture should be vibrated and compacted before the initial setting, and the plastering and finishing operations should be completed before the final setting. Since the setting time of concrete is not clearly specified or applicable in the relevant standards, one of the following factors or the combined influence can cause plastic shrinkage cracks in concrete during the process of forming, setting and hardening: ① Standard laboratory conditions and engineering site The difference in setting time of concrete caused by environmental differences; ② the poor adaptability of admixtures and cement or the short storage period of cement in the cement silo after grinding, resulting in too short setting time of concrete; ③ transportation time, transportation time of concrete mixture The interval time to the project site is long; ④The difference in the ability of forming and maintenance operators to control the setting time;

2.3 Hazards

The occurrence of cracks has a great influence on the durability and usability of the concrete cast-in-situ slab; although the load test of the cast-in-situ slab with plastic crack distribution meets the requirements of use, its bearing capacity is lower than that of the cast-in-situ slab without crack distribution; this kind of If the crack is not dealt with effectively, it will also cause complaints and thus social impact.

2.4 Treatment measures

①The degree of cracks is relatively light: the number of cracks is small, the length is short, and the width of the cracks is small; it can be sealed by filling the cracks. ②Severe degree of cracks: the number of cracks is large, distributed in the whole plate, the length is long or short, and the crack width is large; if it is found early (generally after the final setting of the concrete or the day after the pouring, it is found and can be found), It should be chiseled and re-cast. ③ The degree of cracks is between light and serious: after comprehensively considering the requirements of each responsible subject, the cracks are filled with carbon fiber after sealing, and pouring the superimposed layer is also one of the treatment methods.

3 Conclusion

The setting time of concrete will affect the quality of the solid concrete after hardening. At present, there are countless quantitative indicators of the setting time of normal concrete, and the setting time of concrete after compaction is controlled without specific portable equipment to support the operation. In order to avoid the adverse effect of setting time on the quality of solid concrete, it should be applicable and practical. , molding requirements); control the quality of cement, admixtures and concrete mixtures for concrete from the source, and pay attention to the control of setting time as much as the strength of concrete; the molding operation of concrete is handed over to professional companies with skilled operation (similar to scaffolding rental). Installation professional company) or professional team completed; process inspection and concrete mixing, transportation, pumping, molding follow-up; scientific research, instrument manufacturers cooperate to develop a portable test instrument for concrete setting time after the compaction of the engineering field test .