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I “Delayed Setting” in Concrete
Excessive setting time in concrete refers to the condition where the concrete has not reached final set even after exceeding the normal setting time. The setting time for ready-mixed concrete typically ranges from an initial set of 6 to 8 hours and a final set of 8 to 12 hours under laboratory conditions.If the concrete setting time exceeds 24 hours, it is considered delayed setting; if it exceeds 48 hours, it is classified as super-delayed setting.
The phenomenon of delayed setting in concrete can be divided into two scenarios:
1.Overall Severe Delayed Setting
This is often caused by the use of admixtures. The inclusion of inappropriate retarding components (many of which are significantly affected by temperature and other factors, leading to noticeable changes in setting time) or the use of admixtures in quantities exceeding the normal dosage can lead to excessive delayed setting of concrete.
2.Local Severe Delayed Setting
If the majority of the concrete in slabs or walls sets normally, but some areas experience delayed setting, the causes may include:
The admixture was added using the late addition method, resulting in uneven mixing of the concrete and local concentration of the admixture;
Water was added on-site, reducing the cohesiveness of the concrete, leading to bleeding or segregation, and the compaction process caused local concentration of the paste, increasing the water-cement ratio and making the admixture relatively excessive;
Sediments in the admixture pool containing retarding components are not easily mixed uniformly, leading to local excessive delayed setting in the concrete.
II "Hard Crust" Phenomenon in Concrete
After pouring the concrete, the surface may appear "hardened," but the interior remains in an unhardened state, forming a "sugar core," hence the term "hard crust" phenomenon. This is often accompanied by cracks of varying degrees, which are difficult to smooth out with a trowel.
This phenomenon frequently occurs in hot and dry weather conditions. The surface is not truly hardened; rather, it is largely due to the rapid evaporation of moisture, causing the concrete to lose water and dry out. The strength of the surface concrete is reduced by about 30%, and further watering and curing are of little help.
In addition to climatic factors, the composition of the admixture and the type of concrete admixture also have a certain relationship. The formation of a hard crust is more likely when the admixture contains sugar and similar retarding components; the use of mineral powder is more pronounced than fly ash.
Solutions:
1. Appropriately adjust the admixture formula, using phosphates as retarding components instead of sugar, lignosulfonate, glucose, sodium gluconate, etc.;
2. Use fly ash as an admixture, which has better water retention properties than mineral powder;
3. If fine cracks appear on the surface, secondary vibration can be used to eliminate cracks before the concrete reaches its initial set to prevent the formation of through cracks;
4. The most effective method should be construction and curing measures, that is, to avoid direct sunlight on the concrete as much as possible. Freshly poured concrete can be cured using spray and watering methods.
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