Causes and countermeasures of abnormal setting of concrete
I. Situations and causes of abnormal setting of concrete
Abnormal setting of concrete includes two situations: setting time is too long and setting too fast. The following are its common causes :
1. Reasons for long coagulation time
-Cement factors
Insufficient gypsum content in cement will cause the tricalcium aluminate in cement to not be inhibited by gypsum in time, resulting in a longer cement setting time. If cement is stored for too long or gets damp, its activity will decrease, which will also prolong the setting time of concrete.
-Admixture factors
Too much retarder will significantly prolong the setting time of concrete. The poor compatibility of water reducer with cement may lead to poor workability of concrete, which will indirectly affect the setting time.
-Mixing ratio factors
If the water-cement ratio is too large, excess water will slow down the hydration reaction of cement and prolong the setting time of concrete.
-Environmental factors
When the ambient temperature is low, the hydration reaction of cement slows down and the setting time of concrete is prolonged.
2. The reason why the coagulation time is too short
-Cement factors
If the tricalcium aluminate content in cement is too high, the cement hydration rate will be accelerated, resulting in a too short setting time of the concrete.
-Admixture factors
Too much early strength agent will promote the rapid hydration of cement and shorten the setting time of concrete. If the admixture contains chloride salts such as calcium chloride, it will accelerate the setting of cement.
-Environmental factors
If the ambient temperature is too high, the hydration reaction of cement will be accelerated and the setting time of concrete will be shortened.
-Others
In addition，abnormal coagulation of concrete may also occur if the mixing time is too long or too short, or if the raw materials contain impurities that affect coagulation.
II. How to judge whether the concrete is abnormally solidified
1. Observe the appearance of concrete
-Surface condition
The surface of normally coagulated concrete will gradually lose its luster and change from wet to dry. If the concrete surface remains wet for a long time without any signs of coagulation, it may be that the coagulation time is too long; if the surface dries up quickly, hardens, or even cracks in a short period of time, it may be that the coagulation time is too fast.
-Color change
Of concrete will gradually darken during the setting process. If the color changes abnormally, such as being lighter for a long time, it may be that the setting is slow; if it quickly darkens and is accompanied by heat, it may be that the setting is too fast.
2. Test the physical properties of concrete
-Slump loss
Through the slump test, if the slump loss of concrete is too fast, it becomes difficult to flow in a short time, which may be due to too fast setting; if the slump loss is too slow and it maintains a large fluidity for a long time, it may be due to too long setting time.
-Penetration resistance
Use a penetration resistance meter to test the penetration resistance of concrete. When the time for the penetration resistance to reach a certain value is significantly deviated from the normal range, it indicates that the concrete setting is abnormal. For example, the standard stipulates that at a certain temperature, the penetration resistance corresponding to the initial setting time of concrete is 3.5MPa, and the penetration resistance corresponding to the final setting time is 28MPa. If the actual test time is significantly different from the standard time, it can be judged as abnormal.
3. Evaluate the strength development of concrete
-Early strength
If the early strength growth of concrete within the specified age is significantly lower or higher than the normal level, there may be abnormal coagulation. Under normal circumstances, the strength of concrete should reach 30% - 40% of the design strength in 3 days. If the actual strength is too low, it may be caused by too long coagulation time; if it is too high, it may be caused by too fast coagulation.
-Strength growth law
Normal concrete strength increases with age in accordance with certain rules. If the strength increases slowly or even stops in the later stage, it may be that the abnormal setting in the early stage affects the continuous cement hydration.
4. Analyze the setting time of concrete
-Determination of actual setting time
Use standard setting time determination methods, such as using a Vicat instrument to determine the initial and final setting times of cement paste. Make cement paste in concrete for testing. If the test result is significantly different from the standard setting time of cement, and excluding cement quality problems, it can be considered that the concrete setting is abnormal.
-Compare with empirical values
Compare the concrete setting time experience data with the same mix ratio and the same construction environment in the past. If the setting time of this concrete deviates significantly from the experience value, further inspection is required to see if there is any abnormality.
III. Preventive measures for abnormal concrete setting
1. Raw material selection and control
-Cement:
Choose cement with stable quality and good performance, and conduct strict inspection on each batch of cement, including setting time, strength and other indicators. Avoid using cement that has been stored for too long, damp or expired.
-Admixtures
According to the performance requirements of concrete and the construction environment, select the appropriate admixture and strictly control its quality and dosage. Conduct an adaptability test between the admixture and cement before use to ensure that the admixture and cement can be well matched.
-Aggregate
Control the mud content, mud block content and impurity content of aggregates to avoid the impurities in aggregates affecting the setting properties of concrete. At the same time, ensure that the aggregates are well graded to ensure the workability and density of concrete.
2. Mix ratio design and optimization
-Reasonable design of water-binder ratio:
According to the strength grade and durability requirements of concrete, a reasonable water-binder ratio is determined through experiments to meet the workability requirements of concrete and avoid abnormal coagulation caused by excessive or insufficient water-binder ratio.
-Optimize the amount of mineral admixtures:
Reasonable use of mineral admixtures, such as fly ash, slag powder, etc., according to the type of cement and concrete performance requirements, determine the appropriate admixture type and dosage to avoid the admixture affecting the setting time of concrete.
3. Construction process control
-Mixing and transportation
Strictly control the mixing time and speed of concrete to ensure that the concrete is mixed evenly. During transportation, take effective moisture and heat preservation measures to prevent the concrete from losing moisture or changing temperature too much, which will affect the setting time.
-Pouring and vibrating
Concrete pouring should be carried out continuously to avoid long-term interruptions. Vibration should be dense to prevent quality problems such as honeycombs and rough surfaces, but excessive vibration should also be avoided to prevent concrete segregation and affect its setting properties.
-Environmental condition control
When construction is carried out in high temperature seasons, cooling measures are taken, such as cooling the raw materials and adding ice water to the concrete; when construction is carried out in low temperature seasons, insulation measures are taken, such as using greenhouses and heating water, so that the concrete can solidify in a suitable temperature environment.
4. Quality inspection and monitoring
-Raw material testing
Each batch of raw materials entering the site is strictly inspected, including the setting time of cement, the performance of admixtures, the mud content of aggregates, etc., to ensure that the quality of raw materials meets the requirements.
-Concrete performance testing:
During the concrete production process, regular tests are conducted on concrete performance, such as slump, setting time, strength, etc., to identify problems in a timely manner and adjust the mix ratio or construction process.
-Construction process monitoring:
Strengthen the monitoring of the concrete construction process, including mixing, transportation, pouring, vibration, maintenance and other links, to ensure that the construction process meets the requirements of the specifications and avoid abnormal concrete coagulation due to improper construction.
IV. Treatment of abnormal setting of concrete
1. The setting time is too long
-Adjust the mix ratio
Appropriately reduce the water-cement ratio to reduce the delaying effect of excess water on cement hydration. At the same time, according to the characteristics of cement and concrete performance requirements, reasonably adjust the type and dosage of admixtures, such as reducing the amount of retarder or replacing admixtures with better compatibility with cement.
-Adopt the ambient temperature Heatingandinsulationmeasures,suchastheuseofgreenhouses,heatingequipment,etc.,raisetheambienttemperaturetotherangesuitableforconcretesolidificationandacceleratethehydrationreactionofcement.
-Add accelerator
Adding an appropriate amount of coagulant to concrete, such as calcium chloride, sodium sulfate, etc., can promote cement hydration and shorten the setting time. However, it is necessary to control the amount of coagulant to avoid adverse effects on the later performance of concrete.
2. The setting time is too short
-Adjust the admixture
Reduce the amount of admixtures that accelerate the setting, such as early strength agent, or add an appropriate amount of retarder to extend the setting time of concrete. At the same time, check the quality and compatibility of admixtures and replace them in time if there are any problems.
-Lower the ambient temperature
The temperature of the concrete pouring and curing environment can be lowered by measures such as sunshade and water spraying to slow down the hydration rate of cement. When constructing in hot seasons, concrete pouring can be carried out in the morning and evening when the temperature is lower.
-Adjust the type or amount of cement
If the tricalcium aluminate content in cement is too high, causing it to set too quickly, you can consider changing the type of cement and choosing cement with a lower tricalcium aluminate content. Or you can appropriately reduce the amount of cement and increase the amount of mineral admixtures, such as fly ash, slag powder, etc., to delay the setting time.
Concrete that has already shown abnormal setting and cannot be adjusted by the above methods, such as concrete that seriously fails to meet the strength standards or does not meet the performance requirements, should be scrapped and re-prepared to ensure the quality of the project.
