Controlling the loss of concrete slump involves a multifaceted approach; measures must be targeted based on the specific causes of the loss to achieve immediate and effective results. The following key measures should be adopted:
1. If ready-mixed concrete experiences rapid slump loss due to unforeseen circumstances—rendering it unpumpable—the "delayed addition" method for pumping agents may be used. After adding the pumping agent, the concrete mixer truck must rotate rapidly for two minutes; the concrete may only be used after testing confirms the slump meets requirements. The dosage for this delayed addition must be determined beforehand through testing and should not be applied arbitrarily or repeatedly; exceeding the dosage limit can prevent the concrete from setting for several days.
2. Finer cement particles increase water demand, which inevitably accelerates slump loss. To mitigate this loss, compatibility tests between the cement and the pumping agent must be conducted to determine the optimal dosage of the latter.
3. Raw materials for pumped concrete—including cement, admixtures, aggregates, and pumping agents—must be carefully selected to ensure they meet quality standards. It is essential to understand their properties and performance, increase the frequency of re-testing, and maintain relatively stable supplier relationships. Concrete mix proportions should be determined through testing and continuously optimized based on production experience to achieve the best mix design, optimal aggregate gradation, minimal silt content, and minimal slump loss.
4. The slump of pumped concrete decreases more rapidly over time and experiences greater loss as temperatures rise. Concrete should be placed into the forms within 60 to 90 minutes after discharge from the mixer. The rate of slump loss accelerates after 60 minutes—a phenomenon particularly pronounced during hot-weather construction. Ideally, concrete should be placed within 90 minutes of discharge when the daily average temperature is below 25°C, and within 60 minutes when it exceeds 25°C. Production and placement teams must maintain close communication and coordination, dispatching trucks based on transport distance and pouring speed to avoid excessive queuing or delays. Concrete that has exceeded its initial setting time must be discarded and cannot be used; this applies even if the mix is thinned by adding water-reducing agents. Generally, concrete without set-retarding admixtures cannot be used after 5 to 8 hours; for concrete containing set-retarding admixtures, the usability limit is determined by the specific retardation period.
5. Concrete mixing time must be strictly controlled, as proper control effectively minimizes slump loss. For pumped ready-mixed concrete, mixing times should be determined by the concrete grade: no less than 30 seconds for C10–C15; 40 seconds for C20–C25; 50 seconds for C30–C40; and 60 seconds for grades above C45. Concrete with special requirements must be mixed for a minimum of 60 seconds.
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