concrete state is to accurately identify whether the concrete meets the construction requirements (such as pumping, pouring and vibrating) through the detection and evaluation of "fluidity-cohesiveness-water retention". 1
. "Gradient detection" method of fluidity slump
(expansion) is the key index to measure the fluidity of concrete.
When the slump or expansion exceeds the design range, it indicates that there is a deviation in the working performance of concrete, which may be caused by factors such as water-binder ratio, admixture dosage or aggregate gradation. The slump (10-220mm) reflects the flow ability of concrete, but the flow uniformity should be judged in combination with the expansion (500-750mm). For example, a slump of 200 mm but a spread of 500 mm × 450 mm (difference > 50 mm) indicates potential segregation (uneven distribution of aggregate).
2. Inverted slump cone test of "dynamic observation" method
of cohesiveness:
after being filled with concrete, it does not collapse or flow within 5S, and only a small amount of bleeding (≤ 50 mL) on the surface, indicating that the cohesiveness is good; In case of aggregate subsidence and mortar stratification, the sand ratio shall be increased by 1% -2% or the fly ash content shall be increased (5%).
Spade stirring test:
stir the concrete for 3 times with a spade. If the concrete can be kept as a whole (no loose aggregate) and spread in a fan shape after landing (no aggregate accumulation at the edge), the cohesiveness is up to the standard.
3. "Bleeding rate quantification" method
of water retention: bottom bleeding:
observe the slurry precipitation of concrete mixture to judge the water retention. If the bleeding depth at the bottom is ≤ 5mm (bleeding rate ≤ 2%) within 30 minutes after the slump cylinder is lifted, it is good, and if it exceeds, the water retention is poor, and the surface laitance may be caused
by too low sand rate or excessive admixture:
the surface laitance thickness is ≤ 5mm (accounting for the section thickness ≤ 2%) 1 hour after pouring. Excessive thickness (e.g. 10 mm) will result in a lower strength of the surface layer (5-8 MPa lower than interior), requiring a reduction in the amount of cement or an increase in the proportion of coarse aggregate.
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