by In order to save costs, modern concrete is mixed with a large amount of admixtures (fly ash or mineral powder), or there is no fly ash or mineral powder but too much admixture in cement.
After vibration, the surface of the concrete is prone to slurry floating, excessive surface floating slurry, water evaporation leaving holes, and excess water causing local water cement ratio to be too high, resulting in low surface strength of the concrete.
In addition, the surface strength of concrete cannot increase even at low temperatures.
The mixing plant uses C30 concrete, which has a rebound of 3-5 megapascals lower, and even lower.
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