To address this question, we must first determine whether water accumulation in the foundation pit affects concrete quality. According to Article 7.5.2 of GB/T14902 "Ready-Mix Concrete" standard: Concrete mixer trucks must completely drain water from mixing tanks before loading, and it is strictly prohibited to add water to the concrete mixture after loading. Additionally, Article 7.2.5 of DB21/T1304 "Technical Regulations for Ready-Mix Concrete" specifies that water addition is forbidden during pumping operations. These provisions clearly indicate that water should not be added or allowed to accumulate in ready-mix concrete before or during pouring. Therefore, pouring concrete is strictly prohibited when water remains in the foundation pit.

There is water accumulation in the ready-mix concrete before or during the pouring. The direct effect of water on the concrete is to increase the original water-cement ratio of the ready-mix concrete. Then what hidden dangers will be brought by the increased water-cement ratio? The main problems are as follows.

(1) The concrete mix ratio issued by the test unit is changed, and the strength, water resistance grade and slump of the concrete will be affected, that is to say, the quality of the concrete can not be guaranteed, which is a serious quality hazard.

(2) If the water-cement ratio is too large, the fluidity of concrete will be increased, that is, the self-flow of concrete will be increased. The fluidity of concrete will be increased, and the segregation phenomenon will occur in concrete, and the quality of concrete can not be guaranteed.

(3) Given that concrete has a significantly higher density than water, the poured concrete tends to push water to the surface during construction, creating localized depressions. These depressions cause water to dilute the cementitious materials in the concrete, resulting in areas with negligible strength. This leads to stress concentration in the concrete, which significantly compromises the overall quality of the pour.

According to the analysis of the above situations and reasons, it is not allowed to pour concrete when there is water accumulation. So how can we prevent this phenomenon from happening?

(1) Strengthen management. In the place where there is water, the water must be drained before the concrete is poured.

(2) Pay attention to the change of weather. When there is precipitation, try to avoid pouring concrete, and do not affect the quality of concrete because of the pursuit of construction period.

(3) Enhance communication. During concrete pouring, promptly contact the ready-mix concrete supplier if rain occurs. If possible, leave construction joints. If pouring cannot be stopped, immediately consult the ready-mix concrete technicians to adjust the mix ratio and minimize rainwater's impact on the concrete.

(4) Try to adopt the method of two vibrations to pour concrete. When the concrete is about to set, the liquid concrete can be re-vibrated with a flat vibrator or rolled with an iron drum to eliminate the water on the surface and reduce the cracks on the surface of the concrete.

(5) After the concrete pouring is completed, the curing should be strengthened. If conditions permit, plastic film covering or spray curing should be adopted. If the poured concrete meets the conditions of large volume concrete, the construction should be carried out according to the construction requirements of large volume concrete.

The above phenomena indicate that concrete pouring should be avoided whenever possible when encountering water accumulation in the foundation pit or rain/snow during concrete pouring. When unavoidable due to various reasons, thorough preparation, rational organization, and meticulous construction should be implemented to minimize adverse impacts and deliver qualified projects to the construction entity.