Today, almost all concrete poured on construction sites is ready-mixed concrete, especially during foundation works. Because foundation concrete pours are large in volume and the site is open, with no obstacles above ground blocking the extension of the delivery pump pipe, the advantages of ready-mixed concrete are fully demonstrated. During the construction of a unit project, since the foundation works pose a safety hazard as long as they are not backfilled within a day, the client, the contractor, and the supervision unit all hope to complete the foundation works as soon as possible. However, we have found that many contractors, during foundation construction—especially for concrete with a column-and-pier structure—start pouring concrete without first draining all the standing water from the foundation pit.

This raises the question: can concrete be poured when there is standing water in the foundation pit?

▲To answer this question, we must first ask whether standing water in the foundation pit affects the quality of the concrete or the concrete itself. According to Article 7.5.2 of GB/T 14902 "Ready-Mixed Concrete": before loading, the water in the mixing drum of the truck mixer shall be drained completely, and after loading, adding water to the concrete mixture in the mixing drum is strictly prohibited. Meanwhile, Article 7.2.5 of DB21/T 1304 "Technical Specification for Ready-Mixed Concrete" stipulates: adding water during pumping is strictly prohibited. From these provisions, it can be seen that ready-mixed concrete must not have water added or be exposed to standing water before or during pouring—that is, when there is standing water in the foundation pit, pouring concrete is not permitted.

The direct impact of standing water before or during the pouring of ready-mixed concrete is that it increases the water-cement ratio of the original ready-mixed concrete. So what hidden dangers does an increased water-cement ratio bring? The main problems are as follows.

▲It alters the concrete mix proportion issued by the testing unit, affecting the concrete's strength, impermeability grade, slump, and other properties—meaning the quality of the concrete cannot be guaranteed. This is a serious quality hazard.

▲An excessively high water-cement ratio increases the fluidity of the concrete, i.e., the self-flowing property of the concrete increases. Increased fluidity will cause segregation of the concrete, and once segregation occurs, quality certainly cannot be guaranteed.

▲Because the density of concrete is much greater than that of water, when concrete is poured, it will "push" the standing water to the surface of the concrete, creating "pits" in some places. In these pits, the standing water dilutes the cementitious materials in the concrete, leaving almost no strength in the concrete there, which leads to stress concentration in the concrete and likewise has a huge impact on the quality of the concrete pour.

From the above situations and cause analysis, pouring concrete is not permitted when standing water is present. So how should we prevent this phenomenon from occurring?

▲Strengthen management. Where standing water is present, all water must be drained before concrete is poured.

▲Pay attention to weather changes. When precipitation is expected, avoid pouring concrete as much as possible; do not compromise concrete quality in pursuit of the schedule.

▲Strengthen communication. When pouring concrete, if it rains, contact the ready-mixed concrete supplier promptly. If a construction joint can be left, leave one; if pouring cannot be stopped, communicate immediately with the ready-mixed concrete technicians to adjust the mix proportion in a timely manner and minimize the impact of rainwater on the concrete.

▲As far as possible, adopt a two-stage vibration method for pouring concrete. When the concrete is about to reach initial set, re-vibrate and liquefy it with a plate vibrator, or roll it with an iron roller, to eliminate standing water on the surface and reduce surface cracking.

▲After the concrete pour is completed, curing should be strengthened. Where conditions permit, use plastic film covering or spray curing. If the poured concrete meets the conditions for mass concrete, construction should follow the requirements for mass concrete.

▲From the above phenomena, it can be seen that when there is standing water in the foundation pit or when rain or snow occurs during concrete pouring, pouring should be avoided as much as possible. When it cannot be avoided for various reasons, full preparation, reasonable organization, and careful construction should be employed to minimize adverse effects and deliver qualified projects to the client.