What is the Allowed Time for the Concrete Discharge?

 

What is the Allowed Time for the Concrete Discharge? There is always confusion on site about the required time for the discharge of concrete. Some say it is 60 minutes, 90 minutes, 120 minutes, etc. Which is it? One time, we had cast the wall below ground. It was in the second basement, and due to the distance of the wall from the truck mixer outside the building, it took several minutes to reach the wall using the bobcat. The consultant found out that the concrete had expired and he said to the project engineer, “Engineer, the concrete has expired!” He then told the project engineer, “You have to return the concrete or throw it out from here!” The project engineer replied, shouting “No! As per the standard time… Blah! Blah! Blah!” And so on and so forth. You would probably be confused listening to this, if you heard the two voices, as they barely differed from each other. The consultant took the delivery notes of the concrete and he said, “Engineer, I will raise an NCR about this because it has become common now and has happened several times already. I can’t accept this anymore!” And the project engineer replied “Ok, engineer, it’s your right. But I will discuss this issue with the resident engineer.” Now, the annoying thing was that no one seemed to prevail here. If you’re like me, then what would you do in this situation? The question is: “What is the basis for calling the concrete expired?” Perhaps you came here because you want an answer to this question too. Well, hold on, because you will find the answer. Where can we find the answer? It is a common practice that we should always find the standard that explains exactly this kind of site-critical issue. Here it is. In ASTM C94, it says that “Discharge of the concrete shall be completed within 1 ½ hours or before the drum has revolved 300 revolutions, whichever comes first, after the introduction of the mixing water to the cement and aggregates.” It is clear to this standard that it should only take 90 minutes for the concrete to be discharged from the mixer, otherwise the concrete can be considered “expired.” Why is it important to discuss? When concrete is not discharged within the time limit and is extended beyond the limit, there are a few consequences that might affect the soundness of the concrete. The first one is the forming of honeycomb. Why does this happen? Because, if the concrete is not discharged within the time limit, the concrete tends to become stiff and this will cause honeycombing. Voids in concrete will then be formed. And then the concrete becomes permeable. The second consequence is the creation of cold joints. If there is a huge time gap from the old poured concrete to the new concrete, the old poured concrete will become stiff and this will prevent it from properly knitting with the new batch of concrete, and there will then be a cold joint. And the third is (in my perception) low concrete strength. Now you might be asking “How will you get low concrete strength?” This is because once the concrete becomes stiff, the site crews will then react. They will say, “It is very hard to vibrate and pick the concrete.” Then they will tell the mixer operator to add “water” to the concrete. This will, therefore, cause a change in the water-cement ratio. In fact, it is not allowed to add water to the concrete at site. You as a quality engineer or even a site engineer or project engineer should ensure that the standard must be followed on site, especially for concrete. Because concrete is the carrier of all the loads on a building, whether this be a dead load or live load, and it must resist forces of nature as well, like strong winds and earth movement, you have to ensure the soundness of the poured concrete, and then the rigidness of the concrete structure will follow.