Why do cast-in-place beams crack easily? The reason has finally been found!

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Many engineers hold a fatal misconception: they think that for cast-in-place beams, as long as the concrete grade is sufficient and the vibration is done properly, everything will be fine. Completely wrong! What concrete beams fear the most is not a lack of strength, but deformation and timing issues. Before even strengthening the body, the back bends under its own weight – how can it not crack? Today, Zhang Bo from Concrete Home will explain the techniques for preventing cracks in cast-in-place beams in detail. Don’t let your cast-in-place beams fail due to poor construction methods! Why do cast-in-place beams crack easily? The three \"killers\": first, transverse deformation. Beams that are several dozen meters long will sag at their mid-span due to their own weight plus the loads associated with construction. If the pouring sequence is incorrect, the beam will experience uneven stress, and cracks will start at the mid-span. Second, thermal stress. Water it at noon in summer; the surface becomes scorching hot, and the heat of water absorption inside is also high. The difference in temperature between the inside and outside causes cracks to form. Third, the timing of tensioning is incorrect. Tensioning should be carried out once the concrete has fully hardened; by then, sufficient shrinkage stress has accumulated, and cracking will occur upon tensioning. First tip: Choose the right time – pour the cast-in-place beam from the middle toward the ends, and it’s best to do this when the temperature is lower during the day. Start watering at four or five in the morning to avoid the high temperatures at noon. Pouring sequence: two transfer pumps and two distribution arms, pouring simultaneously from the mid-span of the beam toward both ends. Why? When pouring from one end to the other, the part that was poured first has sunk, while the part poured later is still on top; this results in uneven stress at the midpoint, making cracks more likely to occur. Concrete is poured simultaneously from the mid-span toward both ends, ensuring symmetry on both sides, uniform deformation at the mid-span, and more rational stress distribution in the beam. Second trick: tension in stages; don’t wait until it’s completely rigid before applying tension. This is the key to preventing cracks in cast-in-place beams. The traditional approach is to tension it to the design value at once, and then apply further tension once the concrete has reached its strength. But by this point, the compressive stress has accumulated enough, causing it to crack upon being pulled. Modified three-stage tensioning: Stage 1: Early pre-tensioning (when the strength reaches about 40% of the design value); the end formwork is removed, while the side formwork and inner formwork remain in place but are loosened, followed by early pre-tensioning. The purpose is to prevent shrinkage or temperature cracks in cast-in-place beams. The tension force is generally 30%-40% of the design value, with the specific value determined through calculation. At this point, the concrete has not yet fully hardened; by applying a little pressure to it, the shrinkage stress can be ‘suppressed’ and prevented from causing cracks to form. Phase 2: Initial tensioning (when the strength reaches over 60% of the design value) is carried out with the formwork in place, tensioning the prestressed tendons to 60%-70% of the design value. The template is still in place, so there’s no risk of it deforming. Stage 3: Final tensioning (the strength and elastic modulus reach over 80% of the design values); all formwork is removed, and all prestressing tendons are tensioned to the design tonnage. By this time, the concrete is strong enough to withstand it. Three-stage tensioning, with each stage requiring careful attention. Preventing cracks in cast-in-place beams cannot be achieved simply by using \"high-quality cement\" or carrying out more vibration. It is a systems engineering approach: choosing the right time, pouring from the center toward the edges, and carrying out tensioning in three stages – only by ensuring that each step is done properly can cracks be controlled. Especially with staged tensioning, many people find it troublesome and prefer to complete it all at once for simplicity. As a result, it saves effort but creates more cracks, making repairs later even more troublesome.