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<p>Bleeding water significantly impacts the quality and properties of concrete, both in its plastic and hardened states. Understanding these effects is crucial for ensuring optimal concrete performance.</p>
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<h3 class="wp-block-heading">Effects of Bleeding on Plastic Concrete</h3>
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<p><strong>Volume Change</strong><br>Mixing cement, aggregates, and water creates a dispersed state of particles in plastic concrete. However, this state is unstable as the heavier particles, such as cement and aggregates, sink due to gravity, displacing the lighter water upwards. This settlement continues until the particles compact and densify, leading to a reduced volume of the plastic mixture, though the material volume remains constant.</p>
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<p>In general construction, this minor sedimentation is not a significant concern.</p>
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<p><strong>Post-Bleeding Expansion</strong><br>Post-bleeding expansion occurs due to physical and chemical reactions during the concrete's setting stages. The gel formed on cement grains during hydration can cause the concrete to expand slightly within the first day. This expansion typically ranges from 0.05% to 1% for Portland cement pastes with a 0.38 water-cement ratio by weight, reducing significantly after the initial day.</p>
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<p><strong>Plastic Shrinkage</strong><br>Plastic shrinkage, also known as setting shrinkage, happens when the concrete loses free water before hardening. This water loss occurs through bleeding and surface evaporation, influenced by factors like air temperature, humidity, wind velocity, and concrete temperature. If the surface evaporation rate exceeds the bleeding rate, the concrete undergoes plastic shrinkage, resulting in tensile stress and cracking. Preventive measures include minimizing surface evaporation and using concrete mixes with higher bleeding characteristics, fibers, evaporation retarders, shades, windbreaks, and plastic sheets for coverage.</p>
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<p><strong>Water-Cement Ratio</strong><br>The concrete's water-cement ratio changes due to bleeding. As solid materials settle, water is displaced upwards and may evaporate. Excessive bleeding can increase the surface concrete’s water-cement ratio, weakening it. If concrete is troweled before the bleed water surfaces, it can trap water underneath, leading to a weaker zone. Proper bleeding can reduce the overall water-cement ratio of the mix.</p>
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<p><strong>Placing and Finishing</strong><br>Normal bleeding aids in placing and finishing by keeping the surface moist. However, visible bleed water must evaporate before finishing to prevent defects like dusting, scaling, and blisters. Excessive bleeding between concrete layers should be managed by removing the water and top layer of low-strength concrete before the next layer is placed.</p>
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<h3 class="wp-block-heading">Effect of Bleeding Water on Hardened Concrete</h3>
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<p><strong>Strength and Density</strong><br>The strength of hardened concrete is closely linked to its water-cement ratio. As particles settle, water is expelled, reducing the water-cement ratio and hence increasing strength. Uneven consolidation leads to slightly higher strength at the bottom compared to the top. Differential settlement between the paste and aggregates can weaken the upper portion due to increased water content.</p>
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<p><strong>Paste-Aggregate Bond</strong><br>Bleed water can weaken the paste-aggregate bond by collecting around coarse aggregate particles during differential settlement. This issue can be mitigated by re-vibrating the concrete after some bleeding occurs.</p>
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<p><strong>Paste-Steel Bond</strong><br>Bleed water tends to collect under reinforcing steel and other embedded items, leading to potential air voids and weak points. Minor collections are not significantly harmful, but excessive water can reduce embedment strength, paste-steel bond, and even promote corrosion in the presence of moisture, carbonation, or chlorides.</p>
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<p><strong>Durability</strong><br>Concrete mixtures are designed to be durable against environmental exposure. While normal bleeding is not detrimental, excessive bleeding can increase permeability and the water-cement ratio, undermining the concrete's resistance to freeze-thaw cycles, deicer scaling, and aggressive chemicals. Surface dusting and reinforcement corrosion are additional risks associated with excessive bleeding water.</p>
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<p>By understanding and managing the effects of bleeding water, construction professionals can enhance the quality and durability of concrete structures, ensuring they meet the required performance standards.</p>
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