Bleeding in Fresh Concrete

Bleeding occurs when water from the concrete mix rises to the surface because the mix's solid components fail to retain all water as they settle. This separation causes water to collect at the top of the newly laid concrete. The extent of bleeding can be measured by assessing total settlement or the decrease in height per unit height of concrete.

Rising water can become trapped beneath large aggregate particles or reinforcement, creating voids that increase horizontal permeability if they align. Additionally, when ascending water carries fine cement particles to the surface, it forms laitance—a weak, porous layer with a dusty texture. This layer creates a fragile plane that compromises adhesion with subsequent concrete layers and can form blisters if it traps air bubbles.

Laitance is a weak, porous layer that forms on the concrete surface when rising water transports fine cement particles upward. Removing laitance by brushing and rinsing is crucial because it creates a fragile plane that weakens adhesion between concrete layers and can trap air bubbles or bleeding water, forming blisters that compromise structural integrity.

Bleeding decreases with finer cement, which improves water retention in the mix. Adding substances with higher alkali or tricalcium aluminate content also reduces bleeding. Calcium chloride is another effective additive, despite potential drawbacks. These modifications help stabilize the mix and minimize water separation at the surface.

Air entrainment effectively reduces bleeding in concrete by improving the mix's ability to retain water and stabilize the structure. By incorporating air bubbles into the concrete, this technique enables a quicker and more efficient finishing process immediately after casting, while simultaneously minimizing water separation and surface defects.

The severity of bleeding is quantifiable by measuring the total settlement or by calculating the decrease in height per unit height of concrete. These measurements provide a numerical assessment of how much water has risen to the surface, helping engineers evaluate the quality of the concrete mix and determine if mitigation strategies are needed.

Bleeding is influenced by factors affecting workability, including cement fineness, water content, and mix composition. Understanding these factors affecting workability helps engineers design concrete mixes that minimize bleeding while maintaining adequate workability for proper placement and consolidation during construction operations.