Bleeding in fresh concrete occurs when water from the mix rises to the surface. This happens because the mix's solid components fail to retain all the water as they settle, leading to separation where water collects at the top. The severity of bleeding can be measured by assessing the total settlement or by noting the decrease in height per unit height of concrete.
Bleeding can cause several issues in the concrete structure. Sometimes, the rising water gets trapped beneath large aggregate particles or reinforcement bars, creating voids within the concrete. If these voids align, they increase the concrete's horizontal permeability. As well as this, if the ascending water carries fine cement particles to the surface, it forms a weak, porous layer called laitance. This layer has a dusty texture and can create a fragile plane that adversely affects the adhesion with subsequent concrete layers.
To prevent structural weaknesses, it is essential to remove laitance by brushing and rinsing it away, as it can also form blisters if it traps air bubbles or additional bleeding water. Bleeding can be mitigated by using finer cement, adding substances with high alkali or tricalcium aluminate content, or introducing calcium chloride, despite some potential negatives. Air entrainment in concrete mix effectively reduces bleeding, facilitating a quicker and more efficient finishing process right after casting.
