If water from the concrete mix rises to the top of the newly laid concrete, it is bleeding. The cause is the mix's solids failing to retain all water as they settle.
The extent of bleeding is quantifiable by measuring the total settlement or the decrease in height per unit height of concrete.
Sometimes, rising water gets trapped beneath large aggregate particles or reinforcement, creating voids.
If these voids align, they can make the concrete more permeable horizontally.
If rising water transports a large amount of fine cement particles, a weak, porous layer known as laitance forms on the slab's surface, leaving a dusty texture that can lead to a fragile plane that compromises the adhesion with subsequent concrete layers.
Removing laitance by brushing and rinsing is crucial because it forms blisters if it traps air bubbles or bleeding water.
Bleeding decreases with finer cement, higher alkali or tricalcium aluminate content, or the addition of calcium chloride, despite the potential drawbacks.
Additionally, air entrainment effectively reduces bleeding, thus enabling a quicker finishing process immediately after the concrete is cast.
