Talk to people who are not in the precast industry about bleeding concrete, and it may evoke visions from a horror movie. But we’re not talking about types A, B or O-negative in this case. We’re referring to bleed water.
Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic. The way you work that bleed water can affect the durability of your products.
The biggest factor in bleed water rates is the water-to-cement ratio. A higher ratio can lead to excessive bleeding. The cement type and fine aggregates can play a role in determining the bleed rate. The fewer fines you have in your mix, the more bleeding will occur. Factors also include concrete height and pressure. The relationship between bleed water and concrete heights starts off as linear, but eventually becomes nonlinear at increased heights.
There are also different types of bleeding:
- Normal bleeding refers to a uniform seepage of water over the entire surface of the structure.
- Channel bleeding refers to water rising through particular paths.
Not all bleed water will reach the surface of the concrete. Some bleed water may rise and remain trapped under aggregates and reinforcing. This results in the weakening of the bond between the paste and those elements.
The goal is not to necessarily eliminate bleed water, but rather to manage it to ensure the concrete’s quality. By allowing free water to migrate to a surface and evaporate, the water/cement ratio of the structure decreases, thus decreasing capillary porosity and increasing its density and durability. It can also be useful to aid in finishing operations and reduce plastic shrinkage cracking.
In addition, it is important not to begin finishing operations before most of the bleed water has evaporated. Working the water back into the mix will raise the water/cement ratio in the top surface. That can also result in an increase in permeability. If this surface is exposed to traffic or aggressive environments, this may cause premature delaminating, blistering and cracking.
The use of supplementary cementitious materials can decrease bleeding rates especially when using finer blends. Fly ash can be effective in reducing bleed rates. Silica fume has the largest effect on reducing bleeding.
Micro fibers used in concrete have also been shown to slow down bleed rates as they control the speed of migration of water to the surface while inhibiting the settling of solid particles.
Ways to reduce bleeding in concrete include:
- Reduce water content. Use lower slump mix
- Use finer cements
- Increase amount of fines in the sand
- Use supplementary cementitious materials
- Use air entraining admixtures
Generally, water reducers will decrease bleeding but they may actually end up increasing this rate based on their chemical composition. Air entraining admixtures can also reduce bleeding by increasing cohesion in the fresh concrete and slowing segregation.
If you have any questions on this topic or any other technical topic, please contact one of the Technical Department staff at NPCA at (800) 366-7731.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Sustainability and Technical Education.