Bleeding water, or ” bleed water,” is the clear water that appears on the top surface of freshly placed concrete, mortar, grout, or paste after placement.
What Causes Bleed Water?
Bleed water occurs due to the settlement of solid particles, such as cement and aggregates, within the wet concrete. This sedimentation leads to the upward movement of water, resulting in the phenomenon known as bleeding or water gain.
Normal vs. Excessive Bleeding Water
A moderate amount of bleed water is typical and does not negatively affect the quality of plastic or hardened concrete. However, excessive bleed water can adversely impact concrete performance, creating challenges during construction and in the final product.
Controlling Bleeding Water
Proper mix proportioning, appropriate ingredient selection, correct placement equipment, and sound construction practices can effectively control bleeding to a desirable level.
Basics of Bleeding Water in Concrete
Freshly mixed concrete is a combination of suspended concrete constituents. Once agitation stops, solid materials settle, and any excess water rises to the surface, forming bleed water. Although the volume of materials remains unchanged, the height of the hardened concrete is reduced due to evaporation of the surface water.
Types of Bleed Water Accumulation
- Normal Bleeding: Uniform seepage of water over the entire concrete surface.
- Channel Bleeding: Water rising through localized channels, possibly carrying fine particles, especially in mixes with low cement or high water content.
Bleeding Rate and Capacity
- Bleeding Rate: The speed at which bleed water rises to the surface, influenced by the permeability of the concrete paste. Expressed in cubic centimeters of water per second per square centimeter of surface.
- Bleeding Capacity: The amount of bleed water a concrete mix can discharge, related to the depth of settlement. Expressed as a ratio or percentage of total decrease in height relative to the initial height, or as a percentage of mixing water.
Duration of Bleeding
The bleeding duration depends on the depth of the concrete section and the setting properties of cementitious materials. Thinner slabs bleed faster than deeper sections. Concrete that sets quickly will expel less bleed water compared to slower-setting concrete.
Increasing and Reducing Bleeding Water
Increasing Bleeding Water
To increase bleeding water for purposes such as preventing plastic shrinkage cracking or improving surface finishing in dry weather, consider:
- Adding more water.
- Reducing sand fines.
- Decreasing cementitious materials.
Reducing Bleeding Water
To reduce bleeding water, which may be necessary for various reasons such as improving finishing operations and minimizing weak concrete formation, consider these strategies:
- Lowering water content, water-cement ratio, and slump.
- Increasing cement content to reduce the water-cement ratio.
- Utilizing finer cementitious materials.
- Adding more fines to the sand.
- Incorporating additional cementing materials like fly ash, slag, or silica fume.
- Using blended hydraulic cement.
- Adding chemical admixtures to reduce the water-cement ratio.
- Employing air-entrained concrete.
Conclusion
Managing bleed water effectively is crucial for ensuring quality concrete. By understanding the causes and measures for controlling bleeding water, concrete performance can be optimized, leading to more durable and robust structures.