By Claude Goguen, P.E., LEED AP
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.
NPCA file photo
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.
Can you do online training courses on concrete and offer certificate
How does air velocity, humidy and temperature affect bleeding of concrete
Thanks for the comment Zoel. Claude Goguen, P.E., LEED AP, one of our technical engineers, has provided the following answer: “While bleeding is primarily attributable to water content and poorly graded aggregates, air velocity, temperature and moisture can have an effect. This effect varies based on what kind of concrete structure you are pouring. Air movement and humidity will influence bleed water rates to a higher degree on a large slab than it would with a deep structure with smaller exposed area. Bleed rates will be dictated among other things by setting time. Cold temperatures will slow setting, and as a result can extend the bleeding period. High humidity and lack of air circulation can also contribute to prolonged bleeding. As long as ambient conditions allow bleed water to remain on the concrete surface and not evaporate, the longer the bleeding period may extend.
ACI 302.1R-04 Guide for Concrete Floor and Slab Construction recommends using “concrete approaching the highest as-placed temperature permitted by the contract documents.” It is important to note that while this may reduce bleeding, it may lead to other issues such as reduced long term strengths. There are also recommendations to use heaters or fans to move the air and evaporate excess surface moisture while concrete is in plastic stage. This must also be done cautiously as accelerated evaporation may also lead to plastic shrinkage cracking. If using heaters, avoid using non-vented heaters, as they may accelerate carbonation of the surface, which can create a soft, dusty, chalky surface.”
thanks for team work
if slump retention factor is greater than does it cause bleeding.
Bleeding
Bleeding, known also as water gain, is a form of
segregation in which some of the water in the mix
tends to rise to the surface of freshly placed concrete.
This is caused by the inability of the solid
constituents of the mix to hold all of the mixing
water when they settle downwards, water having
the lowest specific gravity of all the mix constituents.
We are thus dealing with subsidence,
and Powers4.10 treats bleeding as a special case of
sedimentation. Bleeding can be expressed quantitatively
as the total settlement per unit height of
concrete or as a percentage of the mixing water;
in extreme cases, this may reach 20 per cent.4.112
ASTM C 232-09 prescribes two methods of determination
of total bleeding. The rate of bleeding
can also be determined experimentally.
The initial bleeding proceeds at a constant
rate, but subsequently the rate of bleeding decreases
steadily. Bleeding of concrete continues
until the cement paste has stiffened sufficiently to
put an end to the process of sedimentation.
If the bleeding water is remixed during finishing
of the top surface, a weak wearing surface,
consisting of laitance, will be formed. This can be
avoided by delaying the finishing operations until
the bleed water has evaporated, and also by the
use of wood floats and avoidance of overworking
the surface. On the other hand, if evaporation
of water from the surface of the concrete is faster
than the bleeding rate, plastic shrinkage cracking
may result (see p. 424).
Some of the rising water becomes trapped on
the underside of coarse aggregate particles or of
reinforcement, thus creating zones of poor bond.
This water leaves behind air pockets or lenses,
and because all the voids are oriented in the same
direction, the permeability of the concrete in a
horizontal plane may be increased. Hence, ingress
of an attacking medium into concrete is facilitated.
A horizontal zone of weakness may also
be created. The formation of such zones was confirmed
by means of tensile tests in the direction
of casting and at right angles to it.4.65 Trapping
an appreciable amount of bleed water must be
avoided also because of the danger of frost damage,
especially in road slabs.
Some bleeding is unavoidable. However, in
high elements, such as columns or walls, as bleed
water moves upwards, the water/cement ratio in
the lower part of the element is reduced, but the
water trapped in the upper part of the now stiffer
concrete results in an increased water/cement ratio
there, and hence in a reduced strength (
Quite educative. Thanks
Physical properties of materials to mix design
Thank you for the comment Jemba. Are you asking if we have resources about physical properties of materials to mix design? I’d recommend that you continue to search around precast.org. We have recently done two article series about SCMs in Concrete and on Concrete Mix Design that I think would interest you. If you need assistance on anything specific, I’d be happy to forward your questions to our technical services engineers.
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