Excessive bleeding of Concrete -Causes, effects, and control

Almost all freshly placed concrete bleeds. As aggregate and cement particles settle, they force excess mixing water upward. The process continues until settlement stops, either because of solids bridging or because the concrete has set.

The total amount of bleeding or settlement depends on mix properties, primarily water content and amount of fines (cement, fly ash, fine sand). Increasing water content increases bleeding, and increasing the amount of fines reduces bleeding. Amount of bleeding is also proportional to the depth of concrete placed. More bleedwater rises in deep sections than in thin ones.

Bleeding usually occurs gradually by uniform seepage over the whole surface, but sometimes vertical channels form. Water flows fast enough in these channels to carry fine particles of cement and sand, leaving "wormholes" in the interior or sand streaks at the form face. Channels are more likely to form when concrete bleeds excessively.

Channels that reach the surface are open paths for deicing solutions to penetrate the concrete. This leads to freezing and thawing damage and rebar corrosion.

Concrete weathers and disintegrates most severely at the tops of walls, piers, and parapets. One reason for this is water gain that's caused by bleeding. Sometimes bleedwater can't entirely evaporate because it has been trapped near the top surface by setting. This raises the water-cement ratio, increases permeability, and lowers strength. The effect has been noted in laboratory load tests of full-sized columns where failure almost always was near the column top.

Excessive bleeding also causes some other problems in deep sections:

* Heavy laitance accumulation at horizontal construction joints. This plus the higher water- cement ratio near the surface can cause a plane of weakness at the joint.
* Bond loss at aggregate and rebar surfaces. Channels form at the surface of coarse aggregate particles (Figure 1) or water collects beneath rebars.
* Unsightly sand streaks (Figure 2) caused by bleedwater rising at the form face. Concretes that bleed excessively can cause problems even before they're placed. When these mixes are pumped, pressure in the pumpline forces water and cement in front of the concrete. With less paste to lubricate the line, rock jams form and the line plugs.

Never float or trowel concrete while there's bleedwater on the surface. That's the cardinal rule of finishing. Strike off and bull float the concrete before bleeding begins, then wait for bleeding to end and bleedwater to evaporate before finishing. Excessive bleeding causes excessive waiting by the finishers. Unless the bleedwater is removed, delays increase finishing costs. But getting on the concrete too soon reduces surface quality. Finishing before bleedwater has evaporated can cause dusting, craze cracking, scaling, and low wear resistance. Working bleed-water into the surface also increases permeability; water, deicing salts, and other harmful chemicals can enter the concrete more easily. Highly permeable concrete increases the possibility of rebar corrosion too. Floating or troweling concrete prematurely can cause surface defects even if there's no bleedwater on the surface. Sealing the surface before bleeding stops traps bleedwater beneath the surface. Blisters may form (Figure 3), or the whole surface may peel off later because of a weak, very porous concrete layer beneath the sealed surface.

Be particularly careful to control mix variables that affect bleeding when concrete slabs will be placed on a nonabsorbent base or during cold weather. Plastic vapor barriers or tightly compacted clay soils beneath the slab aggravate bleeding problems (Figure 4). Because the base absorbs no water, more excess water comes to the surface.

Low temperatures prolong the bleeding period. A combination of cool concrete and rapidly drying surface in a heated enclosure can cause blistering or surface scaling.

If the concrete is already in place and bleeding too much, fans may speed evaporation and permit earlier finishing. Another solution is dragging a rubber hose slowly over the entire surface; concrete should be stiff enough so that only water is removed. In small areas, a single pass with the tilted edge of a trowel removes the excess water.

Excessive bleeding can be avoided. Don't add too much water to the concrete. Most of the water added to make placing easier bleeds out of the concrete. Any time saved during placement will be lost while waiting for the bleedwater to evaporate. Place concrete at the lowest possible slump. If you need a higher slump to speed placement, consider using a superplasticizer. Add additional concrete fines to reduce bleeding. The fines may come from a number of sources:

* Use a more finely ground cement. Concretes made with high early strength (Type III) cement bleed less because the cement is ground finer than normal (Type I) cement.
* Use more cement. At the same water content, rich mixes bleed less than lean mixes.
* Use fly ash or other pozzolans in the concrete.
* If concrete sands don't have much material passing the No. 50 and 100 sieves, blend in a fine blow sand at the batch plant.
* For air- entrained concrete, use the maximum allowable amount of entrained air. Consider using an air- entraining agent whenever excessive bleeding is a problem. Entrained air bubbles act as additional fines. Air entrainment also lowers the amount of water needed to reach a desired slump.