I just had a conversation with a producer who is experiencing grout leakage, sand streaking, and the appearance of crack like fissures, often called “worm trails” in the finished product. This producer uses ready-mixed concrete, and they recently reduced the size of their coarse aggregate from 3/4” rock to 3/8” pea gravel. The problems that this producer is seeing are mostly the result of excessive bleeding in the concrete mix.
What is meant by the term “bleeding” in concrete? Bleeding is the term used from the result of gravity that occurs when heavier materials in the placed concrete settle causing the lighter materials (mostly water) to rise toward the surface. Bleed water is often important to finishers when placing floors, driveways, patios, etc., and it allows the concrete to be properly finished. In precast, the concrete pieces are so large that the bleed water cannot easily escape. This leaves the “worm trails” that form along the casting walls that nearly always are vertical and they have the appearance of a crack. Bleeding also causes scalling and crazing at the horizontal surface due to the higher water percentage in the paste as the water migrates to the surface.
There are three things that a producer can do to improve this condition and reduce the amount of “bleeding” that is occurring. First, the water to cement ratio (w/c) needs to be as low as practical. Water is used to cause Portland cement to react and become a paste that hardens within a few hours. Cement requires about one pound of water for every four pounds of cement to completely hydrate. That is a w/c of 0.25. Any additional water is called water of convenience. Also, a w/c of 0.40 is required to even begin to develop a slump for placement. Most concrete mixes have about one pound of water for every two pounds of cement, which is twice the amount of water that is necessary for cement hydration. Chemical admixtures called water reducers allow less water to be necessary to provide flowability, and this will also help reduce the amount of bleed water.
The second modification that a producer can do to improve/reduce the bleed water is to increase the amount of fines in the mix. A volumetric concrete mix design begins with a determination of the coarse aggregate ratio. For conventional concretes, this is typically 60-65% of the absolute volume of all materials. If the producer wishes to reduce bleeding in the concrete, and they have reduced the water to cement ratio to a point where further reductions begin to have an adverse effect on economy, then the next step is to lower the coarse aggregate ratio gradually. A mix design formula following ACI 211 for the volumetric proportioning of concrete is the method used. If the producer already has a properly proportioned mix, then begin by reducing the coarse aggregate by 25 pounds and increasing the amount of fine aggregate by the same amount. This is not an exact relationship, but it will help the producer to achieve a mixture with a lower potential for bleeding.
The third component that a producer can do to reduce the effects of excess bleed water is to add an air entraining admixture (AEA). The AEA does two things: it takes up a small amount of volume formerly occupied by other materials, and it creates some fluidity to the mix that allows for less water needed for the convenience of placement. In effect, and AEA can be described as tiny ball bearings in the mix allowing it to be more flowable.
Don’t let excessive bleed water continue to affect your concrete’s appearance and quality. It is possible to take a few simple steps to solve the root cause of the underlying problem. The result will wow your customers with a better looking precast concrete product that is more durable and higher quality.
