With all that is known about concrete, it seems to me that we are too frequently revisiting the same problems, trying to solve them in much the same ineffective way. This is very true of the "bughole;" the name given to a small regular or irregular void, ranging from microscopic in size to 1-inch in diameter, found at the formed surface of concrete.
Before we can hope to solve the problem we must first understand how a bughole is formed. Under certain conditions entrapped air and free water found in fresh concrete migrate to the formed surface of the concrete and form bubbles of air or water. Later, after the concrete hardens and the free water evaporates, there remains at the surface visible empty void spaces, commonly known as a bugholes.
Entrapped air is the biggest source of bugholes
Entrapped air is a natural occurrence in concrete. Air can be entrapped in concrete in two ways: 1) the inability of the cement paste to fill all the voids between the aggregate particles and 2) during the mixing of the concrete, air can be entrapped in the cement paste. Entrapped air is different from entrained air. Entrained air is intentionally incorporated into concrete primarily for freeze/thaw protection. The bubble size of entrained air is typically between 10 and 100 microns in diameter, whereas entrapped air is 1,000 (1 mm) and larger in diameter and offers no freeze/thaw protection. The entrapped air content for most concretes can range between ½ % and 3%, but the typical range is between 1% and 1½ %.
The problem with "FREE" water:
|Closing the surface... A clean release from the forms will leave the surface of the concrete intact and prevent the void areas just below the surface from becoming bugholes.|
As long as entrapped air and free water are well distributed throughout the concrete, bugholes are not a problem. It is only when this balance is disturbed and the entrapped air and free water are allowed to migrate to the formed surface of the concrete that bugholes are formed. In a close examination of concrete that has a bughole problem, two things are obvious. First, the bugholes are at the surface or just below the surface, behind a thin skin of dried cement paste. Second, there are a disproportionate number of voids in this zone as compared to the interior of the concrete.
Form Coatings: Sometimes more harm than good.
Form release agents cannot control the amount of entrapped air and free water in a concrete mix. Entrapped air and free water are only controlled by the concrete mix design and those materials which make up the concrete. However, a good form release agent can go a long way in preventing the formation of the surface blemishes that can become bugholes. A high quality form release agent that produces a smooth surface for the concrete to be cast against reduces form drag, and thereby prevents the entrapment of air pockets between the form face and the formed concrete surface. A clean release from the forms will leave the surface of the concrete intact and prevent the void areas just below the surface from becoming bugholes. If there is a thin skin of dried cement paste left on the forms, the release was not clean and either the form release agent performed poorly or the forms were dirty or damaged prior to the pour.
|This long, horizontal void is indicative of form drag, caused by a poor form release.|
High quality Form Release Agents: The first line of defense.
There is a better way. L&M founder, Larry Schwietz, discovered over forty years ago that chemically active release agents were better. He used his findings to develop our first product, a chemically active form release agent, DEBOND FORM COATING. Today, DEBOND is one of the industry's leading chemically active form release agents.
Simply put, DEBOND works this way; A very thin application of DEBOND can be lightly sprayed onto the face of the formwork and is then allowed to dry for a few minutes. Once the fresh concrete poured into the form comes in contact with the DEBOND treated form surface, the alkalis within the concrete react with DEBOND's active ingredients on the form surface, producing a very thin, slippery film between the form face and the concrete. This soap film is very slippery and not only assures a clean release but also reduces form drag, allowing voids trapped between the form face and the concrete mass to easily slide up the side of the form face and out of the concrete. Please pardon advertisement, but it is something you need to know.
Sometimes the answer is in the admixtures:
In the late 70's and early 80's bugholes in formed concrete were a hot topic. It was during this time a new type of chemical concrete admixture came upon the scene in the form of superplasticizers. It is commonly known that superplasticizers allow concrete to be placed at a higher slump without an increase in water. It does this by changing the rheology characteristics of the cementitious material in the concrete, thereby allowing for greater aggregate mobility. This newfound mobility can come at a cost. While the aggregate has greater mobility, so does the entrapped air. It is a well-known fact that air in concrete has a tendency to migrate to the source of internal vibration. Entrapped air with a high degree of mobility will very quickly move to the source of internal vibration. These two facts can create a condition in which bugholes can be problematic. The solution lies in two areas, 1) the amount and type of vibration and 2) the makeup of the concrete mix.
Very early on in the use of superplasticized concrete two things were obvious, 1) superplasticized concrete is not self-consolidating and 2) it cannot be consolidated like ordinary concrete. I recommend that you refer to ACI 309R and ACI 309.2R as excellent sources for more in-depth information about the techniques and problems associated with the consolidation of superplasticized and non-superplasticized concrete.
Another excellent source is ACI publication SP-96, Consolidation of Concrete, Steven H. Gebler, P.E., editor, 1987. In this publication there is a paper by Lars Forssblad entitled, "Need for Consolidation of Superplasticized Concrete Mixes." In this paper Forssblad recommends using an internal vibrator "one size" smaller than that used for ordinary concrete. He also recommends that the time of vibration be reduced by about 50%. With this information in hand and the knowledge that air in concrete migrates to internal vibrators, we can start to formulate a strategy to reduce bugholes forming at the surface of superplasticized concrete.
Here is something else you should know. ACI 309.R addresses another key point in the control of bugholes. I quote, "Form vibrators (externally attached) tend to draw mortar to the form and, when used in combination with internal vibration, have proved effective in reducing the size and number of air voids on the surface."
|Bugholes are unsightly, costly to repair and unnecessary!|
As Paul Harvey would say, "and now for the rest of the story." The rest of the story is the concrete mix design. With a better understanding of the basic causes of bugholes, we can adjust the concrete mix design to reduce the levels of entrapped air and free water and limit their ability to produce bugholes. There are several basic concrete concepts that will aid us in this effort. The first of which is the air produced by an air-entraining agent for freeze/thaw protection is generally helpful in reducing bugholes. You should be aware that some water reducing admixtures could entrain air voids the size approaching that of entrapped air. Air bubbles of this size will not provide proper freeze/thaw protection, but they can produce bugholes. Concrete mixes that are lean (low in cement content) and ones that have poor workability also have a high incidence of bugholes. Fine aggregate (sand) in a mix design plays an important role in determining mixing water requirements and the level of entrapped air. As the amount of sand increases, the specific surface area of the mix increases. As the specific surface area increases mixing water requirements likewise increase. A mortar requires more mixing water than a concrete. It is a well-established fact that as the sand content in a concrete increases the amount of entrapped air increases. Sand blends that are coarse (deficient in aggregate particles passing the #30 sieve) generally produce higher levels of entrapped air than sand blends that are well graded.
Our recommendations include,
- For superplasticized concrete, reduce the size of the internal vibrators and the duration of vibration.
- Use external form vibrators to draw mortar to the form face. This will help fill any voids that form at the form face, thereby reducing the formation of bugholes.
- Do not let the internal vibrator come too close to the form face. If this happens the entrapped air can migrate to the form face and become bugholes.
- Maintain form faces in good repair and keep them clean.
- Choose a high quality form release agent that can be applied in thin coats and which provides a predictable, clean release.
- Familiarize yourself with ACI publications on the subject, in particular ACI 309R and ACI309.2R.
© 2001 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2001.