I read with interest your article titled “Finishing Lightweight Air-Entrained Concrete” which discussed causes of delamination of lightweight concrete slabs. The theme that seems to be implied in the article (“No problem if you follow standard practice”) and the line in the table of contents (“Be wary of heavy trowels”) seemed to indicate that two questionable conclusions had already been made:

  • Any delamination of an air-entrained lightweight concrete surface is due to the contractor not following standard practice.
  • Use of riding trowels equipped with pans is the main cause of de-lamination.

ACI Committee 302, “Construction of Concrete Floors,” has established a task group to investigate all of the possible causes of delamination of air-entrained lightweight concrete slabs. As co-chairman of the ACI 302 lightweight concrete finishing task group, I believe it's important that your readers get a more balanced perspective on delamination of air-entrained lightweight concrete.

The article states that the advent of pan floats makes it possible to begin finishing operations before the concrete is sufficiently firm. It's true that the large area of the pans distributes the weight of the riding power trowel, thus providing a very low applied pressure to the slab surface. This means the concrete is sufficiently firm to support the trowel weight. This would be true for regular weight concrete as well, since there is a difference between (weight) load and load-per-unit area. Later, the article says the weight of the trowels produces a concentrated load that is “capable of producing overly compacted surface finishes—sometimes down to half the thickness of the concrete slab.” In the article, Jack Gibbons reports petrographic studies showing compacted surfaces that are ½ to ¾ inch deep, which he considers to be abnormal. No cited data supports the claim of overly compacted surfaces down to half the slab thickness, nor is there an explanation of the phrases “overly compacted surface finish” or “sufficiently firm.” What are the criteria used for defining an “overly compacted surface finish?”

Second, the article says “Adding air entrainment in the range of 6% to 7.5% (and sometimes higher) is needed,” then implies that any resulting delaminations at these air contents are the finishers' fault because of improper power-trowel timing. Air contents greater than 3% are likely to cause delaminations in normal weight concrete. This was demonstrated by a study conducted by an ACI 302 task force in 1997 with results reported in the January 1998 issues of both CONCRETE CONSTRUCTION and Concrete International. Your lightweight article concludes, without data, that the air content is not an issue, even though test data indicates that it is an issue for normal weight concrete. Where is the proof that high air content is not a problem for lightweight concrete?

Your article also claims that nothing has changed in the science of lightweight aggregate concrete mixes, but most contractors I have talked to strongly disagree with this statement. Concrete proportions and ingredients have indeed changed dramatically. There have been changes in the amounts and types of admixtures and supplementary cementitious materials used in both normal weight and lightweight concretes.

The ACI 302 task group is considering effects of concrete ingredients on plastic concrete properties—plus the temperature, relative humidity, and wind velocity at the jobsite—as possible contributory causes for de-lamination of lightweight concrete:

  • Air contents: Entrained air is used in lightweight concrete to help produce the required maximum density and fire resistance. But when is there too much air?
  • Supplementary cementitious materials: Lightweight concrete mixes designed for strength and economy contain more supplementary cementitious material such as fly ash and slag than they did in the past. These materials slow setting times, which can delay bleed water, which makes the delayed bleeding due to high air contents more problematic. Also, the supplementary cementitious materials are no longer in the 15 to 20% range but are moving upwards to 50% of the total cementitious content. The increased use of materials that lengthen setting times also makes the concrete more susceptible to top-down setting. That is, surface concrete exposed to drying and higher temperatures sets faster than the concrete below.
  • Retarding effects of admixtures: Many water reducers increase concrete setting time. These effects are even more pronounced when water reducers are used with supplementary cementitious materials. The dosage rate of the admixtures is based on ounces per 100 pounds of portland cement. If supplementary cementitious materials are used to replace some of the cement, but the admixture dosage isn't changed, longer set retardation can be expected. In addition, some water reducers increase air content.

All factors that can affect the occurrence of delaminations, including materials, environment, and finishing tools and techniques, should be considered when attempting to explain the cause or causes. The above listed factors, and perhaps more, can cause premature surface crusting and delayed bleeding, giving a false indication that the concrete is ready to start finishing operations.

Before drawing conclusions, it may be useful to review findings from the previously mentioned ACI 302 task-force study on delamination of normal-weight concrete. The findings included:

  • Pan floats used on non-air-entrained concrete did not cause delaminations.
  • Pan floats used on non-air-entrained concrete—without first using walk-behind trowels to ensure that the concrete was ready for pan floating—did not cause delaminations.
  • Pan floats used on non-air-entrained concrete with a dry shake applied at a rate of 1 pound per square foot did not cause delaminations.
  • Pan floats used on non-air-entrained concrete “while there was still some sheen on the surface” did not cause delaminations.

After reviewing these findings for normal weight concrete, an attempt to develop or collect similar data for lightweight concrete should be made. The article draws (questionable conclusions based on selected (limited) opinions and no sufficient test data.

The task group has presented a plan for studying these factors. When these tests are conducted, Concrete Construction as well as other interested parties will be invited to attend.

Readers should be advised to investigate the proposed mix submitted on any project and determine if there is any ingredient or condition that may cause delayed bleeding and a top-down set. This is new territory and the contractor should be advised to proceed cautiously with full awareness of ingredients from the ready mix producer.

Dennis Ahal. Ahal Contracting, St. Louis. Past president of the American Society of Concrete Contractors Task group co-chairman, ACI 302 lightweight concrete task group