ACI 309.2R-98 Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces

ACI 309.2R-98 Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces. 2.6—Consolidation Concrete consists of coarse aggregate particles in a matrix of mortar, and irregularly distributed pockets of entrapped air. If the concrete is air entrained, an additional evenly dis- tributed system of entrained air bubbles is present. The vol- ume of entrapped air in unconsolidated concrete may vary from about 5 to 20 percent depending on the workability of the mixture, size and shape of the form, amount of reinforc- ing steel, and method of depositing the concrete. The pur- pose of consolidation is to remove as much of this entrapped air as practical. Vibration is the most common method of consolidation. It causes very rapid movement of the concrete mixture parti- cles and briefly liquefies the mixture, thus reducing the inter- nal friction. When vibrated, concrete becomes fluid and through the action of gravity seeks a lower level and denser condition as entrapped air rises to the surface and is expelled. It compacts laterally against the form and around the rein- forcing steel. In practice, vibration is normally continued un- til the entire placement acquires a uniform appearance and its surface just starts to glisten or large bubbles cease to ap- pear. A film of cement paste should be discernible between the concrete and the forms. These visual indicators are not necessarily an accurate indication of good consolidation. ACI 309R provides guidance on judging the adequacy of vibration. Undervibration is far more common than overvibration, and may be caused by the following: 1. Use of an undersized, underpowered, or poorly main- tained vibrator; 2. Excessive or haphazard spacing of vibrator insertions; 3. Inadequate vibration during each insertion; 4. Failure of the vibrator to penetrate into the preceding layer; and/or 3.1—Honeycomb Honeycomb (Fig. 1) is a condition of irregular voids due to failure of the mortar to effectively fill the spaces between coarse aggregate particles. Where bridging of the coarse ag- gregate particles or stiffness of the mixture is a cause of hon- eycomb, vibration may assist in overcoming the bridging by increasing the flowability of the concrete. Factors that may contribute to honeycombing are: congested reinforcement, segregation resulting in insufficient paste content, and im- proper fine aggregate to total aggregate ratio, improper plac- ing techniques, rapid stiffening of hot concrete, difficult construction conditions, and insufficient consolidation ef- fort. Changes in construction practices and in mixture...