Good Vibrations

Honeycomb, subsidence cracks, cold joints and excessive surface air voids are common surface blemishes associated with ineffective concrete consolidation. In addition to being unsightly, formed surfaces with blemishes may not conform to the requirements of the contract documents. When this occurs, surface blemishes become defects and subsequently must be repaired. Along with the expense of repairing surface defects, it is difficult and sometimes impossible to make repairs match the color and texture of the surrounding concrete. By understanding the causes and using effective consolidation techniques, contractors can avoid repair costs and aesthetic disputes due to consolidation-related surface blemishes.

Stages of consolidation

Consolidation using an internal vibrator occurs in two stages: 1) leveling and 2) de-aeration. During the first stage, concrete is temporarily liquefied due to the rapid oscillatory motion transmitted to the concrete by the vibrator. Due to the energy imparted to the concrete, coarse aggregate particles become suspended, large voids between aggregates fill with mortar and concrete settles due to gravity.

In the second, or de-aeration stage, the remaining entrapped air bubbles rise to the surface and escape, especially the large bubbles. It is not possible to remove all the entrapped air bubbles; however, vibration should continue until the cessation of large air bubbles occurs.

Use the largest and most powerful internal vibrator, systematically vibrate the concrete area through the full depth of the lift being sure to penetrate the previously lift and limit the distance between insertions so the area visibly affected by the vibrator overlaps the adjacent just-vibrated area. Continue vibrating until the coarse aggregate particles become embedded, a thin film of mortar forms on the top surface and along the form faces, and large air bubbles stop escaping from the surface.

Also, listen to the pitch or tone of the vibrator. When the vibrator is first inserted into the concrete, vibrator frequency drops but then increases and finally becomes constant when the concrete is free of entrapped air bubbles.

Many times, untrained operators only level the concrete and fail to complete the de-aeration phase of consolidation resulting in unwanted surface blemishes. It is important for operators to understand the stages of vibration and indicators of well-consolidated concrete.

Honeycomb

Honeycomb occurs when mortar fails to completely fill the spaces between the coarse-aggregate particles leaving irregular voids or stony zones in the concrete. Typically, honeycombing occurs along the form face and in many cases is limited to the region between the form face and reinforcement. Causes include: congested reinforcement and narrow forms, insufficient paste due to segregation of the concrete, improper fine to coarse aggregate ratio, improper placement procedures (excessive concrete free fall and lift thicknesses), trying to place stiff and dry concrete, difficult construction conditions, and insufficient consolidation efforts.

Subsidence cracks

Sometimes called plastic settlement cracks, subsidence cracks occur when additional settlement or consolidation occurs after the freshly placed concrete has been vibrated and finished. Causes of delayed settlement include excessive bleeding and incomplete concrete consolidation. Cracks form in the plastic concrete because of some type of restraint that restricts concrete settlement. Subsidence cracking in columns and walls commonly occurs over fixed items such as tie bolts and reinforcement and in columns and walls where settlement is restrained by wedging or arching of the concrete. Deep concrete sections are more prone to subsidence cracking.

This form of plastic cracking can be minimized by using proper placing and consolidation procedures and essentially eliminated by revibration. Revibration is not harmful, and it can mend subsidence cracks and reconsolidate concrete after delayed settlement has occurred. If the vibrator can be inserted into the concrete and removed without leaving a hole, it is not too late to revibrate the concrete.

Cold joints

Cold joints are different than lift or layer lines. Lift lines occur when successive lifts of concrete are not completely knitted together. In many cases, lift lines occur because of color variations between loads of concrete and do not indicate there is a joint or discontinuity in the concrete. In contrast, a cold joint is a discontinuity between two lifts of concrete. They occur when, due to a concrete placement delay, the earlier lift of concrete hardens sufficiently to preclude knitting and bonding of the two lifts. Except for the unsightly appearance, cold joints are typically not a concern unless the concrete is unreinforced or an environmental tank.

Surface air voids

Commonly called bugholes, surface air voids are small, regular or irregular cavities, usually less than 58 inch in diameter, caused by entrapment of air bubbles in the surface of formed concrete. Bugholes are normal for vertical cast-in-place concrete and not considered a defect unless voids exceed the maximum size specified by the contract documents.

Minimize bugholes by using: smooth, impermeable formwork; the thinnest coat possible of an appropriate release agent; limited lift thicknesses; high-frequency vibrators; and proper vibrating procedures with sufficient periods of vibration to de-aerate the concrete.

Contractors will save time and money when formwork consolidation it done right. The key to avoiding repair and aesthetic disputes is both knowing why surface blemishes occur and how to properly vibrate the concrete.

References: ACI 309R-05 Guide for Consolidation of Concrete, American Concrete Institute, www.concrete.orgACI 309.2R-98 Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces, American Concrete Institute, www.concrete.org

Kim Basham is president of KB Engineering LLC, which provides engineering and scientific services to the concrete industry. Basham also teaches seminars and workshops dealing with all aspects of concrete technology, construction and troubleshooting. He can be reached via email at KBasham@KBEngLLC.com.

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