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Minimizing Field Fixes of Cast-in Anchors

Posted by Jesus Barredaalmost 6 years ago

By: Jesus A. Barreda, P.E. and Andy Howell, E.I.T.

Cast-in anchors,ConcreteConsolidation,FieldFixes,BestPractices,Anchor Channel

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The combination of new construction practices, new technologies, and more sophisticated designs is allowing for the construction of buildings in record times. Spans get longer, columns get smaller, slabs get thinner, and lighter construction materials are used. As a result, the construction process has become more complex and best practices have evolved to solve the disputes of modern business. A common field issue encountered in building construction is the formation of air pockets (entrapped air) in the concrete at the anchorage zone.

Air pocket formation occurs due to improperly or no concrete consolidation. Repairing cured concrete with entrapped air is time-consuming, tedious, and can turn out to be quite expensive. Adopting best practices to ensure proper concrete consolidation is performed at the anchorage zone is one of the most effective ways to minimize field issues due to improper concrete consolidation. A couple of extra seconds at each connection point can pay dividends, avoid additional coordination and additional work to an already complex and fast-track work environment where the project’s schedule leaves room for a minimal margin of error.

Proactivity goes a long way. Implementing best practices that ensure adequate concrete consolidation will ensure the anchorage will perform the way it was designed. During pre-construction meetings, emphasize the importance of adequate concrete consolidation at the anchorage zone, especially if the cast-in anchor is near a confined space or congested zone. Try it out in your next project!


Figure 1: Pictures of Conditions with Improper Concrete Consolidation at the Anchorage Zone.

Concrete
Concrete is the combination of four basic components:
·       Water
·       Cement
·       Sand (small aggregate)
·       Rock (large aggregate).

When mixed together, hydration, or curing, occurs, where the cement paste acts as a glue binding all of the surrounding aggregates. When concrete is poured, it contains entrapped air. If the entrapped air is not removed and the concrete is hardened in this way, it will negatively impact the expected properties of the concrete. Moreover, it will create serviceability, esthetics, and other issues such as subsidence cracking, and placement lines (1).

Entrapped Air in Concrete
When concrete is first poured, entrapped air can occupy up to 20% of the concrete volume (1). The amount of entrapped air varies depending on the concrete’s workability. As a rule of thumb, concrete compressive strength improves by about 5% for every 1% of air removed (3). Figure 2 illustrates the decrease in strength of the designed concrete compressive strength per percentage of entrapped air. 


Figure 2: Loss of Concrete Compressive Strength Through Increase in Entrapped Air (2).
 
Concrete Consolidation
Concrete consolidation is defined as the process of removing entrapped air from freshly placed concrete. Several methods and techniques are available, the choice depending mainly on the workability of the mixture, placing conditions, and degree of air removal desired. Some form of vibration is usually employed (1).

Figure 3: Pouring and Vibrating Concrete at the Edge of the Slab.
 
Purpose of Concrete Consolidation
Proper concrete consolidation helps avoid air pocket formations, honey combs, and removes the entrapped air in the concrete. Adequate concrete consolidation helps ensure the concrete will reach its designed properties as well as helping the concrete reach its designed strength. Moreover, adequate concrete consolidation helps enhance the bond of the concrete with the reinforcing bars and increases the general durability of the concrete. Finally, it helps decrease the permeability and helps minimize it’s shrinkage and creep characteristics. Figure 4 provides pictures of jobsites with proper concrete consolidation at the anchorage zone. 

Figure 4: Pictures of Conditions with Proper Concrete Consolidation at the Anchorage Zone.
 
The Process of Concrete Consolidation
Although there are different ways to consolidate the concrete, the most effective way to remove entrapped air is vibration. Vibration consists of subjecting freshly placed concrete to rapid vibratory impulses which liquefy the mortar and significantly reduce the internal friction between aggregate particles (1). Figure 5 illustrates a construction member vibrating the concrete at the anchorage zone using a standard immersion vibrator.



Figure 5: Construction Member Vibrating the Concrete at the Anchorage Zone.

Consolidation via standard immersion vibrators is best described as consisting of two stages—the first comprising subsidence or slumping of the concrete, and the second a de-aeration (removal of entrapped air bubbles). The two stages may occur simultaneously, with the second stage underway near the vibrator before the first stage has been completed at greater distances (2). Figure 6 illustrates the two-stage process to best consolidate concrete.  

If in doubt, always err on the side of more vibration.  


Figure 6: The Process of Concrete Consolidation (2 & 4).

Per ACI 309R-05, the procedure for internal vibration shall be as follows (1):
·       Concrete should be deposited in layers compatible with the work being done.
·       The maximum layer depth should be limited to 20”.
·       The depth of the layer should be nearly equal to the vibrator head length.
·       The layer should be as level as possible so that the vibrator is not used to move the concrete laterally, as this could cause segregation.
·       The vibrator should be systematically inserted vertically at a uniform spacing over the entire placement area.
·       The distance between insertion should be approximately 1-1/2 times the radius of influence and should be such that the area visibly affected by the vibrator overlaps the adjacent just-vibrated area.

MINIMIZING FIELD FIXES OF CAST-IN ANCHORS

Depending on the amount of entrapped air, the field fix for the anchorage may vary from a simple patch with high strength non-shrinkable grout, installing post-installed headed rebars, or re-pouring a large amount of concrete. The field fix becomes more cumbersome and costlier for areas where anchors are highly loaded and reinforcing bars are used.

To avoid/minimize the number of conditions with entrapped air at the anchorage zone, be proactive. During pre-construction meetings, discuss the following with the concrete contractor:

a)    Require concrete consolidation at each anchorage point (the GC will thank you later!).  Add extra emphasis on heavily reinforced zones, congested areas, confined spaces, areas with minimum reach, and zones where the formwork prevents visibility.
 
b)    Share the best practices and make sure the field personnel know to consolidate the concrete.

The Hilti Anchor Channel Engineering team handles such field issues. Here at Hilti, we focus on creating a better future. Our hope is that this article will create awareness and will help minimize the number of occurrences of this common and costly field issue.  

References:
1)    Guide for Consolidation of Concrete, ACI 309 R-05
2)    Cement Concrete and Aggregates Australia 2006 Compaction of concrete (NSW Australia) 1-7
3)    Design and Construction of Concrete Floors, George Garber. Elsevier Ltd, 2006.
4) The Why and how of consolidating concrete, by Bruce A. Suprenant

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