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Top Bar Effect in bond in Beams and Slabs

 
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Fri Sep 11, 2020 4:38 pm    Post subject: Top Bar Effect in bond in Beams and Slabs Reply with quote

Top Bar Effect in bond in Beams and Slabs

A factor has to be adopted to the development length when the depth of fresh concrete below the bar during casting is greater than 300 mm. it is because experiments have shown that the excess water (often used in the mix for workability) and entrapped air invariably rise towards the top of the concrete mass during the consolidation of concrete by vibration and tend to get trapped beneath the horizontal reinforcement, thereby weakening the bond at the underside of these bars (Jirsa and Breen 1981). This effect is called the top cast bar effect.

Thus, top-cast bars have lower bond strengths than bars cast lower in a member. ACI 318 defined top bars as horizontal bars placed in beams and slabs so that more than 300 mm of concrete is cast below the bar and accounts for this effect by multiplying the development length of a top bar by an arbitrary factor of 1.3. It has to be noted that Note that this is applicable to concrete with less than 100 mm slump.  (reduced from 1.4 since 1989).  Also see pp. 269 of my book Design of RC structures, Oxford University Press, 2013.

The same is also mentioned in IS 456, only for lap splices in Clause 26.2.5.1 (1), and it has to be noted that the multiplication factor it adopts is still  1.4! I do not know how many designers use this clause.


Top bar effect in Self compacting concrete



If more fluid concrete (for example, Self compacting concrete,where the slump will be greater than 150 mm)is used, we normally expect no top bar effect, as no vibrator is necessary for SCC. However, some researchers have found the same top bar effect in such concretes also, even though the use of silica fume in High-Performance Self Compacting Concrete may reduce this effect (Dybel et al., 2018)

Best wishes
NS
References:
Jirsa, J. O., and J. E. Breen. Influence of Casting Position and Shear on Development and Splice Length—Design Recommendation, Research report no. 242-3F, Center for Transportation Research, University of Texas at Austin, Nov. 1981, 46 pp.

Jaunty, P. R., Mitchell, D. and Mirza, M. S., (1988). “Investigation of top bar effects in beams.” ACI Materials Journal, 85 (3), 251-257.

Dybel, P., Walach, D., and Ostrowski, K., The Top-Bar Effects in Specimens with a Single Casting Point at One Edge in High-Performance Self Compacting Concrete, Journal of Advanced Concrete Technology, 16:2018:282-292
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