Wrong Myths On Column Construction – A Challenge To Overcome

Introduction

What is Column? Column is a vertical compression member which transmits load of the structure to foundations (Fig 2). They are reinforced by means of main longitudinal (vertical) bars to resist compression and/or bending; and transverse steel (closed ties) to resist shearing force (Fig 3).

Typical Loads to be considered for Column Design (i) Dead Load: Any permanent load acting on the column, e.g. self-weight of column, weight of beam

(ii) Live Load: Any non-permanent or moving load

(iii) Earthquake Load: Depends on the seismic zone where building is located. Higher is the zone, more is the load

(iv) Wind Load: Depends upon the wind speed, height & location of building. Also terrain and adjacent structures play a role in determination of this load

5. Lapping should preferably be done in the centre part of column with a min lap length of 57 times the dia of rebar(c). So if you are using 16 mm rebars then lap length will be 3 feet.

6. The ends of the ties must be bent as 135° hooks. The length of tie beyond the 135° bends must be at least 10 times diameter of steel bar used to make the closed tie; this extension beyond the bend should not be less than 75 mm (Fig 7a).

If this guideline is not followed then the tie/ring holding the vertical rebars have a higher probability of opening up during an event like earthquake. This consequently may lead to failure of column .

7. Minimum grade of concrete to be used for building a RCC column is M20.

8. Minimum percentage of steel to be used in a RCC column is 0.8% of cross-sectional area of column.

Note on Honeycombing Honeycombs are hollow spaces and cavities left in concrete mass on surface or inside the concrete mass where concrete could not reach. These look like honey bees nest (Fig 8).

Honeycombs which are on sides are visible to naked eyes and can be detected easily as soon shuttering is removed. Honey combs which are inside mass of concrete can only be detected by advanced techniques like ultrasonic pulse velocity testing or rebound hammer test.

Honeycomb is formed mainly due to: a) Improper vibration/compaction

b) Less cover to reinforcement bars

c) Construction joints (joints upto which a stage of construction is done) are the typical positions where honeycombs are observed. This is due to non-treatment of construction joints (cleaning of laitance and loose cement slurry from joint using wire brush/chipping) before resuming construction.

d) Improper mix proportioning of various components of concrete and/or improper gradation of aggregates are also responsible for such honeycomb formation.

Remedies for Honeycombs in Concrete: • Strictly speaking wherever honeycombs are observed concrete should be chipped off at that location and the portion should be re-built after applying fresh concrete. Honeycombs as a defect not only reduces the load bearing capacity but water finds an easy way to reinforcement rods and corrosion starts. Corrosion is a process which continues through reinforcement rods even in good concrete, which results in loosing grip between rods and concrete, which is very dangerous to safety and life of concrete structures.

• It will not be out of context to point out that by applying superficial cement plaster on the honeycombs can be a temporary solution of hiding them, but is never safe/advisable.

• At beam-column junction, concrete with 20 mm and below graded aggregates can be used with slightly more water and cement to avoid honeycombs.

• Use of needle vibrator for proper compaction of concrete helps in reducing honey combs. The fresh concrete should be thoroughly vibrated near construction joints so that the mortar from new concrete flows between large aggregates and develops proper bonding with old concrete.