• In this article we are looking at the correct compaction of concrete to walls. It will be part of a series on the methodology of concrete placement.

    These articles highlight common defects picked up by FCM’s Clerks of Works during their site inspections.

    Our Clerks of Works are your "eyes and ears on site", helping to get it right first time.



    Incorrect compaction resulting in honeycombing and voids (not to mention the tie wires left in the bottom of the formwork).

    With so many construction projects being built around a concrete frame it is imperative that quality inspections are carried out during the process. A Clerk of Works, who carries out quality inspections on behalf of the client is a must have to ensure that standards are being maintained.

    After concrete has been mixed, transported and placed, it contains up to 20% by volume, entrapped air in the form of large voids. If this air is not removed by proper compaction, the presence of these voids will:

    1. Reduce the strength of the concrete - For every 1 % of entrapped air, the strength falls by about 5 to 6 %. So a concrete with, say, 3 % voids will be about 15-20 % weaker than it should be.

    2. Increase the permeability, which in turn reduces the durability. If the concrete is not dense and impermeable, it will not be watertight, it will be less able to withstand mildly aggressive liquids, and any exposed surfaces will weather badly; in addition, moisture and air are more likely to get to reinforcement and cause it to rust.

    3. Lead to blemishes on the surface, such as blowholes and honeycombing.

    Blow Holes


    Unsightly blow holes in finished concrete.

    Blow holes are individual, generally rounded, cavities on vertical surfaces of concrete, generally less than 10 mm across.

    They are caused by air in the concrete being trapped against the form face, sometimes due to insufficient vibration.

    Some blow holes are almost inevitable unless a permeable formwork material is used. The occurrence of blow holes can be minimised by using a suitable release agent on the surface of the formwork and the use of adequate vibration. In addition, the concrete should have adequate workability.

    Blow holes are a cosmetic problem only and will not affect the long-term performance of the concrete

    Cold Joints


    Cold Joint caused by pouring wet onto set concrete along with honeycombing, voids and debris.


    A cold joint is a plane of weakness in concrete caused by an interruption or delay in the concreting operations. It occurs when the first batch of concrete has begun to set before the next batch is added, so that the two batches do not intermix.

    Voids reduce the contact between the concrete and the reinforcement and other embedded metals; the required bond will then not be achieved, and the reinforced member will not be as strong as it should be.

    One of the duties of a Clerk of Works is to ensure that pockets which may contain debris such as wire ties are cleared out prior to the concrete pour.

    Fully compacted concrete will be dense, strong, durable and impermeable. Badly compacted concrete will be weak, non-durable, honeycombed and porous. The air must be removed.

    Lift or Pour Lines


    Lift or pour lines due to poker not penetrating previous layers of concrete.

    Lift lines (sometimes referred to as pour lines) appear as colour changes along the boundaries between successive lifts within a pour.  Can mistakenly be referred to as cold joints.

    Lift lines are typically caused by a slight change in w/c ratio between each pour/batch and darker fine material being brought to the surface with the bleed water before the subsequent pour is placed.

    To reduce the likelihood of it occurring ensure:

     - consistency in the concrete supply,
     - the concrete is well mixed,
     - the pour concrete at a lift rate of not less than 2m rise per hour,
     - the time between placing batches is minimised.

    Poker Burn


    Poker burn.

    Poker burn is recognised by dark coloured marks on the concrete surface, caused by the insertion vibrating poker bruising the formwork (typically timber) face leaving a scar that is then transferred to the concrete cast surface.  Building contractors need to check that their operatives are properly qualified and carry out their own quality control inspections.

    To prevent it occurring, avoid poker contact with the form face.

    Also avoid touching the reinforcement with the poker.

    Provided that all the concrete is still fresh, vibrating the reinforcement should not do any harm and could improve the bond. The danger lies in the vibrations in the reinforcement being transmitted into parts of the section where the concrete may have stiffened, in which case the bond may be affected.

    Most concrete is compacted by means of internal poker vibrators that fluidise the concrete and permit the entrapped air to rise to the surface.

    External vibrators bolted to the formwork are occasionally used for in situ concrete though their main use is for precast concrete.

    Slabs are best consolidated by vibrating beam compactors. Deeper slabs should be compacted with poker vibrators and finished with a vibrating beam.

    Internal vibrators which sometimes called spude or poker vibrators are usually applied to compact concrete in beams, walls, columns, and slabs.

    Horizontal movement of vibrators should be avoided to prevent concrete segregation.

    The head of the poker should be allowed to sink under its own weight and penetrate the previous layer of concrete by 150mm to avoid pour lines showing on the exposed concrete.

    The poker should remain in the concrete for between 5 and 15 seconds – but until the air bubbles have stopped coming to the surface. Too long and the ballast will settle to the bottom leaving just fines at the surface.

    The compaction radius of the poker head is typically in the region of 0.5m.

    The poker should be lifted vertically and slowly at the rate of approx. 300mm (1ft) in 3 seconds and placed into the concrete again so that the vibration radius’ overlap for full compaction.

    Correct use of poker


    See video below for correct use of a poker vibrator.
    Ignore the sales pitch, but the methodology is correct.


    Steve Farrell
    T: 07714234380
    E: stevefarrell@fcmltd.co.uk
    L: linkedin.com/in/stevefarrellfcm

    Fox Curtis Murray are a building consultancy specialising in providing Clerks of Works services and Quality Control inspections.

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