14 Oktober, 2010

Honeycomb and Void Repairs.

Reinforced concrete - one of the most commonly used construction materials - is a combination of concrete (cement and aggregates) and reinforcing steel. The cement mortar portion of this composite material (approx 25% of the concrete) acts as a paste or glue that binds all of the many sized aggregates together. When properly designed and placed these three materials create a unique composite that can be cast into thousands of different infrastructure applications. In order to have a reinforced concrete beam, column, slab or wall function properly, the concrete must be properly consolidated in the forms, i.e. it must completely encapsulate the reinforcement and be free of voids or "honeycombs". Voids are completely empty or hollow spaces in the form in which no concrete exists. Honeycombs based on a Portland Cement Association definition, are voids left in concrete due to failure of the cement mortar to effectively fill all the spaces between the coarse aggregate particles.  Severe honeycombs (sometimes known as rock pockets) occur when an excessive amount of aggregate is found without the presence of cement paste. (See Figure 1)
Honeycomb in Concrete Column
Figure 1: Honeycomb in a column from excessive lift depth.


 Void created by rebar congestion
Void created by rebar congestion
Primary causes of voids or honeycombs in reinforced concrete:

  • Rebar Congestion - if rebar is placed too close together or too close to formwork it will trap the larger pieces of aggregate while the mortar in the mixture may or may not pass through.  Other causes related to rebar congestion include excessive reinforcement splices that prevent the concrete from properly filling the forms.
  • Mix Design - Improper mix design can lead to low workability, early stiffening or an aggregate that is too large to properly consolidate the concrete for a given application. A good mix design should take in consideration the issues noted for rebar congestion and lift depth.
  • Lift Depth - When single concrete placements or "lifts" are too deep, proper vibration can become very difficult or impossible. Excessive lift depths can also allow too much free-fall of the concrete that can create a separation of the cement mortar and aggregate as the aggregate impacts the reinforcing steel when falling through the forms.
  • Inadequate Vibration - When the concrete is properly vibrated it acts more like a liquid allowing it to better settle in the form, consolidate around the reinforcement and completely fill the forms. It also helps in releasing any of the air voids in the mix to the surface. Improper vibration can be related to:
    • Too small or large a vibrator for the size of the pour and mix design
    • Too low a frequency or amplitude of the vibrator for the size of the pour and mix design
    • Too short or long an insertion time of the vibrator in the concrete in a single location
    • Too wide of a spacing between each insertion of the vibrator
    • Lift depths too deep to actually vibrate the concrete
    • Congested reinforcing that will not allow a standard vibrator to reach all areas required
  • Form Leaks - Leaks in the formwork can allow the cement paste to escape out of the form leaving behind only unbonded aggregate and rock pockets.

    Repair Process
    There are several factors to consider when repairing honeycombs and voids:
    • Void size and depth
    • Access to the repair area
    • Rebar details and congestion within the repair area
    • Quality assurance (how do you determine actual void size and prove the problem is fixed)
    • Costs from a "repair or replace?" perspective ... which is more cost effective?
    These critical items will clearly define the following key steps and sequential order of repairing a void which are:
    1. Define the size of the repairs
    2. Define the depth of the repairs
    3. Is shoring required?
    4. Select the type of removal
    5. Select the material for repairs
    6. Select the proper placement technique of this material to ensure filling of the void AND create a composite bond with the substrate.

    Defining the size and depth
     NDT Testing
    Figure 2: NDT Testing measuring response time in a concrete section. A void will appear as a shorter response time.
    The actual repair process starts with defining the removal geometry - i.e. size and depth of the area requiring repair. Methods used can include chipping, sounding and Non Destructive Testing (NDT) such as Impulse Response or Pulse Echo testing. These tests measure response times or frequency of a wave or impulse though a concrete section. If a void exists, the wave or travel time is of course greatly affected. (See Figure 2)  In many cases the area of repair is going to be larger than what is noticeable to the eye from the surface of the concrete. This difference is based on several items:
    • The presence of unseen voids.
    • The concrete materials around the void may not be sound and typically must be removed deep enough to reach a sound substrate.
    • The final repair shape is normally larger than the rough shape of the actual void. Per ACI and ICRI repair specifications the final surface shape should be rectilinear in shape vs the rough, multi-shaped void left after removal of unsound concrete. Rectilinear can be defined as a combined series of rectangular and/or square shapes. (See Figure 3)
    • Additional removal requirements, in relation the depth of the repair, must be considered. Typical repair specs call for a uniform depth throughout the repair area and where there is any exposed reinforcing steel (50% of the bar exposed), the concrete should be removed at a minimum of ¾" behind the bar. (See Figure 4)
     Final repair shape
    Figure 3: The final repair shape (in yellow) is often larger than the rough shape of the actual void.
    Careful thought must be given to the size, depth and location of the void as removal may affect the structural capacity of the element being repaired. If enough material is missing or is being removed from critical sections of a beam or slab, shoring should be installed when the void is detected or prior to removal. This is especially true with gravity elements such as columns. For example, any chipping or removal on a column  inside the structural core (any section inside the horizontal rebar ties) will dramatically affect the load carrying capacity.  As a rule of thumb, it is better to err on the side of safety and shore the element.

    Removal Techniques
    Removal of the unsound material can be accomplished using hydrodemoltion techniques, or typically with 15# chipping hammers.  Why use such a small hammer?  When using anything greater than a 30# chipping hammer, the impact forces of the bit will microfracture the substrate and damage it. Also in may cases due to the concentration of rebar in the void, a smaller hammer is easier to accomplish the "dental" like removal without breaking the bond of the concrete around the rebar outside the repair area. Chipping is done until a uniform depth and shape is achieved and a "Fractured Aggregate Profile" is present in the substrate. This chipping profile is achieved when the aggregate in the concrete is sound and so well bonded that it will shear in half while being chipped with a 15# hammer.  At this point it is clear that the substrate is sound as the cement paste is well bonded to the aggregate.  This is a typical gauge to determine when sound substrate has been reached.
    Hydrodemolition on the other hand utilizes water pressure up to 50,000 psi to literally explode the cement in order to remove it. This is advantageous in situations where the concentration of rebar is so high that it is impossible to remove the concrete with a chipping gun. It also requires a water collection process due to the volumes of water required which can be difficult in some environments.
     Proper Surface Preparation
    Figure 4: Proper surface preparation.
    The last step of surface preparation requires that the concrete substrate has an "open pore structure". During chipping or Hydrodemolition, the pores of the concrete have been impacted and filled with concrete dust and slurry. Removing this is imperative as the bond between the repair material and substrate can only be achieved by forcing the repair material into these pores. The concrete can be properly prepared by abrasive blasting or high pressure water after demolition.

    Repair Material and Placement Techniques
    Now that the size and depth of the repairs are know, the next step is to determine the optimum repair material and the best technique for placing it into the void.  For most honeycomb and voids repairs, the Form and Pump Technique (See Figure 5) is the best choice to fill the repair area and ensure a good bond to the existing concrete.  Form & Pump, as the name suggests, pumps the repair material into a closed form. This guarantees two key requirements noted above. One is making certain that the repair material has entirely filled
    Form & Pump 2
         Figure 5: "Form & Pump"
    the formwork and is consolidated around all rebar. The second relates to the fact that once the form is full and all air voids removed, it is pressurized. This assures that the repair material has been literally driven into the open pores of the existing prepared concrete. This will achieve a bond sufficient to make the two materials act as one. Use of vent tubes and bleed locations in a Form & Pump repair operation are important in order to make sure that material is reaching all the sections of the repair area. Selection of repair material must consider the required flowability and aggregate size for void size, shape and rebar clearances.  For smaller, more minor honeycombs or voids, a Form and Cast in Place placement technique and vibration may be an option for a repair method.

    Quality Control
    Lastly, depending on the size and complexity of the repair, the same NDT type test methods can be used to make sure that no voids still exist.  Similar to finding a void in concrete, the testing can be used to measure the frequency or travel time through the concrete.  Results can be compared to initial testing and confirm a void free section

     Before Column - Fig 5A
     During Column - Figure 5A
     After Column - Figure 5A
    Figure 5 A-C

    Case Studies

    New High Rise Building Column Repairs
    A new twenty story high rise building project was being constructed. During inspection it was discovered that several columns and shear walls located throughout the structure had various levels of honeycombing.  There appeared to be two possible causes of the honeycombs:
    1. The lift depth for placements was excessive and segregated the cement and aggregates as they fell through the formwork and rebar
    2. The vibration process was very difficult to accomplish in the bottom 4 feet of the elements due to the lift depth.
    For most cases, it was determined that the unsound material would need to be removed inside the structural core (area inside the horizontal reinforcing bars). As such, columns and shear walls were shored up prior to the removal of material.  Removal was done using 15# chipping hammers and the surface was high pressured washed to open the pores.  Form & Pump or Form and Cast in Place was then used to fill the repair area depending on repair size and geometry.  NDT type tests or sounding was conducted to ensure that no voids or honeycombs existed after the forms were removed. (See Figure 5 A-C)

    Army Base Incinerator Wall Repair
     Before Project - Figure 6
     After Project - Figure 6
     Figure 6 A-B
    A very thick, heavily reinforced concrete incinerator wall located on an army base in the southeastern region of the United States had a large void that was observed after the forms were removed.  The combination of excessive reinforcing steel, a through wall penetration and placement techniques appeared to have caused the void.

    • The repair process began by defining the size and depth of the repair area through a combination of chipping, sounding and NDT Testing (which was larger than what appeared to the eye). The next step was to define the removal geometry with the final surface shape being rectilinear.  Material was then removed using 15# chipping hammers and hydrodemolition.  The repair area surface was then prepared to open the pores of the concrete. Form & Pump was used to fill the void with repair material. Finally, QC was again performed using NDT testing to make sure that the no voids existed after the forms were removed. (See Figure 6 A-B)

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