- 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 ProcessThere are several factors to consider when repairing honeycombs and voids:
These critical items will clearly define the following key steps and sequential order of repairing a void which are:
- 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?
Defining the size and depthFigure 2: NDT Testing measuring response time in a concrete section. A void will appear as a shorter response time.
- Define the size of the repairs
- Define the depth of the repairs
- Is shoring required?
- Select the type of removal
- Select the material for repairs
- Select the proper placement technique of this material to ensure filling of the void AND create a composite bond with the substrate.
Figure 3: The final repair shape (in yellow) is often larger than the rough shape of the actual void.
- 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)
Removal TechniquesRemoval 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.Figure 4: Proper surface preparation.
Repair Material and Placement TechniquesNow 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
Figure 5: "Form & Pump"
Quality ControlLastly, 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
Figure 5 A-C
New High Rise Building Column RepairsA 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:
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)
- The lift depth for placements was excessive and segregated the cement and aggregates as they fell through the formwork and rebar
- The vibration process was very difficult to accomplish in the bottom 4 feet of the elements due to the lift depth.
Army Base Incinerator Wall RepairFigure 6 A-BA 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)