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Section 3: Major Spall Repair and Concrete Replacement

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Description

Use batched concrete for patching large spalls and defects in concrete members, or when removing and replacing large concrete components. Using batched concrete ensures that the patch material properties will be the same or similar to the parent concrete.

Batched concrete is required in structural applications because it becomes more critical that the patch material have similar material properties as the parent concrete. Proprietary bagged mixes may only be used in structural applications if specifically authorized by the Engineer.

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Material

For new construction, make the repair material using the same concrete mix design that was utilized when the damaged member was originally cast. This applies to precast fabrication yards and construction sites where new structures are being built.

In remedial applications, the Engineer will specify which class of concrete should be utilized per Item 421. For repairs, the Engineer should select concrete mixes that closely match the parent material when such information is available via design documents or construction records.

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Repair Procedure

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  • Surface preparation.
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    • Remove any damaged or loose concrete prior to proceeding.
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      • Avoid damage to sound concrete that is to remain in place.
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      • Unless otherwise approved by the Engineer, use only hand tools or power-driven chipping hammers (15-lb. class maximum) to remove concrete.
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  • If more than 1/2 the perimeter of any mild reinforcement is exposed or if the exposed bar exhibits significant corrosion, remove the concrete from around the entire bar.
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    • Provide ¾-inch clearance or 1.5 times the largest sized aggregate in the repair material, whichever is greater, between the steel and surrounding concrete to permit adequate flow of the repair material.

      NOTE: A good rule of thumb is that adequate clearance is attained when you are able to wrap your fingers around the bar. Ensuring that you can grab the bar is a simple but highly effective method of ensuring there is adequate clearance to permit the repair material to flow around the exposed bar.

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    • The Engineer may require that the steel be coated or that corrosion inhibitor be added to the repair material when reinforcing steel has been exposed.
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    • Do not chip around prestressing strand that is exposed anywhere away from the immediate end of the member. Consult the Engineer when patching an area in which prestressing strands have been exposed. When repair dictates that chipping occur around exposed strands, the Contractor must avoid striking the strands directly or otherwise causing damage that could lead to wire or strand breaks.
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    • Use abrasive blasting or other approved technique to remove rust from exposed steel surfaces.
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  • Square the patch perimeters to eliminate feathered edges and to ensure that the repair material will be applied in depths no less than 1/2 inch.
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    • Handheld grinders or saws may be used to square the patch perimeters.
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    • Do not over-cut the patch perimeters at the corners of the repair areas.
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    • When practical, undercut the patch perimeter at an approximate angle of 30 degrees such that the profile will help hold the patch material in place.
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  • Roughen the substrate to ensure that there will be a mechanical bond between the patch material and the parent concrete. Though difficult to quantify and measure, Contractor should attempt to attain a minimum surface roughness profile of 1/8 inch or CSP (Concrete Surface Profile) 6 per ICRI.
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  • If the damage occurs at the end of a member and prestressing strand has been exposed, recess the strands a minimum 3/8 inch using a torch or other approved method. Do not overheat or damage the surrounding concrete.

    NOTE: In the past some Fabricators have opted not to recess prestressing strands in spalled areas so they can serve as dowels for the patch material. While the strands would serve well as dowels in those circumstances, they could be exposed to moisture and chlorides if the patch fails over the life of the structure. For that reason it is more important that the strand be completely recessed. Anchors should be installed to hold the patch material in place.

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  • For practically all batched concrete repairs there will be an adequate amount of exposed steel to provide sufficient mechanical anchorage to the parent material. If the Engineer requires that Contractor install additional ties or dowels, select material and install in accordance with the requirements in Section 3.2 for Intermediate Spall Repair.
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  • Epoxy Anchors.
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    • The Engineer will identify anchor or reinforcing steel type in plans.
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    • Anchor the bars using a preapproved Type III or Type VIII structural epoxy.
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    • Drill a hole 1/8 to 1/4 inch greater than the bar diameter. Make the hole deep enough to permit a minimum 6-inch embedment of the bar.
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    • Remove any contaminants from the hole, including laitance, oil, dust, debris, or other foreign particles.
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    • Just prior to installing the anchor, clean the hole using a high-pressure air compressor equipped with filters to remove all oil from the compressed air.
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    • Dry the concrete surface inside the hole prior to installing the dowel.
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    • Fill the hole approximately 1/3 full with structural epoxy. Twist the bar as it is inserted. For u-shaped bars that cannot be twisted fill the holes approximately ½ full with epoxy prior to insertion.
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      • Ensure that the space between the dowel and the concrete is completely filled with epoxy.
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      • Remove all epoxy from the concrete surface that leaks from the hole after the dowel is inserted.
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    • Where supplemental reinforcement is installed, ensure minimum cover of 1 ½ inch.
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    • Substrates must be clean and sound. Remove any contaminants, including laitance, oil, dust, debris, or other foreign particles.
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    • Just prior to patching, blast the repair area using a high-pressure air compressor equipped with filters to remove all oil from the compressed air.
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  • Mixing.

    Produce repair material in accordance with the approved methods for batching concrete. In order to ensure an adequate mix, batch a minimum of one cubic yard of concrete to patch the damaged area even if less volume is required to complete the repair.

    Ensure that concrete is fluid enough when it is placed that it can be adequately consolidated around reinforcing steel, anchors, and other tight places inside the forms.

    Remove the largest coarse aggregate from the material if the patch depth will be less than 3 inches over a significant portion of the damaged area, the spall occurs in an area that is highly congested with prestressing strands or mild reinforcement, or there is any other reason for concern regarding adequate consolidation. Remove the large aggregate using a 3/4-inch sieve or another method approved by the Engineer.

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  • Application.

    Hot and cold weather application.

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    • The temperature of the patch material and the concrete substrate at the time of application must be between 40°F and 95°F.
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    • Do not apply patch material when the ambient temperature in the shade is below 40°F and falling. Patch material may be placed when the ambient temperature in the shade is 35°F and rising or above 40°F.
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    • Shade the repair material components and the patch substrate if the ambient temperature is above 100°F.

    Convey the material to the repair area using approved concrete delivery equipment.

    Apply the repair material over an SSD substrate.

    Obtain an SSD condition using one of the following methods:

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    • Pond water over the substrate for a minimum of 24 hours.
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    • Several minutes before patching, apply high-pressure water blast to the surface for a brief period (1 or 2 minutes depending on the porosity of the concrete). An SSD condition is achieved when the surface remains damp after being exposed for 15 minutes.

    Surface may be damp, but must be free of standing water. Remove all free (ponded) water just before placing repair material.

    Do not use a proprietary epoxy bonding layer in lieu of an SSD substrate unless approved by the Engineer. If use of a proprietary bonding agent is authorized, mix it in accordance with the manufacturer’s requirements. Use only TxDOT approved Type V or Type VII material (refer to DMS 6100Epoxies and Adhesives).

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  • Forms.

    Prepare and install the forms prior to mixing the repair material.

    Ensure that forms are tight enough to prevent grout leakage.

    Place the repair material in the forms while the concrete substrate is still SSD. If the parent concrete is no longer SSD, remove the forms and re-spray the surface with a high-pressure water blast.

    Consolidate the material by rodding, vibrating, and/or hammering the forms. Do not over-vibrate the mix.

    If required by the Engineer, make concrete test cylinders to determine the compressive strength of the patch material. If the same concrete mix is being used for production work in another location, the results of compressive strength testing for that work may be used.

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  • Curing.

    Cure batched concrete repairs for a minimum of 72 hours. For most batched concrete applications, the material should be cured by leaving the forms in place during the entire curing period. Place wet mats on exposed sections and over the openings used to place the material.

    Do not allow concrete surfaces to become dry during the specified moist curing period. Ensure that wet mats are kept wet during the entire cycle.

    Insulate the patch material to ensure that there is adequate heat for curing if ambient temperature is expected to fall below 50°F. If using artificial heating methods, do not heat the repair material to above 130°F.

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Commentary

Batched concrete is typically the best choice when patching deep spalls and in structural applications. Particularly in new construction, mix designs can be selected to ensure that the material properties will closely match the substrate.

Failures at the bond line between the patch material and parent concrete are a common problem due to stresses that develop as a result of loading or differential thermal expansion and contraction between the patch and parent material.

To that end, using repair material that has a comparable coefficient of thermal expansion and a comparable or lower modulus of elasticity is critical for the long-term success of a repair when significant stresses are likely to develop.

Typically, it is not feasible to determine the modulus of elasticity and coefficient of thermal expansion in a member that has already been cast. The best solution is to use the same mix design for the repair material as that used when the damaged member was originally cast, ensuring that the material properties will be similar.

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