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Section 5: Taking, Marking, and Patching Cores

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Description

Cores may be taken from concrete members for a variety of reasons, including verification of compressive strength, investigation of potential concrete material problems (e.g. segregation or bleeding), or examination of specific defects (e.g. cracks or cold joints). This section covers proper taking and marking of cores, and patching of the core holes.

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Selection Criteria

Select core locations and have them approved by the Engineer. Check fabrication sheets so, if possible, cores are taken with minimum impact to mild reinforcement. Check design sheets or shop drawings so do not take cores through prestressing strands unless specifically approved by the Engineer.

Take four-inch outside diameter cores when feasible. When approved by the Engineer, take smaller cores in highly congested areas to avoid impact to mild reinforcement or prestressed strands.

When possible, take cores through the entire width of the concrete section.

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Taking Cores to Check Compressive Strength

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  • Take at least two cores from a member if design cylinders reveal a potential deficiency in the required 28-day compressive strength.
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  • Evenly space the cores along the member(s) in question. Typically, take the cores through the webs or sidewalls of prestressed concrete girders.
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  • Ensure cores taken for testing compressive strength have lengths at least twice their diameter.
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  • There can be no mild reinforcement or prestressed strands in the cores if they will be used for testing compressive strength.
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Taking Cores to Investigate Potential Concrete Material Problems

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  • To check questionable batches of concrete, take the cores through the worst section or sections. Also take a control core from the same member where the concrete was placed from a different batch and there are no apparent problems.
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  • If checking for bleeding or segregation, take one core through the problem area and another core through the top of the member directly above the first one. Again, take a control core from an unaffected section of the member.
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Taking Cores to Investigate Specific Defects

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  • Take cores directly through the problem areas when investigating specific damage or defects. It typically will not be necessary to take control cores in these types of situations.
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  • If investigating a cold joint, take the sample such that approximately half the core is above the joint and half is below the joint.
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Marking Cores

Draw two arrows on the core locations BEFORE a core is taken. Point both arrows straight up, and drawn them on each side (left and right) of the core. In most cases the petrographer will need to cut in the vertical orientation, so it is important that the core be marked such that both sides will indicate the “up” direction after cutting.

After a core has been taken, write additional information on each side (left and right) of the sample. Include the following information:

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  • Structure No. (existing structure) or CSJ (new construction).
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  • For new construction, name of Prime Contractor if at jobsite or Fabricator if in Precast Concrete Plant.
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  • Member ID and location.
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  • Core number. Also, take photographs and notes indicating from where in the member the core was taken and why.

Again, most cores are cut vertically. Write all of the above information on both sides of the core so each part of the sample can be properly identified if it is cut.

Include a standard TxDOT Form 202 for each set of cores taken from a member. Request that the TxDOT Inspectors fill out Form 202 as needed so hard copies of the completed forms can be sent directly with the samples. Also send copies of applicable concrete mix design worksheets and batch tickets with the cores when they are available.

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Patching Cores

As with all large patches, utilize preapproved bagged cementitious repair material or batched concrete to patch core holes when feasible. Follow the requirements set forth in the section on Intermediate Spall Repair for implementing the work.

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