Fire Scene Evidence Collection Guide

Ignitable Liquid Residue & Fire Debris

Concrete

Detailed Procedure:

Concrete can be a productive surface if the concrete is untreated. Common ignitable liquids poured on concrete and ignited often leave an intermixed and mottled black, brown, and gray stain. Try to sample areas that have expansion joints, lally columns, pre-fire cracks, or areas where an ignitable liquid may have spread under items sitting on the concrete.

If spalling is present, do not assume that it is proof of ignitable liquid use. Spalling can be caused by other conditions and events. Photograph the spalling.

To collect concrete:

  1. Select a productive sampling area:

    • Within an area of ignitable liquid indicators, such as a suspected pour pattern

    • An area where ignitable liquid may collect, such as the lowest point on the floor or at the base of a wall

    • Expansion joints

    • Joint with columns

    • Pre-fire cracks (usually display interior sooty edges)

    • Edge of a burn pattern or suspected pour pattern

    • Junctures with furniture and walls

    • Near floor drains

  2. Photograph the pattern area on the concrete.

  3. Wear eye protection, breathing protection, and wear new, unused, clean latex or nitrile gloves. Use protective gloves (properly cleaned or new) if necessary to prevent cuts.

  4. Squeegee the area, if necessary, to remove excess water.

  5. Lay a new, clean paper towel or cloth over the concrete area that is being sampled. This will minimize the spraying of the concrete fragments as you break it up.

  6. Using a mallet, hit the paper towel or cloth to break up the concrete. Use a chisel if necessary. Break up the concrete to a depth of about 1/2 inch to 1 inch and pulverize it into small pieces. If sampling a joint or crack, shatter and sample the concrete from ½ inch away on both sides of the joint or crack.

  7. Using gloved hands or a clean tool (ie: trowel), gather chipped fragments into a new, clean metal paint can. Leave at least 1/3 headspace in the can.

  8. Clean the lip of the can using a new or properly cleaned screwdriver or other suitable tool.

  9. Tightly close the can by gently tapping it with a rubber mallet to seal it. Do not dent the can because it may compromise the seal.

  10. Seal the container with evidence tape. Initial and date the tape.

  11. If possible, sample any base material (ie: soil) beneath the concrete sample area using the appropriate procedure for that material. Place the base material in a second can and note on the evidence label which concrete sample it is associated with.

  12. If possible, collect a comparison sample of concrete that is not suspected to contain ignitable liquid residue. To collect a comparison sample, locate a protected area of concrete away from the suspected pattern area. Change gloves and follow the same procedure described above, collecting the fragments with a new or properly cleaned tool into a different new, clean metal paint can.

  13. Label each container with identifying information, including case number, date, exhibit number, a brief description, and your name.

  14. Store the item in a secure location, such as a locked evidence van or your vehicle, until you are able to transport it to the laboratory.

In some cases, the absorbent method may be appropriate on a hard surface such as concrete. The absorbent method uses material such as lime, diatomaceous earth, or non-self-rising flour spread over the area and left to stand for 20-30 minutes. Then, the absorbent material is collected. A comparison sample of unused absorbent should also be submitted to the laboratory in a separate, properly labeled container.

Laboratory testing of concrete for ignitable liquid residue:

Gas chromatography mass spectrometry can be used to identify ignitable liquid residue in the sample.

Source:

IAAI. A Pocket Guide to Accelerant Evidence Collection, 2nd Edition. Massachusetts Chapter, International Association of Arson Investigators, 1999. Online version: http://www.interfire.org/res_file/aec.asp#con

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