App D_Fairchild_GPR Investigation.pdf

Geophysical Survey LLC

711 S. Tacoma Street

Kennewick, Washington 99336

June 18, 2020

Devin Newton

Tetra Tech

711 Tank Farm Rd Ste 110

San Luis Obispo, CA 93401

Re: Concrete Investigation

Fairchild Air Force Base

Spokane, Washington

Mr. Newton:

Geophysical Survey LLC conducted a concrete investigation at Fairchild Air Force Base in Spokane, Washington on June 9, 2020. The objective of the investigation was to determine rebar spacing, estimate rebar diameter and estimate the thickness of concrete.

Methodology

Ground-Penetrating Radar

Ground-penetrating radar (GPR) uses a transducer to transmit FM frequency electromagnetic energy into the concrete. Interfaces in the concrete, defined by contrasts in dielectric constants, magnetic susceptibility, and to some extent, electrical conductivity, reflect the transmitted energy. The GPR system then measures the travel time between transmitted pulses and arrival of reflected energy. Buried objects such as rebar, post-tensioning or conduit can cause all or a portion of the transmitted energy to be reflected back towards a receiving antenna.

The dielectric constant and magnetic susceptibility of the medium primarily control the velocity of the EM energy. Values of EM velocities, for depth calculations, are determined by measurement and from knowledge of the subsurface medium.

The depth of investigation is a function of the transmit power, receiver sensitivity, frequency of the antenna, and attenuation of the transmitted energy due to the medium.

The maximum depth of investigation may vary significantly depending on the age of the concrete. Fresh concrete is relatively conductive and the GPR signal attenuates quickly reducing signal penetration and resolution. Depth of investigation is also affected by highly conductive material, such as rebar that essentially reflect all the energy. The method cannot “see” directly below areas of highly reflective material because all of the energy is reflected.

Concrete Investigation

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FIELD SURVEY

GPR Data Acquisition & Processing

1.6 GHz antenna data was collected on the surface of the concrete slabs throughout the building. Data were collected at 60 scans/foot with a 10 nano-second window

(approximately 18 inches with a dielectric constant of 9). Radar data were collected in two orthogonal direction at five areas throughout the building: the mechanical room, machine shop, locker room, office, and hanger bay.

GPR data were processed using Radan 7.0 software from GSSI. Data were corrected for zero time and a finite impulse filter (background removal) was applied. Data migration was used to determine a precise velocity.

RESULTS AND INTERPRETATION

Mechanical Room

Figure 1

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No rebar reinforcement was detected in the concrete slab in the mechanical room shown in Figure 1. The scan area was a five foot by six foot area. The image below, GPR Data

1, is a radar profile collected running west to east and shows the depth of concrete at approximately six inches. A depth of six inches is consistent throughout the mechanical room. The reflectors in the image are utility lines at and below the bottom of the concrete.

GPR Data 1

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Machine Shop

No rebar reinforcement was detected in the concrete slab in the machine shop. The scan area was a six foot by twenty four foot area. The slab is approximately fourteen inches thick as shown in GPR Data 2. The scan area included three joints where the slabs on either side of the joints were suspected to have differences in depth and rebar. The depth of concrete is consistent throughout the scan area and no changes in depth or reinforcement were detected among the different slabs.

GPR Data 2

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Locker Room

Rebar reinforcement was detected in a portion of the concrete slab on the south side of the locker room. The image below, GPR Data 3, shows the spacing between the rebar is twelve inches. The rebar runs in one direction north to south and extends out approximately four feet from the south wall. No rebar was detected in any other areas of the concrete slab in the locker room. The depth of concrete is approximately nine inches.

Concrete thickness is consistent throughout the locker room.

GPR Data 3

Radar does not directly measure the diameter of a rebar, cable or conduit. Due to the signal wavelength, any object under 2 inches in diameter is a “dot” with no visible size.

However, where two or more intersecting bars are found and it can be assumed that these bars are touching each other. This is the case, for example, when a tied mat of rebar is used. The diameter of the top bar can then be measured by subtracting the measured depth of the top bar from that of the intersecting bar. Rebar size was not able to be determined because no intersecting rebar was detected in the scan area.

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Office

Figure 2

A reinforcing mesh was detected throughout the concrete slab in the office. The welded wire is spaced 6 inches apart in two orthogonal directions. The concrete slab is five inches thick. The two images below, GPR Data 4 and GPR Data 5, are radar profiles of the concrete in one direction. Image GPR Data 4 is raw data, GPR Data 5 is migrated data.

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GPR Data 4

GPR Data 5

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Hanger Bay

No rebar reinforcement was detected in the concrete slab in the hanger ay. The area scanned was a section of the concrete slab on the west end of the hanger bay. The scan area was ten feet out from the wall and ran along the wall shared with the men’s restroom and locker room. The image below, GPR Data 6, is a radar profile in one direction and shows the slab is fourteen inches thick.

GPR Data 6

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CLOSURE

Geophysical surveys performed as part of this survey may or may not successfully detect or delineate any or all subsurface objects or features present. Locations, depths and scale of buried objects or subsurface features mapped as a result of this survey are a result of geophysical interpretation only, and should be considered as confirmed, actual, or accurate only where recovered by excavation or drilling.

Geophysical Survey LLC performed this work in a manner consistent with the level of skill ordinarily exercised by members of the profession currently practicing under similar conditions. No warranty, express or implied, beyond exercise of reasonable care and professional diligence, is made. This report is intended for use only in accordance with the purposes of the study described within.

Respectfully, Geophysical Survey LLC

Mark Villa L.G.

Geophysicist