16-CHRI_217021-244555_Concrete_Apron_Void_and_Rebar_Detection_Report-GPRS_09-10-18.pdf

GPR Investigation of Concrete Slab

Prepared For: HDR, Inc

Prepared By:

Peterson Jean-Baptiste

Project Manager-Miami, Fl

9/10/2018

Sept 10th, 2018

HDR, Inc Attn: Josiah Berg Site: Hospital Street, Christiansted, St Croix 00820, USVI (Fort Christiansvaern)

We appreciate the opportunity to provide this report for our work completed on the week of September 10th 2018 at the above address.

PURPOSE

The purpose of this project was to locate potential voided areas along a 14,000 Square-foot sidewalk and map the rebar pattern in up to (5) 4’x4’ sections of the sidewalk. On site, the client identified the sections needing to be scanned and agreed that the rebar patterns would be marked with markers, while any identified voids would be marked with paint.

EQUIPMENT

• 2300 MHz GPR Antenna. The antenna is only approximately 4”x3.5” and rolls over the surface. The antenna needs a reasonably smooth, unobstructed surface for scanning so we would not be able to scan within 1.75” of obstructions such as walls and metal tracks unless they are removed prior to our work. The data is displayed on a screen during the scanning and marked on the surface in real time. GPR works by sending pulses of energy into a material and recording the strength and the time required for the return of the reflected signal. Reflections are produced when the energy pulses enter into a material with different electrical properties from the material it left. The strength of the reflection is determined by the contrast in signal speed between the two materials.

The total depth achieved can be as much as 18” or more with this antenna but can vary widely depending on the conductivity of the materials and other factors such as the spacing of the reinforcing. Depth accuracy depends on the ability to obtain an accurate dielectric for the concrete or other material being scanned through. No harmful radiation is emitted and the work can be performed at any time with people in close proximity. For more information, please visit: Link

• 1600 MHz GPR Antenna. The antenna is approximately 6”x9” and rolls over the surface. The antenna needs a reasonably smooth, unobstructed surface for scanning so we would not be able to scan within 3” of obstructions such as walls and metal tracks unless they are removed prior to our work. The data is displayed on a screen during the scanning and marked on the surface in real time.

GPR works by sending pulses of energy into a material and recording the strength and the time required for the return of the reflected signal. Reflections are produced when the energy pulses enter into a material with different electrical properties from the material it left. The strength of the reflection is determined by the contrast in signal speed between the two materials. The total depth achieved can be as much as 18” or more with this antenna but can vary widely depending on the conductivity of the materials and other factors such as the spacing of the reinforcing. Depth accuracy depends on the ability to obtain an accurate dielectric for the concrete or other material being scanned through. No harmful radiation is emitted and the work can be performed at any time with people in close proximity. For more information, please visit: Link

• 400 MHz GPR Antenna. The antenna is mounted in a stroller frame which rolls over the surface. The surface needs to be reasonably smooth and unobstructed in order to obtain readable scans. Obstructions such as curbs, landscaping, and vegetation will limit the feasibility of GPR. The data is displayed on a screen and marked in the field in real time. GPR works by sending pulses of energy into a material and recording the strength and the time required for the return of the reflected signal. Reflections are produced when the energy pulses enter into a material with different electrical properties from the material it left. The strength of the reflection is determined by the contrast in signal speed between the two materials. The total depth achieved can be as much as 8’ or more with this antenna but can vary widely depending on the conductivity of the materials. Conductive soil types such as clay may limit our depths to 3’ or less. As depth increases, targets must be larger in order to be detected and nonmetallic targets can be especially difficult to locate. Depths provided should always be treated as estimates as their accuracy can be affected by multiple factors. For more information, please visit: Link https://www.gp-radar.com/Mini-XT-Data-Sheet.html https://www.gp-radar.com/GSSI-Antennas-Brochure.html https://www.gp-radar.com/GSSI-Antennas-Brochure.html

PROCESS

Our process begins by using GPR to collect initial scans throughout the area. These scans are used to calibrate the equipment and determine the type of slab, reinforcing patterns, maximum depth penetration, and any other potential limitations. Each location is then scanned in a grid pattern in 2 directions. The GPR data is interpreted in real time and anomalies in the data were located and marked on the surface. GPRS covered all accessible areas as directed on-site. The select field data and site photographs were retained for this report as reference. Additionally, data from previous projects has been included as supplemental reference for void-like data for comparison.

LIMITATIONS

Please keep in mind that there are limitations to any subsurface investigation. The equipment may not achieve maximum effectiveness due to conditions in the concrete or soil such as moisture content, age of the concrete, reinforcing size and spacing, and a variety of other factors. No subsurface investigation or equipment can provide a complete image of what lies below. Our results should always be used in conjunction with as many methods as possible such as consulting existing plans and drawings, visual inspection of above ground features, drilling or cutting as far as possible, exploratory excavation or potholing, visual inspection of above ground features, and utilization of services such as Dig Alert/Underground Service Alert.

In addition, void mapping is not a definitive process. A void will be the highest amplitude negative response in the

GPR signal but there are materials other than air that may cause strong negative responses. Therefore, there may be false positives in our findings. There also may be voids that are not able to be detected for a number of reasons.

GPR can determine the approximate boundaries/edges of voids that are detected along with the approximate depth to the top of the void but cannot determine the depth to the bottom of the void (volume).

At this site, the 1600 MHz GPR signal was able to penetrate through the slab completely; however, the depth on the 400 MHz GPR antenna was limited up to 4’/5’. This limitation was due to the reinforced concrete pavement that weakened the GPR’s signal.

FINDINGS

Based on the areas scanned, the GPRS investigation found no apparent voiding or subsurface anomalies associated with voiding on this visit. Additionally, the soil and reinforced concrete slab for this particular site allowed for maximum GPR depth penetration of about 4’-4.5’ in most areas. Furthermore, some utility-like anomalies were noted with the site contact during the scan process; however, these utilities were not marked on the concrete pavement on this particular project. Based on the GPR investigation the slab was found to be approximately 8” deep in most areas. The slab is layered with wire mesh that are spaced approximately 6” apart; however, the data anomalies/reflections created by the wire mesh weakens as you approach the seawall (see data images below).

Lastly, the rebar spacing was found to be approximately 4’ on center on the east side of the property, running perpendicular to the sea side wall. Additionally, the rebar spacing on the west side of the property was inconsistent in most areas (see data images below).

Additional exploration measures (i.e. limited destructive testing via hammer drill and bore scope) may be required in various locations in order to confirm or deny the presence or absence of potential voids beyond the data presented herein.

The following pages will provide photos and further explanation of our finding.

Subsurface Anomalies Investigation GPR Data Reference

The GPR data screenshot above is from a previous project site with no apparent void like subsurface anomaly shown as typical data produced while scanning using our standard 1600 MHz antenna. The images below show data from areas where varying degrees of voids (highlighted in red) were located using GPR and verified using limited destructive testing via hammer-drill and borescope beneath a slab-on-grade.

Distance

D ep th

Concrete

Sub-grade

Dielectric Change

Fort Christiansvaern

Site Map

The four arrows below are identifying the sections where the wire mesh and rebar pattern was marked. The different colors coincide with the data images below. Notice on each of the data screenshots below, the depth scale is on the left and the distance of the scan is across the top, forming a cross section view of the subsurface.

The consistent gray coloring beneath the surface is indicative of a slab without any voids. Voided areas will show up as a consistent black color that is very dominant; small inconsistent black patches should not be mistaken for a void (see red arrows below).

GPR Data Screenshots and Photos

GPR data screenshot for green arrow on Site Map above. The black line serves as a divider to show where one scan ended and another began. The red side of the above screenshot shows the slab with wire mesh, while the blue side shows the slab without the wire mesh. This was possible through a method called Cross polarization, which allows wire mesh anomalies to become faint in order to distinguish other anomalies, such as rebars. As you can see, the slab is without voids and this area does not have any rebars.

Picture of location #1: This photo coincides with the data screenshot to the left. The picture shows how the wire mesh pattern was marked on the concrete. The sidewalk was measured and numbered by the site engineer as a reference point; those reference points can also be seen below each area scanned (See black arrow)

Picture of location #2: This photo coincides with the data screenshot to the left. The wire mesh patterns can be seen, along with the numbered reference points. Rebars were not found in this location.

GPR data screenshot for blue arrow on Site Map above. The data indicates that voids are not present as evidenced by the consistent grey coloring. This area did not have any rebars present. The red arrows identify the wire mesh that is approximately 6” apart.

GPR data screenshot for purple arrow on site plan. The data shows that this area is once again without any voids. The arrows are used to distinguish between the wire mesh(blue) from the rebars (red) above at about 4-5” deep.

Picture of location #3: This photo coincides with the data screenshot to the left. The wire mesh was marked in black markers while the rebars were made distinguishable with red crayons.

GPR data screenshot for orange arrow on site plan. The black arrow indicates the bottom of the slab between 8-9”. Once the 1600 MHz antenna was cross polarized, the wire mesh

(blue arrow) disappeared and the rebar (red arrows) remained

Picture of location #4: This photo coincides with the data screenshot to the left. The wire mesh was marked in black markers while the rebars were made distinguishable with red crayons.

GPR data screenshot with no apparent void-like anomaly.

Although, it would seem that the wire mesh data anomaly weakens as you approach the seawall (red side). The black arrow shows the direction of the scan from left to right. (See image to the right to see where this data was taken)

Site Picture. This photo coincides with the data screenshot to the left. The black arrow shows the direction of the scan. The concrete area within the red parenthesis contained more cracking than other areas so it was heavily focused on.

GPR data screenshot with no apparent void-like anomaly. This was taken with the 400 Antenna on the east side of the sidewalk. The wire mesh and rebars (red arrow) are seen clearly on top. However, the data starts to get fuzzy passed

4’, which indicates that the GPR signal was not penetrating below that depth, only reflections.

Site Picture. This photo coincides with the data screenshot to the left as the 400 antenna is seen going over the rebar (red arrow)

The arrows on this site map coincided with the arrows that are on the data images below. These are data screenshots of the slab that were taken in random locations to show that those locations had no apparent void-like anomalies. Notice on each of the data screenshots below, the consistent gray coloring beneath the surface, indicative of a slab without any voids.

Fort Christiansvaern GPR Data Screenshots and Photos

The arrows on this site map coincided with the arrows that are on the data images below. These are data screenshots that were taken, with the 400 Antenna to see below the slab, in random locations to show that those locations had no apparent void-like anomalies. Notice on each of the data screenshots below, the consistent gray coloring beneath the surface, indicative of a slab without any voids. Again, note that in some areas the wire mesh and rebar pattern is more consistent than others, and when the data starts to get indiscernible passed 4’, that indicates that the GPR signal was not penetrating below that depth.

CLOSING

GPRS, Inc. has been in business since 2001, specializing in underground storage tank location, concrete scanning, utility locating, and shallow void detection for projects throughout the United States. I encourage you to visit our website (www.gprsinc.com) and contact any of the numerous references listed.

The results within this report are based on the field findings from the GPR Investigation conducted. The data included within only represents a portion of the data collected from this site as a representative sample. GPRS appreciates the opportunity to offer our services, and we look forward to continuing to work with you on future projects. Please feel free to contact us for additional information or with any questions you may have regarding this report.

Signed, Peterson Jean-Baptiste

Project Manager—South Florida

Direct: 786-566-6295

Peterson.Jean-Baptiste@gprsinc.com www.gprsinc.com http://www.gprsinc.com/ http://www.gprsinc.com/