concrete & pavement case studies

7/28/2019 Concrete & Pavement Case Studies

1/13

Concrete & Pavement Case Studies

Electric Cable in Concrete

Conquest survey in an warehouse.

Overview

When cutting or coring concrete, embedded electrical cabling poses a challenge. This

example from a client shows the power of powerline detection combined with GPR imaging

to define active power cables.

Problem

Locating live electric cables embedded in concrete is a concern for both operational safety

and building rennovation. Confirming the as-built construction at the design stage allows

planners to optimize placement of new infrastructure. If concrete structures have to be cut or

cored for facility upgrade, damage to live power cabling creates an operator safety hazard

and disrupts critical building functions. Avoiding such surprises is the goal!!


2/13

GPR Contribution to Solution

A contractor in Mississauga, Canada used Conquest GPR to image concrete for many

years. Embedded conduits were normally identified by the meandering response pattern

which did not align with normal reinforcing structure. On this occasion, he found an

anomalously strong response from a reinforcing member but no indication of the expectedconduit.

Sensors & Software had just introduced the next generation of Conquest GPR with the

integrated PCD (power cable detection) sensor and the new system was tested on this site.

Collecting a standard Conquest data grid resulted in the images below. The GPR depth slice

showed a strong response at position 2.25 which fit into the regular reinforcing pattern visible

in the depth slice. The PCD detector image showed that a strong powerline response was

aligned with this strong GPR response suggesting that the rebar and a power cable were co-

located.

Conquest GPR depth slice showing an unusually strong response from a reinforcing element aligned

along numeric position 2.25.


3/13

Conquest - PCD image showing live current.

Subsequent coring and chipping confirmed that an electric cable was located close to a rebar.

The enhanced Conquest GPR response was the combined response of the rebar and the

electrical cable.

Results & Benefits

This case study demonstrates the power of Conquest with PCD; combining the two sensor

observations provides more definitive indication of embedded live power cables. Some

key observations are:

Conquest imaging quickly defines reinforcing structure Users can be effective with only a few hours of training Experienced users can identify unusual responses but explanations may be ambiguous Adding the PCD sensor provides a simple, reliable indicator reducing uncertainty

GPR responses vary greatly depending on the target being sought and the host material. GPR

response variability can be challenging to new GPR users. When learning about GPR, the best

practice is to review several similar case studies to develop an understanding of variability.

Check for other insightful information on the resources tab to learn more. Use Contact Us orAsk-the-Expert to reach our Application Specialists who can help you tap into Sensors &

Software's vast array of technical information.


4/13

Power Main under Slab-on-Grade

Meeting room where power supply line was located adjacent power panel area. Conquest screen

shows real-time mapping data from the power cable (PCD) sensor.

Overview

A recent building renovation required mapping the precise location of the main power

line entering the building under the slab-on-grade floor. Conquest with PCD (Power Cable

Detection) provided a quick and easy solution.

Problem

Many low rise buildings with concrete slab-on-grade floors have poor documentation of

utility connections that were placed under the slab. GPR provides a powerful way of mapping

out the sub-slab installations. In this case study, the lead-in from the street to the building was

not well defined at the front of the building. The power feed disappeared under the concrete

floor and emerged at a panel in the center of the building.


5/13


The contractor selected a Conquest concrete imaging GPR with a PCD (Power Cable

Detector) sensor to attempt to locate the power cable path. A major benefit of the Conquest

approach was the speed of data acquisition and the fact that very little disruption was required

to day-to-day business activity. The added benefit of the integrated PCD sensor that can detectlive power cables simplifies data analysis.

An initial 8' x 8' Conquest grid was established in a meeting room adjacent to the power panel

since this was the probable route for the cable. The PCD data were collected simultaneously

with the GPR data and the results superimposed on a floor plan as seen below.

The GPR data defined tha slab reinforcing and gave a suggestion of a deeper feature. The

PCD data were very definitive and clearly showed the presence of the power cable alignment

as show below.

GPR and PCD imaged data superimposed on building floor plan. The GPR data (left) shows the

distribution of the reinforcing in the slab-on-grade. The PCD response (right) shows the classic

lozenge-shaped response of a twisted parallel wire.


6/13

With this initial result in hand, a series of smaller grids and traverse lines were run in the rest

of the room and the next adjacent room. The power line was even mapped on the sidewalk

outside and tied into the utility street locate marks.

Results & Benefits

The use of Conquest GPR with PCD demonstrates the power of combining multi-sensor

data to solve a practical day-to-day construction problem. Some key benefits are:

Conquest operation is simple and intuitive Users can be effective with only a few hours of training The integrated GPR and PCD provide a high level of confidence in the postioning of live

power cables

Systematic search protocols are available as best practice guides Grid mapping with superposition of depth images on as-build or floor plans simplifies data

analysis and reduces poor interpretations



practice is to review several similar case studies to develop an understanding of

variability. Check for other insightful information on the resources tab to learn more. Use

Contact Us or Ask-the-Expert to reach our Application Specialists who can help you tap into

Sensors & Software's vast array of technical information.


7/13

Road Pavement Forensics

This custom Noggin configuration is capable of carrying multi-GPR sensors in a ground couple fashion

at highway speeds on a legally-licensed platform.

Overview

Subsurface structural failures generate gradual degradation in the pavement structure that

works its way up to the surface. GPR pavement forensics surveys can detect unusualsubsurface conditions prior to a surface failure. A survey of a multi-lane highway constructed

with asphalt over concrete shows growing surface failures directly above joints in the

underlying concrete.

Problem

The challenge is to identify zones of incipient failure to plan remedial action. Firefighting

emergency repairs are costly and disrupt maintenance budgets. Technology is needed that can

define subsurface conditions rapidly and inexpensively at highway speeds without roadclosure.


Surveys for undocumented and non-visible repair zones were conducted on Highway 401 inToronto, Ontario to plan maintenance and road upgrades. The highway in this area is 40

years old and has seen many upgrades and repairs. Two sites of surface degradation are

marked on the Google Earth map, below.

RoadMap uses multiple Noggin GPR systems to acquire precisely geo-referenced GPR. Data


8/13

are collected concurrently from multiple GPR sensors with differing frequencies and/or

different path alignments. Synchronized video augments the GPR data, and all data are

digitally recorded for later review and analysis.

The location of sites mapped with the RoadMap system.

The road construction in the area consists of a 200 to 250 mm layer of asphalt pavementoverlying concrete slab which in turn overlies granular material. Multiple passes over several

lanes over a distance of about 14 km indentified numerous zones of past repairs and structural

issues which were neither on record nor visible from the surface.

Site 1" shows a prior repair zone where part of the entire concrete slab has beenremoved The concrete slab to the immediate left is also tilted.


9/13

At Site 1 a previous repair and tilting concrete slab to the left of the repair are clearly visible in this

GPR section. The visible surface cracks bracket the repair.

Site 2 indicates a different construction repair character. There were no clear surface

expressions of the repair at the time of the survey.

Another repair zone on Hwy. 401 - Site 2. In the repaired section, the steel reinforcing and possibly

the concrete slab have been removed.

In both instances, the RoadMap data were acquired at 10 mm station interval. Depths were

determined from the localized point scattering diffractions and validated against subsequent

core sampling.

Results & Benefits

The GPR road and pavement forensics investigation demonstrates the power of GPR

for detecting and characterizing subsurface road conditions. Some key benefits are:

GPR can detect a wide range of changes in road material conditions Exploration to depths in excess of 1m are practical Operation of ground-coupled systems at highway speeds is practical Geo-referenced and synchronized video are essential to delivering quantifiable results.


10/13




Check for other insightful information on the resources tab to learn more. Use Contact Us or

Ask-the-Expert to reach our Application Specialists who can help you tap into Sensors &


Post-Tension Cable Characteristics

Example of Conquest GPR 3D visualization of rebar and post-tensions cables embedded in a concrete

slab.

Overview

Post-tension (PT) cables are used in concrete construction to allow thinner slabs and greater

span lengths between support columns. These cables, composed of steel wires inserted into aplastic sheath, are subsequently "tensioned" and grouted after the concrete is poured.

Problem

Contractors want to avoid hitting PT cables when drilling or cutting for rehabilitation or

renovation. The tremendous cable tension makes cutting a cable dangerous for the

operator and compromises of the structure's integrity. GPR can readily locate embedded metal

structure; identifying PT cables when a rebar mat is present is challenging.


11/13


Unique aspects of PT cables allow GPR imaging to identify these features with more

certainty. A prior understanding of construction practice is extremely helpful. The following

are two common features of PT cables:

They droop lower in the slab between support columns The cables are often bundled together in a grouping

In GPR images, thes attributes indicate patterns to look for. First, if there is a systematic

cable grouping, looking for that pattern in the GPR image is highly diagnostic. Second,

drooping cables will appear at different locations on GPR depth images from the

adjacent depths. Scanning multiple depth slices looking for features that plunge or rise is

diagnostic.

Conquests multiple depth image displays are specificaly designed for the field operator to

quickly look for these characterstics on site.

A typcial group pattern from PTcables bundled together in a grouping.


12/13

GPR cross section collected along a traverse directly over a PT cables in a concrete deck structure

showing the PT cable drooping between support columns

Discerning a post-tension cable from rebar generally requires scanning a larger area toproperly understand the layout of structural elements in a slab.

In the below example a 2'x 8' Conquest grid scan captured several cables in one grid.

3 Depth Slice showing the rebar.

8Depth Slice showing the PT cables.

When in doubt, expanding the survey area grid size to get the big picture always helps.


13/13

Results & Benefits

This case study shows how understanding the structure can enhance the diagnostic value of

GPR for concrete imaging. Some key benefits are:

Grid imaging is key to seeing the unique attributes of PT cables. Real-time depth imaging is essential for cost-effective site evaluation Conquest provides a compact, portable easy-to-use on site imaging capability. Operation is

simple and intuitive

Interactive viewing of depth slices is highly beneficial marking out a target and profiling along its axis can define the droop pattern unique to PT

cables.

Imaging over larger areas to see patterns and trends is a best-practice.

GPR responses vary greatly depending on the target being sought and the host material. GPRresponse variability can be challenging to new GPR users. When learning about GPR, the best


Check for other insightful information on the resources tab to learn more. Use Contact Us or

Ask-the-Expert to reach our Application Specialists who can help you tap into Sensors &


concrete & pavement case studies

Documents