Integrity Assessment for Drill Shafts Foundation in Public and Private Works with Available Technologies in the Twenty-First Century

Abstract

The evaluation of the concrete integrity in large-capacity piles (drilled shafts) associated with engineering works, designed to receive service loads from public or private structures, represents an especially important responsibility that is shared among designers, construction engineers, and inspectors. Additionally, this responsibility also lies in the hands of the promoters who may not choose to conduct the investigation for the integrity of the concrete cast in situ with the non-destructive tests available at the presentation date of this chapter. Integrity control tests allow one to know in more detail about the continuity of the concrete pouring in each analyzed element and promptly alert the engineers and managers involved in the site about the existence of anomalies due to flaws or construction defects. This information is available with different types of graphics, data, and tomography images that guide us on the continuity of the emptied concrete in the whole body of the piles. The methodologies on display make it easier to search anomalies in 2 dimensions (2D) and 3 dimensions (3D) for possible flaws and defects that may occur in the cap, shaft and the tip of the drilled shaft. These are techniques that allow the structural and geotechnical engineers to visualize what was previously invisible to the eyes and uncertain to everyone’s knowledge.

1. Introduction

The objective of this article is to present information on three of the nondestructive instrumentation tests, available at the beginning of the twenty-first century, to safely evaluate the integrity of the casting in piles and in large-capacity piles. These tests protect the lives of the users in the structures, where they are used and preserve financial investments used for the construction of public and private works. The use of these tests mitigates the effects of differential settlements, reinforcement corrosion, and functional failures that may occur in the future due to anomalies in the quality of the poured concrete.

In the case of public works, simple or mixed bridges (vehicular and/or with built-in Metro-type mass transit systems) are among the largest, most expensive, and most complex structures that currently exist, which makes them a crucial transportation asset invaluable for the local and regional connectivity of many countries. When the bridges fail, the results can quickly become a human and economic catastrophe for any country due to the isolation and interruption of the movement of people and the different services for their development.

According to international standards, all large-capacity pile or pile foundations involved in vehicular or mass transportation bridges, docks, wind turbines, buildings for multifamily housing, offices, shopping centers, land retention systems for excavations of more than three basements, discontinuous stabilization systems, stations for mass transportation systems, and piles for transmission towers should be evaluated with the integrity tests available. Failing to evaluate the foundation elements represents an extremely considerable risk for these works.

When a single (“mother”) pile is built with simple (type 1) or coaxial (type 2) reinforcements (which do not have redundancy of additional elements in their caps), and the execution times required for works (public or private) are accelerated to achieve the goals, this situation can lead to anomalies if they are not reviewed properly due to failures and/or defects in any of the construction stages of the foundations. Any variation in the materials and/or construction processes affects the final integrity of the piers and piles.

With the support of the integrity tests available to date, it is possible to verify and adapt them after their construction with the necessary corrective measures in the event of anomalies due to defects or failures in the head, shaft, or tip from the pile. So far in the twenty-first century, modernized nondestructive testing equipment and increasingly complete calculation programs have been developed making it possible to facilitate the interpretation of the results based on the experiences accumulated by the different specialists who use these tests in different continents.

This article presents the tests based on current ASTM standards, as well as the terminology and evaluation criteria established by the committee of experts that make up the institute of deep foundations (Sellountou, Amir, and others from the DFI – Deep Foundations Institute) [1].

Some experiences collected by Centeno Pulido and Centeno Werner; In Refs. [2, 3] are included in multiple trials carried out with two of these technologies available for public projects (several kilometers of bridge sections, 8 stations on Line 2 of the Panama Metro), industrial buildings, public and private buildings in the city of Caracas, and on the Gamboa bridge of the interoceanic railway (Balboa-Pacific/Colón-Atlantic) in the Panama Canal zone.

In the face of earthquake threats, integrity tests allow geotechnical and structural engineers to mitigate the risk of functional failure due to the presence of partial discontinuities (faults or defects) in piles. They specify the location of uncoated areas to avoid reinforcement corrosion and evaluate the condition of the pile in the face of overloads, lateral loads, wind loads, earthquake loads, and variations in the admissible capacity that may occur in infrastructure elements due to combinations of regulatory loads.

These tests make it possible to evaluate the quality and integrity of the piles poured with concrete (drilled shaft) according to the construction procedures used by the different contractor companies for public and private works.

Before all nondestructive tests that currently exist (CHUM, CSL, PIT, and TIP) were available in Venezuela, only the Testcrete equipment, developed by the Gamma Test company in Israel under Gamma-Gamma technology, was used with frequency Preiss K [4]. This nondestructive test was widely used by the engineers Gustavo Pérez Guerra, José Bernardo Pérez Guerra, Roberto Centeno Werner, Carlos Rodríguez Álvarez, and Mr. Maxim Silberg, all of whom are members of a pioneering group in the use of this technology in Caracas, Venezuela. They formed an expert work and research team that conducted the auscultation of the important project piles during the 1980s.

These tests also allowed improvements in the techniques available for safely excavating and pouring piles using increasingly better equipment from piling companies. They also made it possible to timely repair some of the foundations that were compromised with failures and/or construction defects in diverse types of soil profiles and decomposed rocks, dry or under the groundwater level.

At the time, the Gamma-Gamma test (Testcrete) made it possible to verify the integrity of the excavated and cast piles (cast in situ) by means of measuring the concrete density profile with respect to depth. It was also able to demonstrate the importance of reviewing the time when foundations were built for important works.

Minor or major inclusions of soils and/or bentonite bubbles in the concrete mass were promptly detected. These detections frequently occurred in the head (higher % due to concrete contaminated with bentonite or sediments), tip (less than the head and caused by wall collapse), and shaft (the lowest % of the 3 cases).

A historical case to determine the integrity of the foundations was made in an important banking tower in Venezuela. The Gamma-Gamma test was used at the time and allowed the early detection of the existence of a bentonite bubble trapped in the shaft of the pier, which covered an important part of its section. The defect was detected at a depth of −11 meters from the surface.

At first, the piling company questioned the existence of the instrumented anomaly, arguing its extensive experience in the execution of piles and denying the certainty of what the performed test indicated. After overcoming the uncomfortable situation, all of the logistics were organized by the inspection management, and the ground around the pile was excavated with the help of metal shoring to safely reach the affected site.

The size of the defect was determined by the test and caused great concern to the promoter and owner of the work, who supported all the investigations conducted by the inspection and requested to review all the piers to be built in the work with these tests. The repair of the defect made it possible to adapt the foundation. Not having conducted these control tests would have generated long-term structural pathologies in the building due to differential settlements and corrosion of the uncoated reinforcements.

It was concluded that it was always necessary to leave the reinforcement hanging 1 m from the bottom of the excavation to avoid the “bullet point” effect caused by soil falling to the bottom and difficulty in lateral exit of the concrete. Another condition that was verified with this test was the contamination that occurs at the top of the pile in the first 50 cm of the concrete with bentonite when it rises and reaches the surface.

Given the difficulty of managing import permits for the radioactive source (Cesium 137) and the costs associated with this test, it was decided to seek new nondestructive testing technologies that do not use radioactive sources.

The Testcrete test made it possible to demonstrate the importance of being able to safely review the foundations on time, clearing up doubts about the quality of the castings, and creating the confidence needed to continue with the construction of the superstructure. 38 years have elapsed from 1985 to the date of the drafting of this article.

Modern technologies continue to be developed globally to assess the integrity of piles. The PET integrity test, the cross hole ultrasonic monitor (CHUM), and the thermal integrity profiling (TIP) test.

Research has been conducted in Florida, USA to measure concrete coatings through correlations with temperatures (°F) using cable and temperature sensors, as well as in other countries. Research has also been done in Europe using °C with the use of optical fiber distributed in the armor of a circular pile (London, England, 2017). The piles were instrumented in parallel with other instruments such as extensometers, strain gauges, Osterberg cell load test, and CHUM tests based on research papers by Gray Mullins et al. [5] and, Yi Rui et al. [6], respectively.

Between 2004 and 2020, the automation of equipment based on most of these techniques (CHUM, PET, and TIP) has made significant progress, making it easier for inspecting engineers to use it in the field and helping with rapid data processing for timely issuance of quality control reports in the field and in the office. The techniques have matured enough, and there is a truly clear protocol established by the DFI [1].

Currently, there are computer programs developed by means of algorithms that allow the presentation of numerical and graphic results for better interpretations of the anomalies due to failures and defects. The most up-to-date software includes the DFI-approved protocol.

There are engineering works, where piles or piers are used in complicated geological-geotechnical environments. In some instances, as is the case of the bridges for the transport of automotive traffic and mass transport trains, there are some special foundations that are located on rivers that are exposed to the processes of progressive soil erosion (scour foundations) due the effect of the floods of currents and to the varied angles of attack of the water flow on them. Further, information on this topic can be found in papers presented by Eng. Marco Falcon Ascanio and Eng. Mauro Nalesso, Institute of Fluid Mechanics of the UCV, Caracas, Venezuela [7] and Magdi M. from Sudan Africa [8].

Hydraulic erosion by scour often exposes the shaft of piers and piles and leaves them partially uncovered with the risk of partial loss of soil around the area that partially contributes positive lateral friction and determines an important part of the capacity of total load considered in the original design. When turbulent flows occur, incipient corrosion and deterioration processes can also be generated in the event of defect anomalies (discontinuities) due to impacts with rounded pebbles on the head and on the upper shaft of the pile.

Many pile works are subjected to load conditions due to lateral and shear forces. They require careful design and construction to provide sufficient flexural stiffness when required. They must offer sufficient metallic reinforcement and be protected with an adequate coating (concrete) to be able to resist shear stresses, axial load, and moments during their useful life.

Lateral load conditions are frequent in bridges with the same or different diameters, in stabilization walls with discontinuous pile systems (contiguous, tangent, and secant), and in buildings located on fills with variable thickness close to slopes with significant heights. Some considerations on this type of solution with secant piles were presented at the XI Conference Gustavo Pérez Guerra, SVDG 2010, published by Engineer Nelson Rodríguez Delgado [9].

The most recent earthquake-resistant engineering codes, developed in California USA, through research carried out by the University of San Diego in conjunction with the California Department of Transportation (Caltrans) “Seismic Design Criteria” and the U.S Department of Transportation — Federal Highway Administration FHWA-AASHTO use the Load and Resistance Factor Design (LRFD) methodology exposed in several editions and complemented in its latest available edition No.9; May 2020 (mandatory use in the USA and in other countries since 2010).Publication made by Juan Murcia et al. [10], Hassan Muhammad [11], V.R. Sturrup ACI-SP-82 [12] and WaDOT-USA [13].

The LRFD method is based on three states: 1. Service limit state, 2. Stress and fracture limit state and, 3. Extreme event limit state (earthquake, winds, flooding, boat, or vehicle impacts). There are combinations of permanent and transitory loads for each state.

The AASHTO 2020 LRFD methodology allows the design of large-capacity foundation piles (drilled shaft), taking into account full-scale dynamic tests carried out in the laboratory, with loads from beams and columns supported on two types of piles. It is possible to evaluate the behavior under conditions of dynamic loads due to earthquakes for this type of structure.

LRFD works with the service limit state. It allows us to consider the forces, moments, and displacements that govern the design through the resistance of the foundations and the superstructure of bridges. It also considers the allowable stress design for the diverse types of foundations (see: Mn DOT LRFD [14], State of California DOT - Caltrans [15]).

The new AASHTO-LRFD design manual (May 2020) [16] combines ACI-318-19; [17] with Geotechnics. It presents complementary considerations that ACI-318-19 does not do separately since this regulation is only of a structural nature. This information can be found in the course presented by the National Highway Institute (NHI) in April 2007 (LRFD) [18].

2. Integrity testing by ECO (PET)

The Pile Echo Tester (PET) or echo integrity test, also known as the “sonic”, “low strain integrity test”, or pile integrity test (PIT), is a “nondestructive” test that is carried out in accordance with the ASTM-D5882 [19] standard and consists of applying a series of impacts repetitive at the top of the pile by means of a light hammer equipped with a plastic/nylon tip.

The pile must be topless and clean in the horizontal plane without soil, dust, grease, or debris to cushion the impact of the hammer so that the compression wave can develop well at the time of testing. The signal is recorded by means of an accelerometer that is activated with each impact of the hammer on the head and receives the signal back after traveling the entire pile from the head to the tip (round trip).

It is particularly important that the accelerometer is well adhered to the concrete surface of the head. The site to be evaluated must be the center of the pile and separated by at least 6″ from where the blows are made. Good adherence of the accelerometer with the putty must be guaranteed to allow intimate contact between the accelerometer and the clean concrete (tip). Only in this way can the wave be recorded well throughout its round-trip trajectory. Several sites of the pile section can be analyzed, avoiding coincidence with the main reinforcement. It is a fast and cheap test and is the most widely used in piling works around the world. Due to its low cost, it is an extremely popular test among builders and inspection engineers.

The result is an echo reflectogram that presents the impedance of the compression wave with respect to depth. From the test, the speed of the wave is obtained by means of the quotient between the detected length and the total round trip time divided by 2. The speed can be correlated with the quality of the poured concrete. The test determines the location of the pile toe in rock or hard ground with good accuracy, except in some exceptional cases, and allows us to qualitatively rule out whether there is any significant interruption of the concrete in the shaft of each pile.

In the case of driven piles, the position of the joints (in elevation) between the segments of the driven pile must be well known so as not to confuse the received signal when it passes through a joint since it would present it as if it were a defect (impedance change in the reflectogram).

When there is an anomaly due to failure or defect in the pile shaft (generated by some inclusion of major earth, air, or bentonite), the round trip of the wave is modified in the echo response of the compression wave. The site where the anomaly occurs can be determined, but not its size in the sectional area or the shape of the volume where it occurs.

There are many types of reflectograms or “echo curves” that can be recovered on a job site during PET testing, depending on the type of pile, the concrete used, the location of the rock, and the rate of setting time. A gallery of reflectogram types with their analysis is detailed in work published by Webster et al., [20] and by Liqun Liang and Frank Rausche; [21].

The shaft of the pile generates friction on the walls (skin friction) to the wave during its round trip to the accelerometer. As a result, the reflectogram has a limitation that depends on the slenderness (ratio between length and diameter) and the types of strata.

It is important to carefully review the sign convention used in the equipment to be able to interpret the start and arrival of the compression wave (“X” axis) in order to fully understand whether it is an anomaly due to excess (over volume) or defect (empty) with respect to the “Y” axis (depth). See Figures 1 and 2.

Figure 1.

PET/PIT test with reflectogram graphic by PDI (Ohio – USA) or pile test (England) in a tested pile. It shows an interpretation of how the bottom of the pile was left. A bottleneck anomaly (necking) at the tip of the pile is represented. Accelerometer and hammer. A cellular phone or laptop is needed to register the data.

Figure 2.

PET (record low strain) on a driven pile evaluated. The jump is observed right in the middle (it coincides with the joint between 12 m precast segments of the pile). Left; [22]. Presented by Amir Joram from Piletest - Israel. Right reflectogram with impedance measured by Centeno [2] on an excavated and cast-in-place pile. Mistakes in the pile tip below the theoretical length [twenty]. It also shows the Fourier transform curve in green (heterogeneous pulse soils and in rock (asymptotic flat area).

3. Crosshole ultrasonic monitor (CHUM) test

The crosshole ultrasonic monitor (CHUM) or crosshole sonic logging (CSL) is nondestructive and is governed by the ASTM D-6760-14 standard, [23]. It is also known as a sonic transparency test.

The frequency emitting equipment must work between 50 kHz and 60 kHz. This is the best frequency available to achieve conclusive and proportional results to the investment and cost of the equipment. There are other equipment that work with higher frequencies between 100 kHz and 250 kHz, but larger drives are required and are not very profitable due to their price, in addition to needing to handle exceptionally large files that are uncomfortable to interpret.

The two most important parameters that are measured are the FAT (First Arrival Time - red curve), which corresponds to the first arrival wave of the ultrasonic pulse from the emitting hydrophone to the receiving hydrophone (time in milliseconds), and the energy reduction (green curve) that is measured in decibels (dB) see Figure 3 prepared by Joram Amir [22, 24] from Piletest (Figure 4).

Figure 3.

Cross hole ultrasonic monitor (CHUM). Source: Amir Joram [22, 24].

Figure 4.

Equipment for cross hole ultrasonic monitor (CHUM) test. Ø 225 cm diameter Centeno-Rodríguez y Asociados s.a. Panamá metro line 2 from 2016; [3]. Pile excavation and the concrete bridge are in operation.

Defining the faults and/or defects in a pile is a process that requires making several measurements with the hydrophones from the bottom of the pile to the surface. Combinations of readings are made between the pipes installed in all directions. It requires experience and dedication to be able to correctly interpret the obtained results.

Opposite diagonals (the most important) and chords (complement) that are formed between the different configurations and combinations from one tube are measured, with the remaining group of tubes installed. All the tubes go inside the reinforcement of the pile before concrete pouring.

The number of pipes that must be installed in the pile is estimated as a whole number that results from dividing the diameter of the pile (cm) by 30 cm. For example, if we have a pile of 215 cm internal diameter in the reinforcement or 225 cm external diameter (excavation), a minimum of seven tubes is required, with 21 combinations of readings between the seven tubes. Joram and Erez Amir, Israel [25] and [22]. Placing fewer control tubes causes blind areas in the evaluation due to lack of access. This has been verified in multiple works around the world (Figure 5).

Figure 5.

Number of sonic tubes and all combinations.

The tubes should be made of steel, preferably 3 mm thick and with a minimum diameter of 1.5″ or up to a maximum of 2″, to facilitate the passage of the hydrophones. The tubes must be joined with the help of threaded sleeves (male–female), or rubber joints with clamps, that allow the coupling of the segments of steel tubes available in the market. The most important thing is that the pipe has well adhered to the cage with the wire and that the joints guarantee watertightness without water leakage.

The pipes are not placed outside the reinforcement (cage) because they can hinder the process of lowering the cage into the excavation and can bend, deviate, and/or open between their joints. PVC pipes should not be used in piers or piles because they tend to debond after a few days during the hardening of the concrete (high setting temperatures) and produce a separation (air ring) through which the ultrasonic waves escape, altering the test results (FAT and RE).

It is especially important to avoid the use of grease at the sleeve joints with each run of threaded pipe because this alters the travel of the ultrasonic waves, and the hydrophone can become contaminated with the grease. The joining between armature bodies is an activity that requires extreme care to be able to join the cages very well and connect all the sonic tubes together. Good practice leads to conducting a precise installation work that avoids deviations and openings of the joints between the tubes. Preferably, male–female steel tubes (quick coupling) or flexible rubber sleeves with their clamps should be used. Threaded sleeves are more difficult to join between two cages when they are being lowered and take time out of the construction process. Security is required during this activity to avoid accidents with the hands of the workers.

When the (FAT) increases and the (RE) decreases in the same place of the pile, it is due to the existence of an anomaly. Thresholds are then determined to conclude if it is a failure (contamination of the concrete) or a defect (discontinuity due to a hole or strangulation with the earth). Below is a photo showing an example of a defect detected with the CHUM test, located at the head of a bridge foundation pile. It is a precise test that checks the defect of the concrete that traveled from the bottom to the top on the surface (poor or contaminated concrete area). We proceeded to scarify, clean well, and replace with high-resistance fluid mortar and epoxy anchor.

Figure 6 shows a typical defect due to interrupted emptying (voids) [3, 24]. The head of the pile was completely scarified and the contaminated concrete was removed in a thickness of 50 cm until reaching the sound concrete. This activity is frequently conducted in the construction of piles. The scarification thicknesses are proportional to the length of the pile and to the existence of concrete contaminated in the upper part with soil or bentonite. The longer the pile is, the scarification thickness on the head may increase (Figure 7).

Figure 6.

Example of the void and soil inclusion in the head.

Figure 7.

Source deep foundation institute (DFI). October 2019 [1]. Green: CLASS A (ACCEPTABLE); Yellow: CLASS B (CONDITIONALLY ACCEPTABLE); Red: CLASS C (ANOMALY); Fault Anomaly: (15–30%) FAT RE (9–12 dB) 50% or more combinations at the same depth; Default Anomaly: (>30%) FAT RE > (12 dB) 2 combinations minimum at the same depth.

Previously, cascade-type (waterfall)images were used to represent the graphic output of the test. Currently, the FAT (first wave arrival time) and RE (energy reduction) curves are also represented in the cascade-type base. Other options also show the previous two, including speed curves (French standard) and PSD curves (Chinese standard). The software we use for research is updated to July 2020 and considers the protocol of the deep foundations institute [1] and Carlos Fernández Tadeo [26]. Centeno-Francisco uses this most recent methodology and highlights his experience gained during multiple ultrasound integrity tests conducted in various works in Panama [3].

Below are three examples evaluated by the author of this article showing the results of typical graphs (waterfall) of the CHUM assay. A case of a healthy pile (left) and two cases with anomalies due to failure and defects in the shaft and tip of a Ø2.25 m pile are presented (Figures 8–10).

Figure 8.

Good (L), shaft defect (pile tip) (C), and defect and a dirty tip (R) [3].

Figure 9.

CHUM tests for the Panama Canal interoceanic railway bridge. LRFD (load and resistance factor design) AASHTO. Piles in the top head were subjected to daily cyclical loads due to the passage of the locomotive and many wagons (2 floors with 40 pc containers each) [3] September 2020. Centeno-Rodríguez y Asociados s.a.

Figure 10.

Example of 2D and 3D tomography (longitudinal profile). Two photos with examples of anomalies detected in the head and shaft of two piles using the CHUM test (cross hole ultrasonic monitor). The author of this article is photographed beside an especially important anomaly detected in the shaft of this pile (−5.00 m depth and 1.5 m height)). No concrete for steel bars (covering) [3].

It is possible to determine the position of a defect or a failure in the concrete of the pile with the CHUM test. When it is possible to safely excavate to the depth where the defect or failure anomaly occurs, it is possible to verify that it is in the place where the test indicated it. Tipping is done before digging. Arriving at the site, the affected area is cleaned very well and repaired in a timely manner, avoiding future damage to the infrastructure of the project. This procedure avoids altering the admissible load capacity and mitigates the risk of future corrosion in the reinforcement steel bars of the piles in contact with soil, weathered rocks, and water (if it had not had an adequate coating).

This test has been recognized by most pile companies, owners, and project managers for many years. Initially, some field operators thought that their results were inconclusive and that they delayed production. Today, serious piling companies recognize its effectiveness and support its use since it allows correcting defects that may have occurred during construction. Future legal problems between the parties are avoided (Figure 11).

Figure 11.

Some digraphs or slices evaluated between −0.20 m and − 2.50 m are presented, indicating the distribution of % anomalies through images with speeds less than 3000 m/sec. Red color indicates water and a probable vacuum. Cuts are presented in various places, where the anomaly is recorded. Gray color (excellent), green (good - limit), blue (poor), and red (void). Tomography analysis conducted by the author of this article using Piletest ® special tomography software (licensed). Israel-England.

4. Limitations of the CHUM test

1. The main limitation is that it does not allow the integrity of the concrete to be measured in the area of the thickness of the external coating to the steel because the sonic tubes are inside the reinforcement (it does not cover the extrados of the reinforcement – 7 to 10 cm outside the steel cage).

2. A minimum of 7 days is required for the concrete to set and harden. The setting temperature of the concrete must be low enough (< 65°C) so as not to damage the hydrophones, and thus be able to operate without damaging the internal electronics. Measuring within less than 5 days could present false positives in the signals. From experience, the FAT and reduction energy values would change if the CHUM test were conducted again, after more than 15 days. The idea is to work between 7 and 10 days for the first reading.

3. When drilling with core-drill equipment for repairs and grout injections in the affected area, there is previously a healthy hard concrete in the unaffected areas and located before reaching the site with the defect. Placing a grout and/or mortar under pressure, still setting after 7 to 10 days in the removed healthy area, will present less resistance than the original concrete placed because the grout does not normally contain the coarse aggregate. The CHUM signal could then present some FAT and reduction of energy (RE) variations that show a partial improvement and not 100%, where the original defect/failure detected with the test occurred (heterogeneous mixture of concrete and grout).

4. It is only possible to measure at the tip of the pile up to the height, where the sonic tube has been left in the reinforcement. Lowering the tube further can buckle the sonic tubing if it rests on the tip. It is not possible to measure the area without the tube. If the reinforcement is lowered further to the bottom, the concrete will not fill the tip well and a “bullet point” effect can be generated in the pile. Normally, the bottom armor is left hanging between 20 cm and 50 cm in many cases.

5. Integrity test with thermal profile (TIP)

This integrity test is the newest of the three tests presented in this article and is known as thermal integrity profiling (TIP). It is governed by the ASTM D-7949-14 standard. Ref. [27]. It was developed at the University of South Florida by Dr. Gray Mullins and others. Ref [5] (other useful references: Ryan Allin [28], Gina Beim [29]). For 16 years, important advances have been made with this methodology, and its use has become popular. It is used to control piles on public transportation bridges in the state of Florida (Clearwater-Tampa) and several other US states, as well as in some other countries.

The TIP test is another reliable alternative for the control of concrete integrity in piles and piles excavated and cast in place (drilled shaft - cast in place). It is also frequently used to assess the quality of the pouring in micropiles, secant pile screens, discontinuous screens, and jet grouting columns, among others.

The test consists of making a continuous record of the setting temperatures of the cement throughout the entire pile while the curing and hardening of the concrete occurs in the first 48 hours.

The values of all temperatures that develop in the concrete mass during curing are recorded. Temperatures are recorded by strategically located thermal sensors distributed throughout the armor (cage). The tubes and sensors are distributed in the same way in the section as in the CHUM or CSL test; the number of vertical lines of sensors is obtained by dividing the diameter of the reinforcement (cm) by 30 cm. The approximate whole number of the resulting quotient is always used.

For piles of 2.15 m internal diameter of the cage or 2.25 m diameter excavation, seven vertical lines are used and proportionally separated from each other on the perimeter (215 cm/30 cm = 7). For 1.20 m piles, four vertical lines (120 cm/30 cm = 4) are used. The tubes can be made of PVC or steel and with the same spacing characteristics as in the CHUM test, in case of using a sensor that travels through the tube. There exists the alternative of leaving expendable sensors (they are consumed (fungible) and cannot be recovered) with their use inside the poured concrete of the pile.

Regarding the “debonding” effect of PVC with concrete, this material (PCV) does not affect the results. It also does not require the tubes to be filled with water to perform the test. The temperature that is measured is the one that is registered at each advance height during the setting of the cement inside the tube. If the tubes are made of steel, they can also be verified with the CHUM test after 7 days of casting (Figure 12).

Figure 12.

Distribution of 10 tubes per stack (3.00 m) TIP test [30]. Execution of the test and recording sensor used [5]. Photos: Work conducted by PDI - Ohio USA.

Lower temperatures are recorded near the surface and higher temperatures are recorded as you go deeper into the stem. On the right, the temperature in different sectors is presented. It was determined that the reinforcement was slightly out of phase or displaced with respect to the perforation with the help of other tubes with little coating. In Figure 13, it is easy to visualize higher temperatures in areas with greater coverage on the left (red point) and lower temperatures (blue point). On the right there is another image that shows the longitudinal section of a pile, indicating the profile with the distribution of temperatures of the pile between the surface and the toe. Gray Mullins [5].

Figure 13.

Distribution of temperatures in the concrete of a pile with respect to the ends (head and tip). A pyramidal temperature distribution is observed under the pile head and tip (Gray Mullins) [5].

The colder regions can be associated with inclusions of soil or hollow zones without concrete (colder). In the upper part (head) and in the bottom (tip), the temperatures are also lower because they are in contact with air above and with earth or rock in contact with the area near the tip.

When the temperatures recorded during setting are above the average values in the diverse types of concrete, these may be related to an excess volume of the concrete mass (bulges) associated with hotter areas due to having more cement. At the tip of the pile and at the top of the pile, it has been determined that there are two zones that always present important changes in temperature (colder zones) that generally correspond to a height proportional to a diameter or a length equal to 40% of the square root of the temperature measured in degrees Fahrenheit. They are affected by their proximity to the ground and/or rock (at the tip) and to the atmosphere (head).

Two important concepts are managed in the test during cement curing (concrete setting) according to research conducted at USF - Fl, USA by Professor Gray Mullins and others [5]:

1. Heat production, which represents the amount of heat produced during the exothermic reaction of cement curing and depends on the amounts of materials and the type of cement used in the mix design (cement, water, sand, and stones) for used concrete.

2. Heat diffusion, which is just as important as the heat production that occurs around the pile in the different soil and rock strata. It also depends on the position of the groundwater level in the subsoil.

The flow of heat that is generated and diffused involves simultaneous mechanisms of conduction, convection, and radiation. Conduction is the mechanism that dominates the transport of heat. The heat capacity in soils is based on the volumetric fraction of the three phases: solid, voids (gas), and water.

There are two parameters that affect the dissipation of heat from the concrete mass to the soils: the dissipation of heat around the pile and the resistance offered by the layers in being heated. The denser the soil and hard rock around the pile, the easier the conductivity of heat in the soil or rock; they require more energy for these to be heated. Hence, there are few hours available for the heat to be maintained and transmitted efficiently during its production. It is important to take measurements during the first 48 hours after pouring.

The TIP test also allows detection if the reinforcement has been well supported or deformed against any of the walls of the excavation and planting. It also detects the presence of failures and defects in any part of the pile. It allows the evaluation of the perimeter area of the concrete pouring (covering) outside the reinforcement. Regarding the CHUM-CSL method (only effective in the internal zone of the reinforcement), TIP does allow us to be able to evaluate a little more of the extrados of the tubes with more precision. The Gamma-Gamma test also allows a little more evaluation of the thickness of the coating on the back of the reinforcement based on the density of the concrete, but less than TIP (Figures 14 and 15).

Figure 14.

Determination of the pile geometry (radius profile (inches) from the TIP test by correlation with the temperatures (°F) measured in a sound pile. Image “rendering” [13]. Correlation: Pile radius (inches) = [(0.81*T(°F)) - 51.3]. (average temperatures). WaDOT. Mullins Gray.

Figure 15.

Examples of TIP curves with temperature dissipation in two piles, one showing a bulge (L) and one showing a local defect at the top (R). Reference: PDI Webinar; OH.USA; June (2020) by Ryan C. Allin [28].

The TIP test allows the engineer to estimate the shape of the shell (radius profile) with respect to the depth. From the data and with the software developed by the USF and PDI, the resulting geometry from the pouring of concrete in the pile or pile can be graphed using computer algorithms. They are image generators based on the integration of data recorded in all the tubes and temperature curves during the first 48 hours of setting.

Automated sensors that automatically record all setting temperatures can be used with a datalogger (TAG) and a recorder per tube (TAP). They can then store the data in the cloud and be consulted remotely from anywhere in the world or from the office at any time. PDI technology available.

The recorded temperature depends fundamentally on the type of concrete used in the mix design, the cement content, diameter, length of the pile or pile, and setting time (hours). The larger the diameter of the pile and the length, the higher the setting temperature of the cement in it will be, and it will remain hot for more hours. A graph is presented below to better understand the results.

6. Limitations of the TIP test

1. The optimal time to conduct the test is indicated in the window of Figure 16; [20]. It is variable and depends on the diameter of the pile and the type of concrete mix. The smaller the diameter of the pile, the less time is available to collect the data. The greater the diameter of the pile or pier, the more hours of setting are achieved (> volume of concrete and cement content). Reliable data can be measured between two and six days (depending on the size of the pile and pile). The data is less accurate after this time due to the significant decrease in temperatures. It is recommended to automate the measurements as much as possible with the help of the TAG data logger and the TAP devices (sensors) so as not to lose valuable information.

2. The rehearsal should be scheduled in good time. The installation and measurement in the field must be fast and efficient so as not to lose the valid time range to be able to read the temperatures during the entire setting. For piles with diameters of 1.2 m to 1.8 m, it is available from day two to day four of setting to measure. For piles with diameters of 2.0 m to 3.0 m, it is available from day two to day six of setting to measure. Between day seven and day ten, there is still some of the setting heat, but the data is not as precise, see Figure 16 (x-axis). Reading is not recommended below 120°F (49°C). The threshold can be extended up to 100°F (38°C) in some, but it is not usual. Measurements after 48 hours to verify any injection repair process must be conducted with another test, such as CHUM or CSL.

3. It can be measured with PVC or metal pipes, but they must be empty and without water. It does not matter if there is a punctual detachment or separation of the PVC pipe with the concrete. This condition does not affect the temperature or the results. Water in the tubes prevents the thermal sensor from working well and can reduce the temperature.

4. Correlations between temperature and concrete maturity compressive strength can be made but are not an intrinsic component. They depend on the mixture design and other stochastic variables.

Figure 16.

Different expected temperatures in concrete with mix design: 280 kg/cm2 (4000 psi) 28 Mpa; for different diameters (1.2 m to 3.0 m), temperatures (80°F and 160°F) and setting time (0 h to 145 h) 6 days. Washington Department of Transportation Final Report (2011) [13], page 155. (Brent Robbinson PDI-OH-USA).

7. Conclusions

Pile integrity tests help the quality manager of a project to make reliable decisions so that their work allows them to lead and have the necessary technical arguments to inform the professionals involved in the design and construction log of the piles. It is used to check whether an adequate or defective construction process has occurred, avoiding negative consequences that cause future performance problems for the work. The quality of the integrity of the piles determines the payment of the elements that are accepted or rejected in a work. Expansion on this subject can also be read with other related articles: Bottiau [31], Camp [30], Grosse [32], and Mark Gaines [33].

The CHUM or CSL integrity test has more than 30 years of development and has been successfully applied on five continents. It is a highly documented test with hundreds of thousands of piles, and piles evaluated in many countries around the world. It is considered a very mature and reliable technique to identify, where the anomalies due to failures and/or defects in piles are located. The TIP integrity test has only been on the market for eight years and is beginning to be documented more frequently.

Controls with the CHUM, CSL, PET, and TIP tests should always be compared with the construction log before rejecting or approving a pile. PET is the most limited of the three tests. Certifying a pile with PET is not recommended for bridges and buildings with large- capacity piers (> Ø1.20 m). Load testing can always be used as a complementary method when a major defect anomaly is observed in a complex and inaccessible position of the shaft or on the tip.

The number of tests required is not defined based on single regulations. Each country has its criteria based on the requirements of structural designers and geotechnical engineers, who will determine the number of piles that must be evaluated on site for each project based on their location, service loads, and redundancy of elements for each column. In mass transportation, works located in seismic zones, where there is only one pile (mother) per column; all piles must be evaluated without exception. In other works, where there is redundancy of piles due to foundations (bridges and buildings), 25% of all piles are evaluated. If more than 20% have defects or failures, the amount is doubled to 25% more. If there are more than 20% with defects or failures, all the piles in the work must be evaluated without exception. In the USA, a greater number of piles are evaluated than in Latin America based on geotechnical and structural criteria. The resistance of any chain is that of the weakest link.

According to the world CHUM test statistics, most defects and failures occur at the head and toe of piles; a lower percentage occurs in the shaft of the piles. Each project has its difficulties, and it will only be possible at the end of the pile and pier work to know in detail how these percentages vary. It is preferable to always work with the FAT (time) and not with the velocity of the ultrasound waves because the tubes are never perfectly aligned and their distance changes with respect to the depth frequently and depends on the installation.

Acknowledgments

To the engineers Roberto Centeno Werner, Carlos Rodríguez Álvarez, and José Bernardo Pérez Guerra for their comments and transmission of previous experiences as pioneers and as users of nondestructive tests (Gamma-Gamma) in Venezuela. To Jack Mulcrone for reviewing this paper.

This chapter is an English translation of the partial original article wrote by the authors: Centeno Pulido F. and Centeno Werner R. “Evaluación de la integridad en pilotes y pilas para obras públicas y privadas con tecnologías disponibles a comienzos del siglo XXI. In Academia Nacional de la Ingeniería y el Hábitat, ANIH Boletín 54 – Memorias del Congreso Venezolano de Geotecnia 2020. Parte 2; 2022. p. 42–56”.