Underwater Rebar Locator

Figure 3-48. Overloading of concrete pile during pile driving.

3.6.2.6 Shrinkage. Shrinkage or contraction can occur from moisture or temperature changes. Hardened concrete that looses internal water due to evaporation will shrink. Any temperature decrease of the concrete will cause contraction. The major cause of microcracks within concrete is from high temperatures generated from the normal hydration of cement. The concrete hardens at a high temperature and later cools to ambient temperatures. Precast concrete members that have been steam cured are particularly susceptible to microcrack formation. If the shrinkage or contraction is restrained, internal stresses may develop in sufficient magnitude to cause significant cracks in the structure.

Variations in atmospheric temperature cause a change in temperature of a hardened concrete mass, which results in volumetric changes. Provisions must be made to permit this expansion and contraction process to take place. Failure to do so will result in contraction stresses (tension), which may cause cracking, or expansion stresses (compression), which may lead to spalling.

3.6.2.7 Swelling. Concrete that increases in moisture content by absorbing water or increases in temperature will swell or expand. Typically, swelling by water absorption is not a concern unless precast dry-concrete members are used. Temperature increases from daily and seasonal changes may cause cracking in some concrete members.

3.6.2.8 Other Deterioration Factors. The preceding has discussed deterioration in concrete caused by improper selection or proportioning of concrete materials, faulty construction methods and procedures, and attack by environmental forces. Of equal importance, and a major cause of much concrete deterioration, is poor design of concrete structural details.

A few examples of poor design and construction details that contribute to concrete failure and deterioration are:

• Congestion of reinforcing steel

• Lack of adequate cover for reinforcing steel

• Abrupt change in size of section

• Reentrant corners

• Lack of chamfers and fillets at corners

• Rigid joints between precast units

• Construction joint leakage

• Poorly designed scuppers, drips, and curb slots

• Inadequate drainage

• Too little gap at expansion joints

• Incompatibility of materials or sections 3.6.3 Concrete Inspection Procedure

3.6.3.1 Visual Inspection. Levels I and II visual inspection of concrete waterfront structures should proceed as shown in Table 3-6.

3.6.3.2 Level III Nondestructive Inspection of Concrete. The qualitative data obtained from visual inspections are sometimes inadequate to accurately assess the condition of the structure. In these instances, quantitative data obtained from nondestructive testing instruments can assist the facilities engineer in determining the condition of the structure. Three specialized instruments have been developed for underwater inspection of concrete structures. These instruments are the:

• Magnetic rebar locator - used to determine the location and orientation of rebar in concrete structures and to measure the amount of concrete cover over the rebar.

• Rebound hammer - used to evaluate the surface hardness of the concrete and obtain a general condition assessment.

• Ultrasonic system - used to obtain a general condition rating and indication of overall strength of the concrete based on sound velocity measurements through a large volume of the structural element.

Each instrument consists of an underwater sensor connected to a topside deck unit through an umbilical cable. The deck unit contains the signal conditioning electronics and data acquisition system. To operate thé instruments, the diver has to position the underwater sensor on a previously cleaned portion of the structure surface and a person topside must operate the data acquisition system in order to collect and store the data. Each instrument is in-dependendy operated and provides unique information to help assess the condition of the concrete structure.

3.6.4 Equipment and Tools Required

To perform a thorough inspection, the marine growth on the structure must be removed. A "Barnacle Buster" or pneumatic chipping gun is an efficient method of removing marine growth from concrete surfaces. Various types of high-pressure water jet cleaning systems are also effective. Exercise care in the use of these methods because they may further damage a deteriorating concrete structure. If minimal marine growth is found in the splash/tidal area, small hand tools, such as wire brushes and scrapers, are sufficient. Refer to Section 2.3 for information on equipment for removing marine growth. A hammer for sounding and an accurate water-depth gauge will unülhwaikh inspection procedures

Table 3-6

Concrete Structure Underwater Inspection Checklist

Checkpoint

Description

1 Inspect the structure beginning in the splash/tidal zone. This is where most mechanical and biological damage is normally found.

2 Clear a section about 18 to 24 inches in length of all marine growth.

3 Visually inspect this area for cracks, abraided surface spalling, or mechanical damage, and exposed reinforcing steel.

Sound the cleaned area with a hammer to detect any loose layers of concrete hollow spots in the pile, structure, or soft concrete. A sharp ringing noise indicates sound concrete. A soft surface will be detected, not only by a sound change, but also by a change in the rebound, or feel, of the hammer. A thud or hollow sound indicates a delaminated layer of concrete, most likely from corrosion of steel reinforcement.

Descend, visually inspecting the pile or structure where marine growth is minimal, and sound with a hammer.

Inspect in greater detail the base of mass structures, such as foundations, quaywalls, breakwaters, or bridge piers. These types of structures are prone to undermining by wave and current action, which, if not rectified, could lead to failure of the structure.

At the bottom, record the water depth along with any observations of damage on a Plexiglas slated.

After returning to the surface, immediately record all information into the inspection log.

NOTE: If signs of deteriorations are found, then a Level III inspection, involving either nondestructive or destructive tests, may be required. Refer to the Level III Test Procedures for Concrete Inspection for mechanical and electrical test methods.

Exposed Area Under Pier or Along Wharf or Dolphin Assembly

Check pile caps and bearing, batter, and fender piles for damaged or broken members, cracks, and spalling of concrete, rust stains, and exposed reinforcing steel.

10 Sound the piling or structure with a hammer to detect any loose layers of concrete or hollow spots. A sharp ringing noise indicates sound concrete. A soft surface will be detected, not only by a sound change, but also by a change in the rebound, or feel, of the hammer. A thud or hollow sound indicates a delaminated layer of concrete, most likely from corrosion of steel reinforcement

NOTE: If signs of deteriorations are found, then a Level III inspection, involving either nondestructive or destructive tests, may be required. Refer to the Level III Test Procedures for Concrete Inspection for mechanical and electrical test methods.

be required. Record observations on a Plexiglas slate with a grease pencil. Use underwater video cameras for permanent visual documentation.

3.6.4.1 Magnetic Rebar Locator. The magnetic rebar locator shown in Figure 3-49 is based on a commercial instrument that detects the disturbances in a magnetic flux field caused by the presence of magnetic material. The magnitude of this disturbance is used to determine the location and orientation of rebar in concrete structures and to measure the amount of concrete cover over the rebar.

The system consists of an underwater test probe, an umbilical cable, and a topside data acquisition unit (DAU) including printer.

The test probe consists of two coils mounted on a U-shaped magnetic core. A magnetic field is produced in one coil and the disturbance-induced magnetic field in the rebar is measured in the other coil. The magnitude of the induced current is affected by both the diameter of the rebar and its distance from the coils. Therefore, if either of the parameters is known, the other can be determined.

By scanning with the probe until a peak reading is obtained, the location of the rebar can also be determined. A maximum deflection of the meter needle will occur when the axis of the probe poles are parallel to and direcdy over the axis of a reinforcing bar, thus indicating orientation.

The underwater rebar locator is calibrated for rebar that varies from No. 3 to No. 16 in size. The meter can be used to measure the depth of concrete cover over rebar in the range of 1/4 to 8 inches thick, or conversely, it can measure the diameter of the rebar. The best accuracy (±10 percent) is obtained for concrete cover less than 4 inches thick.

Figure 3-49. Underwater rebar locator system.

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Data Acquisition Unit

Umbilical Cable

L'nderw.uer Test Probe

Printer

L'nderw.uer Test Probe

Printer

Data Acquisition Unit

Umbilical Cable

• System Limitations. The presence of other metallic objects in the vicinity where the measurements are being made can affect the operation of the rebar locator. For example, in heavily reinforced structures, the effect of nearby rebar cannot be eliminated and accurate depth readings are difficult or impossible.

If the separation of two parallel rebars is at least three times the thickness of the concrete cover, this effect can be neglected.

The presence of rebar perpendicular to the axis of the underwater probe has less effect on the measurement of concrete cover than that of parallel rebar, and in most instances it can be ignored.

3.6.4.2 Rebound Hammer. The underwater rebound hammer system, shown in Figure 3-50, is a surface hardness tester which can be used to obtain a general condition assessment of concrete. The system consists of an underwater rebound hammer, an umbilical cable, and a topside data acquisition unit (D AU) including printer. The rebound hammer is mounted in a waterproof housing which contains an electrical pickup to sense the position of the rebound mechanism. The umbilical connects the underwater rebound hammer to DAU that contains the signal conditioning electronics and data acquisition system.

The rebound hammer correlates the rebound height of a spring-driven mass after it impacts the surface of the concrete with the compressive strength of the concrete under test. The spring-driven mass slides on a guide rod within the tubular housing. When the impact plunger is pressed firmly against the concrete surface, a trigger releases the spring-loaded mass causing it to impact the plunger and transfers the energy

Underwater Umbilical System

I nibilical I'able

Data Acquisition Unit

Underwater Rebound Hammer

Printer ft

Figure 3-50. Underwater rebound hammer system.

to the concrete surface. The mass then rebounds and the rebound height is correlated to the surface hardness of the concrete.

WARNING Do not operate the rebound hammer with the impact plunger in contact with human body parts; serious injury can result.

A general calibration chart that relates the rebound number to cube compressive strength for the underwater rebound hammer is shown in Figure 3-51.

The pressure housing has a depth rating of 190 feet and it is pressure compensated at 5 psi over the ambient pressure. Air is supplied to the rebound hammer from a scuba tank through the umbilical cable via an external pressure regulator to maintain the positive pressure differential inside the housing.

• System Limitations. The following characteristics of concrete can affect the correlation of the rebound number with the actual surface hardness and should be understood before using the instrument:

1. Higher rebound numbers are generally obtained from smoother surfaces and the scatter in the data tends to be less. Minimizing the data scatter increases the confidence in the test results. Therefore, underwater concrete surfaces must be thoroughly cleaned and smoothed with a carborundum stone (or similar abrasive) before measurements are taken.

2. Water-saturated concrete tends to show rebound readings approximately 5 points lower than for the same concrete tested dry. This affects the comparison of data taken above and below the waterline.

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  • markus
    Which instrument are underwater construction?
    4 months ago

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