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Guidelines to Consider in Structural Assessment of Existing Building

 

 

When an abandoned building is set for renovation and restoration, changing the occupancy into a massive one or even a newly constructed building had an obvious bad workmanship and execution after the concrete casting and a noticeable physical defects are shown, whatever the reason it may be, the client often hires a Structural Engineer or team of engineers in evaluating the integrity of the structural condition. This task is quite challenging for Structural Engineers as it involves extensive evaluation and investigations to identify where the defects are coming from and how to address each of the physical and structural defects. This is to make sure that the building is physically and structurally safe and sound. At the end of the day, we will give our professional advice and recommendations. Doing so, it is important that we know how to deal with it.

Structural assessment in an existing building is usually taken by a team of Civil/Structural Engineers. Here are the guidelines to consider in the assessment of the structural condition of the existing building.

1. Site Inspection, Sampling, and Testing

An ocular site inspection is a must to have an initial assessment of how deep the scope of evaluation and investigation it may be. Along this process, the study of the performance, durability, and strength of the material used in an existing structure should be considered. This can be achieved through the following:

1.1 Initial Site Survey

An initial site survey should be conducted to carry out a visual inspection, gathering of data and information on the condition of the building. The photographs and locations of the physical defects are being recorded and identified. The engineer will prepare a detailed plan which includes every aspect of architectural and structural considerations. This is to carry out in order to optimize all the aspects of work.

1.2 Investigation, Sampling, and Testing

To determine the physical, mechanical and chemical properties of concrete, a comprehensive in-situ and laboratory tests program should be prepared. The physical samples were obtained from the structural elements in representation and to cover the area under investigation. So that the site will not further disturb, non-destructive testing should be performed. Sampling and site testing should be carried out which may include:

        • Drilling of concrete cores in the structural member into consideration.
        • Carry out a concrete cover survey using a digital cover meter to determine cover to reinforcement.
        • Extract concrete samples to check the chloride content.
        • Conduct the ultrasonic and resistivity test to determine the quality and rate of steel corrosion respectively.
        • Measure the depth of broken surfaces of the concrete

The concrete samples collected on-site should be taken to the laboratory for further analysis. This is to obtain the compressive strength and density of the existing structural members and to further verify the additional information and data needed.

1.3 Final Site Investigation

The working team should carry out the final investigation and site inspection to verify the outcome of the results from the collected samples. The site survey, observation and recommendation should be recorded in a detailed report. The detailed report of the team should be submitted accordingly to the client and should include the following points mentioned in Section 2.

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2. Evaluation, Investigation & Test Results

2.1 Visual Inspection

A visual inspection of the concrete member in consideration should be carried out to determine the extent of the cracks and to determine other damages and defects. The following are the common inspection observations which are usually present in concrete during site observations:

        • Cold Joints
        • Concrete Honey Comb
        • Hairline Cracks in Slabs and slab soffits
        • Exposed steel reinforcement due to insufficient concrete cover
        • Column misalignment due to bad formworks during casting

Figure 2.0 Common Defects in Concrete

2.2 Mapping of Defects

Defects mapping is basically a key-plan of defects found in the site inspection. The defects found during the visual inspection should be marked on the plans accordingly. The type of defects and their location should be mentioned clearly in the plan. This is to easily locate the observed defects for further reference. Refer to the image below for the sample defects mapping.

2.3 Concrete Cores

A number of cores should be considered and should be taken in different locations. During the coring process, care should be taken into consideration to avoid further damage to the existing structures. To avoid cutting of steel rebar, an electromagnetic cover meter should be used to locate the existing rebar prior to drilling. The rotary core cutting machine with hollow diamond bits cooled with fresh water should be used in drilling.

The obtained core samples should be clearly marked and protected to avoid damage. These samples will be transferred from the site to testing laboratories for physical and mechanical examination to determine the actual densities and compressive strengths of each sample.

2.4 Cover Meter Surveys

The inspector should conduct the cover survey at selected areas in the concrete structures in considerations. This is to indicate the depth of reinforcement. A low concrete cover should be identified and recorded to make sure if additional cover is needed. The thickness of the concrete cover can be measured using an electromagnetic cover meter.

2.5 Depth of Carbonation

As stated in section 2.4, the concrete cover should be check on-site to make sure that it is sufficient enough so that the carbonation should not reach the steel reinforcement throughout the life of the structure.  Carbonation is one of the reasons for the corrosion of steel reinforcements.  It is important for the engineers to measure the depth of carbonation to determine the rate and cause of corrosion.

The depth of carbonation can be measured on a freshly exposed surface of the concrete by chipping the drilled hole used in concrete sampling. This can be tested by applying a chemical indicator. The difference in alkalinity between the carbonated an un-carbonated concrete can be seen if the color changes. The indicator to be used is a solution of 1% phenolphthalein in diluted ethyl alcohol which changes each color from transparent to purple-pink as the PH value of above 10 rises. Consequently, the outer carbonated layer of concrete retains its natural color while the un-carbonated stained pink. This test can be applied only on freshly exposed surfaces free from dust.

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Tested Member

Locations Carbonation (mm) Ave. Actual Concrete Cover(mm)

1st Floor Columns

1

20 30
2nd Floor Beams 2 15 25
Typical Floor Columns 3 10

35

Table 2.5: Sample Carbonation Results

Referring to Table 2.5 above, the maximum depth of carbonation for column and beam is 20 mm and 15 mm respectively. These are less than the average concrete cover in tested structural members. Therefore, the reinforcement is protected and no corrosion presence caused by carbonation.

2.6 Concrete Dust Samples

Dust samples are also considered in the structural assessment of the existing building. These samples should be taken to determine the chloride contents of the concrete. This can be determined using the Sherwood Model 926 Chloride Analyzer. The results are usually expressed as the chloride iron content (CI) by weight of the concrete samples including the aggregate. These can be converted to the chloride ion by weight of cement after determination and knowing the cement content. The CI by weight is useful to enable comparisons against the recommended level in accordance with the code of practice.

The concentration of the chloride, alkalinity and the type of cement used can contribute to the chloride content in concrete. It is critical to the life of the steel reinforcement as it may rust as a result. However, the maximum chloride content should be tolerable according to the standard set by code specifications mentioned in CIRIA 2002. The actual chloride content of each dust sample was determined according to the code set forth in BS1881: Part 124: 1988.

2.7 Ultrasonic Pulse Velocity

The ultrasonic pulse velocity tests are used to verify the quality of concrete. Concrete with pulse velocity greater than 4000 m/s is considered to be of good quality. The ultrasonic pulse velocity test can perform when a pulse of longitudinal vibrations is produced by an electro-acoustical transducer held in contact with one surface of the concrete under test. After traversing a known path length in the concrete, the pulse of vibrations is converted into an electrical signal by a second transducer and electronic timing circuits enable the transit time of pulse to be measure.  The velocity criterion for the quality grade of concrete can be check using a direct and in-direct transmission according to the tables below taken from BS EN 12504-4-2004. The recorded ultrasonic pulse velocity in each core sample is then be classified accordingly.

No.

Pulse Velocity in Core Probing

Concrete Quality Grading

1

Above 4500

Excellent

2

3500 to 4500

Good

3

3000 to 3500

Medium

4

2000 to 3000

Poor

5

Less than 2000

Very Poor

Table 2.7.1 Velocity Criterion for Concrete Quality Grading ( Direct)

 

No.

Pulse Velocity in Core Probing

Concrete Quality Grading

1

Above 4000

Excellent

2

3000 to 4000

Good

3

2500 to 3000

Medium

4

1500 to 2500

Poor

5

Less than 1500

Very Poor

Table 2.7.2 Velocity Criterion for Concrete Quality Grading ( In-Direct)

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2.8 Electrical Resistivity Measurements of Concrete Surface

The surface resistivity measurement provides useful information about the state of concrete quality. By evaluating the electrical resistance of concrete, it is possible to assess the probability of reinforcement corrosion. Because the corrosion of steel reinforcement in concrete is an electro-chemical process that creates a current flow causing the metal to dissolve. Electric current is passed through the outer probes and the potential drop is measured by the inner probes. From the current and voltage drop, the resistivity of concrete can be measured. The concrete resistivity meter replaced the Rapid Chloride Permeability Test and surface resistivity for ASTM C1202 and AASHTO T277. The Surface Resistivity test is a much quicker and easier test for estimating concrete permeability. It is a proven method that can replace the more laborious rapid chloride permeability test.

2.9 Electro Half Cell Potential Measurement Test

Since corrosion is an electro-mechanical process, it is possible to obtain an indication of whether the steel reinforcement in concrete is in the corroded stage. The state of steel reinforcement can be verified by measuring electrical potentials by means of the Electro Half Cell Potential Measurement Test. One terminal of high impedance milli-volt meter can be connected to a point on the reinforcement and the other terminal to a half-cell or copper sulfate or silver chloride which place in contact with the concrete surface. The half cell usually consists of a copper electrode immersed in an electrotype of copper sulfate solution. Corrosion is likely to be negligible in readings less than -200 mV.

3. Interpretation of Test Results, Conclusion and Recommendation

Based on the physical and mechanical properties of concrete and as a result of the above data and laboratory tests, the next task of the engineer involved in the assessment of the structural integrity of the existing building is to summarize all the results and findings. He/she should itemized and breakdown clearly these findings and come up with the conclusion of the overall safety of the structure. A recommendation is also required after the interpretation and conclusion has been stated. Structural strengthening in members affected is most likely the outcome of the structural assessment.

4. Remedial Work

In case of structural strengthening is required as per the engineer’s recommendation. Remedial work should be planned out in order for the structure to be structurally sound. A required method of repair should be considered and the procedure on how the repair should be done. Watch out for the upcoming article on how to address the structural integrity defects as a result of the structural assessment.

References: ACI 318-11, ACI 417R 1991, ASTM D512-89 1999, BS6089, 1981, BS5328 PART 1, BS1377 PART 3 1990, CIRIA REPORT 2002.


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1 Comment

  1. suresh

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