Rebound hammer test: A popular NDT in the evaluation of concrete strength- Part 2

Previously we have discussed about the history and development of rebound hammer, and the operating principle. Here we will talk about the limitations of the use of rebound hammer in the assessment of the strength of concrete. 

In real practice, the number of influencing factors are very large and most of them are uncontrolled. Many factors may affect the test result to different degrees to the overall results. The influencing factors are identified and quantified by many researchers. The main influencing factors are summarised below:

Type of aggregates- Type and grading of aggregates have significant influence on the rebound hammer values. A difference of 7 points in rebound values were reported when concrete are made with limestone and gravel coarse aggregate, harder aggregates results high rebound value.

Age- Upper surface of the concrete reacts with the carbon-di-oxide resulting carbonation making the surface of the concrete harder. Due to the effect of carbonation, error may reach up to 50%.

Moisture content- A difference up to 10 rebound values between dry air concrete and saturated concrete can be observed on site where dry air concrete results in higher rebound value.

Surface regularity and roughness- Increases the variability in the measurements. Concrete just under the hammer crushes when the hammer impacts. For this reason, it is advisable to flatten the surface before conducting the rebound hammer test.

Near surface properties- Factors like type of framework used during the casting, curing conditions etc. that affect near surface properties, influences the measurement of rebound value. The presence of aggregates, air bubble or voids near the surface affect the measurement to a great degree.

Rigidity of the structure: Rebound value is lower if rigidity of the member of the structure is smaller. The rebound value measured on a core is lower than that directly provided on the structure. It is advisable to apply a pressure of 7 MPa to a concrete cube to keep it steady during the rebound hammer testing.

Rebound hammer device- Rebound value depends on the direction of testing (horizontal, upwards, or downwards) and type of hammer used for testing as well. Many types of rebound hammer exists in the market which are used for different cases, N-type, L-type, M-type, and P-type. The N-type rebound hammer is used for normal strength concrete having the impact energy of 2.207 Nm. The L-type rebound hammer is used for testing of small or thin walled (less than 100 mm) concrete members. The impact energy of L-type hammer is 0.735 Nm. The impact energy (29.43 Nm) and size of the M-type hammer is much higher than the N-type hammer and is used for high strength concrete pavement. For the low  strength concrete, such as lightweight or fresh concrete, mortar, P-type hammer is used having the impact energy of 0.833 Nm.

Stress state: Stress state influences the rebound value measurement, although in normal practice this is likely to be small in comparison with many other variables.

Rebound hammer test is considered to be one of the least reliable application in strength assessment of concrete. This is where misuse is most common as, unfortunately, a strength estimate by rebound hammer is frequently addressed by engineers. The accuracy in the estimating concrete strength depends entirely upon the assessing the influencing factors. The accuracy of the rebound hammer test in assessing compressive strength of concrete is within the range of ±15-20% for a properly calibrated hammer and for test specimens cast, cured and tested under laboratory condition, where in practical field, it is unlikely that the accuracy becomes lower, within the range of ±30%. Although it may be possible to address for one or two influencing factors in the relationship which may not be identical on site, the accuracy in predicting strength will decline as a consequence. For this reason, in practice, it is advised to use the Schmidt rebound hammer as a device of assessing relative concrete quality and uniformity, rather than a device for strength estimation.

Rebound hammer test: A popular NDT in the evaluation of concrete strength- Part 1

Good to see you again. This time, I will go through a series explaining some popular non-destructive tests (NDTs) of concrete strength assessment. 

Rebound hammer test can be considered as a method of measuring surface hardness of the concrete. The increase in the hardness of concrete with age and strength encouraged the development of the test method to measure the hardness of concrete. The use of hardness testing method in concrete can be traced back to the early twentieth century after the introduction of the Brinell indentation method for metals. The test method is based on the rebound principle consisting of measuring the rebound of a spring driven hammer mass after its impact. Ernst Schimdt in 1948 introduced spring impact device of concrete by rebound principle. To date, it is the most popular in-situ testing method for concrete due to being inexpensive and its relatively simple use and fast operation. With the development of this device, the hardness measurement became much easier. The measurement can be read directly on the scale of the device. 

The technique provides an easy measurement of rebound of impact hammer and the operation is quite simple. During the operation, a hammer impacts the concrete at a fixed energy with the help of a spring and rebounds from the surface of the concrete after the impact. Energy of the hammer is applied by tensioning of spring and it is independent of the operator. The test method consists of measuring the rebound of the hammer after its impact with concrete and correlating the rebound value with compressive strength of concrete.

When the hammer impinges on plunger, a compressive wave propagates into the concrete through the plunger. The plunger deforms elastically during the stress wave propagation. When the compression stress wave reaches the other end of the plunger (i.e. the concrete), part of the energy is absorbed in the concrete and the rest of the stress wave is reflected back in the plunger. The reflected compression wave returns to the free end of the plunger and causes the hammer to rebound. From the theoretical point of view, the rebound of the hammer is dependent on the energy absorbed during the impact. The energy absorbed in concrete results from both elastic deformation and plastic deformation (local crushing) of the concrete. When the acceleration of the plunger is brought to rest, the elastic deformation of the concrete recovers. A residual set is formed in the concrete under the tip of the plunger. The absorbed energy is dependent on the properties of the concrete at the vicinity of the tip of the plunger. The relationship between rebound value and concrete strength depends on the strength, stiffness and damping capacity of concrete in the vicinity of the tip of the plunger of the rebound hammer. Concrete with high strength, high-stiffness will absorb less energy than the concrete with low strength and low stiffness. Therefore, for two concrete mixes with same strength but different stiffness, the rebound values can be different even if the strengths are equal and vice versa.

Rebound hammer is the most popular NDT in this field. The test is affected by high number of factors, most of them are uncontrolled, thus affecting the overall assessment of the strength of concrete. We will talk about that in the next episode. 

Assessment of concrete strength in existing structures: Cores and NDTs

Previously I discussed about the necessity of assessment of concrete compressive strength in an existing structures. I tried to present some of the testing methods to assess the concrete strength. The methods were non-destructive test (NDT) methods as they offer no or little damages to the structures. I presented the limitations of using NDTs in this field.

The direct determination of strength assessment of concrete requires concrete specimens to be taken from the structure that can be tested destructively. The samples are known as cores. Core testing is the most direct and reliable method to estimate concrete strength in a structure. The examination and compression testing of cores cut from hardened concrete is a well-established method for strength estimation. The principal limitation of core testing are those of cost, possible damage to the structural integrity, and the whole procedure is time consuming. The number of core that can be taken from a structure is usually limited. Core testing may not always be possible at all parts of the structure. NDTs are considered to be advantageous to supplement the core test. NDTs allow more economical evaluation of concrete in the structures. As NDTs measure compressive strength of concrete indirectly, a valid relationship between test result and compressive strength must be established following a valid statistical procedure.

European Standard (EN 13791 2007) gives methods and procedures for the assessment of the in-situ compressive strength of concrete in structures by coring and provides guidance on the use of NDTs for the strength assessment. Use of NDTs with limited number of cores introduces two sources of uncertainty. One is due to the calibration that is performed from a limited number of cores and another is due to the lack of precision of NDTs. EN 13791 offers two approaches; Alternative 1-Direct correlation with cores and alternative 2-Calibration with cores for a limited strength range using an established relationship. Alternative 1 requires minimum of 18 cores where at least 9 pairs of data is required for alternative 2. According to ACI 228.1R 2003, use of NDTs in the practical field should be preceded by the preparation of calibration curve from the same concrete that is under investigation (ACI 228.1R. 2003 ).

In conclusion, it can be stated that core testing is a must for the assessment of concrete strength in existing structures. NDTs are useful to minimise the number of cores to be taken and to reduce the uncertainty in compressive strength assessment across the structure. NDT allows the collection of data in a short time at a low cost. NDTs are be used in determining the statistically most reliable results of core tests with limited expenses and time. 

Assessment of concrete strength in existing structures: Why

Hi everyone. Today I will present an introductory overview of my research project. My research is related to the assessment of concrete strength in existing structures. My today's discussion will be based on answering the question why assessment is required. I will also add how the concept grew and developed over time. 

Concrete is the most widely used construction materials all over the world, we all aware of that. But concrete properties varies from sample to sample. Concrete is generally composed of aggregate, cement, and water. Properties of constituting materials along with their mixing ratio play important role in determining final mechanical properties of concrete. Placement of concrete, environmental condition, and quality control during each operation also contribute to final property of concrete. All of these factors lead to the variability in concrete property. 

We sometimes hear about the collapse of different structures. These collapses result tragic loss of lives as well as economy. Most of these collapse occurs during construction and operation of the structures. Again, earthquake is continuously threatening in many parts of the world. Most of the old structures were not built considering the threats of earthquake. With the advancement of knowledge and technology, earthquake and other environmental loading is more clear to us. On the other hand, existing structures sometimes undergo structural modification. A structure can be subjected to changed loading condition. All these factors lead to the assessment of capacity evaluation of the structures. Moreover, with the progress of time, people are  focusing on how to make the most use of limited resources. So, extending life time of existing structure has gained special attention in recent years. It is now preferred to retrofit the old structures rather than demolishing and rebuilding. So, assessment becomes necessary to evaluate the performance of existing structures. Strength of concrete plays important role in carrying out accurate assessment of the structures. 

Testing in concrete structures is a century old practice. The testing began with the implementation of Brinell hardness method for concrete and cement paste. Later on several testing methods has been developed over time. These tests is carried out in existing structures without damaging original structures. That is why they are known as non-destructive tests. Non-destructive tests (NDTs) for compressive strength estimation of concrete allows the concrete to be tested without any damaged or creating any adverse effect on the structural integrity in concrete structure. 

With the help of NDTs, measurement can be performed directly on the structural concrete. Among the commonly used NDTs, some tests are completely non-destructive and some are partially destructive. Partially destructive tests offer some damage on the surface of concrete but they do not affect the structural integrity. Rebound hammer test, ultrasonic pulse velocity (UPV) test, resonance frequency test, and maturity test are completely non-destructive. Partially destructive tests include pull out test, cut and pull out (CAPO) test, penetration resistance test, break off test, and pull off test. All of these tests are generally classified as non-destructive tests.

NDTs are influenced by different properties of concrete and its constituting materials, and several factors related to the environment. NDTs provide indirect approach for compressive strength estimation of concrete. NDTs generally assess the easily measureable properties of concrete. Measured value is then related to uniaxial compressive strength. Efficiency and quality of strength estimation depend on the sensitivity of strength to the tests, measurement error and range of variation of test results as compared to the variation of strength. Being influenced by several factors, NDTs provide compressive strength of concrete within their own limitations which result in high degree of uncertainty. This uncertainty in strength estimation in turn results in poor evaluation of existing concrete structure.

We do not want to live in dark in present time especially in time of science and technology. So, the project focuses on the reduction in uncertainty in assessing the strength concrete. Hope, this clears the query. See you soon with more insights.

Where the cliff greets the sea

Come

Surf the clouds

Race the dark

It feeds from the runs undone

Meet me where the cliff greets the sea. (Nightwish-élan)

Those some lines seem perfectly written. It is a some place where you surely would want to fly spreading your hands. You certainly would feel the void in head. Cold breeze  would help you to be a stone. Open calm ocean would guide you to a surreal world. It's a feeling of trance, you are not in a mood of dancing, but you would want to enjoy flamenco or tango or ballet. You would never want to get back to real. You would want to be friend with seagulls. Seagulls would beckon as if they would teach you to fly. Deep soft sound of ocean would try to seduce you, wind would whisper in your ear. You would be out of balance, you would have feeling of scare, you would be overwhelmed with amusement. 

I was talking about "Cliffs of Moher". 200 meter directly above the Atlantic ocean, this place will always attract you. 

Sorry for the quality of the picture. All these are taken in low quality mobile phone.

International Mother Language Day

Language changes with time. It is like river, always changing the course. It is reported that as many as half of the world's 7000 languages are expected to be extinct by the end of this century. That means one language dies out in every 14 days. To cherish the cultural heritage and diversity and to promote the preservation and protection of all languages used by the people of the world, "International Mother Language Day" day is celebrated every year on 21st February. The day was proclaimed by UNESCO in November, 1999. It represents the day in 1952 when the students demanding for recognition of their language, Bangla, as one of the two national languages of the then Pakistan, were shot and killed by police in Dhaka (Capital of present Bangladesh). 

This day has great significance to Bangladeshi culture and is remembered as Language Movement Day or Language Martyrs' Day. Shaheed Minar (Martyr Monument) is the national monument established to commemorate those killed during the language movement. People, wearing black badge, go to Shaheed Minar in bare foot with flowers in hand to pay homage to the martyrs early in the morning by singing song of 21st February. At present, all national, mourning, cultural and other activities have been centered on the Shaheed Minar.

    

                             Shaheed Minar (Martyr Monument)

Movement for equal status of Bengali language also took place in the Indian State of Assam. On May, 1961, number of people were killed in police firing while demanding state recognition of Bengali.

One of the most precious and at the same time most fragile treasures of mankind is language. Let it mark on the pages of human history. Let it flow. Let the children fall asleep listening to the bedtime stories in their mother tongue. Let them dream in their mother tongue. Happy mother language day. 

Introduction (শুরুর কথা)

It is not easy to write. I think ability to write is a special gift. Someone may think of many things but while it comes to the question of writing or speaking, output is very little. But of course things are not same for all. Otherwise, no writer, storyteller would born. Human civilization are influenced by those people. They live from century to century, country to country. These people are making the shape of the society starting from the beginning of human civilization. Actually what I am trying to say is I am not good at writing. I do not need to tell you this, eventually you will find it. Anyway, welcome to my blog. 

Let me start with the introduction. My name is Md Shah Nur Alam Sourav, yes, it is a large name. I am from Bangladesh. I was born in a small village in the district of Mymensingh. It was very early in the morning in my grandfather’s house (surely I cannot remember). The village is very beautiful surrounded by rivers, green fields and trees. I am running from my family since my childhood. When I was 11 years old, I left my home for study. I completed my Bachelor’s degree in civil engineering from Bangladesh University of Engineering and Technology. And now here I am in this project thousand miles away from home.

My project here is related to the concrete strength assessment in the existing building. World is moving towards the idea of reusing and sustainability. From that concepts, instead of constructing new structures and demolishing old ones, present structures are being provided importance for its safety against new use and design condition. Concrete strength is happened to be one of the most important factors in assessing those old structures. Many building collapse from time to time led to the necessity of assessment of concrete strength in structures. One of the most recent and dangerous collapse happened in Dhaka in 2013 killing more than thousand people made the engineers and researchers of this field to rethink about it. Several methods are in practice but till today there remains some questions about the reliability of these methods. This led me to work on the reduction of uncertainty in concrete strength assessment of existing building. I started here from mid November. 

When I came out of the door of the airport here in Dublin, I was welcomed by a cold breeze. At least I felt it cold. A new country, a new environment. I was kind of afraid, kind of excited, it was the feeling of exploration, discovering the new. Everything I see looks new to me, even when I see the sun, it looks different. Of course it is different, it seems that sun is tired of his age, it cannot stand straight, it lost all his power of youth or sun does not want to make any friendship with this part of the world so it keeps distance. I was grown up seeing the sun dictating from the above with its heat, power. It sometimes makes me sad when I do not see the sun continuously for some days. People there wake up with the rise of the sun, lifestyle is dependent on it. Even in winter, if there appears no sun in some day, life becomes stuck. It seems that energy of life has to be extracted from the sun.

It is difficult to express your inner thoughts in an informal way specially if you want to add some literary value. I hope you will overlook the mistakes. I do not know what is coming next. Thank you for your patience.