Safety Methodology Assessment in Civil Engineering – Assignment

By : PUTIKA ASHFAR KHOIRI (putikaa@civil.eng.osaka-u.ac.jp) / ID : 28J16118

Estimate the causes of crack in concrete structure based on Japan Concrete Institute (JCI) guideline

Introduction

There are many concrete structure worldwide. To evaluate crack on concrete structure is important and need to be considered in order to determine crack repairing method. Simple and systematically investigation to determine cause estimation of the crack has been done by Japan Concrete Institute. They made a procedure of investigation of cracks to the application of repair and strengthening. Practical Guideline for investigation, Repair and Strengthening of Cracked Concrete Structures has been developing since 1980 until their last updated version in 2013. The contents covers the practicable investigation of cracked concrete members or structures, cause estimation, evaluation, judgement and necessity of repair or strengthening, selection of the most effective repair and strengthening method. The main objective of the investigation is to collect data for estimation the cause of cracking and judge the necessary repair method and strengthening. The procedure from the investigation of cracks is shown below in Fig. 1

Crack investigation

1. Standard Investigation The investigation is carried out in short period by investigating the visual inspection and the existing documents of the structure. The objective of the investigation is to collect data for the estimation of the causes of cracking of a structure or its members. This is also necessary for subsequent evaluation of cracks, judgement of necessity of repair and strengthening. The investigation can be done without any experiments or long term observation. We can do standard investigation by investigating the documents of the building or visual observation of the structure.

1.1 Documents investigationWhat documents that we need to investigate? Below are the items we need to investigateThe engineering drawing of the building1. Design report and specification of the building

This include engineering drawings of the structures, structural design calculation arrangements and rules that is used to build the structure. The design reports also have the construction specification of the building like materials used, dimension, steel bar layout drawing, etc.

2. Construction recordThe construction record means the condition of the building when its constructed. Recent standards of ISO 9000 series promote contractor and owner to storage the construction records for 5 to 10 years. Therefore, the construction records are including the materials used, mixture proportions of concrete, placing of concrete, experimental data of quality control (e.g. tensile test, etc.) and the environmental conditions where the buildings placed (e.g. ground profile condition, etc.)

3. History of past investigation (repair and strengthening)Recorded past investigation is very important and valuable to determine the cause of crack. We can know the history of the daily and routine inspection done by inspector. Daily inspection covers the date at crack occurrence, the crack propagation and the claim for the residents, while routine inspection covers defect and deterioration of concrete which are difficult to recognize by daily inspection.

4. Climate and geographical conditionsWe can obtain the climate conditions from the metrological agencies or observations. Temperature, relative humidity, wind directions and wind velocity, etc. For the building close to seashore, we also need the wave height and wave directions also the occurrence of past storm surge or flood events in those area.

5. Ground conditions and profileThe investigation of the ground profile is depending on the geographical location of the building. Necessary assessment of the building place and observation of the building environment is necessary. The treatment of the structure depends on the surrounding location (e.g. near the seashore, cold district, etc.). For example, in the location that near to the seashore we have to consider the impact of the wave height and re-bar corrosion by the chloride.

1.2 Visual observation

On visual observation we can identify the location of the crack, crack width, crack pattern and the existence of penetrating crack by using eye observation, photo and simple apparatus

to measure the crack width. We can use crack scale to measure the crack width or microscope.

Fig. 2 The example of crack scale

1. Crack widthCrack width is defined as the width measured at the surface of the structure perpendicular to the direction of the crack. From the crack width we can estimate the effect of the crack width to the structure.

2. Location of crackThe location of crack could be the indicator of the cause of crack. Drying shrinkage of concrete, expansion of concrete by temperature rise and leakage of water can cause the crack. Crack that penetrate through the structural member are closely related to the degree of serviceability inconvenience due to cracks. It is necessary to confirm whether crack penetrate throughout the member of the structure.

1. Crack PatternThe pattern of the crack might be different due to the causing of the crack. It can cause by the chemical reaction or propagation of salt which made a corrosive way.

2. Detailed Investigation

When the cause of cracking is not possible to determine within the scope of standard investigation. Detailed investigation usually needs longer time to identify the cause of crack than the standard investigation and more expensive. We can do detailed investigation by :

1. Laboratory InvestigationBring some sample of concrete to the laboratory and identify the cause of crack by some experiments.

2. On-site investigation The example of on-site investigation:For on-site investigation there is non-destructive test and destructive test. The test is used to estimate the location and depth of the crack. The crack depth can be estimated by ultrasonic method by knowing the time receiving of the reflected ultrasonic waves through the concrete width. 1. Measurement with clip gage (non-destructive test)

Strain is defined as the ratio between the length of an element before and after a force has been applied to it. In concrete strain can be seen in the change of concrete or reinforcing steel due to temperature, curing and forces applied during its use. Deformation within the concrete will cause the end bars to move relative to each other.

The tension in the wire between the bars will change accordingly thus altering the resonant frequency of the wire.

2. Laser measurement (non-destructive test)We can identify crack by induced and scaning laser to the concrete. The wave is penetratring through the concrete and reflect if there is any hole or crack inside the concrete.

3. Rebound hammer method (destructive test) We can measure the rebound distance by using rebound hammer apparatus and estimate the strength of the concrete. If the hardness of the surface if high, the rebound distance is large. For test hammer method, the standard usually uses 20 poles sample to measure. concrete hardness, but in case of JCI they use 150 poles samples

Fig. 3 Rebound hammer test illustration4. Infrared thermography method (non-destructive test)

Infrared thermography is equipment or method, which detects infrared energy emitted from concrete, converts it to temperature, and displays image of temperature distribution. The principle of the method is the surface of the concrete wall are heated by the solar radiation and thus the temperature gradient become constant. In the morning, the sunlight is received by the concrete surface, so the temperature of the surface concrete is higher than the inner site. The condition of the temperature is opposite at night-time when the temperature of the inner side is higher than the surface sides.

Fig. 4 Temperature distribution of building cracks identification by infrared thermography

5. Impact elastic wave method (non-destructive test) We can investigate if there is any voids or crack on the concrete by penetrating vibration wave through the concrete width. Then we will get two conditions: 1. If there is a crack on the concrete, then the period of the penetrating wave will be

shorter and the peak of the wave spectrum peak of will move due to the higher frequency value, and also have no single peak.

The frequency of the propagation (ft) can be calculated by f t=C p

2TWhen we can’t estimate the frequency of the propagation (ft), we can estimate the crack by the concrete thickness (T), while (Cp)is the propagation velocity of elastic wave on concrete (in longitudinal direction)

Fig.5 Wave response on concrete cracks by impact elastic wave method2. If there is no crack on the concrete, the vibration wave will reach the bottom of the

concrete. The wave spectrum has only a single peak and the peak position of the frequency value is lower.

We also have to consider about: Structure materials, loadings on the structure, environmental conditions around the structure, foundation condition, structural performance and deformation and vibration of the structure

Classification of Cracks

1. Classification based on the crack generation period, regularity and extend of the crack

Before we classify the crack to the major classification of crack, we better know the estimation of crack category based on their generation period, regularity and extend of the crack to simplify the estimation of crack causes (see table 1).

Table 1 Classification based on the generation period, regularity and extend of cracks

Pattern of cracksCause EstimationGeneration

Period Regularity Extend

Few hours to one day

Yes Reticular B2, B3Surface layer A8, B2, B3, B5, B14, B16, B17Penetration B2, B3, B4, B10, B16, B17

NoReticular B8Surface layer A1, B5, B7, B8, B13, B17Penetration B4, B10, B17

Few days

YesReticular Surface layer A2, A10, B15, D5Penetration A2, A10, B16

NoReticular A4, B9Surface layer B7, B9Penetration

More than several ten days

Yes

Reticular A6, A9, B2, B3, D2

Surface layerA6, A7, A9, A10, B2, B3, B11, B12, C1, C2, C7, C8, D1, D3, D5

PenetrationA9, A10, B2, B3, B4, B10, B18, C1, D2, D4, D5, D6

NoReticular A3, A4, A6, B1, B9, C3, C4, C5, C6Surface layer A3, A4, A5, A6, B9, C3, C4, C5, C6, D7Penetration B4, B10, B18, D6

1.1 Classification based on the extend

The extend of crack is defined as reticular for mesh type cracking, as surface layer if the depth of the cracks is limited to the surface region and as penetration if the cracks continue through the section.

1.2 Classification based on the regularity

The severity of a crack can be characterized in terms of its direction, width, and depth; cracks may be longitudinal, transverse, vertical, diagonal or random. Different risks for cracking exist for cured versus uncured concrete, and for reinforced concrete.We have to investigate and determine whether the shape of the crack is regular or not in its direction, width and depth.

1.3 Classification based on the generation period

The generation period of crack formed after concrete placing from a few hours to more than 10 days. We can investigate the cause of the cracking by its generating period.

2. Classification based on the deformation and limit considerations

It is required to investigate the deformation factors of concrete (drying shrinkage, expansion, settlement, bending and shearing). From the table 2, we can see if the cracks region shows the occurrence of shrinkage, expansion, settlement, bending and shearing. It is required to investigate material, member (such as beam, column, wall, slab, etc) and structure (including roof and foundation) for cause estimation.

Table 2 Classification based on the deformation and limits of consideration

Deformation factors of concrete

Limit of consideration Cause estimation

ShrinkageMaterial A1, A2, A4, A9, A10, B1, C1, C3, C4, C5Member A2, A9, A10, B2, B3, B8, B14, B15, B17, B1, C2, C3, C4, C5Structure A9, B2, B3, B8, B15, C1, C2, C3, C4, C5

ExpansionMaterial A3, A5, A6, B1, C1, C3, C4, C5, C6Member A7, B1, B12, B18, C1, C2, C3, C4, C5, C7, C8Structure A7, C1, C4, C5

Settlement, bending and shearing

Material A5, C1

MemberA8, B4, B5, B6, B7, B9, B10, B11, B12, B13, B16, B17, C1, C2, D1, D2, D3, D4, D5, D6, D7

Structure B6, C1, D1, D2, D3, D4, D6, D73. Classification based on mixture proportions (table 3)

Mixture proportion Standard Cause estimation

RichCement content per unit volume of concrete is more than 350 kg/m3

A2, A6, A9, A10

Poor Cement content per unit volume of concrete is less than 350 kg/m3 A8, C3, C6, C7, C8

4. Classification based on weather condition during placing concrete (table 4)

Weather condition StandardCause

estimation

High temperature The daily men temperature during placing is higher than 250C and the ambient temperature at placement is higher than 250C

A2, B2, B8, B17

Low temperature The daily mean temperature at placing is lower than 40CA8, B7, B9, B13, B16, D7

Low Humidity The humidity is lower than 60%A4, A9, B8, B17

Major estimation of crack causes

This procedure is a method to estimate the cause of crack and obtain data for the evaluation of the crack. There are so many causes of cracks based in their category from A to D. Major classification of causes is divided into 5 sections, which can be seen in table 5

Table 5 Major cause of cracking

Major Classification

Sub Classification Sub-sub Number Cause

A. Materials Used Materials Cement A1 False setting of cement

A2 Heat of hydration of cement

A3 Abnormal expansion of cement

AggregateA4 Clay inclusion in aggregate

A5 Low quality aggregate

A6 Reactive aggregate (alkali-aggregate reaction)

Concrete

A7 Chloride in concrete

A8 Settlement and bleeding of concrete

A9 Drying shrinkage of concrete

A10 Autogeneous shrinkage of concrete

B. Construction

Concrete

MixingB1 Non-uniform dispersion of admixture

B2 Long-time mixing

Transport and placing

B3Change of mix proportion at pumping

B4 Innapropriate placing sequence

B5 Rapid placing

Compaction B6 Innapropriate compaction

CuringB7 Loading or vibration before hardening

B8 Rapid drying during initial curing

B9 Early age frost damage

Construction joint B10 Innapropriate joint treatment

Steel Arrangement of steel

B11 Innapropriate placement of reinforcement

B12 Lack of cover

FormworkFormwork

B13 Deformation of formwork

B14 Water leakage (from formwork, into subgrade)

B15 Early removal of formwork

Support B16 Settlement of support

Others Cold joint B17 Innapropriate joint or discontinuity

PC grout B18 Insufficient grouting

C. Environment Physical Temperature

and humidity

C1Change of enviromental temperature and/or humidity

C2Difference of temperatures and humidity between two surfaces of member

C3 Repeated cycles of freezing and thawing

C4 Fire damage

C5 Surface heating

Chemical

Chemical reaction C6 Chemical reaction of acid and/or salt

C7 Corrosion of embedded steel due to carbonation

C8Corrosion of embedded steel due to chloride attack

D. Structure and external force

Load

Long-term load D1

Long-term load within design load

D2 Long-term load over design load

Short-term load

D3 Shor-term load within design load

D4 Shor-term load over design load

Structural design D5

Insufficient cross sectional area or quantity of steel

D6 Differential settlement of structureSupport condition D7 Freezing heave

E. Others Others

We can determine the causes of cracks by following the procedure of crack estimation. The crack estimation is based on the crack generation period, mixture proportion, weather conditions and concrete deformation. We can do visual inspection by hearing the sounds of concrete when we knock it. If the concrete is weak, the compositions of cements and aggregate is not in a good proportion because of the false setting of mixture or there is a hollow inside the concrete.

Another way to estimate the cause of cracking on concrete

Concrete can deteriorate for a variety of reasons, and concrete damage is often the result of a combination of factors. The following summary discusses potential causes of concrete deterioration and the factors that influence them. material limitations, design and construction practices, and severe exposure conditions can cause concrete to deteriorate, which may result in aesthetic, functional, or structural problems.

Evaluating cracks causes and status

It is important to identify the primary concern in regard to any cracking. The main concerns are whether the cracks are affecting structural integrity, caused by inappropriate design, aesthetically unacceptable, or reducing durability. We can only identify the primary concern after evaluating a crack thoroughly. The type of cracking provides useful information to help understand a crack’s effects on structural stability, Summary of the different type of concrete cracks and their possible causes can are presented in table 6.

Table 6 Types of crack and their cause

Before Hardening

PlasticEarly frost damagePlastic shrinkagePlastic settlement

Construction movement

Formwork movementSubgrade movement

After Physical Shrinkable aggregates

Hardening

Drying shrinkageCrazing

ChemicalCorrosion of reinforcementAlkali-aggregate reactionsCement carbonation shrinkage

Thermal Freeze/ thaws cycleExternal seasonal temperature variations

StructuralAccidental OverloadCreepDesign loads

A crack’s status is critically important. Active cracks may require more complex repair procedures that may include eliminating the actual cause of the cracking in order to ensure a successful long-term repair. Failure to address the underlying cause may result in the crack’s repair being short-term, making it necessary to go through the same process again. Dormant cracks are those not threatening a structure’s stability, but those responsible for the structure must address durability issues and take appropriate action if aesthetics are a priority. A crack’s environmental conditions influence the extent to which it affects its structure’s integrity. Greater exposure to aggressive conditions increases the possibility of structural instability. Cracks’ sizes range from micro-cracks that expose the concrete to efflorescence to larger cracks caused by external loading conditions. Noting cracks’ sizes, shapes, and locations can aid in determining their initial causes.

1. Cracks before hardening

1.1 Cracking in plastic concrete

Cracks that form in plastic concrete can be categorised as either plastic shrinkage cracking or plastic settlement cracking. Both of these types result from the bleeding and segregation process that occurs when fresh concrete is placed. Such cracks usually appear from one to six hours after concrete placement.

1.2 Plastic shrinkage cracking

As the concrete’s heavier particles settle due to gravity, they push the water and lighter particles toward the surface. This is called bleeding. If you fail to monitor the temperature, wind, and humidity conditions properly the evaporation rate of the surface water may exceed the bleed rate, drying out the concrete’s superficial layer and therefore shrinking it due to dehydration. The concrete beneath the surface layer is still well hydrated, however, and maintains its volume. This applies opposing tensile forces to the lower part of the drying concrete on the surface, causing a cracked concrete profile. These plastic shrinkage cracks are usually shallow and only from 1 to 2 mm in width, which means you cannot repair them with the injection method. They may, however, self-heal through continual cement hydration or by the precipitation of calcium carbonate from the concrete. If the cracks are wider than 2 mm and do not self-heal, it is important that you repair them with a suitable coating or flood-grouting product to stop them from penetrating the full depth of the concrete slab. If they do become active their reaction to stresses may result in further cracking that weakens the structure either directly or by exposing its reinforcement steel to contaminants that will in time corrode it.

1.3 Plastic Settlement Cracking

The settlement process is a major factor in concrete’s strength at different levels as it forms. Plastic settlement cracking can occur as a result of such restraints to the consolidation of the fresh concrete as the use of steel reinforcing bars or formwork. As the concrete bleeds, the water works its way to the surface. Sedimentation then occurs as the aggregate and cement move downwards under the force of gravity. This separation forms a weaker layer of concrete near the surface. If such restraints as steel reinforcing bars are close to the surface and insufficiently covered with concrete the concrete bends back around the restraint and cracks at the apex. Deeper sections of concrete lead to greater separation between the sediment and the water, so it is important to ensure that you cover all superficial restraints adequately to reduce the amount of cracking.

2. Cracks after hardening

Cracking in hardened concrete can result from any one of many causes. These causes include (a) drying shrinkage, which is the main cause, (b) thermal stresses, (c) chemical reactions, (d) weathering, which involves heating and cooling and is linked to thermal stresses, (e) the corrosion of steel reinforcing, (f) poor construction practices, (g) construction and structural overloads, (h) errors in design and detailing, (i) externally applied loads, and (j) poor loading and storage practices. It is important to understand the factors that influence the above causes of cracking in order to eliminate the cause and select the correct repair method. The following sections explore the causes of cracking in hardened concrete in more depth.

2.1 Drying Shrinkage

This is the main cause of cracking in hardened concrete. This cracking takes place near the restraints due to volume changes in the concrete. When concrete is exposed to moisture it swells and when it is exposed to air with relatively low humidity it shrinks, such air drawing water out of its cement paste, which is cement and water. If the shrinkage could occur without restraint no cracking would result, but in most cases the requirements of structural support makes this impossible. This cracking is the result of a combination of factors that influence the magnitude of the tensile stresses that cause it. These factors include the amount and rate of shrinkage, the degree of restraint, the modulus of elasticity, and the amount of creep. Additional

Evaluating factors to be aware of include the type of aggregate, water content, binder type, and the concrete’s mix proportions and mechanical properties. The amount and type of aggregate and the cement paste are the main influences on the amount of drying shrinkage. To minimise the amount of shrinkage it is best to use a stiff aggregate in high volumes relative to the cement paste. The rate of shrinkage increases with the volume of cement paste. The aggregate provides internal restraints to shrinkage. As the outside of the concrete cools more quickly than the inside it shrinks, and the pressure caused by the inner section’s lack of shrinkage produces tensile stresses that, when exceeding the concrete’s tensile strength, cause the concrete to crack to relieve the pressure.

2.2 Thermal stresses

Volume differentials are likely to develop in the concrete when different temperatures occur across a concrete section. The concrete then cracks when the tensile stresses imposed by a change in volume differential exceed that of its tensile strength. Thermal stresses usually

cause cracking in mass concrete structures, the main cause of the temperature differentials being the influence of the heat of hydration on volume change. The heat of hydration is the amount of heat released during the cement’s hydration, causing a temperature differential to occur between the concrete structure’s centre and exterior as a result of either greater exterior cooling or greater heat hydration in the centre.

2.3 Chemical reactions

Chemical reactions in concrete can be due to the materials used to make it or materials that may have come into contact with it after it has hardened. The cause of the cracking is the expansive reactions between the aggregate and the alkalis in the cement paste. The chemical reaction occurs between active silica and alkalis, producing an alkali-silica gel as a by-product. The alkali-silica gel forms around the surface of the aggregate, increasing its volume and putting pressure on the surrounding concrete. This increase in pressure can cause the tensile stresses to increase beyond the concrete’s tensile strength. When this occurs the concrete cracks to relieve the pressure.

2.4 Structural loads

It is important to pay close attention to the way you load, transport, and unload pre-cast concrete, and how you secure it in place. At any one of these stages the pre-cast concrete modules can become subject to stresses that overload their structure. If these stresses occur in the concrete’s early ages they may result in permanent cracks. You need to employ lifting procedures that disperse the load across the structure in order to reduce the risk of overload stresses. Pre-tensioned beams may present cracking problems at the time of stress relief, especially in beams that are less than one day old. You need to pay particular attention to the storage of materials and operational equipment during the construction phase, as these may generate loads that exceed those that the structure was designed to withstand.