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The slabs-on-ground does not require high protection against water penetration and moisture in arid regions that does not have soils or have any sort of drainage problems.

This precaution is not necessary if the area does not have irrigation activities going on nearby. This is the case unless the building code of that regions requires treatment.

But for all site conditions other than mentioned above, it is essential to have water proofing or damp proofing for the slabs constructed on grounds.

As per ACI 302.IR-89, there is no need for damp proofing or water proofing nor vapor retarders on sites that are well drained or those sites under the following conditions:

1. The site has water table level very low from the ground surface
2. A substratum of coarse aggregate that is freely draining is installed
3. A floor covering is provided to the slab that is not affected by the moisture.

Damp Proofing of Slabs–on–Ground

The damp proofing of slabs constructed on ground can be done either below or above the surface. Mostly, the damp proofing is installed below the surface of the slab. Mostly plastic film of vapor retarders is employed below the slab surface as a damp proofing membrane.

The figure-1 below shows the installation of a vapor retarder over the surface of gravel substrate. The vapor retarders are sealed to the foundation wall as shown. This will provide continuity in the placement.

In order to avoid the penetration of sheet, brick chairs are employed other than the wire chairs.

Fig.1. The plastic vapor retarders are placed above the grave substrate that is tamped.

When wood flooring has to be provided above the slabs on ground, the damp proofing is done over the surface of the floor. The damp proofing over the slab top surface is permitted only under the following conditions, when:

1. The water table is at least 12 inches below the slab surface
2. Installation of footing drains is carried out

The slabs-on ground with wooden flooring on the top will ask for damp proofing unless the above conditions are satisfied.

The figure-2 below shows the installation of wooden flooring over the slabs that are constructed on ground.

The wood flooring span sleepers will separate the surface from the bottom slab as shown in figure-2. The damp proofing can be applied to the slab only under the sub soil conditions mentioned above.

Fig.2.Wooden Flooring Above Slabs-on-Ground

In the figure-2, an air space is created between the wooden flooring and the slabs constructed. This must be vented at the junctions so that the water penetration due to water vapor migration is avoided. This will help in maintaining an equal relative humidity both above and below the surface of the wooden flooring provided.

The top surface damp proofing of the slabs constructed on ground will be interrupted by the partitions and the columns present. Vapor retarders constructed will be interrupted by these extra elements bringing a loss of integrity of the damp proofing system.

The figure -3 below shows the same. Under such situations, the best alternative is to have damp proofing or water proofing under the slab.

Fig.3. The column interrupting the vapor retarder placed above a slab-on-ground

Risk in Using Vapor Retarders as a Waterproofing Solution

The substitution of vapor retarders instead of water proofing will welcome risks. The performance of the vapor retarder is dependent on the integrity of the film as well as the way it is seamed. The maintenance of a vapor retarder seam integrity in field condition is very difficult to attain in a slab on ground construction.

The use of wooden or vinyl coverings too make the vapor retarders into risk. Other risk parameters are the use of wall – sensitive adhesives. Under these situations, it is most preferred to have a heavy-duty panel or water proofing to be installed than waiting for the loss caused by the complete replacement of moisture damaged floor.

As shown in figure-2, the attachment of sleepers to the slabs-on-ground must be carried out with the help of water resistant adhesives instead of mechanical anchors.

The use of nails and other mechanical anchors will result in puncture of vapor retarders that is located on the top surface of the slab.

Considerations in Damp Proofing Slabs on Ground

When the top surface of the slab is subjected to damp proofing, the slab should have a substratum that is of a granular material. This layer of properly graded aggregate will help in preventing the rise of moisture (mainly by the principle of capillary action).

It is advised not to have a vapor retarder below the slab surface by using suspenders and other belts along with having a top surface damp proofing. The residual moisture in the concrete slabs can result in vapor pressure that can disband the vapor retarder and result in rupture.

It is not appropriate to have too many floor finishes and coatings. This is because the concrete is moisture sensitive. Moisture sensitive adhesives can be used as a mode of moisture resistance.

Liquid applied coatings in concrete slabs on ground have resulted in moisture in concrete. The following conditions can result in high level moisture contents:

1. For those slabs that are cast on a grade with no proper functioning of the under-slab vapor retarders.
2. Conditions when the suspended slabs are cast over the non – vented steel formworks.
3. The conditions where the suspended slabs are constructed over the occupancies that possess higher relative humidity. These includes elements like commercial kitchen and swimming pools.
4. The installation of suspended slabs over the crawl spaces that are unvented.
5. The slabs with lightweight aggregates.
6. The slabs that used a water cement ration in excess of 0.55 (w/c = 0.55).
7. The slabs that cured in a period less than 90 day.

The manufactures of the floor finishes or damp proofing coatings highly follow the requirement of testing the concrete slab before the installation of any membranes. This testing will ensure that the moisture content in the slab is within the susceptible limits and won’t result in any failure.

The Calcium Chloride Test as specified by ASTM F1869 is used to determine the Moisture Emission Rate (MVER). This is measured in pounds per thousand square feet within 24 hours. The accepted range of MVER is three to five pounds.

The internal relative humidity in the structure is obtained by means of a hygrometer by placing probes in concrete. This is as per ASTM F710 and the established rate is 75 percent.

This result can be achieved within 1 month of drying time per inch of the concrete element tested. It recommends the installation of vapor retarders in the direction of concrete pouring.

Materials Used in Bridge Construction

Stones, Timber, Concrete and Steel are the traditional materials that are used to carry out bridge construction. During the initial period, timber and stones were used in the construction, as they are directly obtained from nature and easily available.

Brick was used as a subgroup construction material along with stone construction. Stones as construction materials were very popular because of its durable properties. Many historic bridges made from stones are still present as a symbol of past architectural culture.

But some of the timber bridge have been washed away or are in the stage of degradation due to their exposure to the environmental conditions.

As time passed, the bridge construction has undergone more development in terms of materials used for construction than based on the bridge technology.

The concrete and steel are manmade refined materials. The bridge construction with these artificial materials can be called the second period of the bridge engineering. This hence was the start of modern bridge engineering technology.

Modern bridges make use of concrete or steel or in combination. Different other innovative materials are being developed so that they can well suit with the bridge terminologies.

Incorporation of fibers which comes in the category of high strength gaining materials is now incorporated for the construction of bridges. These materials are also used in order to strengthen the existing bridges.

Stones for Bridge Construction

For a long time in the history, the stone has been used in and as a single form. They are mainly used in the form of arches. This is because they possess higher compressive strength.

The use of stones gave the engineers ease of constructing bridges that are aesthetically top and high in durability.

When considering the history of bridge construction with stones, the Romans were the greatest builders of bridges with stones. They had a clear idea and understanding of the load over bridge, the geometry as well as the material properties. This made them construct very larger span bridges when compared with any other bridge construction during that period.

The period was also competitive for Chinese. China had also developed large bridge called the famous Zhuzhou Bridge. The Zhuzhou bridge is the world’s known oldest open-spandrel, stone and segmental arch bridge. Nihonbashi is the most famous stone bridge in Japan. This is called as the Japan Bridge.

Fig.1: The Zhuzhou bridge, China

With time, the stone bridges have proved most efficient and economical due to the durability and low maintenance guaranty it provides throughout its life period.

Timber or Wood for Bridge Construction

The wood material was used highly in the construction of bridges, unlike today, where it is used for the construction of building works and related. Nowadays, steel and concrete grant a higher range of work flexibility, that the use of wood and timber for mega works diminished.

But, there are innovations related to the preservation of wood, which has helped to increase the demand of wood in structures.

Wood as an engineering material has the advantage of high toughness and renewable in nature. They are obtained directly from nature and hence are environmentally friendly.

The low density of wood makes it gain high specific strength. They have an appreciable strength value with a lower value of density. This property makes them be transported easily.

Some of the disadvantages related to wood as a construction material are that it is:

• Highly Anisotropic in Nature
• Susceptible to termites, infestations, and woodworm
• Highly combustible
• Susceptible to rot and disease
• Cannot be used for High temperature

There are a variety of timber bridges around the world. Figure-2 shows the Mathematical Bridge located in Cambridge. Another bridge is the Togetsu-Kyo Bridge over the Katsura River in Kyoto.

Fig.2: The Mathematical Bridge, Cambridge

Fig.3. The Togetsu-Kyo Bridge, Japan

Steel for Bridge Construction

 

Steel gain high strength when compared with any other material. This makes its suitable for the construction of bridges with longer span. We know that steel is a combination of alloys of iron and other elements, mainly carbon.

Based on the amount and variation of the elements, the properties of the same is altered accordingly. The properties of tensile strength, ductility and hardness are influenced by the change in its constitution.

The steel used for normal construction have several hundred Mega Pascal strength. This strength is almost 10 times greater than the compressive and the tensile strength obtained from a normal concrete mix.

The major inbuilt property of steel is the ductility property. This is the deformation capability before the final breakage tends to happen. This property of steel is an important criterion in the design of structures.

Fig.4. The Hachimanbashi Bridge

The first iron bridge, Danjobashi Bridge which was built in 1878 in Japan. The figure-4 below shows the Danjobashi Bridge. Danjobashi Bridge was relocated to the present location and was named as Hachimanbashi Bridge in 1929.

It has great historical and technical value as a modern bridge. The bridge was honored by the American Society of Civil Engineers in the year 1989.

The chemical composition and the method of manufacture determines the properties of structural steel. The main properties that are to be specified by the bridge designers when it is required to specify the products are:

• Strength
• Toughness
• Ductility
• Durability
• Weldability

When we mention the steel strength, it implies both the yield and the tensile strength. As the structures are more designed in the elastic stage, it is very essential to know the value of yield strength.

Yield strength is mostly used as it is more specified in the design codes. In Japan, the code recommended is designed for ultimate strength. For example, SS400 designated by the ultimate strength of 400MPa. This is an exception.

The property of ductility is very much relied on by designers and engineers for the design aspects related to the bolt group designs and the distribution of stress at the ultimate limit state conditions. Another important property is the corrosion resistance by the use of weathering steel.

Concrete for Bridge Construction

Most of the modern bridge construction make use of concrete as the primary material. The concrete is good in compression and weak in tensile strength. The reinforced concrete structures are the remedy put forward for this problem.

The concrete tends to have a constant value of modulus of elasticity at lower stress levels. But this value decreases at a higher stress condition. This will welcome the formation of cracks and later their propagation.

Other factors to which concrete is susceptible are the thermal expansion and shrinkage effects. Creep is formed in concrete due to long time stress on it.

The mechanical properties of concrete are determined by the compressive strength of concrete.

The reinforced or the prestressed concrete is used for the construction of bridges. The reinforcement in R.C.C provides the ductility property to the structure. Nowadays, ductility reinforcement is provided as an additional requirement mainly in the earthquake resistant construction.

RCC is nowadays made from steel, polymer or other combination of composite materials. Much sustainable materials is available that can take the role of cement. This is a new innovation in sustainable bridge construction.

When compared with RCC bridge construction, prestressed concrete is the most preferred and employed. A pre-compressive force is induced in the concrete with the help of high strength steel tendons before the actual service load.

Hence this compressive stress will resist the tensile stress that is coming during the actual load conditions. The prestress is induced in concrete either by means of post tensioning or by means of pretensioning the steel reinforcement.

Many disadvantages of normal reinforced concrete like strength limitations, heavy structures, building difficulty is solved using prestressed concrete.

Also Read: Dutch inaugurate the 3D Printed Reinforced Concrete Bridge Designed by Technical University of Eindhoven 

Composite Materials in Bridge Construction

 

Composite materials are developed and used for both the construction of new bridges as well as for the rehabilitation purposes.

Fiber reinforced plastic is one such material which is a polymer matrix. This is reinforced with fibers which can be either glass or carbon. These materials are light in weight, durable, high strength giving and ductile in nature.

New solution and materials are encouraged due to the problems of deterioration the steel and concrete bridges are facing.

Another material is the reactive powder concrete (RPC ) that was developed in Korea. This material is a form of high performance concrete that is reinforced with steel fibers. This mix will help to make slender columns for bridges of a longer span. This also guarantees durability extensively.

Composite materials are used in the repair of bridge columns and any other supporting elements to improve the ductility and the resistance against the seismic force.

Epoxy impregnated fiberglass are used to cover the column (columns that are non-ductile in nature). This is an alternative for the steel jacket technique.

Corrosion is the slow but continuous eating away of metallic components by chemical or electrochemical attack. That this is costly and destructive will be vouched for by any motorist.

Three factors govern corrosion.

• The metal from which the component is made.

• The protective treatment the component surface receives.

• The environment in which the component is kept.

Corroded structural elements of Nandu River Iron Bridge, Nandu River, Hainan, China

All metals corrode to a greater or lesser degree; even precious metals like gold and silver tarnish in time, and this is a form of corrosion. Prevention processes are unable to prevent the inevitable failure of the component by corrosion; they only slow down the process to a point where the component will have worn out or been discarded for other reasons before failing due to corrosion. Let's now look at the three ways in which metals corrode.

This occurs in two ways:

a) The oxidation of metals in the presence of air and moisture, as in the rusting of ferrous metals.

b) The corrosion of metals by reaction with the dilute acids in rain due to the burning of fossil fuels (acid rain) - for example, the formation of the carbonate 'patina' on copper. This is the characteristic green film seen on the copper clad roofs of some public buildings.

This occurs when two dissimilar metals, such as iron and tin or iron and zinc, are in intimate contact. They form a simple electrical cell in which rain, polluted with dilute atmospheric acids, acts as an electrolyte as generated and circulates within the system. Corrosion occurs with (depending upon its position in the electrochemical series) being eaten away.

Other metals, in addition to iron and steel, corrode when exposed to the atmosphere. The green corrosion-product which covers a copper roof, or the white, powdery film formed on some unprotected aluminum alloys is clear evidence of this.

Fortunately the reactivity of a metal and the rate at which it corrode is not related. For example, although aluminum is chemically more reactive than iron, as soon as it is exposed to the atmosphere it forms an oxide film which seals the surface and prevents further corrosion from taking place. On the other hand, iron is less reactive and forms its to oxide film more slowly.

Unfortunately, the iron hydroxide film (rust) is porous and the process continues unabated until the metal is destroyed.

Once 'rusting' commences the action is self-generating - that is, it will continue even after the initial supply of moisture and air is removed. This is why all traces of rust must be removed or neutralized before painting, otherwise rusting will continue under the paint, causing it to blister and flake off.

It has already been stated that when two dissimilar metals come into intimate association in the presence of an electrolyte that a simple electrical cell is formed resulting in the eating away of one or other of the metals. Metals can be arranged in a special order called the electrochemical series. This series is listed in Table 1 and it should be noted that, in this context, hydrogen gas behaves like a metal.

Metal	Electrode
Sodium	-2.71 Corroded (anodic)
Magnesium	-2.40
Aluminium	-1.70
Zinc	-0.76
Chromium	-0.56
Iron	-0.44
Cadmium	-0.40
Nickel	-0.23
Tin	-0.14
Lead	-0.12
Hydrogen (reference potential)	0.00
Copper	+0.35
Silver	+0.80
Platinum	+1.20
Gold	+1.50 Protected ( cathodic)

If any two metals come into contact in the presence of a dilute acid, the more negative metal will corrode more rapidly and will be eaten away.

We have seen that failure of a component may take place due to corrosion arising from electrolytic action between two different phases in a microstructure, or between two different materials in a fabricated structure. Failure of a component may also occur as a result of the complementary effects of chemical corrosion and mechanical stress. The methods of stress application may vary and this will affect the extent of corrosion which occurs. Forms of corrosion in which stress plays a part can be classified as follows:

In a cold-worked metal the pile-up of dislocations at crystal boundaries and other points increases the energy in those regions so that they become anodic to the rest of the structure. Consequently, corrosion takes place in these regions of high energy and the locked-up stresses give rise to the formation of cracks which grow progressively with the continuance of corrosion.

A similar process may take place in components in which unequal heating or cooling has given rise to the presence of locked-up stresses, as, for example, near to welded joints.

As might be expected, any component which is subjected to alternating stresses and is working in conditions which promote corrosion may fail at a stress well below the normal fatigue limit (3.72). The action of the corrosive medium will tend to be concentrated at any surface flaw and behave as a focal point for the initiation of a fatigue crack. Once a crack has been formed it will spread more rapidly as a result of the corrosive action combined with alternating stress.

It is allied to corrosion fatigue and occurs particularly where closely fitting machine parts are subjected to vibrational stresses. In steel this form of corrosion appears as patches of finely divided ferric oxide (Fe₂0₃).

It refers to the combined effects of mechanical abrasion and chemical corrosion on a metallic surface. Mechanical wear can be caused by the impingement of entrained air bubbles or abrasive particles suspended in the liquid.

The impingement of such media may lead to the perforation of any protective film existing on the surface. This film may be an oxide, which is cathodic to the exposed metal beneath. This type of corrosion is encountered in pump mechanism turbine and tuber carrying sea-water.

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☞Cathodic Protection of Reinforcement in Reinforced Concrete Structures

Portland cement consists mainly of compounds of calcium silicate and calcium aluminate, the calcium silicates are predominate being between 55% and 85%. There is also tricalcium aluminate, 7% to 12% and ferrites 6% to 10%.

It is made by burning at high temperature a mixture of chalk and clay in a rotary kiln. The clinker is ground, and gypsum is added to control the set. BS 12 limits the amount of sulphur (expressed as SO3) to 3.5%. The fact that Portland cement contains sulphate is important when investigating the possibility of sulphate attack on the concrete or mortar.

The hydration of the cement (the addition of water), results in a complex chemical reaction accompanied by the evolution of heat.

Revised British Standards for cement were published in 1991 and included BS 12: Portland cement and BS 4027: Sulphate-resisting Portland cement.

The new designations for Portland cements likely to be used for repair are as follows:

• Portland cement-class 42.5; to BS 12:1991 (CEM 1)

• Portland cement-class 52.5; to BS 12:1991

• Portland cement-class 42.5R; to BS 12:1991

• Sulphate-resisting Portland cement-class 42.5; to BS 4027:1991

• Masonry Cement: BS 5224:1995—ENV 413–1

The letter R denotes high early strength.

The revisions were mainly concerned with methods of test and terminology and were intended to agree with the European Standard for cement (ENV 197–1). Minor changes in composition were also introduced.

If a cement equivalent to ‘ordinary Portland is required, then this should be ordered as ‘Portland cement—class 42.5, to BS12:1991”. If a rapid hardening Portland cement is required, equivalent to ‘rapid hardening Portland cement’, then a Portland cement-class 52.5 to BS 12:1991, or

Portland cement-class 42.5R to BS 12:1991 should be ordered. The above listed cements are the ones used almost exclusively for repair work.

In 1990 and 1991, a completely revised edition of BS 5328:1991: Concrete, was issued in four Parts.

Blended cements consisting of mixtures of Portland cement and pulverized fuel ash (pfa) and Portland cement and ground granulated blast furnace slag (ggbs) are used in concrete for special purposes, but I have not come across their use in repair mortars and normal repair concrete.

The principal characteristics of Portland cement are as follows.

1. A very fine powder, particle size 1–50 microns.

2. The paste (cement and water) is highly alkaline, having a pH of about 13.5. This high alkalinity is relevant to the occurrence of alkali aggregate reaction.

3. The setting time (initial and final) is in the range of 45 minutes to 10

hours.

4. Both setting, and hardening (rate of gain of strength) are affected by temperature; an increase in temperature speeds up the chemical reaction between the cement and the mixing water.

5. Portland cement provides a comparatively high compressive strength to concrete and mortar. Tensile strength is only about 10% of the compressive strength.

6. The compounds which are responsible for the cementing action of the cement paste are mainly the calcium silicates (the C2S and the C3S).

7. It is the hydration products of the cement which, other things being equal, determine the strength of the concrete/mortar. The hydration products are very complex chemical compounds, the principal

compounds are calcium silicate gel, calcium hydroxide (about 20%) and tricalcium aluminate hydrate. Calcium hydroxide (Ca(OH)2) is liberated by the hydrolysis of the calcium silicates. The various hydration products hydrate at different rates, but the hydration is rapid to start with and then slows down.

8. The two major factors which influence the rate of gain of strength are its chemical composition and its fineness. With modern cements the increase in strength after the first 28 days is likely to be very small and should generally be ignored.

9. The amount of water in the mix (usually referred to as the water/ cement ratio) is a vital factor in determining the strength, permeability and absorption of the concrete/mortar. For higher strength and durability the w/c should not exceed 0.50, and for special purposes, 0.40–0.45; this is the free w/c which means the aggregates are saturated but surface dry.

The action of acids on Portland cement

Solutions of sulphates and their effect on Portland cement

Why Concrete Cracks?

Generally, it is assumed that cracks are due to some problems in the foundation, whereas it is not always correct and should not be considered failure of structure or improper design or bad quality work. Generally, 1/16 to 1/4-inch-wide cracks is acceptable limits.

The American Concrete Institute as per ACI 302.1-04 addresses this issue, even the best construction & concreting cannot prevent cracking in concrete, and 0% cracks is an unrealistic thing.

Causes of Cracks in Concrete

Causes of cracks in concrete can be many summarized as:

• Concrete expands and shrinks due to temperature differences
• Settlement of structure
• Due to heavy load applied or
• Due to loss of water from concrete surface shrinkage occurs
• Insufficient vibration at the time of laying the concrete
• Improper cover provided during concreting
• High water cement ratio to make the concrete workable
• Due to corrosion of reinforcement steel
• Many mixtures with rapid setting and strength gain performance have an increased shrinkage potential.

Types of Cracks in Concrete

The following figure shows types of cracks in concrete:

How to Prevent Cracks in Concrete Structures?

Preventive measures to avoid creation of cracks:

Preventive measures must be taken at the time of concreting and later to reduce cracks after concrete formation. Main factors are:

Reduce Water Content in Concrete:

A low water cement ratio will affect the quality of concrete. W/C ratio is weight of water to the weight of cement used. A lower w/c ratio leads to high strength in concrete and lesser cracks.

W/C ratio shall not exceed 0.5 in concreting, which reduces the workability of concrete which can be covered by use of plasticizer or superplasticizer. Less water content increases the durability of concrete.

Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink. Shrinkage is the main cause of cracks, when concrete hardens it evaporates the excess water and thus shrinks, so lesser the water content, lesser is the shrinkage.

Cracking shrinkage in slabs is ½ inch per 100 ft. The shrinkage of concrete pulls the slab apart showing it as cracks on surface.

Proper Concrete Mix Design and use of Quality Materials

The concrete itself must be properly proportioned, and properly mixed. If you use too little cement, you can almost guarantee cracks. Using too much water will make the concrete weak, leading to cracking.

Use good quality aggregates so will produce lower shrinkage concrete. Hard, dense aggregate, using a large top size aggregate and optimizing the gradation of the aggregate is able to reduce the shrinkage of the concrete.

If the aggregate is of poor quality, maximizing the size, gradation, and content may have little effect on the concrete shrinkage. Mixing large aggregate with poor qualities to a mid-size aggregate with good properties may increase the shrinkage of the concrete.

Avoid the use of shrinkage-promoting admixtures (such as accelerators, dirty aggregate which increases water demand and using a cement with high shrinkage characteristics.

Finishing of Concrete Surface

Use proper finishing techniques and proper timing during and between finishing operations. Flat floating and flat troweling are often recommended.

Avoid overworking the concrete, especially with vibrating screeds. Overworking causes aggregate to settle and bleed water and excess fines to rise.

Don’t finish the concrete when there is bleed water on the surface, finishing leads the water back to concrete instead of evaporating thus leading to cracks.

Proper Curing of Concrete

Stop rapid loss of water from surface or drying of concrete due to hydration (liquid concrete converts to plastic and then to solid state) causes drying of the slab, so it’s recommended to cure the slab for several days.

As soon as the concrete on slab sets its general practice to make boundary with mortar on the slab and keep it filled with water. Cover slab with cotton mats soaked with water or spray on a curing compound also prevents loss of water.

The concrete should not be subjected to load during the curing period, which can last up to one month.

Proper Placement and Vibration of Concrete

Properly placed, vibrated, finished concrete reduces the chances of producing cracks. Properly vibrate to release entrapped air which later leads to cracks.

Proper Compaction of Soil to Prevent Settlement Cracks in Concrete

The area below the concrete slab has to be compacted properly and in layers so as to ensure against settlement of soil later. If the soil is left loose it will settle over time and create cracks on surface. This applies in the home as well as constructions on highways.

Providing Control Joints in Concrete

Control joints should be located at regular intervals so as to adjust the shrinkage of concrete. Generally, for 4-inch depth of slab joints are provided 8 to 12 ft. apart. Control joints are pre-planted cracks. An engineer should have an idea that concrete will crack at control joints instead of cracking any other location.

Some Other Preventive Control Measures for Cracks in Concrete:

• Applying good acrylic silicone sealer yearly to concrete works
• Avoid calcium chloride admixtures
• Prevent extreme changes in temperature
• Consider using a shrinkage-reducing admixture
• Warm the subgrade before placing concrete on it during cold weather
• Consider using synthetic fibers to help control plastic shrinkage cracks.
• Repairing Methods of Cracks in Concrete

Various types of Concrete Crack Repair Methodologies:

• Stitching
• Muting and sealing
• Resin injection
• Dry packing
• Polymer impregnation
• Vacuum impregnation
• Autogenously healing
• Flexible sealing
• Drilling and plugging
• Bandaging

To summarize, always prevention is better than cure. Prevention of concrete cracks give good quality, saves time, money and peace of mind to the owner.

Following articles might also be of interest to you:

☞ How to Prevent Plastic Shrinkage Cracking of Concrete?
☞ Cracks in Concrete Floors
☞ How to Repair Cracks in Concrete Floors?
☞ Cracks in Concrete Construction

Cracking is a common problem in concrete construction. Homeowners see it in basement fl oors, garage fl oors, and basement walls. Cracks occur in sidewalks, dams, bridges, and retaining walls. Any crack is a reason for concern and warrants a thorough inspection and investigation.

Pattern cracking

Pattern cracks are common. These cracks tend to be short and uniformly distributed throughout a concrete surface. Pattern cracking can have two causes: It can indicate restraint of contraction on the surface layer by the backing or inner concrete, or it can be due to an increase in the volume in the interior concrete.

You may hear pattern cracking referred to as map cracks, crazing, checking, or D-cracking. D-cracking is often found in the lower part of a concrete slab, usually near a joint in the concrete. If you find moisture accumulation, you could find D-cracking.

Isolation cracks

Isolation cracks appear as individual cracks. This type of cracking indicates tension on the concrete. The tension is usually perpendicular to the cracks. An individual crack can run in a diagonal, longitudinal, transverse, vertical, or horizontal direction.

Crack depth

Crack depth is categorized into four terms: surface, shallow, deep, and through.

Crack width

Crack width ranges from fine to medium to wide. Fine cracks are typically less than 0.04 inch wide. A medium crack would be between 0.04 to 0.08 inch. Wide cracks exceed 0.08 inch.

Crack activity

Crack activity has to do with the presence of a particular factor causing a crack. Determining crack activity is necessary to determine a mode or repair. If the cause of a crack is causing more cracking, then the crack is active. Any crack that is currently moving is considered active. If a specific cause for a crack cannot be determined, the crack must be considered active.

Dormant cracks do not have current movement. Some cracks are considered dormant when any movement of the crack is minimal enough to not interfere with a repair plan.

Code enforcement officers have the authority to require the results of strength tests of cylinders cured under field conditions.

When preparing to pour concrete, installers must do the following:

● Clean all equipment.

● Make sure no debris is in the concrete.

● Make sure no ice is in the concrete.

● Clean the forms.

● Make sure any filler units that are in contact with the concrete are well drenched.

● Make sure there are no deleterious coatings or ice on the reinforcing materials.

● Make sure there is no water in the path of the concrete installation without the consideration and approval of a code officer.

● Make sure no unsound material is present.

When concrete is mixed, it must be mixed to a uniform distribution of materials.

ALL CRACK OCCURRENCE

Cracks can occur before or after concrete cures. The cracking can be structural or nonstructural. It may be hard to determine whether a crack is structural or nonstructural by visual inspection only. A full analysis by a structural engineer is normally required to make a full determination of the type of cracking that is encountered.

Structural cracks tend to be wide. Their openings can increase as a result of continuous loading and creep of the concrete. As a rule of thumb, any crack that could be structural in nature should be treated as a structural defect and receive a full evaluation from appropriate experts.

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☞ How to Prevent Plastic Shrinkage Cracking of Concrete?
☞ Cracks in Concrete Floors
☞ How to Repair Cracks in Concrete Floors?

1. CHANGES OVER TIME

Over the years, the changes in materials and reductions in safety factors make it more important to understand the behavior of reinforced concrete and provide more care. Rules of thumb and empirical methods may have been developed for different conditions and may not be applicable for today’s materials’ properties and design criteria need to be checked to determine whether they are still applicable. Another result of the development of and changes to material properties is that the ultimate limit state is often no longer critical, and a design now often depends on the serviceability limit states, apart from punching shear.

Concrete: There has been a continuous increase in the strength of concrete over the last hundred years; much of the increase has developed since 1980 (see Figure 1). Around that time, the value of blended cements and the use of admixtures was realized. Modern concretes have become complex with almost infinite variations available depending on the requirements.

The understanding of how to change the properties of concrete and reinforcement is developing rapidly. It includes:

• The use of admixtures and blended cements. Admixtures are essential for modern concrete. Self-compacting concrete is one important example. Blended cements allow the control of the rate of strength gain and the amount of heat created.
• The use of stainless steel will increase for situations where durability is paramount.
• The use of higher strength concrete will become more popular for floor slabs, particularly flat slabs. This will result in thinner and longer span slabs.
• Serviceability limit states have already become critical to flat slab design and it will become more common to check vibration of floors.
• The use of fibres will increase; the use of steel fibres has already been proven for ground floor slabs.

All these developments add complexity and cost to concrete construction. Above a cylinder strength of 50 MPa, the stress–strain properties change with increases in strength. Figure 2 shows this change diagrammatically. The concrete itself becomes more brittle as the strength increases, but it should be noted that in flexural members (beams and slabs), the ductility and brittleness are dependent mostly on the properties of the reinforcement. The increase in concrete strength and reduction in overall factor of safety (see Figure 3) have meant that, for many structural elements, the design for the serviceability limit state is becoming more critical than that for the ultimate limit state.

Reinforcement: A similar pattern of change has occurred for reinforcement both in strength and partial safety factors (see Figures 4 and 5).

Figure 5 Reduction in reinforcement overall safety factor during 20th century.

In 1964, the construction of a 35 m high dust bunker for a coal-fired power station included an external concrete cantilever staircase to be built on t the face of the outside wall of the bunker. The construction of the wall meant that the reinforcement required for the stairway would be cast flush with the wall and then bent out after removal of the formwork. At that time, proprietary reinforcement systems for such a situation did not exist and the bars were bent before fixing within the shutter. The radius of bend would have been to a standard of three times the bar diameter. 

After the formwork had been removed, the surface of the concrete was scabbled to expose these bars and the scaffold tubes were threaded over them. The scaffold tubes were then used to lever the bars out of the wall into their final positions. About 30% of all the bars bent out snapped off during the operation. The reason was a combination of factors:

• The bars should have been bent out with a special tool that ensured that the radius of bend was at least three times the bar diameter.
• The particular batch of reinforcement was found to be more brittle (less ductile) than specified.
• The work was carried out at a temperature just above freezing.

The remedial action taken was to drill holes into the concrete and grout in replacement bars.

Comment: The bending out of reinforcement cast into walls is a common procedure and, all too often is done with scaffold tubes that are readily accessible on site. It is regrettable that a proper re-bending tool is not often used which is a reflection of poor understanding of the material’s physical and chemical properties. In the manufacture of reinforcement, special procedures are in place to check the re-bending of bars to ensure that the reinforcement is sufficiently ductile. It is unfortunate that some manufacturers have continually tried to eliminate such tests from the reinforcement standard.

3. TACK WELDING OF REINFORCEMENT

The design of a building with large columns required 32 mm diameter starter bars projecting from the pile caps. The temporary works for construction included tack welding some small diameter bars to the starter bars. When the time came to fix the column reinforcement to the starter bars, the contractor attempted to bend the starter bars to ensure that they would fit into the column shutter with sufficient cover to the concrete face. A large sledge hammer was used to effect this. During this operation, two of the 32 mm diameter bars snapped off.

The reason was that tack welding the small bars onto the larger diameter starter bars changed the molecular structure of the latter. Unlike structural welding, tack welding heats just the local spot, and the heat sink of the main bar cools it very rapidly. The result was that the starter bars became brittle and required only a sharp blow to fail. In the past, tack welding on site was forbidden. Today it is sometimes permitted if carried out by a skilled specialist. Unfortunately once permitted, it is all too easy for a nonspecialist to do this work, believing that it will do no harm.

Comment: Too many people are unaware that tack welding can have significant structural effects. This is another case where a material’s chemical and physical behavior was not properly understood.

4. HIGH ALUMINA CEMENT

High alumina cement concrete has achieved a certain notoriety following the collapse of several buildings in the 1970s. By the end of 1974, up to 50,000 buildings had been reported as suspect and a major effort was made to check their safety. Fortunately, many of the affected beams stood in dry conditions and the chemical deterioration had not reached an advanced stage. The worst affected elements were positioned in damp environments.

Description: High alumina cement is manufactured from limestone or chalk and bauxite (the ore from which aluminium is obtained). The two materials are crushed and fired together using pulverised coal as a fuel. The materials fuse together, and after cooling are crushed and ground into a dark grey powder.

The predominant compounds are calcium aluminates; calcium silicates account for no more than a few percent. The calcium aluminates react with water and the primary product is calcium aluminate decahydrate (CAH₁₀).

One of its main characteristics is that the concrete made with it achieves its full strength after 24 hours compared with 28 days for a concrete with Portland cement. However, its crystal structure is unstable and changes to tricalcium aluminate hexahydrate (C₃AH₆) spontaneously (albeit slowly). This process occurs at room temperature and is accelerated by an increase in temperature. The crystal structure transforms itself to a more compact form, with the result that the cement matrix of the concrete becomes porous and weaker. The extent to which this conversion, as it is known, occurs is largely a function of the:

Degree of conversion: It was found that at the time of the collapses most HAC concrete used in buildings was 90% or more converted. A concrete from a wet mix exposed subsequently to the sun was found to have its strength reduced from 40 MPa at 24 hours to an average of about 10 MPa after less than 10 years. In contrast, concrete from prestressed precast beams with a low water-to-cement ratio and hence a 24 hour strength of 65 MPa from the same building but in a dry environment was found to have retained a strength of about 35 MPa.

5. CALCIUM CHLORIDE

Many reinforced concrete structures have suffered from too much chloride in the concrete mix. This causes the breakdown of the high level of alkalinity. When moisture and oxygen are present, carbonation occurs. This allows the reinforcement to rust and leads to spalling of the concrete surface.

Before 1980, calcium chloride was used extensively for in situ concrete works, frequently without adequate supervision. It was used principally for frost protection and to facilitate the rapid stripping of shutters. However, all too often, too much was added. In the 1980s, the codes of practice and concrete specifications were tightened to ensure that the rusting and spalling should not happen again. The following three examples describe where too much chloride in concrete caused structural failures.

Example 1: A primary school (built in 1952) was shut in 1973 due to extensive corrosion of the reinforcement of factory-made precast concrete beams. This was due to the presence of too much calcium chloride added during the manufacture of the beams to hasten the hardening of the cement. The condensation under the beams accelerated the corrosion by combining with the calcium chloride to produce hydrochloric acid.

Example 2: In 1974, the concrete roof of a school collapsed. The reason was found to be too much calcium chloride in the concrete, causing the reinforcement to deteriorate and eventually fail.

Example 3: An independent investigation of the collapse of a 100 m long pedestrian bridge found the cause to be high levels of calcium chloride in the grout used in the ducts for the prestressed tendons. This led to corrosion and failure of the prestressing tendons.

6. ALKALI–SILICA REACTION

The alkali–silica reaction (ASR) is a heterogeneous chemical reaction that takes place in aggregate particles between the alkaline pore solution of cement paste and silica in the aggregate particles. Hydroxyl ions penetrate the surface regions of the aggregate and break the silicon–oxygen bonds. Positive sodium, potassium, and calcium ions in the pore liquid follow the hydroxyl ions so that electro-neutrality is maintained. Water is imbibed into the reaction sites and eventually an alkali–calcium–silica gel is formed.

Figure 6 Examples of alkali–silica reaction. (Top: From the US Department of Transportation Highway Administration; middle: From Dr. Ideker, http://web.engr. oregonstate.edu/~idekerj/; bottom: From the US Department of Transportation Highway Administration.)

The reaction products occupy more space than the original silica so the surface reaction sites are put under pressure. The surface pressure is balanced by tensile stresses in the centres of the aggregate particles and in the ambient cement paste. At a certain point, the tensile stresses may exceed the tensile strength and brittle cracks propagate. The cracks radiate from the interior of the aggregate out into the surrounding paste.

The cracks are empty (not gel-filled) when formed. Small or large amounts of gel may subsequently exude into the cracks. Small particles may undergo complete reaction without cracking. Formation of the alkali–silica gel does not cause expansion of the aggregate. Observation of gel in concrete is therefore no indication that the aggregate or concrete will crack. ASR is diagnosed primarily by four main features

• Presence of alkali–silica reactive aggregates
• Crack pattern (often appearing as three-pointed star cracks)
• Presence of alkali–silica gel in cracks and/or voids
• Ca(OH)2 depleted paste

In mainly unidirectional reinforced members, the cracks become linear and parallel to the reinforcement. The degree of cracking depends on the amount of confining reinforcement, i.e., links, etc. One major concern was that ASR caused cracking that led bits of concrete to fall off structural elements and hit people below. This led to demolition of the structures in some cases. Examples of ASR effects are given in Figure 6.

7. LIGHTWEIGHT AGGREGATE CONCRETE

During the 1960s, a medium-size civil engineering contractor wanted to join the housing drive, then at its peak. At the time, an Austrian construction firm used crushed brick rubble as aggregate in un-reinforced concrete walls for six- and seven-storey blocks of flats. Inspired by this, it was decided to try to develop a similar form of load-bearing wall with adequate thermal insulation, made of lean-mix plain concrete with light expanded clay aggregate (LECA). A 12-storey block was constructed as a pilot project.

The strength of the wall concrete was reduced in stages—about 2000 psi (14 MPa) at 28 days for the four bottom storeys, 1600 psi (15 MPa) for the next four, and 1200 psi (8 MPa) for the top storeys. The floor slabs were of traditional reinforced concrete, but the roof slab was reinforced LECA concrete with a strength of 3000 psi (21 MPa).

There was no significant adverse feedback from the tenants nor the building authority. The block remained standing and in use for over 40 years. Encouraged by the apparent success, the contractor started promoting the ‘system.’ About the same time, lightweight aggregate concrete was included in the code of practice and a minimum strength of 3000 psi (21 MPa) was

stipulated. This required a richer mix than that used for the walls of the earlier block. The resulting effects of this on the thermal insulation and shrinkage properties of the LECA concrete appear to have been overlooked by the design team.

A few blocks were built for local authorities outside the London County Council area. These were higher than the first block utilising the higher concrete strength required by the code in the walls. Many of the flats were allocated to tenants in poor financial circumstances, who could not afford the charges for the underfloor heating and used paraffin heaters instead.

This, combined with the reduced thermal insulation of the external walls, led to severe condensation. Structurally more important, however, were the diagonal cracks that developed on the top floor of one of the blocks within a short time after hand-over. From their geometry, they appeared to be due to lower shrinkage and greater thermal expansion of the roof slab relative to the wall concrete.

Definitely alarming was the occurrence of horizontal cracks in one of the 200 mm thick internal cross walls connected to the 300 mm thick external wall at right angles. One of these cracks on the 13th floor of a 16-storey block opened suddenly with a noise like a gun shot. The wall was 200mm thick and, according to the design assumptions, carried the floor slabs that spanned about 3.5 m on either side. This meant that the building contained three storeys of unreinforced concrete cross wall, with the load from approximately 3.5 m width of floors plus the roof hanging or cantilevering off the external wall!

Structurally, the only explanation for these cracks seemed to be that the internal wall was drying out, and therefore shrinking and shortening, while the external wall with very little load to carry (at least initially) and exposed to the British weather was not shortening at the same rate.

Discussion: The porous LECA pellets were soaked just before the mixing of the concrete, to prevent them from absorbing water from the fresh mix and thus making it too stiff. They therefore constituted a reservoir of water, over and above that required for the hydration of the cement. This extra water meant that the LECA concrete needed more time to dry out and the 300 mm external walls would have a slower rate of drying out than the 250 mm internal walls even if they had the same environment on both faces.

Comment: These cracks were due to the changed properties of the wall concrete. A proper study of the properties of the materials along with a review of the design would have shown that the two-stage extrapolation from medium rise to high rise and from lean mix, brick rubble, unreinforced concrete to dense, albeit lightweight aggregate, concrete could not be sustained.