The cracking of concrete may be divided into the following categories: 1. Cracking of Plastic Concrete 2. Cracking of Hardened Concrete 3. Cracking Due to Chemical Reactions 4. Cracking Due to Weathering 5. Cracking due to Corrosion of Reinforcement 6. Cracking due to Faulty Construction Practices 8. Cracking Due to Error in Design and Detailing.
Type # 1. Cracking of Plastic Concrete:
The cracking of plastic concrete takes place when the exposed surfaces of freshly placed concrete are subjected to a very rapid loss of moisture due to low humidity, high temperature or wind. Due to this rapid loss of moisture, the surface of the concrete shrinks. Tensile stresses develop in the weak, stiffening plastic concrete due to the restraint developed by the surface below the drying layer of concrete.
These tensile stresses cause shallow, short and discontinuous cracks running in all directions. Sometimes these cracks extend upto the free edge. In un-reinforced concrete slabs they are seen to develop diagonally. The presence of reinforcement may change the direction of cracks.
The plastic shrinkage generally takes place before the final finishing prior the curing starts. These cracks at the surface often are wide enough. On large surface areas and elements and on horizontal surfaces as slabs or floor they extend from few centimeters to many metres. These plastic shrinkage cracks may extend the full depth of elevated thin structural elements.
Control of Plastic Shrinkage Cracks:
The plastic shrinkage cracks can be controlled by reducing the relative volume charge between the fresh surface and the surface below it by preventing the rapid loss of moisture due to dry winds and hot weather. The rapid moisture loss can be controlled by saturating the air above the concrete surface by fog nozzles and covering the concrete surface by plastic sheets at the time of placing and finishing the concrete.
The surface temperature may also be reduced by using sun shades and wind velocity can be reduced by providing wind breakers. Further the concrete work may be done in the night or in the afternoon. Further flat work may be scheduled after the completion of vertical members or walls. After the formation of cracks the best remedial measure is to seal them to prevent the entry of water into them. The sealing may be done by brushing with cement paste or low viscous polymers.
These cracks develop due to the settlement of concrete after placement, vibration and finishing. The green concrete has a tendency to settle especially in deep sections after it has started stiffening. If during this interval the fresh concrete is restrained by the previously laid concrete or reinforcing steel or form work, tie bolts, these restraints may cause cracks or voids near the restraining element.
Factors Affecting the Settlement Cracks:
The crack developed due to the presence of reinforcing steel is affected by the following factors.
The settlement cracking increases:
i. With increase in the size of reinforcement steel bar
ii. Increase in slump, i.e. the greater the slump, higher the cracking
iii. By the decrease in cover thickness, lesser the cover thickness, greater the cracks
iv. Insufficient compaction increases the settlement cracking many folds
v. Highly flexible form or leading base also increases settlement cracking.
The settlement cracking may be checked by adopting the following measures:
i. The concrete should be poured in small thickness and each layer is properly compacted before laying the next layer.
ii. The use of minimum w/c ratio should be adopted i.e., the slump should be as low as practically possible.
iii. The thickness of concrete cover over reinforcement should be adequate.
iv. There should be adequate time interval between the placement and final finishing of concrete.
Type # 2. Cracking of Hardened Concrete:
The volume changes due to moisture movement are the inherent characteristic of the concrete. Upto 1 % shrinkage of volume of cement paste has been observed due to loss of moisture from it. However the internal restraint provided by the aggregate reduces the magnitude of this volume change to about 0.05%. On the other hand an increase in moisture content tends to increase its volume. If these volume changes are restrained by the sub grade or by another part of the structure then the tensile stresses are developed.
When the tensile stresses exceed the concrete tensile strength, then concrete cracks, the cracks may propagate at much lower stresses than are required to cause crack initiation. On walls and slabs the surface cracking occurs due to drying shrinkage when there is higher water content in concrete of surface layers than the interior concrete. The surface cracking appears in the form of a series of shallow closely spaced fine cracks.
In case of massive concrete elements, the tensile stresses are developed due to the differential shrinkage between the surface and interior of the concrete. Due to the quick evaporation of moisture from the outer surface of the concrete, the surface shrinkage is more and causes the cracks to develop. In due course of time these cracks penetrate deeper into the interior concrete.
Factors Affecting the Extent of Cracking:
The extent of shrinkage cracking depends upon the following factors:
i. Degree of restraint
ii. Modulus of elasticity of concrete
iii. Amount of shrinkage, and
iv. Amount of creep.
The extent of drying shrinkage is mainly influenced by the amount and type of coarse aggregate and the amount of water content in the concrete. The shrinkage is found to decrease with the increase in the amount of aggregate and reduction in the water content. Further higher the stiffness of the aggregate, lower the shrinkage of the concrete.
Thus the concrete made with granite or basalt aggregate showed 50% shrinkage than that of the concrete made with sand stone aggregate. Thus the drying shrinkage can be reduced by using the maximum practical amount of aggregate and lowest amount of usable water in the concrete mix.
Prevention of Shrinkage Cracking:
The shrinkage cracking can be controlled by the following methods:
i. By providing properly spaced construction joints
ii. By providing proper steel detailing, and
ii. By the use of shrinkage compensating cement.
Type # 3. Cracking Due to Chemical Reactions:
Cement is the most important constituent of the concrete. Cement usually is alkaline. When water is added to the mix, it turns out to be a strongly caustic solution due to solubility of alkalis from cement. This caustic liquid attacks reactive silica to form alkali-silica gel of unlimited swelling type. This silica gel grows in size. The continuous growth of silica gel exerts excessive pressure and causes cracking.
In case of hardened concrete the alkalis of cement react with acidic compounds in the presence of moisture. Due to these reactions the concrete matrix becomes weak and its constituents leach out. In the case of hardened concrete the silica contained in aggregates and alkalis from cement react to form a swelling silica gel and draws water from other portions of the concrete. This causes local expansion accompanied by tensile stresses. If these tensile stresses are large, they deteriorate the whole structure.
Following measures may be taken to control the silica-alkali reaction:
i. Use of low alkali-cement.
ii. Proper selection of aggregate having low quantity of silica.
iii. Use of pozzolana.
Symptoms of Silica-Alkali Reaction:
Generally following symptoms have been observed:
i. Development of map cracking of concrete.
ii. Leaching out gel from cracks.
This reaction takes place between certain lime stone aggregate and cement as lime stone contains more silica. The reaction forms silica alkali gel. The layers of this gel occur between the aggregate particles and the surrounding cement paste.
The problem may be minimised by adopting following precautions:
i. Avoiding reactive aggregate,
ii. Use of low alkali cement, and
iii. Use of smaller size aggregate.
When sulphate bearing water comes in contact with the concrete, the sulphate penetrates into the hydrated paste of cement and reacts with the hydrated calcium aluminate of cement and forms calcium sulphominate, which subsequently increases to a very large volume. The increase in volume causes high local tensile stresses resulting in the deterioration of the concrete. The use of pozzolana cement resists the sulphate attack better.
Effect of Carbon Dioxide:
When carbon dioxide from the air reacts with the calcium hydroxide (CaOH2) of the hydrated cement paste, it forms calcium carbonate, which occupies smaller volume than the calcium hydroxide, resulting in the so called carbonation shrinkage. The development of carbonation shrinkage may result in significant surface grazing. This situation is serious on freshly placed concrete surfaces kept warm during winter season by improperly vented combustion heaters.
Type # 4. Cracking Due to Weathering:
Following environmental factors cause cracking of concrete:
i. Freezing and thawing.
ii. Wetting and drying, Heating and cooling.
i. Freezing and Thawing:
The greatest damage to concrete is caused by freezing and thawing in all parts of the world except tropical regions. It is the most common weather related physical deterioration.
In the hardened concrete there are various explanations for the frost damage as follows:
(a) The empty space available in the concrete is insufficient to accommodate the additional solid produced due to the freezing of free water held in concrete. The ice formed due to freezing of free water exerts pressure resulting in cracking of concrete. The damage is related to the degree of saturation.
(b) During depressing point of freezing. Capillary water from the concrete comes out on the surface forming ice lenses parallel to the surface of the concrete. This ice lense forming exerts pressure resulting in cracking of concrete.
(c) When the moisture present with in the capillaries of the concrete freezes develops water pressure there, resulting in cracking of the concrete. Similarly if the aggregates used are saturated above the critical degree of saturation, the expansion of absorbed water during freezing may cause the cracking of the concrete.
Following remedial measures may be taken against freezing damage:
1. Use of lowest practical water-cement ratio and total water content.
2. Adequate air entrainment has been found effective to control the freezing damage.
3. Use of durable aggregate also is useful to check the freezing effect.
4. Adequate curing of concrete prior to exposure to freezing conditions is also important.
ii. Wetting and Drying, Heating and Cooling:
Effect of wetting and drying, heating and cooling are given below:
When volume changes due to these effects are excessive, cracks may develop resulting in the disintegration of concrete. The fire and frost-actions also damage the concrete. The damage due to fire and frost appears in the form of general spalling and flacking of concrete from the surface. With the increase in temperature above about 300°C concrete gradually loses strength. If the aggregates used in the concrete have high coefficient of thermal expansion, the damage will be greater.
Type # 5. Cracking due to Corrosion of Reinforcement:
It is the most common and frequent cause for the deterioration of reinforced concrete structures. However the salient features of this aspect have been given here for ready reference.
When reinforcing steel comes in contact with moisture, it reacts with the oxygen of the moisture forming iron oxide and hydroxide. The volume of these oxides is much greater than the volume of the original metallic iron. This increase in volume causes high radial bursting stresses around the reinforcing bars, resulting in local radial cracks.
These splitting cracks may propagate along the bars resulting in the formation of longitudinal cracks parallel to bars or spalling of concrete. These cracks provide easy passage to moisture, air or oxygen, fumes containing chlorides etc. helping to corrosion to continue and causing further cracking of concrete.
Reinforcing steel usually does not corrode in the concrete due to the formation of a tightly adhering protective oxide coating in a highly alkaline environment. This phenomenon is called passive protection.
If this passivity of steel is destroyed either by reducing the alkalinity of the concrete by the reaction of carbon dioxide with alkalis of cement or by the reaction of aggressive chloride ions reaction, the reinforcing bars will corrode. In case the concrete has low permeability, then transverse cracks usually do not contribute to the continuation of corrosion of reinforcement. If the combined density and concrete cover thickness is sufficient to restrict the flow of moisture or air, corrosion either will cease or will slow down.
Reinforcement corrosion can be checked by adopting following measures:
i. To keep the permeability of concrete as low as possible, is the best control measure against corrosion induced splitting.
ii. To keep the protective cover to reinforcement of sufficient thickness.
iii. In very severe exposure conditions, protected coated reinforcement or sealers or over lays on concrete and corrosion retarding admixtures may be adopted.
Type # 6. Cracking due to Faulty Construction Practices:
In concrete structures cracking also results due to faulty practices adopted during the construction period, such as:
i. Adding more water than specified to improve workability. It will not only reduce the strength, but will also increase the ultimate drying shrinkage and settlement of the concrete.
ii. Early termination of curing. It will result in the inducement of increased shrinkage in concrete when it has developed low strength. Incomplete hydration of cement due to inadequate supply of moisture or drying will not only reduce the long term strength, but also the durability of the structure.
iii. In adequate compaction or lack of support of form. Inadequate compaction will reduce the strength. It has been observed that 1% deficiency in compaction will result in 5% decrease in strength. Further these factors will result in settlement cracks before it has developed sufficient strength to support its own weight.
Type # 7. Cracking Due to Over Loads During Construction:
The loads induced in the structure during construction may be far more severe than those experienced in service. These conditions may occur at the early ages, when the concrete is most susceptible to damage and often results in permanent cracks.
A common error is observed when the precast members are not properly supported during transportation and erection. The use of arbitrary or convenient lifting points may cause severe damage. If a heavy or big element lowered too fast, and then stopped suddenly has significant momentum, which is translated into an impact load while stopped. This impact load may be many times more than the dead weight of the element.
Placing of equipment and storage of materials during construction may result in more severe loading condition than any load for which the structure has been designed. The damage from such construction over loads can be prevented if the designer provides information regarding the damages of such loads.
Type # 8. Cracking Due to Error in Design and Detailing:
The design and detailing errors that may cause unacceptable cracking are as follows:
i. Improper selection and/or detailing of reinforcement.
ii. Use of poorly detailed re-entrant comers in walls, precast members and slabs.
iii. Restraint of members subjected to volume changes due to variations in temperature and moisture.
iv. Lack of adequate contraction joints.
v. Improper design of foundations results in differential settlement within the structure.
vi. Re-entrant corners. These corners provide a location for stress concentration and thus are main locations for initial cracks, as in the case of window and door openings in concrete walls and beams.
To keep the inevitable cracks narrow and to prevent them from further propagating, provision of additional properly anchored diagonal reinforcement is necessary. An inadequate amount of reinforcement may result in excessive cracking. A common mistake observed is to lightly reinforce a nonstructural member or element and tying it with the rest of the structure in such a way that it is required to carry a major portion of the load once the structure begins to deform. The nonstructural element will carry a load in proportion to its stiffness.
As this element is not detailed to act structurally, unsightly cracking may result even though the safety of the structure is not threatened. The restrained members subjected to volume changes usually develop cracks. A wall, slab or a beam restrained against shortening even if pre-stressed can easily develop tensile stresses sufficient to cause cracking. Beams should be free to move.
The improper design of the foundation may result in excessive differential movement within a structure. If the differential movement is relatively small, the cracking problem may be only visual in nature. In case there is a major differential settlement, the structure may not be able to redistribute the load rapidly and a failure may occur. However in R.C.C. structures creep allows some redistribution of load.
The structures in which cracking may cause major problem of serviceability need special care in the design and detailing, these structures also need continuous inspection during all phases of construction.
Thermal cracking is associated with thermal shrinkage and volume changes in the concrete. Before discussing the thermal cracking, it is essential to know the thermal properties of concrete. The study of thermal properties of concrete is an important aspect while dealing the thermal cracking and the durability of concrete.
The important thermal properties of concrete are as follows:
1. Thermal conductivity
2. Thermal diffusivity
3. Specific heat, and
4. Coefficient of thermal expansion.
1. Thermal Conductivity:
This is the measure of the ability of a material to conduct heat through it. Thermal conductivity is measured in joules per second per square metre of area of the body, when the temperature difference is 1°C per metre thickness of the body.
The conductivity of concrete depends upon the following factors:
(a) Type of aggregate,
(b) Moisture content,
(c) Density, and
(d) Temperature of concrete.
When the concrete is saturated, the value of thermal conductivity ranges between about 1.4 to 3.4 J/m2.S°C/m. The wet density of concrete made with different aggregates varies from 2240 to 1590, the higher the density of concrete, higher the value of concrete thermal conductivity.
Typical values of thermal conductivity are shown in Table 26.2 below:
2. Thermal Diffusivity:
It represents the rate of change of temperature with in the concrete mass.
It is related with the thermal conductivity by the following relation:
The range of diffusivity of concrete is between 0.002 to 0.006 m2/h.
3. Specific Heat:
It is the quantity of heat required to raise the temperature of a unit mass of a material by one degree centigrade. The value of sp. heat for concrete varies in between 840 to 1170 J per kg per degree centigrade of temperature.
4. Coefficient of Thermal Expansion:
It is defined as the change in unit length per degree of temperature. In concrete it depends upon the mix proportion. The value of coefficient of thermal expansion of hydrated cement paste varies between 11 x 10-6 to 20 x 10-6 per °C and for aggregates it varies between 5 x 10-6 and 12 x 10-6 per °C. It has been observed that lime stones and Gabbros have low values of coefficient of thermal expansion while Gravel and Quartzite have high values. Thus the kind and content of aggregate influences the value of coefficient of thermal expansion of concrete. The average value of coefficient of thermal expansion for concrete may be assumed as 10 x 10-6 per °C.
Thermal Expansion and Shrinkage:
We have learnt that due to the development of heat of hydration the temperature of concrete increases. The thermal changes due to heat of hydration will be important only for few days in normal structures but for mass concrete work, it may last for long. The temperature difference with in a concrete structure results in differential volume change. When the tensile strain due to differential volume change exceeds the tensile strain capacity of the concrete, it develops cracks.
The temperature differentials associated with the heat of hydration affect the mass concrete such as dams, piers, large column etc., whereas temperature differentials due to changes in the ambient temperature can affect any structure as bridges decks, air field pavements, roads, etc. These structures are subjected to diurnal or seasonal changes of temperature. In certain parts and cities of India, the change of ambient temperature in day and night is as high as 30°C and the actual change in temperature of concrete surface is much higher than 30°C. The seasonal changes may be as high as 40°C.
The liberation of heat of hydration of cement makes the internal temperature of concrete to rise during the initial period of curing so that it is slightly warmer than its surroundings. As the concrete cools, it will try to contract. Any restraint on the free contraction during cooling will result in the development of tensile stresses which are proportional to the temperature change, coefficient of thermal expansion, degree of restraint, and modulus of elasticity. The more massive the structure, the greater is the potential for temperature differential and degree of restraint.
Similarly the change in diurnal or seasonal temperatures as mentioned above makes the concrete to expand and contract. As the structures are not free to expand or contract due to restraint at support in case of roof slabs or bridge girders and sub grade reaction in case of air field pavement etc., a considerable tensile stress more than the tensile strength of concrete will be developed, resulting cracking of concrete.
An idea of the tensile stress that could be developed due to the temperature variation can be have from the following example:
Calculate the tensile stress developed in the concrete having the characteristic compressive strength as 36 MPa. Use standard formulae.
From is 456-2000, modulus of elasticity of concrete is given by the relation.
Assuming coefficient of thermal expansion of concrete as 10 x 10-6 per °C and diurnal variation in temperature as 25°C
i.e., the thermal shrinkage strain = 25 x 10 x 10-6
= 250 x 10-6
On the basis of his experimental work, Lowe has suggested that cracks are developed in concrete at a differential strain of 200 x 10-6. In the present case the differential strain is 250 x 10-6, which will cause vary high degree of micro cracking in the concrete.
Further modulus of elasticity = Stress/Strain
∴ 3.0 x 104 = stress/(250 x 10-6)
Tensile stress = 3.0 x 104 x 250 x 10-6
= 7.5 N/mm2
The tensile strength of concrete is 3.5 N/mm2, thus a tensile stress of 7.5 N/mm2 which is more than the double of the tensile strength of concrete. Thus this stress is sure to cause high degree of micro cracks in the concrete. In case of stronger concrete with higher values of modulus of elasticity and higher degree of variation of temperatures, the tensile stress will be much more.
However the concrete of higher compressive strength will have higher tensile strength to with stand the higher tensile stresses. But due to the lower value of creep of high strength concrete, the relaxation in the stress will be smaller. Thus the stronger concrete will crack more than the weaker concrete from this consideration. However cracking of concrete is a complex phenomenon.
Very fine cracks are known as micro cracks. Investigations have shown that very fine cracks in the concrete exist at the interface between the coarse aggregate and cement paste even prior to the application of load on the concrete. These cracks remain stable upto about 30% or more of the ultimate load and then start to increase in length, width and number.
The overall stress under which these cracks develop is sensitive to the water cement ratio of the paste. At this stage the propagation of cracks is slow. At 70 to 90% of the ultimate strength, these cracks open through the mortar (Cement paste and fine aggregate). These cracks pass over (bridge) the bond cracks and a continuous crack pattern is formed. This stage of cracking is known as the fast crack propagation stage. If the load remained sustained, failure may take place with time.
The results of length measurement of cracks are shown in Fig. 26.6. From the figure, it can be seen that there is very little increase in the total length between the beginning of loading and a stress of about 0.85 of the prism strength (10 cm x 10 cm x 300 prism). A further increase in stress resulted large increase in the total length of the crack.
At a stress/strength ratio of about 0.95 (determined on prism) not only the interface cracks but mortar cracks were also observed. The orientation of many cracks was seen roughly parallel to the applied load. On the descending part of the stress/strength curve, the rate of increase in the crack length and width was found large.
It has been observed that crack widths at surface of the concrete play an important role in the durability of concrete structures. The design Engineer should understand this fact and give serious thought to it while designing the structure.
IS-456-2000 has specified the crack width as follows:
1. In general the width of surface crack should not exceed 0.3 mm in members, where cracking is not harmful and does not have any serious adverse effect upon the preservation of reinforcing steel nor upon the durability of concrete.
2. Cracking in tensile zones of members is harmful either due to exposure to the effects of the weather or continuously exposed to moisture or in contact with soil or ground water. In such situations the maximum upper limit of crack width is suggested as 0.2 mm.
3. For aggressive environment such as severe category of exposed conditions, the surface width of cracks should not in general exceed 0.1 mm.
Some specifications, limit the crack widths at the points near the main reinforcement instead of the surface. International pre-stressing Federation has recommended the maximum crack width at the main reinforcement to be 0.004 times the normal cover thickness. If the thickness of the normal cover is taken as 50 mm, then the crack width near the main reinforcement comes to be 0.004 x 50 = 0.2 mm.