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Factors Affecting Durability of Concrete

Concrete durability has been defined by the American Concrete Institute as its resistance to weathering action, chemical attack, abrasion and other degradation processes.

Factors Affecting Durability of Concrete
Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. Construction and demolition waste contribute to solid waste going to landfills. The production of new building materials depletes natural resources and can produce air and water pollution The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters. 

Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete. 

Seawater Exposure:  Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40. 

Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of carrions begins

The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection

Resistance to Alkali-Silica Reaction (ASR):  ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction

Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well. 

Concrete cracking

Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures. 

Surface spalling

Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons.
1.) In cold climates subjected to freezing and thawing, concrete surfaces have the potential to spall if the concrete is not air-entrained.
2.)Too much water in the concrete mix will produce a weaker, more permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).
3.) Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable. 

High Humidity and Wind-Driven Rain: Concrete is resistant to wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not encapsulate concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments. 

Portland cement plaster (stucco) should not be confused with the exterior insulation finish systems (EIFS) or synthetic stucco systems that have become popular but may have performance problems, including moisture damage and low impact-resistance. Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped.

inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.

Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in concrete long after paints have faded due to the sun’s effects. 

Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors.

Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifier’s Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments. 

Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both. 

With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraullic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8% will withstand a great number of cycles of freezing and thawing without distress.

Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcareous aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack. 

Resistance to Sulfate Attack:  Excessive amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete. 

Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate. 

Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environments

Sulfate attack in concrete and mortar

Sulfate attack can be ‘external’ or ‘internal’.
External: due to penetration of sulfates in solution, in groundwater for example, into the concrete from outside.
Internal: due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate, for example.
External sulfate attack
This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include:
· Extensive cracking
· Expansion
· Loss of bond between the cement paste and aggregate
· Alteration of paste composition, with monosulfate phaseconverting to ettringite and, in later stages, gypsum formation The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste
The effect of these changes is an overall loss of concrete strength.

The above effects are typical of attack by solutions of sodium sulfate or potassium sulfate. Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.

Other sources of sulfate which can cause sulfate attack include:
· Seawater
· Oxidation of sulfide minerals in clay adjacent to the concrete – this can produce sulfuric acid which reacts with the concrete
· Bacterial action in sewers – anaerobic bacterial produce sulfur dioxide which dissolves in water and then oxidizes to form sulfuric acid
· In masonry, sulfates present in bricks and can be gradually released over a long period of time, causing sulfate attack of mortar, especially where sulfates are concentrated due to moisture movement
· Internal sulfate attack
Occurs where a source of sulfate is incorporated into the concrete when mixed. Examples include the use of sulfate-rich aggregate, excess of added gypsum in the cement or contamination. Proper screening and testing procedures should generally avoid internal sulfate attack.
· Delayed ettringite formation
Delayed ettringite formation (DEF) is a special case of internal sulfate attack.
Delayed ettringite formation has been a significant problem in many countries. It occurs in concrete which has been cured at elevated temperatures, for example, where steam curing has been used. It was originally identified in steam-cured concrete railway sleepers (railroad ties). It can also occur in large concrete pours where the heat of hydration has resulted in high temperatures within the concrete.

DEF causes expansion of the concrete due to ettringite formation within the paste and can cause serious damage to concrete structures. DEF is not usually due to excess sulfate in the cement, or from sources other than the cement in the concrete. Although excess sulfate in the cement would be likely to increase expansion due to DEF, it can occur at normal levels of cement sulfate.
A key point in understanding DEF is that ettringite is destroyed by heating above about 70 C.
A definition of delayed ettringite formation DEF occurs if the ettringite which normally forms during hydration is decomposed, then subsequently re-forms in the hardened concrete.
Damage to the concrete occurs when the ettringite crystals exert an expansive force within the concrete as they grow.

In normal concrete, the total amount of ettringite which forms is evidently limited by the sulfate contributed by the cement initially. It follows that the quantity of ettringite which forms is relatively small. Ettringite crystals form widely-dispersed throughout the paste. If expansion causes cracking, ettringite may subsequently form in the cracks but this does not mean the ettringite in the cracks caused the cracks initially. 

Conditions necessary for DEF to occur are:
· High temperature (>65-70 C approx.), usually during curing but not necessarily
· Water: intermittent or permanent saturation aftercuring
· Commonly associated with alkali-silica reaction (ASR)
In laboratory tests, limestone coarse aggregate has been found to reduce expansion.
DEF usually occurs in concrete which has either been steam cured, or which reached a high temperature during curing as a result of the exothermic reaction of cement hydration. 

As the curing temperature of concrete increases, ettringite normally persists up to about 70 C. Above this temperature it decomposes. In mature concrete, monosulfate is usually the main sulfate-containing hydrate phase and this persists up to about 100 C. DEF could occur in concrete which was heated externally, eg: from fire.

An ettringite molecule contains 32 molecules of water; ettringite formation therefore requires wet conditions DEF and ASR appear to be closely linked; in one study (Diamond and Ong, 1994) a mortar made using limestone aggregate was cured at 95 C. Subsequent ettringite formation within the paste was scarce and expansion was minimal. However, if aggregate susceptible to ASR was used instead of limestone, ettringite formation and expansion were both much greater. This, and other studies, suggests that ASR is, or can be, a precursor for DEF expansion.

The effect of cement composition on DEF is not well understood. Some factors correlate strongly but the causes are not clear. In laboratory tests, DEF expansion was shown to correlate positively with cement-related factors, including:
a. high sulfate
b. high alkali
c. high MgO
d. cement fineness
e. high C3A
f. high C3S
DEF is still by no means fully understood. For further reading on this subject, try:
The resistance to deformation that makes concrete a useful material means also that volume changes of the concrete itself can have important implications in use. Any potential growth or shrinkage may lead to complications, externally because of structural interaction with other components or internally when the concrete is reinforced. There may even be distress if either the cement paste or the aggregate changes dimension, with tensile stresses set up in one component and compressive stresses in the other. Cracks may be produced when the relatively low tensile strength of the concrete or its constituent materials is exceeded.

Cracking not only impairs the ability of a structure to carry its design load but may also affect its durability and damage its appearance. In addition, shrinkage and creep may increase deflections in one member of a structure, adversely affecting the stability of the whole. These factors have to be considered in design. Volume change of concrete is not usually associated with changes that occur before the hardened state is attained. Quality and durability, on the other hand, are dependent on what occurs from the time the concrete mix has been placed in the mold.

Settlement and Bleeding

Concrete is said to be in a plastic state before it begins to set. The aggregate is dispersed by the cement paste and the particles in the paste are dispersed in the water. After placing, there is a period of settlement when the particles come closer together; most of this settlement usually occurs within an hour or so of placement. Total volume change may, in extreme cases, amount to 1 per cent or more, but it is not of great significance because the concrete is in a plastic or semiplastic state and no appreciable stresses can result from these changes. During settlement, water often appears at the surface, having exuded from the plastic mass. This phenomenon is called bleeding.

Accumulation of water at the top of a mass of concrete is often undesirable; for example, when concrete is placed continuously in a deep form, the upper part can gain progressively more water as the filling of the form progresses, leading to relatively poor quality at the top. On the other hand, the accumulation of some water at the surface is not always undesirable because surface water is required to prevent plastic shrinkage and to lubricate the tools used for finishing the surface. Again, an excess of surface water may lead to a thin layer of slurry on the finished surface and a weak susceptible layer on the surface of the concrete. Care must be taken that finishing does not begin before the bleeding period is over.

Settlement may give rise to structural flaws. A layer of water may be left under horizontal reinforcing bars so that half the area of contact between the steel and concrete is lost. This problem can be eliminated by proper vibration or revibration of the plastic concrete, care being taken not to touch reinforcing. It must not be overlooked, however, that settlement and bleeding do result in a reduction of water content. If not offset by one of the undesirable features discussed, the effect is beneficial to strength, permeability and volume stability.

Plastic Shrinkage

When the evaporation rate exceeds the rate of bleeding and the free settlement period is ended, a hydrostatic tension begins to develop throughout the mass owing to the formation of menisci at the water surfaces in the capillaries. This results in vertical as well as lateral compressive forces and may be manifested in a slab by pattern cracking. It is called plastic shrinkage cracking. Remedial measures may involve sun shades and windbreaks, application of water sprays or application of a curing compound to arrest evaporation.

Nature of Hydrated Portland Cement and Mechanism of Volume Change

Following hydration and hardening, cement consists of a mixture of several compounds, all chemically combined with water in different ways. The compound that has the greatest influence on the characteristics of hydrated cement, including shrinkage, is calcium silicate, which has a large internal surface area of 25 to 50 thousand square yards per pound. This internal surface is composed of the walls of the tiny pores and fissures within the physical dimensions of the specimen. (It is the character of this surface that makes hydrated cement an effective cementing agent and provides the versatility of concrete in forming bodies of high strength and almost any desired shape. When surfaces are very close to each other there is a mutual gravitation-like attraction that forms a strong “weld.” When the internal surface area is high the many strong welds develop the strength and rigidity of the body.)

Thus concrete is not a solid inert mass but a vast number of small pores or capillaries that in total can account for up to 50 per cent of the volume of the concrete. During curing the pores and capillaries are usually full of water and no stresses exist. As drying takes place, three mechanisms cause shrinkage:

1. The unstable nature of newly-formed calcium silicate hydrate results in shrinkage as drying occurs; the exact nature of this mechanism is not clearly understood but it is permanent and irreversible;
2. Compressive stresses are set up in the concrete because of the development of menisci in the capillaries as drying progresses;
3. Energy changes occur at the surface of calcium silicate as the water evaporates. 

These mechanisms (phenomena) acting separately or in combination cause initial drying shrinkage of the concrete. Part of it, 30 per cent or more, is irreversible.

Autogenous Volume Changes and Expansive Cements

Before volume changes resulting from drying or wetting of hardened concrete are discussed, autogenous volume changes should be mentioned because they occur where little or no change in total moisture content is possible and are of particular importance in the interior of mass concrete. Two opposing effects can be produced. As reaction between water and the unhydrated cement proceeds, the actual volume of the solid increases. This causes stresses through the set structure and results in expansion. At later ages, the water available for the reaction will decrease, resulting in self-desiccation of the cement paste and a shrinkage ranging from 0.001 to more than 0.015 per cent.
The increase in volume of some constituents during their formation has been used as the basis for developing expansive cements. Some, specially prepared, undergo relatively large expansions at early age so that if used in concrete that is restrained they develop compressive stresses. Later, when drying occurs, the resulting shrinkage that would have developed is partly or completely offset, and compressive stresses no longer exist in the concrete.

Volume Changes due to Moisture Changes

Although the mechanism of volume change that occurs during moisture change is not fully understood, much has been learned to provide useful information for engineering purposes. When concrete is dried, the first water to be removed causes no change in volume. This is considered to be free water held in rather large “pores.” With continued drying, shrinkage becomes quite large and at equilibrium in 50 per cent RH values in excess of 0.10 per cent have been recorded for some concretes. The above behaviour is somewhat similar to that of wood (in a qualitative manner). Shrinkage values for neat cement paste have been observed in excess of 0.40 per cent; the difference of this value from that of concrete is due to various restraints. A large portion of concrete is made up of relatively inert aggregate (from 3 to 7 times the weight of cement) and this, together with reinforcement, reduces shrinkage. In addition to internal restraints, some restraint arises from non-uniform shrinkage within the concrete member itself. Moisture loss takes place at the surface so that a moisture gradient is established. The resultant differential shrinkage is associated with internal stresses, tensile near the surface and compressive in the core, and may result in warping or cracking.
If concrete that has been allowed to dry in air at 50 per cent RH is subsequently placed in water, it will swell. Not all initial shrinkage obtained on drying is recovered, however, even after prolonged storage. For the usual range of concretes the irreversible part of shrinkage is about 30 to 60 per cent of total drying shrinkage, the lower value being more common. Because shrinkage has such an influence on the performance of concrete structures much work has been carried out to obtain information on the factors affecting it.

Effect of Cement and Water Contents on Shrinkage

Water content is probably the largest single factor influencing the shrinkage of paste and concrete. Typical shrinkage values for concrete specimens with a 5 to 1 aggregate-cement ratio are 0.04, 0.06, 0.075 and 0.085 per cent for water-cement ratios of 0.4, 0.5, 0.6 and 0.7, respectively. One of the reasons is that the density and composition of calcium silicate formed at different water-cement ratios may be slightly different. In general, a higher cement content increases the shrinkage of concrete; the relative shrinkages of neat paste, mortar and concrete may be of the order of about 5, 2 and 1. For given materials, however, and a uniform water content, the shrinkage of concrete varies little for a wide range of cement contents; a richer mix will have a lower water-cement ratio and these factors offset each other.

Properties of Cement

Fineness of cement seems to be a factor in shrinkage and particles coarser than No. 200 sieve, which react with water very slowly, have a restraining effect similar to that of aggregate. Thus, high-early-strength cement, which is finely ground, shrinks about 10 per cent more than normal cement. Low-heat and portland-pozzolan cements shrink a further 20 and 35 per cent, respectively. This is believed to be caused by larger quantities of calcium silicate, the shrinking component, present in them.

Type and Gradation of Aggregate

As stated previously, the drying shrinkage of concrete is a fraction of that of neat cement because the aggregate particles not only dilute the paste but reinforce it against contraction. It has been shown that when readily compressible aggregate is used concrete will shrink as much as neat cement, and that expanded shale leads to shrinkage one-third more than that of ordinary aggregate. Steel aggregate on the other hand, leads to shrinkage one-third less than that of ordinary concrete. In general terms the elastic properties of aggregate determine the degree of restraint offered. The size and grading of aggregate do not, by themselves, influence the magnitude of shrinkage, but an aggregate incorporating larger sizes permits the use of a mix with less cement and hence a lower shrinkage. Increasing the maximum aggregate size and thereby the aggregate content by 20 per cent of the total volume of the concrete will ensure a substantial decrease in shrinkage.

The shrinkage of aggregates themselves may be of considerable importance in determining the shrinkage of concrete; some fine-grained sandstones, slate, basalt, trap rock and aggregates containing clay show large shrinkage. In general, concretes low in shrinkage often contain quartz, limestone, granite or feldspar. Various harmful effects of abnormal shrinkage of concretes, caused by the aggregate and observed in actual structures, have included excessive cracking, large deflection of reinforced beams and slabs and some spalling. It is essential that any new source of aggregate be tested to ascertain whether its use in concrete will cause excessive shrinkage to develop. Any shrinkage in excess of 0.08 per cent is taken to indicate an undesirable aggregate.

Effect of Admixtures

As can be predicted from the effect of water-cement ratio on shrinkage, admixtures that increase the water requirement of concrete increase shrinkage and those that decrease the water requirement decrease it. Calcium chloride in the amount often added as an accelerator – 2 per cent by weight of the amount of cement – may increase drying shrinkage by as much as 50 per cent.
The over-all effect of the use of air entrained concrete is not to increase shrinkage. Some admixtures, if used in somewhat larger than normal doses, do increase shrinkage greatly and care must be exercised in the proportioning.

Rate of Drying

The size of the specimen and conditions of exposure are important in assessing the relevance of the shrinkage problem. Drying of ordinary concrete exposed to an environment maintained at 50 per cent RH will affect moisture content to a depth of 3 in. in one month. Continued exposure to these conditions would be a significant factor in small concrete members but would be of no importance in massive elements.

Carbonation Shrinkage

Another mechanism that will result in shrinkage of concrete is the reaction between carbon dioxide and hydrated cement. Maximum shrinkage occurs when the concrete is at equilibrium in a 50 per cent RH environment. This shrinkage combined with drying shrinkage results in excessive crazing of exposed surfaces such as concrete floors when CO2 levels are high, a condition often found on winter construction projects.
Carbonation during the curing of concrete products is sometimes used to encourage shrinkage and thus reduce shrinkage stresses when these units are incorporated into a structure. Carbonation also reduces permeability, presumably due to deposition of the reaction products in the pores and capillaries.

Creep of Concrete

Creep of concrete resulting from the action of a sustained stress is a gradual increase in strain with time; it can be of the same order of magnitude as drying shrinkage. As defined, creep does not include any immediate elastic strains caused by loading or any shrinkage or swelling caused by moisture changes. When a concrete structural element is dried under load the creep that occurs is one to two times as large as it would be under constant moisture conditions. Adding normal drying shrinkage to this and considering the fact that creep can be several times as large as the elastic strain on loading, it may be seen that these factors can cause considerable deflection and that they are of great importance in structural mechanics.

If a sustained load is removed, the strain decreases immediately by an amount equal to the elastic strain at the given age; this is generally lower than the elastic strain on loading since the elastic modulus has increased in the intervening period. This instantaneous recovery is followed by a gradual decrease in strain, called creep recovery. This recovery is not complete because creep is not simply a reversible phenomenon.

It is now believed that the major portion of creep is due to removal of water from between the sheets of a calcium silicate crystallite and to a possible rearrangement of bonds between the surfaces of the individual crystallites.

Factors Influencing Creep

Concrete that exhibits high shrinkage generally also shows a high creep, but how the two phenomena are connected is still not understood. Evidence suggests that they are closely related. When hydrated cement is completely dried, little or no creep occurs; for a given concrete the lower the relative humidity, the higher the creep.

Strength of concrete has a considerable influence on creep and within a wide range creep is inversely proportional to the strength of concrete at the time of application of load. From this it follows that creep is closely related to the water-cement ratio. There is no doubt also that the modulus of elasticity of aggregate controls the amount of creep that can be realized and concretes made with different aggregates exhibit creep of varying magnitudes.

Experiments have shown that creep continues for a very long time; detectable changes have been found after as long as 30 years. The rate decreases continuously, however, and it is generally assumed that creep tends to a limiting value. It has been estimated that 75 per cent of 20-year creep occurs during the first year.

Effects of Creep

Creep of plain concrete does not by itself affect strength, although under very high stresses creep hastens the approach of the limiting strain at which failure takes place. The influence of creep on the ultimate strength of a simply supported, reinforced concrete beam subjected to a sustained load is insignificant, but deflection increases considerably and may in many cases be a critical consideration in design. Another instance of the adverse effects of creep is its influence on the stability of the structure through increase in deformation and consequent transfer of load to other components. Thus, even when creep does not affect the ultimate strength of the component in which it takes place, its effect may be extremely serious as far as the performance of the structure as a whole is concerned.

The loss of prestress due to creep is well known and accounted for the failure of all early attempts at prestressing. Only with the introduction of high tensile steel did prestressing become a successful operation. The effects of creep may thus be harmful. On the whole, however, creep unlike shrinkage is beneficial in relieving stress concentrations and has contributed to the success of concrete as a structural material.

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