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Factors Affecting StrengthWater/Cement (w/c) Ratio -- The capillary porosity of a properly compacted concrete

is determined by the w/c ratio. If concrete is not properly compacted it may contain

voids which will contribute to its porosity. At low w/c ratios where full compaction is

difficult to achieve, the relationship between w/c and strength is invalid.

There are some problems associated with using the w/c ratio as the primary

indicator of strength. For example, if finer cements and admixtures are used, 7-

and !-day strengths may not represent the true "uality of the cement. #owever,

until some other field test is available, w/c ratio remains the best indictor of

strength and durability.

Time -- The rate of strength gain depends of the w/c ratio, low w/c ratio mixes gain

strength faster than high w/c ratio mixes. As a general rule the ratio of !-day to 7-day

compressive strengths lies between $.% and $.7, and is generally less than $.&. These

ratios are not valid if accelerators or extreme curing temperatures are used.

The Maturity Concept -- The hydration of cement is greatly affected by both the time

and the temperature of hydration, therefore, strength gain is controlled by these two

factors. The concept of 'maturity' is a function of the product of curing time and

temperature. The assumption is that concrete of different mixes, curing times, and

curing temperatures will have about the same strength at the same level of maturity. A

datum or reference point below which no concrete will gain strength is commonly a

value of -$((). The general trend is that as maturity increases compressive strength

increases, especially at low maturity values.

There are a number of limitations on the use of maturity for predictingcompressive strength. *$+ humidity of curing is not considered, *+ only ambient

temperature is considered the contribution of heat of hydration is ignored, *%+

maturity functions are not useful at low values *time should be calculated from

when concrete actually begins to gain strength not at mixing and casting+, *+

invalid over large curing temperature variations, *&+ cement characteristics and

w/c ratio affect strength, and *+ invalid for accelerated concretes.

/// 0evertheless, the maturity concept may be useful in establishing 'after the

fact' strength estimates of concrete.

Cement -- From our previous study we 1nown that the chemical composition and

fineness of cement affect the strength of concrete. 2arly strength comes from )%3 and

later strength from )3. )ement that hydrates more slowly will have lower initial

strength but higher ultimate strength. The degree of fineness also affects the strength

the rate of hydration increases with the increase of fineness.

Aggregate -- 3econd to w/c ratio, aggregate is an important factor affecting concrete

strength. The most important properties of aggregate are shape and texture and the

maximum aggregate si4e. 3ince aggregate is generally much stronger than cement paste

the strength of the aggregate is less important. Texture affects both the bond and thestress level at micro-crac1s. This type of behavior will affect the tensile strength but

will not affect the compressive strength. )ompressive strength depends on the strength

of the aggregate itself.

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5aximum aggregate si4e affects strength in several ways6 larger particles reduce the

specific surface area of the aggregate which leads to a reduction in bond strength also,

larger particles tend to restrain volume changes in the cement paste and therefore

induce some internal stress which will wea1en the concrete. These effects can be offset

by reducing the water content, therefore the net effect of aggregate si4e is small. In

general, at a constant w/c ratio, higher strength can be obtained by using a leaner

mixture. If constant wor1ability is maintained, strength will increase with cement

content.

Factors Affecting the Measured Compressive Strength

)ompression tests assumed that a pure state of uniaxial loading. #owever, this isnot the case, because of frictional forces between the load plates and thespecimen surface. The affect is to restrain the specimen from expanding. As

specimen length to diameter ratio decreases the end effects are more importantresulting in higher apparent compressive strengths. The use of rubber of lubricantbetween the specimen and the loading plate can induce lateral tensile load at theend of specimen. This will cause vertical splitting and reduce apparent strength.

A hard or stiff plate will concentrate stress at the outer edges whereas a softerplate will have higher stress at the center. This same concepts of hard and soft atapplicable to the testing machines themselves. A soft machine will release thestored energy of its deformation to the specimen as it fails whereas a hardmachine will not.

As ld decreases below a value of the strength increases. At ratios above theeffect is more dramatic. Also, this phenomena is significant in high-strengthcement.

3pecimen si4e is important for the simple fact that as the specimens become largerit is more li1ely to contain an element that will fail at a low load..

8ate of loading as discussed above is "uite important to the test compressivestrength. In general, the higher the loading rate the higher the measured strength.The reasons for this are not completely clear, however, it is thought that under

slow loading rates more subcritical crac1ing may occur or that slow loading allowsmore creep to occur which increase the amount of strain at a given load.

5ost concrete specimens are tested in a saturated state. )oncrete that has beendried shows an increase in strength, probably do to the lac1 of lubricating effectmoisture has on the concrete particles. #igher temperatures at the time of testingwill lower the apparent strength of the concrete.

Tensile Strength

There is as yet no standard test for directly determining tensile strength. #oweverthere are two common methods for estimating tensile strength through indirecttensile tests. The first, is the splitting test carried out on a standard cylinderspecimen by applying a line load along the vertical diameter. It is not practical toapply the a true line load to the cylinder because the side are not smooth enough

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and because it would induced high compressive stresses at the surface. Therefore,a narrow loading strip made of soft material is used.

Another way of estimating tensile strength is the flexural test. A specimen beam x x ( inches is molds in two e"ual layers each rodded ( times, once for each in of top surface area. The beam may be vibrated and should be cured in thestandard way. This test tends to overestimate the true tensile strength by about

&(9. This can be explained by the fact that the simple flexural formula used isbased on a linear stress-strain distribution whereas concrete has a nonlineardistribution. This is an important test because it model how a concrete beam isnormally loaded.

ond et!een Concrete and Reinforcement

For bond between steel reinforcement and concrete to be effective, there must bean ade"uate frictional bond between the two materials. As concrete ages andshrin1s there may be a decrease in bond strength, or if the concrete crac1s or is

very permeable, some corrosion of the steel may ta1e place. There are notstandard test for reinforcement, however, a pull-out test has been developed forcomparison of different concretes *A3T5 )%+. The test consist of a inch cubewith a 0o. *$:-mm-diameter+ deformed steel bar embedded in it. The bar isloaded at a rate not greater than &((( lbin;s. The load and slip at recorded atintervals until *$+ the steel is yielded *+ the concrete splits or *%+ a slip of at lest.&mm occurs at the loaded end

"ondestructive #uality Test

This test are useful to6 *$+ "uality control *+ determination of the time for formremoval and *%+ help assess the soundness of existing concrete structures.

Surface $ardness Methods -- s. ?asically, the surface is impacted with a mass and thesi4e of the resulting indention is measured. The accuracy of these type of testsis only ( to %(9.Re%ound $ardness -- The most common nondestructive test is the rebound test.The test measures the rebound of a hardened steel hammer impacted on theconcrete by a spring. This method has the same limitations as the surfacehardness tests. The results are affected by6 *$+ surface finish *+ moisture

content *%+ temperature *+ rigidity of the member being tested *&+carbonation of the surface and *+ direction of impact *upward, downward,hori4ontal+. 5ost useful in chec1ing the uniformity of concrete.&enetration Resistance -- 8esistance of concrete to penetration by a steelprobe driven by a given amount of energy is measured. This test is not affectedby surface hardness or carbonation as the above tests, however, the mixproportions and material properties are still important.&ull'ut Test -- @ull-out test determine the force re"uired to pull a steel insertout of concrete which it was embedded during casting. This test is a measure ofthe shear strength of the concrete which can be correlated with compressivestrength. This test is better than those previously discussed, however, the testmay be planned in advance and the assembly embedded in the concrete duringcasting.ltrasonic &ulse *elocity -- This test is based o the fact that the velocity ofsound is related to the elastic modulus. The device is accurate to about $9.

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The position of the testing e"uipment can affect the measurement, method Agiven the best results. There are several factors which affect this test6 *$+surface smoothness *+ travel path of the pulse *%+ temperature effects on thepulse velocity *+ moisture content *&+ presence of steel reinforcing bars and*+ age of concrete.

Creep of Concrete

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

If a sustained load is removed, the strain decreases immediately by an amount equalto the elastic strain at the given age; this is generally lower than the elastic strain onloading since the elastic modulus has increased in the intervening period. hisinstantaneous recovery is followed by a gradual decrease in strain, called creeprecovery. his recovery is not complete because creep is not simply a reversiblephenomenon.

It is now believed that the ma!or portion of creep is due to removal of water frombetween the sheets of a calcium silicate crystallite and to a possible rearrangementof bonds between the surfaces of the individual crystallites.

Factors Influencing Creep

Concrete that e"hibits high shrinkage generally also shows a high creep, but how thetwo phenomena are connected is still not understood. #vidence suggests that theyare closely related. When hydrated cement is completely dried, little or no creepoccurs; for a given concrete the lower the relative humidity, the higher the creep.

$trength of concrete has a considerable influence on creep and within a wide rangecreep is inversely proportional to the strength of concrete at the time of application of load. %rom this it follows that creep is closely related to the water&cement ratio.here is no doubt also that the modulus of elasticity of aggregate controls the

amount of creep that can be reali'ed and concretes made with different aggregatese"hibit creep of varying magnitudes.

#"periments have shown that creep continues for a very long time; detectablechanges have been found after as long as () years. he rate decreasescontinuously, however, and it is generally assumed that creep tends to a limitingvalue. It has been estimated that *+ per cent of )&year creep occurs during the firstyear.

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 takesplace. he influence of creep on the ultimate strength of a simply supported,reinforced concrete beam sub!ected to a sustained load is insignificant, but deflectionincreases considerably and may in many cases be a critical consideration in design.

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 Another instance of the adverse effects of creep is its influence on the stability of thestructure through increase in deformation and consequent transfer of load to othercomponents. hus, even when creep does not affect the ultimate strength of thecomponent in which it takes place, its effect may be e"tremely serious as far as theperformance of the structure as a whole is concerned.

he loss of prestress due to creep is well known and accounted for the failure of allearly attempts at prestressing. -nly with the introduction of high tensile steel didprestressing become a successful operation. he effects of creep may thus beharmful. -n the whole, however, creep unlike shrinkage is beneficial in relievingstress concentrations and has contributed to the success of concrete as a structuralmaterial.