determining compressive strength of chb

7/26/2019 Determining Compressive Strength of Chb

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Compressive strength is the capacity of a material or structure to withstand

loads tending to reduce size, as opposed to tensile strength, which

withstands loads tending to elongate. In other words, compressive strength

resists compression (being pushed together), whereas tensile strength resists

tension (being pulled apart). In the study of strength of materials, tensile

strength, compressive strength, and shear strength can be analyzed

independently.

Compressive strength is often measured on a universal testing machine;

these range from very small table-top systems to ones with over ! "#

capacity. "easurements of compressive strength are a$ected by the speci%c

test method and conditions of measurement. Compressive strengths are

usually reported in relationship to a speci%c technical standard.

&he 'strain' is the relative change in length under applied stress; positive

strain characterizes an obect under tension load which tends to lengthen it,and a compressive stress that shortens an obect gives negative strain.

&ension tends to pull small sideways deections bac* into alignment, while

compression tends to amplify such deection into buc*ling.

+y de%nition, the ultimate compressive strength of a material is that value of

uniaial compressive stress reached when the material fails completely. &he

compressive strength is usually obtained eperimentally by means of a

compressive test. &he apparatus used for this eperiment is the same as that

used in a tensile test. owever, rather than applying a uniaial tensile load, a

uniaial compressive load is applied. s can be imagined, the specimen

(usually cylindrical) is shortened as well as spread laterally. /tress0strain

curve is plotted by the instrument and would loo* similar to the following1


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234C56735/

8. 9nowing the proportion of each materials.

"any factors a$ect the compressive strength test results of

masonry units, especially the con%nement e$ect that signi%cantly

changes the strength of the similar elements. "any di:culties are

found in researches that aim to characterize the mechanical properties

of constituent material of masonry unit, mainly because concrete

bloc*s have a distinct moulding process in comparison to moulding test

samples. &herefore, some researches utilize samples etracted fromunits, assuring euality of material mechanical properties.

s steel platen restraint and unit geometry cause signi%cant

e$ects on test results, a summary of previous researches is presented.

In these analyses, a better standardization of samples is important due

to the inuence of platen restraint and sample geometry on test


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results. &hus, it is necessary to de%ne a sample that represent the

behaviour of the bloc* under aial compression.

ollow concrete bloc*s and cylindrical samples with plastic consistencyconcrete were moulded in three nominal levels of compressive strength1 8, ?< #=mm> and !< #=mm>. Concrete bloc*s and samples were castaccording to the same steps1 concrete placement, vibration, cure,demoulding and %nishing (top and bottom attening with a thin sulphurlayer)./teel moulds were used to prepare concrete bloc*s. Internal cores wereprovided by two 52/ (epanded polystyrene) bloc*s %ed to the steel mould,as illustrated in @igure 8. &he 52/ units were etracted right after A days. ll

the bloc*s were manufactured in laboratory to assure the desired concretestrength, high process uality and geometric precision. /ome of these stepsare illustrated in @igure ?. Concrete bloc*s and cylindrical samples were

subected to aial compression force in a servo-hydraulic machine and testedin linearly controlled displacement mode. &he controlled displacement modeallowed acuisition of stress-strain graphs, including the softening branch.


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?. Calibrating the compression machine

&he test load was applied through a ? mm thic* rectangular steel

platen (!B mm 8Bmm). 4n the top, this platen was connected to

the loading system by means of a ?Bmm diameter rigid steel cylinder.

4n the bottom, the test samples were supported by the rigid steel

bloc* of the 7niversal testing machine. &he dimensions of the platen

ful%ll the recommendations of t*inson and

the @igure ! illustrates the test set-up.Concrete that constitutes the core bloc*s

was characterized by compression and

tension tests of cylindrical concrete samples(8


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transducers is represented in @igure !b. @or eample, reference line ?

locates a transducer that is disposed on the main face of the bloc*, in

the middle of the hollow. &he strain measurement in cylindrical

concrete samples was performed by two displacement transducers.

Compression and tension tests of bloc*s are illustrated in @igure .

6uring the compression tests of bloc*s, crac*s appeared at a load level

about E


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!. nalysis of test results

&he preliminary set of tests indicates a small value of coe:cient of

determination, always less than F. @or this reason it was de%ned for

each group of test, the uantity of si samples and three bloc*s for

compression and tension test. &he values presented in the tables referto the mean value and coe:cient of variation of each group.&able ? summarizes the compressive and tensile strength of the hollow

concrete bloc* tests. It can be seen that the tensile strength is about

8


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n additional set of tests (named + in the followed tables) with displacement

transducers (@igure ! and @igure ) was carried out to obtain the stress-strainrelationships of the elements. &ables ! and present the relationshipsbetween compressive and tensile strength of bloc*s and samples. &heserelationships can also be observed in the graphs shown in @igure A. @igure Eaillustrates the stress-strain curves of the samples. Galues of mechanicalproperties obtained in sample tests are presented in &able . &he 2oissonratio value obtained for all strength levels is about


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&he proposed analysis gives a reasonable indication of the deformation

behaviour of the bloc*. t lower stress levels the estimated curve become

closer to the eperimental one and the correlation of compressive strength

determines the most di$erences among the relationships.

owever, theoretical curves always present compressive strength values

lower than those obtained eperimentally. It is possible that if a very rigid

steel platen is used in the test set-up, a better approimation can be

obtained.

Conclusions

/everal researches aim to correlate mechanical properties of hollow concrete

bloc*s and mechanical properties of concrete that constitute it. &hey are

limited to single comparisons between compressive strength values obtained

in tests of bloc*s and concrete samples. &he results of this *ind of study

cannot be etended to all situations because many variables inuence them,such as geometry of bloc*s and concrete samples, con%nement e$ects,

moulding or etraction methods and variability of test results.

&his paper presented some preliminary results of a broader research that

targets a detailed analysis of structural behaviour of bloc*s, prisms and

masonry walls.


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determining compressive strength of chb

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