Core Testing - Site Mix

8/20/2019 Core Testing - Site Mix

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COMPRESSIVE STRENGTH TEST ON CONCRETE CORE

Compressive Strength Test on Drilled Concrete Cores is required to determine the strength of

hardened concrete in structure. Following are the specification for drilled concrete cores to be

suitable for compressive strength test:

Diameter of concrete core:

The diameter of the core specimen for the determination of compressive strength in load

bearing structural members shall be at least 3.7 inch!"# mm$. For concrete with nominal

ma%imum si&e of aggregate greater than or equal to '.( inch!37.( mm$) the preferred minimum

core diameter shall be )three times the nominal ma%imum si&e of coarse aggregate but it should

be at least two times the nominal ma%imum si&e of the coarse aggregates.

Length of concrete core:

The preferred length of capped specimen is between '." and *.' times the diameter. +igh lengths

can be trimmed and for specimens having low length) correction factor has to be applied in

compressive test.

Moisture conitioning of concrete core:

,fter cores have been drilled) wipe off the surface drilled water and allow the surface moisture to

evaporate. -hen surface appears dr) but not greater than ' hour after drilling) place cores in

separate bags or non absorbent containers and seal to prevent moisture loss.

/aintain cores at ambient temperatures and protect from e%posure to direct sunlight. Transport

the cores as soon as possible to laborator. Cores can be ta0en out of the bags for a ma%imum tie

of * hrs to permit

capping before testing. 1f water is used for grinding or sawing the core ends) complete these

operations as soon as possible) but not later than * das after drilling. /inimi&e the duration of

e%posure to water during end preparation.

,llow the cores to remain in the sealed plastic bags or non absorbent containers for at least (das after last being wetted and before testing.


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Sa!ing of the ens of concrete core:

The ends of core specimen shall be flat) and perpendicular to the longitudinal a%is. Sawingshould be such that prior to capping the following requirements are met:

a2 ro4ections) if an shall not e%tend more than .* inch!(mm$ above the end surfaces

b2 The end surfaces shall not depart from perpendicularit to the longitudinal a%is b a slope of

more than '.5 d or ':.3d where d is the average core diameter.

Ca""ing of concrete core:

• 1f the ends of cores do not confirm to the perpendicularit and plainness requirement)

the shall be sawed or ground or capped.

• 1f cores are capped) the capping device shall accommodate actual core diameters and

produce caps that are concentric with the core ends.

• The material used for capping shall be such that it6s compressive strength is greater

than that of the concrete in the core.

•

Caps shall be made as thin as practicable and shall not flow or fracture before theconcrete fails when specimen is tested.

• Capped surface shall be right angles to the a%is of the specimen and shall not detach

depart from a plane b more than .( mm.

• /easure core lengths to the nearest .' inch !* mm$ before capping.


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Measurement of concrete core:

• efore testing) measure the length of the capped or ground specimen to the nearest .'

inch !* mm$ and compute this to calculate the length diameter !89D$ ratio.

• Determine the average diameter b averaging the two measurements ta0en at right

angles to each other at the mid height of the specimen.

• /easure core diameters to the nearest .' inch!.* mm$ when the difference in core

diameters does not e%ceed * of their average) otherwise measure to the nearest .'

inch!*mm$.

• Do not test cores if the difference between smallest and largest diameter of the core

e%ceeds ( of their average.

Testing of concrete core:

Test the specimen within 7 das after coring.

Ca#cu#ation of com"ressi$e strength:

Calculate the compressive test of the specimen using the computed cross sectional area based on

average diameter of the specimen. 1f the 89D ratio is '.7( or less) correct the result obtained b

multipling with

correction factors as given below:

L%D Ratio Correction &actor

'.7( ."5

'.( .";

'.*( ."3

'. .57

The value obtained after multipling with correction factor is called corrected compressive

strength) this being equivalent strength of a clinder having 89D ratio of *. The equivalent cube

strength can be calculated b multipling the corrected clinder strength b (9#.

Re"ort of com"ressi$e strength test:


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b2 8ength of the specimen before and after capping to nearest * mm and average diameter of

core to the nearest .* mm or * mm.

c2 Compressive strength to the nearest .' /a when diameter is measured to nearest .* mm

and to the nearest .( /a when diameter is measured to the nearest *mm after correction for 89D ratio.

d2 Direction of application of load with respect the hori&ontal plane of the concrete as placed

e2 /oisture conditioning histor

f2 1f water was used during end preparation) the date and time end preparation was completed

and core was placed in concealed bags.

g2 The date and time when tested

h2 =ominal ma%imum si&e of the aggregates.

T1S F>< C>


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• The strength of concrete measured b test of cores and beams is affected b the

amount and distribution of moisture in the specimen at the time of test.

• These factors shall be considered in planning the locations for obtaining concrete

samples and in comparing strength test results.

(""aratus for Core E'traction of concrete:

• Core drill) for obtaining clindrical core specimens with diamond impregnated bits

attached to the core barrel.

• Saw for trimming the ends of the core. The saw shall have a diamond or silicon

carbide cutting edge and shall be capable of cutting specimens that confirm to the

prescribed dimensions without e%cessive heating or shoc0.

Sam"#es for core testing:

• Samples of hardened concrete for use in the preparation of strength test specimens

shall not be ta0en until the concrete is strong enough to permit sample removal

without disturbing the bond between the mortar and the coarse aggregates.

• ,lso samples that have been damaged during removal shall not be used unless the

damaged portions are removed and the resulting test specimen is having the required

length.

• Samples containing embedded reinforcement cannot be used for the test

Core ri##ing :


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• Core specimen shall be drilled perpendicular to the surface and not the formed 4oints

or edges.

•


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Fig: 1nstrument showing core cutting

Fig: Concrete Core

The strength of a test specimen depends on its shape) proportions and si&e. The influence of

height9diameter +9D2 ratio on the recorded strength of clinder is an established fact. Strength

of core have to be related to the standard clinder strengths) i.e. for +9D ratio of *. Thus core

should be preferabl have this ration near to *. For values of +9D less than ') between ' and *) a

correction factor has to be applied. Cores with +9D ratio less than ' ield unreliable results and

S '55': art#:'"7 prescribes a minimum value as ."(. The same standard specifies the use

of '(mm or 'mm cores. +owever cores as small as (mm are also permitted in the standards.

Eer small diameter cores e%hibit more variabilit in results than larger dia cores) hence their use

is generall not recommended. The general rule adopted for fi%ing the core si&e) besides the +9D

ratio) is the nominal si&e of stone aggregate and the dia should be not less than 3 times the


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ma%imum si&e of stone aggregate. For diameter of core less than 3 times the si&e of the stone

aggregate) an increased number of cores have to be tested.

&o##o!ing are the factors !hich affect the com"ressi$e strength of e'tracte concrete cores:

• Si-e of stone aggregate: 1f the ratio of diameter of core to ma%imum si&e of stone

aggregate is less than 3) a reduction in strength is reported. For concrete with *mm

si&e aggregate) (mm dia core has been tested to give ' lower results than with

'mm dia cores.

• Presence of trans$erse reinforcement stee#: 1t is reported that the presence of

transverse steel causes a ( to '( reduction in compressive strength of core. The

effect of embedded steel is higher on stronger concrete and as its location moves awa

from ends) i.e. towards the middle. +owever presence of steel parallel to the a%is of the

core is not desirable.

• H%D ratio: This has been alread discussed above. +owever its value should be

minimum ."( and ma%imum *. +igher ratio would cause a reduction in strength.

• (ge of concrete: =o age allowance is recommended b the Concrete Societ as some

evidence is reported to suggest that insitu concrete gains little strength after *5 das.

-hereas others suggest that under average conditions) the increase over *5 das6

strength is ' after 3 months) '( after ; months. +ence it is not eas to deal the

effect of age on core strength.

• Strength of concrete: The effect in reducing the core strength appears to be higher in

stronger concretes and reduction has been reported as '( for # /a concrete.

+owever a reduction of ( ( 7 is considered reasonable.

• Dri##ing o"erations: The strength of cores is generall less than that of standard

clinders) partl as a consequence of disturbance due to vibrations during drilling

operations. -hatever best precautions are ta0en during drilling) there is alwas a ris0

of slight damage.

• Site conitions $is.a.$is stanar s"ecimens: ecause site curing is invariabl

inferior to curing prescribed for standard specimens) the insitu core strength is

invariabl lower than the standard specimens ta0en and tested during concreting

operations.

How to Get the Best Results from Concrete Core Testing


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-hen compressive strength tests of laboratorcured clinders fail to meet the specified

acceptance criteria) core tests are commonl used to verif the strength and to obtain acceptance

of the inplace concrete. -hile the process of core testing ma seem straightforward) there are

man details contractors must consider to achieve accurate results.


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,s discussed in ,void the False ,larm for 8owStrength ConcreteG Hanuar *'3 Concrete

Contractor2) ,C1 3'5 uilding Code


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>f course) consolidation techniques for inplace concrete and clinders are not the same and

different levels of consolidation between cores and clinders contribute significantl to strength

differences. ,lso) ,ST/ C#*3 now requires the densit or unit weight of cores to be calculated

before strength testing. Inowing the core densit ma help determine if a lowstrength core test

was due to defective concrete or poor consolidation of the inplace concrete.

,nother ma4or reason wh core strengths are tpicall less than clinder strengths is less

favorable curing conditions i.e.) moisture and temperature conditions2 for strength development

of inplace concrete as compared to the standardG cure requirements for laboratorcured

clinders.

For concrete mi%tures with strengths less than () psi) ,ST/ C3' # requires clinders to be

stored up to #5 hours initial curing2 in a temperature range from ; to 5J F with the balance of

the *5da cure period at a temperature of 73.( 3.(J F. For specified strengths of ;) psi and

higher) the initial curing temperature is ;5 to 75J F. 1n most cases) curing temperatures for in

place concrete is variable and less favorable than standardG cure temperatures resulting in lower

strengths for cores as compared to clinders. See Figure '.

Core #ocation an orientation

Strengths are affected b the location and drilling orientation of cores relative to the structural

element. 1n general) concrete at the bottom of an element is stronger than concrete near the top of

an element or near the top of a lift because of the effects of bleeding and settlement of the coarse

aggregates. leed water decreases strength in the upper portions of walls) columns) beams and

slabs b increasing the water to cementitious materials w9cm2 ratio. Figure * illustrates the top

tobottom strength variation for a wall and also shows concrete is tpicall wea0er along edges

of a unit of deposit or formed 4oints.

,s shown in Figure 3) bleeding creates a wea0 cementaggregate bond) or planes of wea0ness)

under coarse aggregate particles. lanes of wea0ness are alwas hori&ontal in concrete. -hen the

planes of wea0ness are located parallel to the applied load 2 of the testing machine as shown

for Core drilled perpendicular to the casting direction2 strength can be reduced ' percent or


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more as compared to Core , drilled parallel to the casting direction2. ,ST/ C#* requires these

factors to be considered when planning a core test program.

Core han#ing

,lwas e%ercise care when drilling and handling cores) especiall to preserve the moisture

content of cores. The amount and distribution of moisture in cores will affect strengths. ,ST/

C#* requires special handling with regard to moisture preservation and conditioning both in the

field and in the lab before testing. 1n the field) wipe off drill water from the cores upon e%traction

and allow the surface moisture to evaporate up to one hour2. Then place cores into separate

plastic bags and seal them to prevent moisture loss. 1n the lab) ,ST/ C#* requires a testing

facilit wor0er to store cores in sealed plastic bags for at least five das after end preparation to

reduce moisture gradients.

,s with most construction activities) advanced planning is required for a successful coring

operation and lowstrength investigation. -hen possible) minimi&e the number of cores removed

from the structure to avoid the Swiss cheese appearance and remember that proper repair of the

core holes is part of the investigation too.


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Core Testing - Site Mix