how can concrete fail on compression

7/23/2019 How Can Concrete Fail on Compression

1/23

How can concrete fail on

compression?If you have stacking of particles, splittingforces are driving forces for cracking. Crackingstarts at: Imperfections Voids Particles Microcracks Interfaces

Note top and bottom triaxial compression force(C!

"t t#e sides$ %e #ave tensile splitting

C&be vs c'linder


2/23

ilica )&me: *#en silica f&me is added to concrete$ initiall' it remains inert. +nceportland cement and %ater in t#e mix start reacting %it# eac# ot#er (#'drating!$

primar' c#emical reactions prod&ce t%o c#emical compo&nds: Calci&m ilicate,'drate (C,!$ %#ic# is t#e strengt# prod&cing cr'stalli-ation$ and Calci&m ,'droxide(C,!$ a b'prod&ct also called free lime %#ic# is responsible for not#ing m&c# ot#ert#an lining available pores %it#in concrete as a /ller or leac#ing o&t of inferiorconcrete. Po--olanic reaction occ&rs bet%een silica f&me and t#e C,$ prod&cingadditional C, in man' of t#e voids aro&nd #'drated cement particles. 0#is additionalC, provides t#e concrete %it# not onl' improved compressive$ 1ex&ral and bondstrengt# b&t also a m&c# denser matrix$ mostl' in areas t#at %o&ld #ave remained assmall voids s&b2ect to possible ingress of deleterio&s materials.)l' "s#: )l' as# is a po--olanic material. It is a /nel'divided amorp#o&s al&minosilicate %it# var'ing amo&nts of calci&m$ %#ic# %#en mixed %it# portland cement and%ater$ %ill react %it# t#e calci&m #'droxide released b' t#e #'dration of portland

cement to prod&ce vario&s calci&msilicate #'drates (C,! and calci&mal&minate#'drates. ome 1' as#es %it# #ig#er amo&nts of calci&m %ill also displa'cementitio&s be#avior b' reacting %it# %ater to prod&ce #'drates in t#e absence of aso&rce of calci&m #'droxide. 0#ese po--olanic reactions are bene/cial to t#e concretein t#at t#e' increase t#e 3&antit' of t#e cementitio&s binder p#ase (C,! and$ to alesser extent$ calci&mal&minate #'drates$ improving t#e longterm strengt# andred&cing t#e permeabilit' of t#e s'stem. 4ot# of t#ese mec#anisms en#ance t#ed&rabilit' of t#e concrete.


3/23

+t#er po--olan e.g. rice #&sk$ metakaolin$ diatomaceo&s eart# etc.

Fibre Reinforced Concrete:/bres for small cracks. )ibres red&ce crack %idt#. It improves post peak be#avio&r

Concrete loaded in tension *it# /bres

)rom t#e above$ it improves t#e tensile strengt#$ and improve post peak be#avio&r.Increased fract&re energ'

teel !bre reinforcedImproves to&g#ness of concrete)lex&ral strengt# is improved b' &p to 56 b' decreasing t#e propagation of cracksImproves tensile strengt# More economical t#an steel reinforcement7ess prone to corrosion8ives an alternative %a' to reinforce concrete ot#er t#en traditional steel rebar4&t9.t#e' s#o&ld be ever'%#ere

Fibre matri" interaction: st&died%it# p&llo&t test. 0#ere %ill befrictional resistance as 'o& tr' to p&llo&t t#e bar. Pfris act&all' t#e p&ll o&tfrorce after friction. ee di;erentbe#avio&rs observed


4/23

Fibres at di#erentdimensions

mall /bres stop t#e microcracks (increase of prepeakstrengt#! prevents


5/23

non EN 206 concretee.g. sprayed concrete, foam concrete, Concrete with density less than 800kg/m, !efractory concrete. ..

Conceptual models for concrete"#. $egree of hydration %. &sothermal hydration . Semi 'adiabatic( hydration


6/23

0#e cement paste contain t#e cement gel (%it#C,$ Ca+,F and ettringite!$ &n#'drated

cement$ capillar' %ater and air. "t 56 #'dration(i.e. before #'dration!$ onl' &n#'drated cement$%ater and air. *it# ibcreasing #'dration$ gelstr&ct&re is formed and bot# t#e &n#'dratedcement and capillar %ater red&ces %it#increasing @o,.0#e c%ater /lled spaces in t#e fres# concreteare called capillar' pores. *it#in t#e gel itself$t#ere exist interstitial spaces called gel pores.

Capillarr' pores represent t#e gross vol&me%#ic# #as not been /lled %it# #'drationprod&cts.. Capillar' porosit' depends on t#e

%atercement ratio$ and t#e @o,. *#ere t#ecapillar' pores are connected leads to increasein permeabilit' and s&sceptibilit' to t#a%ingand free-ing ,'dration increases t#e solidcontent$ and ne%l' formed prod&cts can blockt#e capillar' pores."s total vol&me of gel increases$ capillar' poresred&ces. +n t#e ot#er #and$ gel pores increasesas it increases %it# gel vol&me

Non evaporable %ater contains c#emicall' bo&nd %ater. P#'sicall' bo&nd %ater (even in t#egel pores! can evaporate.


7/23

,o% can %e approximate t#e @o,G It #asto be somet#ing meas&rable t#at :

,eat prod&ction co&ld be estimated on

site. 4o&nd


8/23

0#o&g# %e said 5.Bg %ater to Egcement. It is never so. o& never getE556. o& can never get a perfectmix

%#ect of initial mi" temperature

on adiabatic temperature rise*#ere t#e initial mix temperat&re is#ig# e.g. sa' 5 degrees$ t#e reactionstarts faster. 7ater #o%ever$ t#e mix(%it# sa' E5 degrees! %#ic# startslo%er %o&ld #ave more reaction

&I' (%I)*@esign criteria for concrete mixes aret'picall'E. trengt# F. *orkabilit' . @&rabilit'

" good concrete is one t#at performssatisfactoril' in #ardened state and also%#ile fres#.

+orkability (consistenc' classes E$ F$ $B!

E. Particle si-e distrib&tion aggregate

F. Max. particle si-e aggregate

. Percentage of fine material (J FA5 Km!

B. "mo&nt of %ater

)ines (J FA5 Km! #as #ig# speci/c s&rface$t#&s t#e' absorb %ater$ and determine t#estickinesstrength: &s&all' speci/ed as e.g. CEF


9/23

o t#e #ig#er t#e norm strengt#$ t#e #ig#er t#e strengt#. 0#e lo%er t#e %ater cementratio$ t#e #ig#er t#e concrete strengt# also

- +orkability" concreteis said to be %orkable if it is easil' transported$ placed$ compacted and/nis#ed %it#o&t an' segregation or bleeding. Ln%orkable concrete needs more %orkor e;ort to be compacted in place$ also #one'combs esistance to 1o% is ca&sed b' friction andinterference bet%een aggregates


10/23

lump test: " mea&re of consistenc'. 0#emo&ld is a cone of "brams %it# opening at t#etop. Its /lled %it# fres# concrete. "fter /lling$t#e cone is lifted slo%l'$ and t#e no%&ns&pported concrete %ill sl&mp. 0#e decreasein #eig#t of t#e fres# concrete is t#e sl&mp. Ver'

sti; concrete #ave no sl&mp. )or CF and C

Compacting factor: e3&ipment is F #oppers(cone s#aped! and E c'linder. 0#e &pper #opperis /lled %it# concrete (%it#o&t an' form ofcompaction!. 0#e bottom of t#e #opper isreleased so it fall to t#e Fnd#opper (%ic# %o&ldbe smaller so it %ill be /lled to over1o%ing!. 0#ebottom of t#is Fnd#opper is released so it fall int#e c'linder. 0#e densit' is calc&lated andcompared %it# densit' of f&ll' compacted (forCE!

Flow table test:0#e 1o% table is %etted. 0#e cone is placed in t#e center of t#e 1o%

table and /lled %it# fres# concrete in t%o e3&al la'ers. ?ac# la'er is tamped E5 times %it#a tamping rod. *ait 5 seconds before lifting t#e cone. 0#e cone is lifted$ allo%ing t#econcrete to 1o%. "fter t#is t#e diameter of t#e concrete (pancake! is meas&red. Class )B ) even CB$ebe test: 0#e apparat&s consists of a metal c'lindrical container mo&nted on a vibratingtable. a sl&mp cone is placed in t#e center of t#e c'linder and /lled in t#e same manner asin t#e standard sl&mp test. "fter t#e sl&mp cone is remove$ 0#e Vebe table is started andt#e time for t#e concrete to remold from t#e sl&mp cone s#ape to t#e s#ape of t#e o&terc'lindrical container is recorded as a meas&re of consistenc'. Lsef&l for CE consistenc'class

Fineness Modulus: The index number, which describes the relative sizes of coarse and fine

aggregates, is called as fineness modulus (FM). Fineness modulus is determined separately by

sieving coarse and fine aggregates through the following set of sieve. (!TM sieve numbers

"##,$#,%#,"&,', with "## being "$#m and being $mm.

The FM is "*"##+sum of cumulative residue retained on sieves. oarse aggregates thus has higher

fineness modulus than fine aggregate. This is because higher number of are retained and these

cumulative continues.

The fineness value and the slump value can be used to determine the amount of sand and gravel


11/23

Fine materials (-$# m) consists of cement, fillers and fine sand, air bubble(air entraining

agents). Minimum amount of fine*m%is related to maximum diameter /maxof aggregate. The larger

/max, the smaller the amount of fines. 0f you use small /max, you1ll need more aggregate.

)ir content affects workability, strength and durability. #* air results in a strength reduction of

about +*.) - you need less water than C. ore coarse

hence less specific area

Durability0#e denser t#e concrete$ t#e more d&rable it become.0#is implies less %ater. 4&t if a;ect %orkabilit'.@&rabilit' iss&es de/ne environmental classes in codes.(dr'$ #&mid= #&mid %it# deicing salt= sea %ater=aggressive9!. 0#e main concerns addressed b'environmental classes in ?&rocode incl&de: >ebarcorrosion (Carbonationind&ced corrosion= C#lorideind&ced corrosion!$ "> "lkaliilica >eaction$ &lp#ateattack (sea %ater!$ 7eac#ing processes (acid attack!=

free-e t#a% attack.(enser cement paste gives higher durability E. 0'pe of cement F. *ater


12/23

Concrete is a p#ase material as in t#e /g&re. eeimportant aspect of t#e p#ases relevant to strengt# oft#e concrete.trengt#


13/23

)raphical approach 4(utch approach9 (e $ree5:0#e fact t#at t#e strengt# ofconcrete increases %it# progress of #'dration$ co&ld %it# t#e fact t#at t#e rate of#'dration increases %it# an increase in temperat&re led to t#e proposition t#atstrengt# can be expressed as a f&nction of time(or age!temperat&re combination.

It is essentiall' t#e area &nder t#e grap# t#atis t#e mat&rit' in degree Celsi&s #o&r. +nl'

temperat&re above a dat&m (reference!temperat&re are taken (sa' E5!. *e expect alinear relations#ip bet%een 7og of mat&rit'and t#e compressive strengt#. ,o%ever$researc#er fo&nd o&t t#at t#e linearrelations#ip expected does not #old for an'aribrar' concrete. 0o make it %ork$ t#e' &se a%eig#ted mat&rit' met#od (b' t#e term Cn! int#e expression %#ic# depend on t#e cementt'pe.

)elpace Ratio ; (egree of hydration 4see 0uly 15)or component of concrete$ aggregate makeover 5 percent. ee t#e expression for gelspace ratio. It simpl' vol&me of gel prod&cedas a f&nction of t#at of li3&id$ air and gel

If %e kno% t#e gel


14/23

E. "diabatic (or isot#ermal! #'dration c&rve (meas&red in lab!F. "ct&al concrete temperat&re (meas&red on site or calc&lated!

o eit#er isot#ermal or adiabatic c&rve (7, and >, in t#e /g&re belo%

Note t#at its same like before 2&st t#at its #eat no% (not strengt#!. )or t#e adiabatic$notice #o% closel' t#e "diabatic #eat looks like t#e adiabatic temperat&re. Itsbeca&se all #eat prod&ced is transferred to concrete. Notice #o%ever t#at t#e processtemperat&re and process #eat is smaller. 0#is is more like t#e act&al sit&ation$ not t#eadiabatic %#ic# is an ideal sit&ation.

)rom t#is #eat$ %e can get @o, b' t#e form&la


15/23

ee linear relations#ips in t#e /g&rebet%een strengt# and @o,. *#ere fmax

is a reference val&e$ and

0

depends on t#e concrete t'pe.

"s #'dration proceeds$ t#e elementparticles are grad&all' connected moreto eac# ot#er. ,aving reac#ed acritical @o,$ t#e relations#ip is 3&itelinear going for%ard. 0#is @o, dependon t#e %LC!= pixel model= nanostr&ct&re models

%#ect of initial mi" temperature on adiabatic temperature rise

0#at it can be &nderstood from t#e microstr&ct&re. "t #ig#er temperat&re$ t#e la'er oft#e reaction prod&ct #as a coarser capillar' pore str&ct&re. 0#e reaction prod&ct seemdenser aro&nd t#e #'drating particle$ and t#is retards s&bse3&ent #'dration. It alsoprod&ces lo% 3&alit' concrete %it# more pores #ence t#e red&ced strengt#. "lso$ non&niform distrib&tion of #'dration prod&cts %ill mean lo%er gel


16/23

7o% earl' strengt# gain is bene/cial on strengt# (even if it %as ca&sed b' &se ofretarders!. 0#ese matter e.g. some people &se steam c&ring on precast so as todemo&ld 3&ickl' and re&se form%ork

Neville: *#ile a rise in c&ring temperat&re speeds &p t#e c#emical reaction and t#&s

bene/ciall' a;ects t#e earl' strengt#. It red&ces t#e lengt# of t#e dormant period sot#at overall$ t#e str&ct&re of t#e #'drated cement paste becomes establis#ed earl'eno&g#.,o%ever$ from da's on$ it ma' adversel' a;ect t#e strengt#. 0#e explanation is t#atrapid initial #'dration appears to form prod&cts of a poorer p#'sical str&ct&re moreporo&s$ so t#at a proportion of t#e pores %ill al%a's remain &n/lled. It follo%s from t#egellastic shrinkage: Concrete is a poro&s material. *#en not all t#e pores arecompletel' /lled an'more$ %e #ave capillar' s&ction. Inside t#e fres#l' cast concrete$t#e #eaviest parts of t#e mixt&re tend to move do%n%ards &nder t#e in1&ence of t#e


17/23

gravitational forces. "s a res&lt$ %ater moves &p%ards. @epending on t#e s&rro&ndingconditions (%ind$ temperat&re$ >,!$ t#is %ater la'er evaporates more or less rapidIn case of a #ig# evaporation rate$ t#e t#in %ater la'er mig#t disappear completel'andplastic shrinkage is ver' likel' to occ&r. 0#e backgro&nd of t#is p#enomenon is t#epresence of an under-pressure in t#e fres# concrete. Imagine 'o&ng concrete as beinga s'stem of a #ig# n&mber of %ater/lled pores. Initiall'$ in t#e %atersat&rated

s'stem$ a #'drostatic press&re is present. *#en t#e s&rface %ater evaporates$ an&nderpress&re develops. 0#is &nderpress&re res&lts in tensile stresses in t#e#ardening mass. If t#ese tensile stresses exceed t#e act&al concrete tensile strengt#$cracking appears. Capillary stresses in pore due to ?xcessive earl' evaporation is t#emain ca&se.

" simple and often &sed meas&re to avoid plastic cracking is keeping t#econcrete %et d&ring #ardening. 0#is is ac#ieved b' s&ppl'ing %ater on top oft#e concreteOs s&rface or b' preventing moist&re loss (evaporation!. )or t#isp&rpose$ t#e concrete s&rface can be covered %it# a carpet$ plastic cover or afoil la'er$ or b' appl'ing a c&ring compo&nd

Chemical shrinkage:0#e vol&me of t#e #'drationprod&cts (t#e gel! is smaller t#an t#e prod&ct of

bot# t#e individ&al reacting components cement

and %ater. 0#is vol&me red&ction is denoted as

c#emical s#rinkage. C#emical s#rinkage represents

itself mainl' b' t#e formation of capillar' pores in

t#e cement stone and #ardl' as an o&ter

deformational c#ange. 0#erefore$ negligible or no

stresses are generated on a macroscale level. 8el

porosit' FW6.

0#e air and %ater is &niforml' distrib&ted over t#e

vol&me of t#e paste. C#emical s#rinkage res&lts ininternal empt' porosit'$ not in external vol&mec#anges. C#emical s#rinkage is more of anincrease in vol&me of pores

3utogenous hrinkage: *it# progress of t#e #'dration process$ t#e %ater availablereacts slo%l'. 0#is process res&lts in Xempt'ingY t#e pore s'stem. "t /rst$ %ater fromt#e largest pores in t#e s'stem reacts$ follo%ed b' %ater from t#e smaller ones. +necan also sa' t#at t#e %ater %it#dra%s itself into t#e smaller pores. 0#is is like a dr'ingprocess$ %#ere no exc#ange of moist&re takes place %it# t#e s&rro&ndings. In t#e?nglis# literat&re$ t#is process is denoted Xself desiccationY. )or lo% %ater cementratios (%cr J 5.A$ t#is Xself dr'ingY res&lts in vol&me red&ction. "&togeno&s s#rinkagemanifests itself mainl' for %atercement ratios belo% 5$B. 7arge a&togeno&s s#rinkageocc&rs in #ig# strengt# concrete.,o% is it related to c#emical s#rinkageG C#emical s#rinkage is t#e reason for t#epores. *it# contin&ing #'dration$ t#ere is demand for more %ater. If t#ere isnOt m&c#$%e #ave s&rface tension %it#ing t#e capillar' pores t#at res&lts in a&togeno&ss#rinkage. 0#ese internal dr'ing ca&ses micro cracks. Concrete %it# lo% %


18/23

ee a plot of a&togeno&s and c#emical s#rinkage %it# time. C#emical is m&c# more.ee t#e second plat %it# %


19/23

In t#e above$ notice t#e bo&nd %ater is in bet%een F cement la'ers. "dsorbed%ater is o&tside so 2&st to&c#ing E la'er of concrete.

(eformation mechanisms e"plained by the &unich modelE. Lp to B56 >,: @eformation d&e to change of surface tension in the solidphaseF. 4et%een B56 and W56 >,: @eformation d&e to dis2oining press&re. 4et%een W56 and E556: @eformation d&e to capillary tension(tension incapillar' %ater!.

Combined a&togeno&s and dr'ings#rinkage

)or NC more dr'ing s#rinkage$ b&t almost no a&togeno&s s#rinkage)or t#e ,C More a&togeno&s s#rinkage$ b&t it can even expand if moist&recome later3utogenous shrinkage

E. C#anges in s&rface tension in solid

particles

F. @is2oining press&re

. ,'drostatic tension in capillar' %ater

3utogenous swelling

E. ?ttringite formation (G!

F. )ormation of cr'stals of calci&m

#'droxide

. MicrocrackingB. ,'dration expansion


20/23

@ightness of concrete: @epending on %


21/23

tressstrain relations#ip of concrete is a f&nctionof time. Creep is de!ned as the increase instrain under a sustained stress. It is af&nction of t#e elastic strain

Initiall'$ %e get an instantaneo&srecover'. "fter%ards$ %e a grad&aldecrease of te strain (time dependent%#ic# %e call creep recover'. ,o%evet#ere is a resid&al deformation %#ic#remain %#ic# %e regard as resid&alcreep. " combination of microstr&ct&rc#ange and moist&re moving o&t. ,en

resid&al strain 0#e larger 'o&r aggregate$ t#e more creep 'o& #ave. 0'pe of aggregate

matters. 7*"C seem to #as #ig#er creep t#an N*C on acco&nt of its lo%ermod&l&s of elasticit'. 7ooks like t#is onl' #as e;ect on t#e initial elasticcreep t#o&g#

,o% does porosit' of aggregate a;ectG 0#e #ig#er t#e porosit'$ t#e lo%er

t#e mod&l&s of elasticit'0#e pict&re on t#e next col&mn is t#e C?4expression for creep. It onl' gives t#e /nalcreep.

Po%er 7a%s expression (as belo%! #elp 'o&comp&te creep in time:

Rela"ation: If restraint is s&c# t#at a stressed concrete specimen is s&b2ectedto constant strain$ t#ere %ill be a progressive decrease of stress in time. 0#is isrelaxation

)rom t#e po%er la%s &sed for creep$%e can get expression for creep rate.Lsing t#is along %it# t#e sti;ness$ %ecan comp&te relaxation.


22/23

Creep and Rela"ation in Hardening concrete: "t t#e #ardening p#ase$t#ere are complication factors arising from t#e fact t#at t#e microstr&ct&re isstill in state of development$ and t#e sti;ness is grad&all' increasing.

In constant stress test #'dration ca&ses ared&ction of t#e creep strain (beca&se t#e

stress


23/23

"ppl'ing t#is kno%ledge for relaxation. tress in earl' age %illred&ce

how can concrete fail on compression

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