TOPICS HIGHLIGHTED CRACKS IN CONCRETE: INTRODUCTION TYPES OF CRACK AND HOW IT OCCURS PREVENT BEFORE IT S TOO LATE!

Transcription

1 TOPICS HIGHLIGHTED CRACKS IN CONCRETE: INTRODUCTION Why Des Cncrete Crack? Nn-structural Cracks Structural Cracks What are the differences between structural and nn-structural crack? TYPES OF CRACK AND HOW IT OCCURS Sulfate attack crack Lading crack Plastic shrinkage crack Drying shrinkage crack D-cracking Alkali-aggregate reactin crack Thermal crack Settlement crack Crrsin crack Crazing Tree grwth crack PREVENT BEFORE IT S TOO LATE! Optimum water rati Curing methd Slid grund Prper usage f material Cntrl jints Reinfrcement steel Cver fr reinfrcement Cating n reinfrcement Cating n cncrete Crrsin inhibitr Cathdic prtectin

2 CRACKS IN CONCRETE: INTRODUCTION Why Des Cncrete Crack? Cracks! At the sight f a crack in cncrete, mst f us panic. Lts f questins asked and few answers. Mst cracks ccur as a result f shrinkage f cncrete. Shrinkage is simply a reductin in the vlume f cncrete as it hardens. If this reductin in vlume were unrestricted, then a crack wuld nt ccur. Hwever, in reality, grund frictin and a number f things such as structural cnnectins inhibit free shrinkage and thus cause cracks. Hw much shrinkage is nrmal? A 100-ft-lng regular-weight cncrete slab nrmally wuld shrink by abut 3/4 inch. In ther wrds, yu shuld expect cracks ttaling in widths up t 3/4 inch in every 100 feet f cncrete. Lightweight cncrete shrinks mre. It is imprtant t nte that cncrete des crack and that this is nrmal. What is nt nrmal is an unsightly and excessive amunt f cracks. Nn-structural Cracks Nt every crack threatens the structural safety f a building. In fact, in many instances, cracks are merely csmetic in nature. These cracks are typically seen in flat wrk such as driveways, pati, walkways and curbs. Typical causes f these cracks are Pr wrkmanship Inapprpriate jint detailing Higher shrinkage f cncrete Smetimes such nnstructural cracks in driveways and sidewalks becme mre than just an eyesre. Tree rts and impact frm vehicles can cause raveling as well as vertical and hrizntal ffsets at the cracks. When these ffsets becme trip hazards, repairs are necessary. Structural Cracks A majrity f structural cracks ccur as a result f the fllwing cnditins: Design deficiency Cnstructin deficiency Settlement r heaving f sil Reinfrcement crrsin Smetimes structural cracks manifest themselves with sme side effects. Drs and windws d nt pen and clse easily. Flrs feel uneven. Vinyl flring

3 tears as a result f crack mvement. Stucc (plaster applied while sft t cver exterir walls r surface) begins t shw new cracks and even interir crners may develp new cracks. Lngitudinal cracks can develp alng the length f the fundatin as a result f crrsin f reinfrcement. What are the differences between structural and nn-structural crack? T summarize what had been discussed abve n their definitins, structural crack refers t crack that develped at the cre r frame that frm the fundatin f the building itself. Nrmally, any types f crack ccurred in this case is very dangerus and must be dealt immediately. Nn-structural as the name itself implies, refers t any parts f the building that desn t belng t the cre r frame f the building fr example wall (except lad bearing wall), driveway, pati, and walkway. Cracks ccurred in this case are nt that threatening if cmpared t structural crack. Still, this desn t mean that it wn t harm the ccupants living inside, as n matter what structure if left nt prperly cared will cause catastrphe eventually. Figure 1: (clckwise frm tp left) i) Wing wall crack f a tunnel (nn-structural) ii) Verandah crack (nn- structural) iii) Clumn crack and spalling (structural) iv) Beam crack f a bridge (structural)

4 Figure 2: Dn t judge a crack by its plaster! Smetimes, peple might cnfuse whether it was the clumn that cracks r the plaster cvering the clumn did.

5 TYPES OF CRACK AND HOW IT OCCURS Sulfate attack crack Sulfate attack can be 'external' r 'internal'. External: due t penetratin f sulfates in slutin int the cncrete frm utside. Internal: due t a sluble surce incrprated int cncrete at the time f mixing. External sulfate attack This is the mre cmmn type and typically ccurs where water cntaining disslved sulfate penetrates the cncrete. A fairly well-defined reactin frnt can ften be seen in plished sectins; ahead f the frnt the cncrete is nrmal, r near nrmal. Behind the reactin frnt, the cmpsitin and micrstructure f the cncrete will have changed. The effect f these changes is an verall lss f cncrete strength. These changes may vary in type r severity but cmmnly include: Extensive cracking Expansin Lss f bnd between the cement paste and aggregate Alteratin f paste cmpsitin Surces f sulfate which can cause sulfate attack include: Seawater Oxidatin f sulfide minerals in clay adjacent t the Bacterial actin in sewers - anaerbic bacterial prduce sulfur dixide Masnry - sulfates in bricks can be gradually released ver a lng perid

6 Figure 3: Image f sulfate attack in cncrete. Ettringite (arrwed) has replaced sme f the calcium silicate hydrate in the cement paste. As a cnsequence, the paste will be weakened. Althugh much f the cement paste here remains apparently unaltered (tp right), if widespread within the cncrete, sulfate attack can significantly weaken the cncrete. Internal sulfate attack Internal sulfate attack ccurs when a surce f sulfate is incrprated int the cncrete when mixed. Examples include the use f sulfate-rich aggregate, excess f added gypsum in the cement r cntaminatin. Prper screening and testing prcedures shuld generally avid internal sulfate attack. Delayed ettringite frmatin Delayed ettringite frmatin (DEF) is a special case f internal sulfate attack. It ccurs in cncrete which has been cured at elevated temperatures, fr example, where steam curing has been used. It can als ccur in large cncrete purs where the heat f hydratin has resulted in high temperatures within the cncrete. DEF causes expansin f the cncrete due t ettringite frmatin within the paste and can cause serius damage t cncrete structures. DEF is nt usually due t excess sulfate in the cement, r frm surces ther than the cement in the cncrete. A definitin f delayed ettringite frmatin DEF ccurs if the ettringite which nrmally frms during hydratin is decmpsed, and then subsequently re-frms in the hardened cncrete. Damage t the cncrete ccurs when the ettringite crystals exert an expansive frce within the cncrete as they grw. If expansin causes cracking, ettringite may subsequently frm in the cracks but this des nt mean the ettringite in the cracks caused the cracks initially. DEF causes a characteristic frm f damage t the cncrete.

7 While the paste expands, the aggregate des nt. Cracks frm arund these nn-expanding 'islands' within the paste - the bigger the aggregate, the bigger the gap. Figure 4: Diagram shwing hw paste expansin prduces a small gap arund small aggregate particles and a bigger gap arund larger particles. Figure 5: Scanning electrn micrscpe image f limestne aggregate particle. The cement paste has expanded and a gap has frmed between the aggregate and the cement paste. This is characteristic f damage t cncrete due t DEF. The aggregate is n lnger cntributing t cncrete strength, since it is effectively detached frm the cement paste. Often, these gaps becme filled with ettringite.

8 Lading crack A lading crack is a result f the lading t which the structure is subjected. A prperly designed structure wuld nt exhibit these cracks, but an imprperly designed structure is very susceptible t this damage. Vertical cracking at the end f a structure is typically due t a cncentrated frce being applied at the tp f a structure which exceeds the shear capacity within the end sectin f the structure. This type crack typically maintains a tight appearance at the tp and at the bttm but may shw a wider gap at apprximately mid-height f the structure. This wuld tend t indicate a bulging effect f the end segment f the structure away frm the remainder f the structure. This crack is illustrated belw: Figure 6: Bulging n end wall when subjected t cncentrated lad Hrizntal cracking within the centre prtin f the structure is typically caused by lateral pressures n the structure which exceed the flexural capacity f the structure. These pressures are nrmally generated by saturated sil cnditins being applied t a basement type structure. When the pressures exerted by the sils retained behind this structure exceed the flexural capacity f the structure, a crack is generated. Observing within the crack, ne will nte that the crack n the expsed face f the structure is cnsiderably wider than the crack n the cncealed face f the structure. Accmpanying this crack, ne will find a measurable amunt f bwing within the structure. This will exhibit itself as a bulge at mid-height int the basement area. This type cracking is illustrated belw:

9 Figure 7: Bulging f wall when subjected t sil behind it Plastic shrinkage crack When water evaprates frm the surface f freshly placed cncrete faster than it is replaced by bleed water, the surface cncrete shrinks. Due t the restraint prvided by the cncrete belw the drying surface layer, tensile stresses develp in the weak, stiffening plastic cncrete, resulting in shallw cracks f varying depth. These cracks are ften fairly wide at the surface. Figure 8: Crack due t plastic shrinkage Drying shrinkage crack Because almst all cncrete is mixed with mre water than is needed t hydrate the cement, much f the remaining water evaprates, causing the cncrete t shrink. Restraint t shrinkage, prvided by the sub grade, reinfrcement, r anther part f the structure, causes tensile stresses t develp in the hardened cncrete. Restraint t drying shrinkage is the mst cmmn cause f cncrete cracking. In many applicatins, drying shrinkage cracking is inevitable. Therefre, cntractin (cntrl) jints are placed in cncrete t predetermine the lcatin f drying shrinkage cracks.

10 Figure 9: Crack due t drying shrinkage D-cracking D-cracking is a frm f freeze-thaw deteriratin that has been bserved in sme pavements after three r mre years f service. Due t the natural accumulatin f water in the base and sub base f pavements, the aggregate may eventually becme saturated. Then with freezing and thawing cycles, cracking f the cncrete starts in the saturated aggregate at the bttm f the slab and prgresses upward until it reaches the wearing surface. D-cracking usually starts near pavement jints. Figure 10: D-cracking ccurred at pavement jints. Alkali-aggregate reactin crack Alkali-aggregate reactivity is a type f cncrete deteriratin that ccurs when the active mineral cnstituents f sme aggregates react with the alkali hydrxides in the cncrete. Alkali-aggregate reactivity ccurs in tw frms alkali-silica reactin (ASR) and alkali-carbnate reactin (ACR). Indicatins f the presence f alkali-aggregate reactivity may be a netwrk f cracks, clsed r spalling jints, r displacement f different prtins f a structure.

11 Figure 11: Cncrete with alkali-aggregate reactivity Thermal crack Temperature rise (especially significant in mass cncrete) results frm the heat f hydratin f cementitius materials. As the interir cncrete increases in temperature and expands, the surface cncrete may be cling and cntracting. This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and center is t great. The width and depth f cracks depends upn the temperature differential, physical prperties f the cncrete, and the reinfrcing steel. Figure 12: Structures in ht climate cuntries face severe thermal crack prblem.

12 Settlement crack Lss f supprt beneath cncrete structures, usually caused by settling r washut f sils and sub base materials, can cause a variety f prblems in cncrete structures, frm cracking and perfrmance prblems t structural failure. Lss f supprt can als ccur during cnstructin due t inadequate frmwrk supprt r premature remval f frms. Figure13: Effect frm settling f sil n the slab abve it The three diagrams belw shw hw settlement n sil causing cracks n the wall resting abve it

13 Crrsin crack Figure 14: Hw different settlements affect the cracking mde Steel reinfrcement, in the alkaline envirnment prvided by cncrete, is in a stable cnditin because a prtective xide layer frms n the steel surface, which stps crrsin. There are, hwever, tw situatins where this passivating envirnment at the reinfrcement can be disrupted. The first is knwn as carbnatin. This is when atmspheric carbn dixide disslves in water t frm carbnic acid, which neutralises the cncrete alkalinity. The carbnatin prceeds thrugh the cncrete cver and eventually reaches the reinfrcement, at which pint the passive layer is n lnger sustained and crrsin ccurs. The secnd disruptive effect is chlride attack. Chlrides may have been cast in the riginal mix, r may be intrduced frm an external surce such as de-icing salts r a marine envirnment. When in sufficient cncentratin at the reinfrcement, they will disrupt the passive film n the steel and als cause crrsin t ccur rapidly. T supprt the crrsin activity there must be xygen and water available and this is nrmally the case in atmspherically expsed cncrete. Crrsin f reinfrcing steel and ther embedded metals is ne f the leading causes f deteriratin f cncrete. When steel crrdes, the resulting rust ccupies a greater vlume than steel. The expansin creates tensile stresses in the cncrete, which can eventually cause delaminatin and spalling. Figure 15: Inside view n crrded reinfrcement grws duble its size

14 Figure 16: Sign f delaminatin in beam Crazing Crazing is a pattern f fine cracks that d nt penetrate much belw the surface and are usually a csmetic prblem nly. They are barely visible, except when the cncrete is drying after the surface has been wet. Tree grwth crack Figure 17: Crack clearly visible when the surface has been wetted Smetimes, the effect f Mther Nature d cause harm t the structure itself. Rts that grw underneath the grund can cause the slab abve it t crack. Wrse cme t wrse is when a tree grwth inside the building, pushing the beams, clumns and walls aside, causing cracks and cllapse eventually. Thugh it might nt cme int cnsideratin during design stage, but this kind f crack if it happened, will bring dwn the reputatin f bth the architects and the engineers themselves.

15 Figure 18: Effects f tree grwth n the structure arund it which shuld be avidable. Imagine what if the rts grw underneath yur cncrete building?

16 PREVENT BEFORE IT S TOO LATE! Optimum water rati Shrinkage is a primary cause f cracking. As cncrete hardens and dries, it shrinks. This is due t the lss, thru evapratin, f excess mixing water. Thus, in mst cases, the wetter the cncrete mix, the greater the shrinkage will be. Cncrete slabs can shrink as much as 1/2 inch per 100 feet. The actual amunt is 1/16th inch fr every ten feet f hrizntal distance. This shrinkage causes frces in the cncrete which literally pull the slab apart. Cracks are the end result f these frces. Cncrete des nt require much water t achieve maximum strength. In fact, a wide majrity f cncrete used in residential wrk, in many cases, has t much water. This water is added t make the cncrete easier t install. It is a labr saving device. This excess water nt nly prmtes cracking, but it can severely weaken the cncrete. Curing methd Rapid drying f the slab will significantly increase the pssibility f cracking. The chemical reactin which causes cncrete t g frm the liquid r plastic state t a slid state requires water. This chemical reactin, r hydratin, cntinues t ccur fr days and weeks after cncrete was pured. Engineers must make sure that the necessary water is available fr this reactin by adequately curing the slab. The use f liquid curing cmpunds, cvering the slab with plastic, wet burlap, and ther methds can be used t cure cncrete. Figure 19: Burlap (arrwed) was used t help the curing f the newly-pured cncrete

17 Slid grund The grund upn which the cncrete will be placed must be cmpacted. Never pur cncrete n frzen grund as nce the ice melt it will cause vid in the sil. If the new cncrete is pured ver sft, uncmpacted sil, a heavy delivery truck will easily bend and crack the cncrete as it passes ver the sft spt. This is why cncrete needs t be pured n slid and cmpacted sils. Figure 20: A wrker using high frequency vibratin t cause the sil t settle int a denser mass Prper usage f material Many peple had wndered why ancient structures are s strng and still standing till nw. Engineers had fund that these buildings were ver-designed r in ther wrds, maximum usage f cnstructin material. Let s take an example f a driveway cncrete slab. A 5-inch thick slab is definitely better in sustaining heavy vehicles than a 4-inch thick slab which is mre likely t crack under lading. Sme cntractrs might suggest that 4-inch is just enugh when cst cmes int cnsideratin but a 5-inch thick is even safer in reality. Thicker cncrete is a gd idea fr better lad bearing structure (fr this case, it was slab). Cracking can be minimized by fllwing ther guidelines as well. Installing prper strength cncrete fr intended use is always a gd practice as cncrete is available in many different strengths.

18 Cntrl jints Prfessinals wh install cncrete driveways install crisp tled lines in the slabs. These are called cntrl jints. Installing adequate cntrl and islatin jints at regular intervals in the slab helps t accunt fr hrizntal and vertical mvement in slabs. These jints can als be frmed with a tl r saw-cut sn after the slab has hardened. The purpse f these jints is t create a zne f weakness where the frces which are pulling n the slab will relieve themselves. Islatin jints allw a slab t mve independently f ther fixed r statinary bjects. The cntrl jints must be deep enugh t perfrm their jb. The minimum depth f the jint shuld be 1/4 the thickness f the slab. Figure 21: (Frm left t right) i) Picture shwing a type f nicely cut cntrl jint ii) A wrker is cutting a cntrl jint n the deck iii) Crack develped beside a cntrl jint iv) Typical manufactured cntrl jint Reinfrcement steel Reinfrcing steel fr residential purpses cmes in tw basic varieties, wire mesh, r rigid reinfrcing bars. The use f reinfrcing steel can help in the event a crack develps. Steel will hld cracked slabs tgether. Withut steel, cracks can grw in size and yu can get ffsets where ne part f the slab is higher r lwer than an adjacent piece. Steel needs t be placed n mre than 2 inches dwn frm the tp f the slab fr maximum perfrmance. Reinfrcing steel is als quite inexpensive. It is usually very easy t prperly install. Steel can significantly enhance the strength and durability f cncrete. In additin t all f the ther measures taken t prevent cncrete frm cracking, steel ffers a lw cst last line f defense.

19 Figure 22: Wire mesh hlds the cncrete tgether frm develping int bigger crack Cver fr reinfrcement The deteriratin f reinfrced cncrete is mainly due t reinfrcement crrsin. The mechanism f this deteriratin is t be reminded. Reinfrcements crrde when they are in cntact with a high amunt f aggressive agents. This is the reasn why, the preventin f reinfrcement crrsin, in structures t be built, is btained mainly by cntrlling the thickness and the quality f the cncrete cver. The cncrete cver thickness arund reinfrcement als depends n the envirnment aggressiveness. But, in additin t the requirements given by the designer, it is significant t cnsider the implementatin (reinfrcements psitining) t estimate the durability f a reinfrced cncrete really in place. Figure 23: Cncrete cver prvides shielding fr reinfrcement frm expsure t air. Hwever, in this case things gne t late and rehabilitatin needed fr a new cver.

20 Cating n reinfrcement When reinfrced cncrete structures are expsed t a very aggressive envirnment, an additinal prtectin can be cnsidered by cvering the reinfrcement steel. The tw types f the mst frequent cvers n steel are rganic catings and metal catings (ht-dip galvanising). These prtective catings which must adhere t steel must als ensure gd a bnd between reinfrcement and cncrete. Figure 24: Picture shws cnventinal reinfrcement bar (tp), cated reinfrcement (middle) and stainless reinfrcement (bttm) Cating n cncrete In sme cases, cncrete reinfrcement crrdes in a slw pace. Therefre, its cver seems physically satisfactry and has neither crack nr delaminatin. But, it is then cnvenient t slw dwn this crrsin rate, even t stp it. The methds which can be prpsed are either cncrete impregnatin with waterprf prducts (sealants) r inhibitrs. Paintings and catings f varius thicknesses can als be applied n cncrete t imprve its resistance t liquid penetratin. It deals, fr example, with either f catings cntaining epxy resin r plyurethane, r with mrtar cntaining mdified hydraulic binder.

21 Figure 25: (left t right) i) Wrkers painting the wall t prevent misture entering the cncrete ii) Smetimes, paint isn t the right chice fr cncrete flrs. Epxy catings specially frmulated fr cncrete are bth prtective and decrative. Crrsin inhibitr The risk f crrsin f a new structure is likely t be due t chlride attack. The latest develpments in crrsin preventin are the use f crrsin inhibitrs in the cncrete mix. There are a variety f generic types available, the principal nes being calcium nitrite and amin alchls. These materials are added t the cncrete and frm a very thin chemical layer n the reinfrcement, which inhibits the crrsin activity. They are cnsumed and will nly wrk up t a given chlride level. It is imprtant t ensure that the crrect dsage level is used and t remember that the inhibitr will have a finite service life.

22 Figure 26: A cut ut frm brchure shwing the benefits f using crrsin inhibitr in cncrete Cathdic prtectin Arguably, the mst established system f all is cathdic prtectin (CP). The principals f CP have been knwn since 1824 when Sir Humphrey Davey first used sacrificial andes t prtect ships hulls. Crrsin is an electrchemical prcess where the crrsin sites are andic and passive sites are cathdic. Cathdic prtectin is by intrducing an ande t the cncrete and making all the steel cathdic. A small direct current is passed between the ande and the reinfrcement t cathdically prtect the reinfrcement. The ande is a permanent additin t the structure and the system is cmputer cntrlled t minimize future mnitring csts. Sacrificial ande systems are based n the use f a galvanic ande such as zinc, which are directly cnnected t the reinfrcement and crrdes in favr f the steel. The cathdic prtectin system is mnitred by using silver/silver chlride reference electrdes, which are buried in the cncrete alngside the cathdically prtected steel reinfrcement. These are cabled back t the cmputer cntrl system, which usually has a mdem link t enable the CP system t be remtely mnitred. As far as the future is cncerned, it will be difficult t predict the next generatin f crrsin preventin systems, simply because revlutinary new ideas are nt discussed until the patents are in place.

23 Figure 27: Schematic diagram shwing the set-up f cathdic prtectin system Figure 28: Example f an verpass in Kingstn, Ontari (Canada) shws the installatin n cathdic prtectin system