DURABILITY of CONCRETE STRUCTURES Assist. Prof. Dr. Mert Yücel YARDIMCI Part- 3 Concrete Cracks This presentation covers the subjects in CEB Durable Concrete Structures Guideline and has been prepared by the graduate students under the supervision of Prof.Dr.Bülent BARADAN in Dokuz Eylul University. 1
Causes of cracks Cracking will occur whenever the tensile strain to which concrete is subjected exceeds the tensile strain capacity of the concrete. The tensile strain capacity of concrete varies with age and with rate of application of strain. 2
TYPES & FORMATION of CRACKS FRESH STATE HARDENED STATE EARLY FROST DAMAGE PLASTIC SHRINKAGE CONSTRUCTURAL MOVEMENTS PHYSICAL CHEMICAL BIOLOGICAL THERMAL CONSTRUCTURAL (MECHANICAL) SHRINKAGE SETTLEMENT FORMWORK MOVEMENT BASE SETTLEMENT SHRINKABLE AGGREGATES DRYING SHRINKAGE BLEEDING CORROSION of REINFORCEMENT ALKALI-SILICA REACTION ACID ATTACK SULFATE ATTACK CARBONATION DELAYED ETTRINGATE FORMATION (DEF) FREEZING & THAWING TEMPERATURE DIFFERENCES EARLY TERMAL SHRINKAGE EXCESSIVE LOADING CREEP INAPPROPRIATE DESIGN SETTLEMENT OF SUPPORT EXT. INT. 3
4 Bending Cracks A A J I I I C K C E F Rust stains B L M N N I I D D I H H G G Cold dilations C O O O O O Relative Shrinkage craks Relative shrinkage craks Late, ineffective dilations Cold dilations Shear crack Bonding cracks K B Bending crack C G See Table 3.1 in CEB
FORMATION of CRACKS & TIMES Loading, service conditions Alkali-Aggregate Reaction Corrosion Drying Shrinkage Early thermal Shrinkage Plastic Shrinkage Plastic Settlement 1 hour 1 day 1 week 1 month 1 year 50 years 1 hour 1 day 1 week 1 month 1 year 50 years 5
TYPES & FORMATION of CRACKS TS 500 (February 2000) (Requirements for design and construction of reinforced concrete structures) 13.3 CRACK CONTROL Cracks that may cause corrosion or may influence the apperance of structures should not be permitted. Max crack width ( max ) 0.4 mm Normal environment and indoors 0.3 mm Normal environment and humid indoors 0.2 mm Humid environment and outdoors 0.1 mm Aggressive environment, indoors & outdoors 6
Shrinkage Shrinkage is the reduction in volume of concrete by different mechanisms. 7
SHRINKAGE CRACKS RESTRAINMENT of SHRINKAGE DEFORMATIONS CAUSE CRACKS. DISADVANTAGES of SHRINKAGE: 1. FORMATION of CRACKS 2. FORMATION of EXTRA STRESSES IN REINFORCEMENT HAZARDS of CRACKS DECREASE TENSILE STRENGTH WITH THE EASIER INGRESS of WATER FREEZE-THAW RESISTANCE DECREASES! DURABILITY AGAINST CHEMICAL ATTACKS 8
TYPES OF SHRINKAGE DRYING SHRINKAGE (Hydraulic shrinkage) LOSS of PORE WATER PLASTIC SHRINKAGE BLEEDING RATE < EVAPORATION RATE AUTOGENOUS SHRINKAGE REDUCTION in VOLUME DUE to HYDRATION of CEMENT THERMAL SHRINKAGE DIFFERENCE of TEMPERATURE IMPORTANT in MASS CONCRETE CARBONATION SHRINKAGE EVAPORATION of WATER in CHEMICAL REACTION 3Ca(OH) 2 +CO 2 CaCO 3 +H 2 O 9
Drying shrinkage The moisture content of hardened concrete has a potentially significant influence on the volume that it occupies, with an increase in the level of moisture producing an increase in volume. Moisture content is controlled by the relative humidity of the surrounding environment 10
Drying shrinkage The reason of drying shinkage is associated with; the development of capillary pressure the drying of calcium silicate hydrate (CSH) gel, a reaction product formed during the setting and hardening of cement. 11
Drying shrinkage Development of capillary pressure In a concrete pore, the two fluids are water and air. A pressure difference develops when two immiscible fluids are present in a capillary and only one is capable of wetting the surface. This attractive capillary pressure (p c ) is described by the Young Laplace equation. As the relative humidity in concrete drops (i.e. as it dries), water in progressively smaller pores will evaporate, causing the value of r in the equation to decrease. This increases the capillary pressure and causes shrinkage. 12
Drying shrinkage Drying of CSH gel The main reaction product of Portland cement is CSH gel, which typically comprises 50% to 60% by volume of a mature Portland cement (PC) paste. The gel is an aggregation of colloidal particles (particles having at least one dimension between 1 nm and 10 μm). Each gel particle is composed of layers of calcium silicate sheets, which are distorted and arranged on top of each other in a disordered manner. The nature of this configuration means that there is much space between the layers, which can be occupied by interlayer water. Water CSH layers The absorption of this interlayer water by the gel leads to swelling, and evaporation of water leads to shrinkage. 13
Effects of aggregates on drying shrinkage Most aggregate materials shrink to a lesser extent relative to cement paste, and although the cement fraction comprises a much smaller volume fraction than the aggregate, it normally has the largest influence on shrinkage. The higher modulus of elasticity of most aggregate relative to cement paste has a restraining effect that limits shrinkage. 14
Effects of aggregates on drying shrinkage There is usually a good correlation between the water absorption capacity of aggregates and the magnitude of drying shrinkage observed in concrete containing it The reason for the correlation between water absorption and shrinkage is the result of the strong relationship between the porosity of aggregate and its stiffness. 15
Effects of w/c ratio on drying shrinkage As the w/c ratio reduces, the modulus of elasticity of the hardened cement paste increases, presenting a greater resistance to shrinkage 16
Effects of different mineral admixtures on drying shrinkage Shrinkage curves obtained from mortars containing a combination of Portland cement and various size fractions of FA is shown below. In all cases, FA has the effect of reducing shrinkage, with the finer fractions producing the least amount of length change. The cause of this effect is that the finer FA fractions permit greater reductions in water content to achieve a given consistence, which means that the mortars with finer ash fractions have lower W/C ratios. However, there may be other mechanisms effective because even the coarsest ash, which is coarser than the PC and requires a higher W/C ratio, produces lower shrinkage than the control mortar. 17
Effects of different mineral admixtures on drying shrinkage Silica fume typically has the effect of very slightly increasing shrinkage at early ages. Later, as ultimate shrinkage is approached, shrinkage is lower at higher silica fume levels. 18
FACTORS THAT INFLUENCE SHRINKAGE INTERNAL EFFECTS CHEMICAL COMPOSITION (CaO, MgO & SO 3 ) CEMENT DOSAGE HEAT of HYDRATION QUANTITY of MIXING WATER MODULUS of ELASTICITY of AGGREGATE EXTERNAL EFFECTS LOW HUMIDITY SPEED of WIND HIGH TEMPERATURES 19
FACTORS THAT INFLUENCE SHRINKAGE DESIGN DETAILS AREA/VOLUME RATIO of STRUCTURAL ELEMENTS V 1 = V 2 Eva 1 < Eva 2 20
FACTORS THAT INFLUENCE SHRINKAGE DESIGN DETAILS AREA/VOLUME RATIO of STRUCTURAL ELEMENTS PERCENTAGE of REINFORCEMENT UNIFORMITY of REINFORCEMENT PLACEMENT UNPROPER CONSTRUCTION JOINTS F 1 = F 2 RIGHT WRONG 21
PREVENTIVE MEASURES for SHRINKAGE CRACKS MIX DESIGN OPTIMUM AMOUNT of CEMENT MINIMUM AMOUNT of MIXING WATER MAXIMUM AMOUNT of COARSE AGGREGATE (GOOD QUALITY) INCORPORATION of MICRO FIBERS CEMENT MINIMUM SHRINKAGE, LOW HEAT of HYDRATION, NOT TOO FINE. GOOD CURING DESIGN DETAILS SUFFICIENT AMOUNT of REINFORCEMENT, UNIFORM PLACING 22
Effect of aggregate content on drying shrinkage 23
Back 24
What is the form of crack formation in plain and reinforced concrete sections due to drying shrinkage? Restraint is important! The crack spacing can be reduced by reducing the bar diameter, improving the bond, increasing the quantity of steel. Reducing the depth of the cover will also reduce the spacing. 25
Strain development due to drying (Eurocode 2 approach) The tensile strength of concrete is substantially less than its compressive strength, usually between 5% and 15%. It develops with time in a similar manner with compressive strength, as shown in Figure 2.18. If the concrete element is restraint, the tensile stress due to shrinkage may exceed the tensile strength capacity of concrete. If this happens concrete cracks! 26
Strain development due to drying (Eurocode 2 approach) 27
Typical plot of drying shrinkage against time No instantaneous deformation due to drying 28
Autogenous shrinkage As Portland cement undergoes hydration reactions, free water will be converted to chemically combined or chemisorbed water on hydrate surfaces. This loss of free water can also lead to shrinkage. This is called autogenous shrinkage. This means that, even if evaporation from concrete is entirely prevented, there will be a considerable reduction in the volume of water present, and shrinkage will occur via the same mechanisms as for drying shrinkage. 29
Reducing the problem of cracking resulting from shrinkage Inclusion of movement joints Using reinforcement for crack control Using steel fibres or macrosynthetic fibres Go to fiber pictures Reducing the cement content The minimum cement content which is required for strength and durability requirements. Reducing the W/C ratio Since the water content of the concrete will be fixed to provide the required consistence to the fresh concrete mix, a reduction in the W/C ratio will lead to an increase in the cement content. For this reason, the use of water-reducing admixtures or superplasticisers is likely to be necessary to allow a reduction in the water content. NOTE: Remember that reduction of the W/C ratio to very low levels, while reducing drying shrinkage significantly, may lead to higher levels of autogenous shrinkage. Selection of aggregate with a high stiffness Using shrinkage-reducing admixtures can be used to modify the surface tension of the pore water. 30
SETTLEMENT CRACKS Reinforcement Cracks Water & cement mortar Coarse aggregate Before Settlement After Settlement Main Causes: Poor grading, too much mixing water, unsufficient compaction, 31
PLASTIC SETTLEMENT CRACKS A Subdivision : Over Reinforcement Deep Sections Causes: Excess Bleeding Rapid early drying conditions Remedy : Reduce bleeding (air entrainment) or revibrate 32
PLASTIC SETTLEMENT CRACKS A B B Subdivision : Arching Top of columns Causes: Excess Bleeding Rapid early drying conditions Remedy : Reduce bleeding (air entrainment) or revibrate 33
PLASTIC SETTLEMENT CRACKS A B C B Subdivision : Change of depth Trough & waffle slabs Causes: Excess Bleeding Rapid early drying conditions Remedy : Reduce bleeding (air entrainment) or revibrate 34
SETTLEMENT CRACKS 35
PLASTIC SETTLEMENT CRACKS 36
PLASTIC SHRINKAGE CRACKS Evaporation Bleeding CAUSE: EVAPORATION > BLEEDING RATE DRYING OF TOP SURFACES 37
PLASTIC SHRINKAGE CRACKS Subdivision : Diagonal Roads & Slabs Causes: Rapid early drying Low rate of bleeding Remedy : Improve early curing D 38
PLASTIC SHRINKAGE CRACKS E D Subdivision : Random Reinforced concrete slaps Causes: Rapid early drying Low rate of bleeding Remedy : Improve early curing 39
PLASTIC SHRINKAGE CRACKS E F Subdivision : Over reinforcement Reinforced concrete slaps Causes: Rapid early drying Steel near surface Remedy : Improve early curing D 40
PLASTIC SHRINKAGE CRACKS 41
PLASTIC SHRINKAGE CRACKS 42
PLASTIC SHRINKAGE CRACKS 43
EARLY THERMAL CONTRACTION G Subdivision : External restraint Thick walls Causes: Excess heat generation Rapid cooling Remedy : Reduce heat &/or insulate 44
EARLY THERMAL CONTRACTION G H Subdivision : Internal restraint Thick slabs Causes: Excess temperature Rapid cooling Remedy : Reduce heat &/or insulate 45
LONG TERM DRYING SHRINKAGE I I Subdivision : Thin Slaps (and walls) Causes: Insufficient joints Excess shrinkage, inefficient curing Remedy : Reduce water content, improve curing 46
LONG TERM DRYING SHRINKAGE Insufficient joints 47
SPALLING (crazing) J Subdivision : Against formwork fair-faced concrete Causes: Impermeable formwork- Rich mixes, poor curing Remedy : Improve curing & Finishing 48
SPALLING (crazing) J K Subdivision : Floated concrete - Slabs Causes: Over-trowelling - Rich mixes, poor curing Remedy : Improve curing & Finishing 49
CORROSION of REINFORCEMENT 50
CORROSION of REINFORCEMENT 51
CORROSION of REINFORCEMENT L Rust Stains Subdivision : Natural Columns & Beams Causes: Lack of cover Poor quality concrete Remedy : Eliminate causes listed 52
CORROSION of REINFORCEMENT Subdivision : Calcium chloride Precast concrete Causes: Excess calcium chloride Poor quality concrete Remedy : Eliminate causes listed L M 53
CORROSION of REINFORCEMENT 54
CORROSION of REINFORCEMENT 55
CORROSION of REINFORCEMENT Electrical pillars with corrosion damage in İZMİR 56
ALKALI-AGGREGATE REACTION N Subdivision : Damp locations Causes: Reactive aggregate plus high-alkali cement Remedy : Eliminate causes listed 57
ALKALI-AGGREGATE REACTION 58
ASR 59
ASR 60
DEF (DELAYED ETTRINGITE FORMATION) 61
THAUMASITE FORMATION Swiss tunnel structures: concrete damage by formation of thaumasite (Romer vd. 2003) 62
LOADING CRACKS PURE BENDING PURE TENSION 63
LOADING CRACKS TORSION CONCENTRATED LOAD Bond Cracks Bending SHEAR 64
LOADING CRACKS SHEAR 65
LOADING CRACKS SETTLEMENT CRACKS Settlement of Support Cracks 66
Influence of various concrete constituent characteristics on bleeding and plastic shrinkage cracking Configurations in concrete structural elements that may cause plastic settlement cracking. 67