Concrete Failure Mechanisms

Advanced Concrete Technology

Failure Mechanisms in Concrete

Failure in concrete: • Concrete is multi-phase composite material consisting of cement paste, aggregates, discontinuities, fluids, etc. The properties of it depend largely on the properties of its constituents. • Concrete failure refers to the failure of concrete to respond under intended or expected environments or conditions (e. g. loads, forces, environmental factors, etc.)

• Concrete being a fairly brittle material, tends to fracture under excessive loads without or very little plastic deformation unlike ductile material. • Failures of concrete structures often imply large deformations, severe honeycombing and cracking with spalling and ultimately collapse.

Main types of concrete failure: • Mechanical: physical impacts, collisions, overloading • Chemical: ASR, sulfate attack, contaminants from soil • Fire: When exposed to extreme temperatures, problems such as expansion and deterioration can occur. • Corrosion: when concrete fails to provide adequate protection from extremities such as road salts, seawater and chlorides, the steel rebar will corrode. Cracks, low cover, porosity and low alkalinity are also other causes of inadequate protection.

Sources of failure Environmental factors such as abrasion/erosion, chemical attacks, bacterial attacks, climatic conditions. These are difficult to be controlled by human. Production factors including type (quality) of aggregates, degree of compaction, w/c ratio, quality and type of cement. ➢ Effects of the sources of concrete failure described gives defect to concrete. Consequences of defects are deformation of concrete surface, cracking of the surface and disintegration of the surface.

DEFORMATION UNDER LOAD • It is a stress strain relationship under normal loading and under sustained loading. • Under normal loading: the first effect of applying a load to concrete is to produce an elastic deformation i.e. as the load increases deformation increases.

• Under sustained loading: the continue application of stress causes a slow deformation known as creep. The increase of deformation is not proportional , as the time passes the deformation is lesser.

Elastic deformation: • When the applied load is released, the concrete does not fully recover its original shape. • Under repeated loading and unloading, the deformation at a given load level increases.

Modulus of elasticity • Defined as the ratio of load per unit area (stress) to the elastic deformation per unit length (strain). • The modulus of elasticity for most concretes at 28 days, ranges from 15 – 40 kN/mm2. 𝑠𝑡𝑟𝑒𝑠𝑠 𝜎 • E= = 𝑠𝑡𝑟𝑎𝑖𝑛 𝜀 • The stress–strain relationship for concrete is non-linear and the material is not strictly elastic. • Three types of E-value are used, namely secant modulus, tangent modulus and initial tangent modulus.

A typical illustration of deformation of concrete subjected to constant load.

Deformation and Failure of concrete under uniaxial loading: Mechanisms of fracture and cracking in concrete; • Concrete can generally support loads up to 60% of ultimate strength without any apparent signs of distress. • As the load is increased above this level, at about 70–90% of ultimate, small cracks appear on the surface. At this stage sustained loads result in eventual failure.

• Cracks spread and interconnect until, at ultimate load and beyond, the specimens are increasingly disrupted and eventually fractured into a large number of separate pieces. • The formation and propagation of small microscopic cracks 2–5 μm long (microcracks) have long been recognized as the causes of fracture and failure of concrete and the marked non-linearity of the stress–strain curve near and beyond ultimate strength.

Comparison between stress-strain curve of steel and concrete

Stages of concrete fracture: The fracture processes in concrete depend primarily upon the applied state of stress and the internal structure of the specimen. Even before loading, intrinsic volume changes in concrete due to shrinkage or thermal movements can cause strain concentrations at the aggregate–paste interface. There appear to be at least three stages in the cracking process;

• Stage I: Within this stage localized microcracks are initiated at the at isolated points throughout the specimen where the strain concentration is the largest. Irrecoverable deformation being small. This shows that these cracks are stable and, at this load stage, do not propagate. As the applied load is increased during Stage I, there will continuous process of stable crack initiation.

• Stage II: As the applied load is increased beyond Stage I, initially stable cracks begin to propagate. There will be gradual transition from one stage to another. During Stage II the cracks propagate but in a slow stable manner. The extent of the stable crack propagation stage will depend upon the applied state of stress, being very short for brittle fractures under predominantly tensile stress states and longer for more plastic fractures under predominantly compressive states of stress.

• Stage III: This occurs when, under load, the crack system has developed to such a stage that it becomes unstable, the

cracks self-propagate until complete disruption and failure occurs. Once Stage III is reached, failure will occur whether or not the stress is increased. This stage starts at about 70–90% of ultimate stress.

Stages of cracking in concrete

Original crack • The interface damage is likely to happen because there are original cracks in interface. • Original cracks refer to the cracks formed in the interface of coarse aggregates and mortar before the concrete is loaded.

Types of original crack • Dry shrinkage

• Cold contract • Volume decrease • Settlement • Plastic shrinkage

• Bleeding path forming

sketch map of original crack

Crack Propagation • Micro cracks initiate in the interfacial zone at very low load levels (as a result of the load being concentrated around the areas of high stiffness, i.e. the aggregates). • These micro cracks propagate further into the mortar, localize, and lead to the ultimate failure of the concrete. • This behavior is depicted in the schematic diagram shown in Figure. As seen in this figure, when the load is below 30 – 40%, the cracks are limited to micro cracks in the ITZ. When the load level increases to about 50%, the micro cracks become larger. At higher loads (~ 75%), these cracks start propagating into the mortar, and then localize to cause the ultimate failure.

Crack initiation and propagation

Main fracture (damage) mechanisms • In concrete, “weaker planes” occur at the interface of the cement mortar and the aggregate: as a result of bleeding, shrinkage, etc.

• The microcracks that appear at the interface tend to propagate along the aggregate surfaces. • These microcracks can combine to form macrocracks. • In addition there can be “mortar cracks” which run through the matrix material, as well as “aggregate cracks” which tend to split apart the aggregates.

Failure under compression: • The propagation of internal microcracks and micro-voids is reflected in the macroscopic stress-strain behavior of concrete.

• For instance, under uniaxial compression, growth of microcracks aligned to the direction of loading leads to stress softening.

There are three types of damage in concrete under compressive load; • Hardened cement damage • Interface damage • Coarse aggregates damage

A. B. C. D.

Undamaged concrete Hardened cement damage Aggregate-paste interface damage Coarse aggregate damage

• Under uniaxial compressive stress evidence suggests that, first, stable cracks are initiated in the matrix parallel to the direction of applied compression. As the load is increased, the cracks multiply and extend in this same direction until, in the vicinity of mineral aggregate inclusions, the fracture path divides and travels around, rather than through, the hard aggregate particles. After final disruption and failure, this fracture mechanism produces isolated particles of aggregate adhering to which are small ‘cones’ of mortar at each end aligned in the direction of maximum principal compressive stress.

Failure mode in compression

• Study of the remnants after concrete failure indicates that the probable mechanism of crack propagation and ultimate failure in concrete under a uniaxial compressive stress is by cracks forming in the cement paste matrix due to the presence of microcracks and flaws and the stress and strain intensification around aggregate particles (ITZ).

• Figures indicate that crack propagation paths may occur at the aggregate–paste interface, in the cement matrix or in the particles of aggregate (as in the case of weak particles).

• Crack development pattern

Compressive strength test • Compressive strength is the capacity of cubical or cylindrical specimens of concrete to withstand axially directed loads pushing on it. • The compressive strength of the specimen is calculated by dividing the maximum load achieved during the test by the cross-sectional area of the specimen. This can be performed by compressive strength testing apparatus. • A concrete cube test or concrete cylinder test is generally carried out to assess the strength of concrete after 7 days, 14 days or 28 days of casting. • ASTM C39/C39M provides Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. • BS EN 12390-3:2019 provides standard compressive strength test of cube and cylinder specimens. For a given mix, testing strength of a concrete specimen is mainly influenced by the following factors:

a. the ratio of the size of the specimens to the maximum size of the aggregates b. the rate of application of the load c. the end restraint due to the platens of the compression machine d. the slenderness ratio of the specimens (i.e. the length to diameter ratio) e. the size of specimen.

Effect of size • As a thumb rule, the cylinder strength will be 0.8-0.84 times cube strength or compressive strength of a cube is about 20% higher than a cylinder made of the same concrete mix. • The main difference between cylinder & cube testing procedures is capping. Cylinder ends are not plane or parallel enough to mate properly with platens of compression testing machines, and thus must be capped with sulphur, neoprene, or other suitable material for proper distribution of the load. • Cube however are not capped but cast in rigid molds with sides that are plane & parallel. When tested they are flipped on their sides so that machine platens mate properly with cube surfaces. • A factor also affecting the cylinder/cube strength ratio is, specimen geometry such as slenderness ratio as cylinders tend to be casted with ratio of length to diameter equal to 2. Given the casting, curing, rate of loading and aggregate grading kept same.

General relationship b/w strength ratio & h/d ratio

• For cube test two types of specimens either cubes of 150mm X 150mm X 150mm or 100mm X 100mm x 100mm depending upon the size of aggregate are used. For most of the works cubical molds of size 150mm X 150mm X 150mm are commonly used. • According to experimental study, it is it is reasonable to assume that the larger the volume of the concrete, the more likely it is to contain an element of weaker strength. As a result, for a concrete specimen of the same batch, it is reasonable to expect that its strength and its variability will decrease as the specimen size increases.

• Strength obtained on 100mm cubes is higher than 150mm cubes and the variation in the values of compressive strength of concrete ranges between 5 to 6%. • The more homogeneous the concrete the smaller will be the size effect. 28-day compressive strength of 100mm and 150mm cubes

• There are quite a few studies available on the strength comparison of 100mm and 150mm diameter cylinder specimens and of 100mm and 150mm cube specimens. • Lessard (1990) found that the compressive strength of cylinders of 150mm diameter by 300mm was about 94% that of cylinders of 100mm dia. by 200mm height. • Baalbaki et al (1992) repeated the experiment on a total of 126 cylinders and found that the strength of the cylinders of 150mm diameter was 93% that of cylinders of 100mm dia..

• Neville (1977) suggested that the strength of the 100mm cubes was about 104% that of the 150mm cubes.

Effect of loading rate • Load is applied continuously at the rate of 0.25±0.05 MPa/s (ASTM C39) or ranging from 0.2 up to 1.0 MPa/s according to BS EN 12390-3 without shock increase corresponding to 140 kg/cm2.min. • With variation in rate of loading on concrete specimen, the strength varies proportionately. • At higher rate of loading, the compressive strength increases. • The increment is from 30% to almost 50% of the original strength. • However, at lower rate of loading, the reduction in strength of concrete cube compared to its true strength is insignificant. • In order to determine the true strength of concrete cube, the obtained strength shall be divided by suitable correction factor.

Cube

Cylinder

Advantages

disadvantages

Advantages

disadvantages

Rigid molds and loading on side eliminates need for capping

More specimens needed due to greater variability in results esp. smaller specimens

Less variation in test results

Cylinders with h/d=2 have weights 1.5 times that of cubes

Lighter, easier to handle

If segregation occurs, cube flipping may give misleading results

Lower variability allows use of less specimens

Capping required

Less materials required for casting

More sensitive to changes Tested in the direction of in aggregate grading, casting platen, h/d variations, etc.

Less storage space needed in curing vessels

Only rigid multi-use molds Single-use and multi-use permitted molds permitted

Capping adds extra material and cost to the testing process

❑ Based on past research, replacing cylinder testing with cube testing is not recommended.

Effect of max. aggregate size experimental facts proved that the larger the aggregate size the smaller the surface area to be wetted per unit mass and therefore if the water/cement ratio is kept constant there will be more water. Concrete samples made with smaller coarse aggregate size have higher strength than concrete samples made with bigger size of coarse aggregate due to the weak bonds in the later resulting from greater heterogeneity, internal bleeding and the development of microcracks.

Failure under tension: • Tensile strength of concrete is approximately one-tenth of compressive strength.

• The fracture process in compression is more stable than for uniaxial tension since the loaded area is less influenced by the cracking. • The behavior is much more unstable under uniaxial tension since cracking reduces the loaded area of the concrete whereas under uniaxial compression the cracks are aligned in the direction of loading and do not significantly influence the loaded area. It is for this reason that the tensile strength of concrete is much less than the compressive strength.

• Under uniaxial tensile states of stress, the fracture path runs essentially orthogonal to the maximum tensile stress direction. • For strong, naturally occurring, dense aggregates the crack path tends to follow the aggregate–paste interface. • With weaker aggregates, including lightweight, the fracture path passes through the aggregate particles.

• The tensile strength of these concretes is dependent, therefore, mainly on the aggregate–paste (tensile) bond strength. • For tensile stresses, stable crack propagation (Stage II) is of short duration, since the cracks propagate very rapidly through the mortar matrix and around the aggregate–paste interface. • Under sustained load, stress-concentrations will cause stress intensification at cracks or weak links such as aggregate-paste interface where pores are existent. • The presence of steel reinforcement reduces crack widths: stiffening effect on the post peak behavior. • At higher applied stress levels (Stage III) cracks begin to propagate around the aggregate particles as the aggregate–paste bond fails. The culmination of Stage III is failure of the element by cracking parallel to the maximum principal compressive stress or orthogonal to the maximum principal tensile stress.

• According to previous studies, the maximum tensile strain for concrete is between 0.00015 and 0.00025; the average maximum tensile strain being 0.0002 corresponding to initial cracking points.

Stress concentration at crack position

Failure mode in tension

Failure Types in Cubes and Cylinders

Failure of concrete under multiaxial loading • Under biaxial compressive states of stress, the alignment of the fracture path in both directions of principal compressive stress produces a crack pattern such that, at final disruption, the ‘cones’ of mortar on aggregate are extended to form complete ‘haloes’ around the particles. • However, the principal problem to be addressed when measuring the deformational and strength characteristics of concrete under multiaxial stresses is the specimen/testing machine interaction. .

Zone 1 type failures are evident over most of the C/T region, with Zone 2 failures near uniaxial stress. Elsewhere Zone 3 failures are produced with cracking orthogonal to the unloaded direction which is the direction of zero stress (minimum compression). Biaxial strength envelops and failure modes

Effect of confinement • Under confinement compressive strength of concrete increases. • Confining pressure acts to prevent crack propagation and leads to more ductile response. • In elastic zone, the behavior of confined and unconfined columns is very similar. On plastification small stress increment causes large radial expansion.

• Confining pressure minimizes the expansion and growth of tensile cracks in concrete & results in higher failure loads.

Stress-strain response of concrete

Typical stress–strain relationships for a concrete under equal biaxial compressive stress using various loading devices.

Stress–strain relationships for a concrete under triaxial compression with various confining pressures.

Volumetric strain–axial stress relationships for a concrete under triaxial compression with various confining pressures.

• Stage I response. For triaxial compression at high confining pressures the mode of failure changes from brittle to ductile as evidenced by the stress–strain relationships and no clear fracture mechanism is visible since the cracks are extremely small and localized. • Stage II response. Defined by the stage at which the volume change relationships attain a minimum value before dilating to failure.

• Stage III (failure mode). The cracking of concrete under multiaxial stress results in the complete disruption of specimens at, or beyond, the ultimate stress level in a manner which is dependent on the type of stress state imposed.

Typical failure mode for concrete under triaxial ‘compression’ at high confining pressure

Confinement by wrapping with FRP • Fiber reinforced polymer (FRP) composites are typically made of fibers such as glass, aramid, and carbon embedded in a polyester or vinyl ester resin matrix. • The strength of FRP depends on the elastic properties of the fiber and matrix, their relative volumes, and length and orientation of fibers within matrix. • GFRP: Made of glass which is not as strong as carbon fiber. It is much cheaper and significantly less brittle. • CFRP: Made of carbon fibers about 5-10μm in diameter. It has high modulus, high tensile strength. • AFRP: Made of aramid fiber. • FRP composites are corrosion resistant, lightweight, and have high strength. • FRPs are commonly used in aerospace, automotive, marine, and construction industries. • Applications include the construction of FRP bridge deck systems, concrete decks with reinforcing FRP rebar, and the strengthening and repair of existing structures.

Confinement by FRP wrap

Property ranges for different type of FRPs

Effects of multiaxial stresses on cube and cylinder specimens

Preparation of FRP wrapped specimens

Wrapped specimens after testing

T H E E N D