Basic Concrete Tests Hardened Concrete Basic Concrete Tests - PowerPoint PPT Presentation

Basic Concrete Tests Hardened Concrete

Basic Concrete Tests Cylinder Compression Splitting Tension Beam Flexure Elastic Modulus Slump Unit Weight Air Content CIVL 3137 2

Cylinder Compression What do we mean when we say “I need 10 yd 3 of 4500-psi concrete”? It’s the uniaxial unconfined compressive strength of concrete cylinders that are made and cured according to either ASTM C31 (field samples) or C192 (lab samples) then tested according to ASTM C39. CIVL 3137 4

CIVL 3137 5

CIVL 3137 6

CIVL 3137 7

ASTM C39 unconfined uniaxial loading cylindrical specimen 6" diameter × 12" high cured 28 days @ 95% relative humidity loaded at 35  7 psi/s loaded using appropriate end caps CIVL 3137 8

Why Cylindrical Specimens? Ideally, you want the stress in the concrete to be uniaxial. Unfortunately, friction between the ends of the specimen and the testing machine imposes lateral stresses that confine the concrete and make it fail at a higher load than it should. In cubical specimens, the lateral stresses are present throughout the specimen. In cylindrical specimens, the concrete at the cylinder mid-height is far enough from the ends to be free of lateral stresses. As a result, cubical specimens fail at a load roughly 25% higher than cylindrical specimens. CIVL 3137 9

Shape Effects CIVL 3137 10

Shape Effects Cube Strength  1.25 × Cylinder Strength Ratio of Cylinder Strength to Cube Strength CIVL 3137 11

Why a 2:1 Aspect Ratio? The 2:1 aspect ratio ensures that the concrete at the mid-height of the specimen is free of lateral stresses. If you use a cylinder with a 1:1 aspect ratio, it would not significantly differ from a cube; there would be at least some confining stress throughout the specimen. If you use a 1:2 aspect ratio, the lateral stresses are so high that the concrete almost can’t fail except by crushing the aggregate particles themselves. CIVL 3137 12

Shape Effects d d 2d d d d/2 CIVL 3137 13

Shape Effects CIVL 3137 14

Size Effects The measured strength of concrete cylinders decreases as the specimen size increases. All concrete contains flaws arising from things like autogenous shrinkage cracks, incomplete cement-aggregate bonds, etc. The strength of a concrete specimen is governed by the weakest flaw within it. The larger the specimen the more likely it is to contain a critical flaw that will precipitate failure at a low load. CIVL 3137 15

Size Effects 3" cylinder  1.07 × 6" cylinder CIVL 3137 16

Loading Rate Effects The faster you load a concrete specimen, the stronger it appears to be. The reasons are not completely clear but one postulate is that slow loading allows small cracks to propagate to failure while fast loading stays one step ahead of the crack growth, allowing a larger load to be applied before the concrete visibly fails. Another postulate is that slower rates allow creep to occur, which increases the internal strains at a given load. Concrete failure is controlled by the strains that develop in the specimen, not the stresses! CIVL 3137 17

Loading Rate Effects CIVL 3137 18

Cylinder Caps Concrete cylinders have end surfaces that are rough and may not necessarily be flat or perpendicular to the direction of loading. If they are tested like that, stress concentrations will cause the cylinder to fail at a lower load than it otherwise would. CIVL 3137 19

Cylinder Caps Platen Cylinder CIVL 3137 20

Cylinder Caps One solution is to grind the ends of the cylinders so they are smooth, flat, and horizontal. This is time consuming and therefore expensive. Another solution is to cap the cylinders with high strength gypsum plaster or molten sulfur mortar. Both are liquid when first applied (to fill in all of the irregularities) and harden into material just as strong as the concrete and with similar stiffness properties. CIVL 3137 21

Cylinder Caps Another option is to use unbonded caps (also called pad caps). These are neoprene rubber pads that are confined within a metal retaining ring and placed over the ends of the cylinder. The pad conforms to the irregular surface of the specimen but is prevented from spreading laterally by the metal retaining ring. CIVL 3137 22

Cylinder Caps Source: https://www.certifiedmtp.com CIVL 3137 23

Cylinder Caps Bonded and unbonded cylinder caps can compensate for cylinder ends that aren’t smooth and plane, but it is difficult in practice to ensure the cylinder ends are exactly perpendicular to the direction of loading. For this reason, testing machines use spherically seated platens to transfer the load from the testing machine to the cylinder. The spherical seats ensure that the line of action of the applied force is vertical even if the cylinder ends are not perfectly horizontal. CIVL 3137 24

Compression Tester CIVL 3137 25

Compression Tester (PLATEN) Corrects for cylinder ends that aren’t horizontal CIVL 3137 26

Failure Types Fixed End Frictionless End Fixed End Frictionless End CIVL 3137 27

Failure Types (ASTM C39) CIVL 3137 28

Compressive Strength P max ��� D � � P max CIVL 3137 29

Basic Tests Cylinder Compression Splitting Tension Beam Flexure Elastic Modulus Slump Unit Weight Air Content CIVL 3137 31

CIVL 3137 32

Splitting Tension Test Source: https://www.quora.com CIVL 3137 33

Splitting Tension Test CIVL 3137 34

Splitting Tension Test 2 P  f t  LD CIVL 3137 35

Basic Tests Cylinder Compression Splitting Tension Beam Flexure Elastic Modulus Slump Unit Weight Air Content CIVL 3137 36

CIVL 3137 37

Beam Flexure Test Third-Point Loading CIVL 3137 38

Beam Flexure Test 6" CIVL 3137 39

Beam Flexure Test Pure bending with zero shear in the middle third L 3 CIVL 3137 41

Modulus of Rupture Based on beam bending formula  PL MOR 2 bd MOR CIVL 3137 42

Concrete Behavior 1 1 Stress-strain behavior becomes E E nonlinear as you approach failure CIVL 3137 43

Flexural vs. Tensile Strength Tensile Strength (f t ) Flexural Strength (MOR) CIVL 3137 44

Basic Tests Cylinder Compression Splitting Tension Beam Flexure Elastic Modulus Slump Unit Weight Air Content CIVL 3137 45

CIVL 3137 46

Elastic Modulus f  c failur e 1 E E 1 CIVL 3137 47

Elastic Modulus f  c 1    0.4 f  1 E c E     2 ,0.4 c   f 0.00005 2 E 1    1 0.00005, CIVL 3137 48

Elastic Modulus   5800 f c  2320 250    6 3.2 10 psi E  0.0007 0.00005 1 E   0.0007,2320 E 1   0.00005,250 CIVL 3137 49

Compressometer CIVL 3137 50

Compressometer pivot rod dial gage CIVL 3137 51

Compressometer L=½H H CIVL 3137 52

Compressometer L − ½  H –  L–  CIVL 3137 53

Compressometer L − ½  L −  pivot rod d/2 d/2 CIVL 3137 54