TESTS ON HARDENED CONCRETE

Properties of Hardened Concrete

• Early volume change
• Creep Properties
• Permeability
• Stress-Strain Relation

I. Early Volume Change (Shrinkage)

What is the problem? >> May result in cracking

Plastic Shrinkage

Due to water loss from fresh paste by evaporation or from suction by dry surface

• Volume decrease ~ 1% when the paste is still plastic
• How to prevent: control water loss

Drying Shrinkage

Due to loss of water and/or cooling.

• 15-30% of dry shrinkage occurs in the first 2 weeks, 65-85% in first year

Can be induced by

- lack of curing

- high water-cement ratio

- high cement content

- low coarse aggregate content

- existence of steel reinforcement

• Depends on size and shape of concrete structure
• May be non-uniform due to non-uniform loss of water

Swelling

May occur if concrete is cured continuously in water after settling. Its effects are relatively small and does not cause significant problems.

II. Stress-Strain Relation

• Concrete is not perfectly elastic
• Rate of loading affects the E value (Non-Linear)

- Aggregate and cement paste individually show a linear stress-strain, but concrete is nonlinear

- Why? Micro-cracking between the aggregate and cement paste interface

• For structural design, we need compressive strength (fc) and modulus of elasticity (E)

Tests performed on hardened concrete are:

• Compressive strength test (most common) – DT
• Modulus of Elasticity – NDT
• Split-tension test – DT
• Flexural strength test – DT
• Rebound hammer test – NDT
• Penetration resistance test – NDT
• Ultrasonic pulse velocity test – NDT
• Maturity test – NDT

II.1. Compressive Strength Test (fc’)

• ASTM C39: cylindrical specimen (6 in. by 12 in.)

- For normal-weight concrete: fc’ range is 21 MPa to 34 MPa (3000 psi to 5000 psi)

• Compressive strength depends on specimen size

- Larger specimens = greater probability for weaker elements, reducing strength

- Larger specimens have less variability and better representation of actual concrete

· How does failure occur? Weakest location of hardened concrete is the interface between cement paste and aggregate

II.2. Modulus of Elasticity – Ec

• ASTM C469 – determine the chord modulus.
• 3-4 loading steps are needed in this method. Same sample is used.
• A useful relationship with strength
• The range is 14 GPa – 41 GPa (2000 ksi to 6000 ksi)

Poisson’s Ratio

• Also determined using ASTM C469
• Range is between 0.11 to 0.21, depending on aggregate, moisture, age, and compressive strength

II.3. Split Tension Test

• Measures the tensile strength on concrete (ASTM C496)
• Cylinder is subjected to compressive load along vertical diameter at a constant rate until fatigue
• Failure occurs along vertical diameter due to tension developed in transverse direction
• Split tension is computed by T = 2p/BLD

T = tensile strength, MPa (psi)

p= load at failure, N (psi)

L = length of specimen, mm (in.)

D = diameter of specimen, mm (in.)

• Tensile strength varies from 2.5 MPa to 31 MPa (360 psi to 450 psi), about 10% of compressive strength

II.4. Flexure Strength Test (ASTM C78)

• Used for measuring Modulus of Rupture (MR)
• Important test for road and airport concrete pavements

Beam specimen of square x-section is loaded into a 3-point loading apparatus·

Calculation of MR

If the fracture occurs in the middle 1/3 of the span

R = PL/(bd^2)

Where:

R = flexure strength, MPa (psi)

P = maximum load applied, N (lb)

L = span length, mm (in.)

b = average width of specimen, mm (in.)

a = average depth of specimen, mm (in.)

• If fracture occurs further outside the middle third, the results must be thrown out

Useful relationships:

• R = (0.62 to 0.83)fc’^0.5 (S.I. units)
• R = (7.5 to 10)fc’^0.5 (English units)

II.5. Rebound Hammer Test (Schmidt Hammer Test)

• Non-destructive test performed on hardened concrete

- A spring-loaded mass hits the concrete’s surface

- A scale measures how far the mass rebounds

- The higher the rebound, the harder the concrete’s surface, and the greater the concrete’s strength

• Use a calibration chart graphs supplied to related the rebound to strength

- 10 to 12 reading are performed per specimen

• The test is used to test the uniformity of the concrete

II.6. Penetration Resistance Test (Windsor Probe Test) -A non-destructive test

• Gun-like device shoots probes into the concrete’s structure

- Performed on each of three holes in a special template.

• An average depth is then found.

- Depth is inversely related to the strength.

– It gives a better estimate than the rebound hammer.

Rebound hammer tests only the surface while penetration resistance test make measurements into the depth of the sample

II.7. Ultrasonic Pulse Velocity Test (ASTM C597)

• Measures the velocity of an ultrasonic wave passing through the concrete
• The length between transducers/the travel time = average velocity of wave propagation
• It is used to detect discontinuities, cracks and internal deterioration in the structure of concrete

II.8. Maturity Test (ASTM C1074)

• Maturity – degree of cement hydration
• Varies with time and temperature
• It is assumed that the strength is a function of maturity for a particular concrete mixture
• Devices are used to measure the temperature of concrete with time 

III. Permeability

Effects the durability of hardened concrete

• Allows water and chemicals to penetrate its surface
• Cause reduced resistance to

- frost

- alkali-aggregate reactivity and other chemicals

- corrosion of steel rebars

• Air voids that affect permeability are obtained from

- incomplete consolidation of fresh concrete

- evaporation of mixing water that is not used for hydration of cement

• Increasing the water/cement ratio has strong effects on permeability

· Other factors affecting permeability:

• age of concrete, fineness of cement particles, air entraining agents

IV. Creep Properties

Creep – The gradual increase in strain, with time, under sustained load

• Long term process (several years)
• Vary with type of structure

- Increased deflection and increased stress in steel

- Gradual transfer of load from concrete to steel

- Losing some of the prestress force in prestressed concrete