Week 6 Strength of Concrete
Page 2: Importance of Concrete Properties
Fresh concrete properties are crucial only in the first few hours.
Hardened concrete properties matter for the remainder of its life.
Page 4: Definition of Strength
Strength defined as the maximum load concrete can carry.
Page 5: Forces/Loads
Measurement units: N (Newtons) or kN (kiloNewtons)
Types of forces: Compression and Tension.
Page 6: Compression and Tension
Concrete resists only compression; tensile actions managed by reinforcement.
Important tensile applications:
Pavement slabs (tensile cracking from drying shrinkage)
Water-retaining structures (minimal cracking for water-tightness).
Page 7: Compressive/Tensile Strength Relationship
Tensile and compressive strength influenced by various factors.
Difference in direct tensile strength (1/10 of compressive strength) and bending tensile strength (1/6 of compressive strength).
Preferably use tested values of specific concrete.
Page 8: Tensile Failure Characteristics
Tensile failure is brittle with minimal deformation.
Failure pathway follows weaknesses like pores or weak interfaces.
Page 9: Compressive Failure Characteristics
Crushing comprises many local failure planes; primarily tensile/shear despite compressive load.
Leads to tensile cracking parallel to compressive stress.
Page 10: Shear Failure
Reinforced beams utilize shear links to prevent shear failure.
Page 11: Factors Influencing Strength
Factors affecting both compressive and tensile strengths:
Cement
Water/cement ratio
Aggregate
Degree of Compaction
Age & Curing Conditions.
Page 12: Cement Effects on Strength
Chemical composition and fineness significantly influence hydration and early strength.
Finely ground cements with high tricalcium silicate content gain strength faster.
Long-term strength differences of Portland cements minimal beyond three months.
Page 13: Strength Development of Cements
Graph details hydration strengths based on cement types and time.
Page 14: Water/Cement Ratio Effects
Hydration: About 0.25g of water combines with 1g of cement (forms 1.25g hydration product).
Excess water creates pores retains resilience when visible, directly affecting concrete strength.
Page 15: Capillary Porosity and Water/Cement Ratio
Graph illustrating capillary porosity in relation to various water/cement ratios.
Page 16: Calculating Free Water Content
Importance of using free water content instead of total water added when calculating water/cement ratio.
Free water includes moisture within damp aggregates.
Incorrect moisture handling may lead to strength variations.
Page 17: Pore Size and Strength Loss
Voids lead to reduced strength; pore sizes influenced by time and water/cement ratio.
Powers' empirical model for strength connects compressive strength and porosity.
Page 18: Concrete Failure Factors
Concrete failure may arise from:
Aggregate particles
Aggregate-cement paste bond
Cement paste.
Page 19: Aggregate Impact on Strength
Aggregates typically stronger than concrete.
Bond between aggregate and paste critical for strength:
Round aggregates yield weaker bonds compared to angular.
Dust coats lower strength; increasing aggregate size decreases strength.
Some aggregates negatively interact with cement alkali, harming durability.
Page 20: Impact of Air Voids
Air voids from poor compaction = significant strength losses (5-6% loss strength per 1% air).
Mix proportions must balance workability and strength needs.
Page 21: Strength & Compaction
Graph showing compressive strength as a percentage of full compaction.
Page 22: Strength Over Time
Concrete strength increases over time if hydration conditions are optimal.
Early strength notable (7-day strength approx 2/3 of 28-day strength for Grade 42.5).
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Page 24: Hydration Conditions
Faster hydrating cements gain significant strength by day 7.
Different cements lead to varied strength development rates.
Long-term strengths similar; must not inform design—base on 28-day values.
Page 25: Essential Conditions for Strength Development
Concrete must remain moist early on for adequate strength development.
Page 26: Temperature Effects on Hydration
Hydration processes vary significantly with temperature; slow in cold and fast in heat.
High-temperature curing is utilized for precast concrete.
Page 27: Strength Testing Methods
Methods per BS.12390:2000 include:
Compressive strength from concrete cubes.
Flexural strength from concrete beams.
Tensile splitting strength from concrete cylinders.
Page 28: Compressive Cube Test
Most widespread strength test;
For tensile strength, both tensile splitting tests are leveraged.
Flexural strength indicates crack formation in beams/slabs.
Page 29: Specimen Sizes
Common specimens:
Cubes: 150mm
Beams: 150 x 150 x 750mm
Cylinders: 150mm x 300mm
If max aggregate size ≤ 20mm:
Cubes: 100mm
Beams: 100 x 100 x 500mm
Cylinders: 100mm x 200mm
Page 30: Curing of Specimens
Specimens typically cured at 20°C; accelerated curing may occur at higher temps.
Demould after 16-72 hours; submerged in 20 ± 2°C water for curing.
Testing commonly at 7 and 28 days, plus other time markers.
Page 31: Compressive Strength Testing Process
Use compression testing machine; measure cube density pre-test.
Load applied gradually; document maximum load and failure type.
Page 32: Strength Calculation
Compressive strength calculated as: [ \text{Strength} = \frac{\text{Max Load}}{\text{Cross-Sectional Area}} ]
Reported to nearest 0.5 N/mm².
Page 33: Flexural Strength Testing
Indirect tensile value; a plain beam subjected to load.
Beam fails in tension due to induced stresses.
Page 34: Flexural Testing Method
Weigh specimen, check dimensions, and density;
Load applied gradually; record maximum load noting failures.
Page 35: Flexural Strength Calculation
Calculate flexural strength using: [ f_{cf} = \frac{F \times l}{2 \times d_1 \times d_2} ]
Where F = breaking load, l = distance between rollers, d1 = breadth, d2 = depth.
Page 36: Tensile Splitting Test
Indirect tensile measure via compression loading along cylinder length.
Splitting occurs across loaded diameter.
Page 37: Tensile Splitting Testing Method
Weigh specimen, check dimensions, and density.
Load applied gradually at specified points.
Page 38: Calculation of Tensile Splitting Strength
Tensile splitting strength calculated as: [ f_{ct} = \frac{2F}{\pi d \times l} ]
Where F = max load, d = diameter, l = length.
Page 39: Additional Resources
See Blackboard for further reading.
Next topic: Durability.
Page 40: End Note
Ulster University contact information.