Concrete lives two lives. For the first few hours it is a fluid that must flow, fill formwork, and surround steel without separating. After that it is a stone-like solid that must carry load for decades. The two lives are connected: almost every hardened property is decided by what happened while the concrete was fresh.
This guide covers the key properties of both stages — what each one means, how it is measured, and the typical values you should expect.
Fresh Concrete Properties
1. Workability
Workability is the ease with which fresh concrete can be mixed, transported, placed, compacted, and finished without segregation. It is the single most-checked fresh property on site because it controls everything downstream.
Measured by:
- Slump test (ASTM C143 / IS 1199) — the site standard. Typical ranges: 25–50 mm (low, for pavements), 50–100 mm (medium, for reinforced beams and slabs), 100–175 mm (high, for congested sections). Full procedure: slump test guide.
- Compaction factor test — for low-workability mixes where the slump is near zero. Values run from about 0.70 (very stiff) to 0.95 (flowing).
- Flow table test — for high-workability and self-compacting mixes; the spread diameter is measured after standard jolts. Full procedure: flow table test guide.
2. Consistency and Cohesiveness
Consistency is the concrete’s fluidity; cohesiveness is its resistance to segregation — the ingredients staying together as one mass. A good mix is both fluid and cohesive.
3. Setting Time
Setting is the gradual loss of plasticity as hydration proceeds:
- Initial setting time — the point after which the concrete can no longer be properly compacted (for OPC, not less than 30 minutes per IS 269).
- Final setting time — when the concrete has hardened enough to resist light pressure (for OPC, not more than 600 minutes).
Measured on cement paste with the Vicat apparatus, and on concrete with the penetration resistance test (ASTM C403). Retarding and accelerating admixtures act directly on these times.
4. Bleeding
Bleeding is the rise of mixing water to the surface as solids settle. A little bleeding is normal; excessive bleeding leaves a weak, porous top layer (laitance) and plastic-shrinkage cracks.
Measured by ASTM C232 — the accumulated bleed water is drawn off and weighed at intervals. Full procedure: bleeding test guide.
5. Segregation
Segregation is the separation of coarse aggregate from the mortar — stone settling to the bottom, paste rising to the top. Causes: over-vibration, dropping concrete from height (keep free fall under ~1.5 m), harsh gap-graded mixes, and overdosed superplasticizer.
6. Air Content
Entrained air (deliberate, microscopic bubbles) protects against freeze-thaw; entrapped air (large accidental voids) is simply weakness. Typical target for entrained air: 4–6% in freeze-thaw exposure.
Measured by the pressure method (ASTM C231) with an air meter, or volumetrically (ASTM C173). Every 1% of extra air costs roughly 5% of compressive strength, so this test matters.
7. Temperature of Fresh Concrete
Fresh concrete temperature controls setting time and early strength development. Hot concrete sets fast and is prone to plastic shrinkage cracking; cold concrete gains strength slowly and can freeze before setting.
Common limits: keep fresh concrete below about 35°C in hot weather and above 5°C in cold weather, and never let it freeze in the first 24 hours. The detailed limits are in the concrete temperature guide.
8. Unit Weight and Yield
Fresh unit weight (about 2,400 kg/m³ for normal concrete) is checked with a container of known volume (ASTM C138). A low yield means you are getting less concrete than batched — usually a sign of excess air or wrong batch weights.
Hardened Concrete Properties
1. Compressive Strength
The defining property — the basis of mix design, grading (M20, M25, C30…), and acceptance. Measured by crushing 150 mm cubes (IS 516) or 150 × 300 mm cylinders (ASTM C39) at 7 and 28 days.
Typical 28-day values:
| Concrete class | Typical use | 28-day strength |
|---|---|---|
| Lean / blinding | Below footings | 10–15 MPa |
| Ordinary structural | Slabs, beams, columns | 20–40 MPa |
| High-strength | Prestressed, tall columns | 60–100 MPa |
| Ultra-high performance | Special structures | 120–200 MPa |
Concrete keeps gaining strength after 28 days — roughly 10–15% more by 90 days with continued curing.
2. Tensile Strength
Concrete is weak in tension — roughly 8–12% of its compressive strength. That is why we reinforce it. Measured by the split-cylinder test (ASTM C496 / IS 5816).
Typical: for M30 concrete (~30 MPa), expect split tensile strength around 2.5–3.5 MPa.
3. Flexural Strength (Modulus of Rupture)
The tensile strength in bending — critical for pavements and slabs on grade. Measured on beam specimens under third-point loading (ASTM C78 / IS 516).
IS 456 gives the handy relation:
fr = 0.7 × √fck (N/mm²)
So for M30 (fck = 30): fr = 0.7 × √30 = 3.83 MPa.
4. Modulus of Elasticity
Stiffness — how much the concrete strains under load. Needed for deflection and crack-width calculations.
IS 456: Ec = 5,000 × √fck (N/mm²)
For M25: Ec = 5000 × √25 = 25,000 MPa (25 GPa). Poisson’s ratio for concrete is typically 0.15–0.20.
5. Creep
Creep is the slow, continuing deformation under sustained load — a column shortens a little more each year under the building’s weight. The creep coefficient typically ranges from 1.5 to 4, higher for young concrete and dry environments.
6. Shrinkage
As concrete dries, it shrinks — roughly 0.0003 strain (300 microstrain) for drying shrinkage. Restrained shrinkage causes the cracks you see in slabs and walls.
7. Permeability and Durability
Permeability — how easily water and aggressive chemicals move through the concrete — governs durability more than strength does.
Durability rules of thumb:
- Water-cement ratio ≤ 0.45 for severe exposure (marine, chemical)
- Minimum cement content and cover per exposure class (IS 456 Table 3/4/5)
- Permeability drops dramatically below w/c 0.40
8. Density of Hardened Concrete
Normal-weight: 2,300–2,500 kg/m³. Lightweight structural: 1,600–2,000 kg/m³. Heavyweight (radiation shielding): up to 3,500+ kg/m³.
9. Fire and Thermal Properties
Concrete is inherently fire-resistant (it does not burn), but prolonged high temperature spalls the surface and degrades strength. Coefficient of thermal expansion: roughly 10 × 10⁻⁶ per °C, conveniently close to steel’s — one reason reinforced concrete works.
How Fresh Properties Decide Hardened Properties
| Fresh-stage decision | Hardened consequence |
|---|---|
| Extra water for workability | Lower strength, higher permeability, more shrinkage |
| Proper compaction | Full strength, dense cover, durable steel protection |
| Controlled bleeding | Sound surface, good bond to reinforcement |
| Right air content | Freeze-thaw durability without strength loss |
| Adequate curing | Strength gain continues; surface resists cracking |
| Correct temperature | Normal setting; no thermal or plastic-shrinkage cracks |
Frequently Asked Questions
What are the properties of fresh concrete?
Workability, consistency, setting time, bleeding, segregation, air content, temperature, and unit weight. Each is measurable with a standard test.
What are the properties of hardened concrete?
Compressive, tensile, and flexural strength; modulus of elasticity; creep; shrinkage; permeability/durability; density; and thermal/fire behaviour.
What is the relation between modulus of rupture and compressive strength?
IS 456 gives fr = 0.7√fck. For M30 concrete this is about 3.8 MPa.
Why does concrete gain strength after 28 days?
Hydration continues as long as moisture is present. With proper curing, concrete typically gains another 10–15% between 28 and 90 days.
Final Thoughts
Think of fresh and hardened concrete as one story, not two subjects: every property of the hardened structure was locked in during those first hours. Test the fresh concrete, compact it properly, cure it patiently — and the hardened properties largely take care of themselves.
Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.
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