Fresh and Hardened Concrete Properties
Key Takeaways
- Concrete performance begins with compatible proportions of cementitious materials, water, aggregates, air, and admixtures plus proper placement and curing
- Fresh properties such as workability, air, temperature, density, segregation, and setting affect constructability and the hardened material
- Specified compressive strength f′c is the design value, required average strength f′cr is a mixture-proportioning target, and a strength test is a defined measured result
- A single cylinder result is not automatically a strength test or a complete acceptance decision under ACI 318-14
- Creep under sustained stress and load-independent shrinkage increase long-term deformation and can create redistribution or cracking when restrained
Fresh and Hardened Concrete Properties
Three strength labels:
f'cis specified for design,f'cris a higher required average used to proportion a mixture for variability, and a strength test is a defined result from sampled concrete. They answer different questions.
Constituents and Fresh Behavior
Concrete combines cementitious materials, water, fine and coarse aggregate, intentional air where required, and chemical or mineral admixtures. Water supports hydration and workability, but the water–cementitious-material ratio strongly influences strength and permeability. Adding uncontrolled water at placement may increase slump while reducing strength and durability and increasing shrinkage.
| Fresh property | What it indicates | What it does not prove alone |
|---|---|---|
| Slump/workability | Consistency and ease of placement under the test conditions | Compressive strength or water content by itself |
| Air content | Entrained/entrapped air in the sampled mixture | Adequate freeze-thaw durability without other requirements |
| Temperature | Placement and hydration environment | Future curing quality |
| Unit weight/yield | Density and batch-volume consistency | Complete mixture acceptance |
| Bleeding/segregation | Stability of the fresh mixture | Hardened uniformity if ignored |
Placement must fill the form and surround reinforcement without harmful segregation or voids. Consolidation removes entrapped air but excessive vibration can segregate a susceptible mix. Finishing before bleed water is addressed can damage the surface. Curing preserves moisture and temperature needed for hydration; poor curing can reduce surface strength and durability even when mixture proportions were correct. Cast-in-place concrete properties therefore depend on delivery, placement, consolidation, finishing, and curing as well as batch design.
Worked Fresh Unit Weight
A calibrated container has volume 0.500 ft³. Its net mass-equivalent weight of fresh concrete is 72.5 lb. The fresh unit weight is
γ_c = 72.5/0.500 = 145 pcf.
If 20 ft³ of that concrete is placed, its fresh weight is
W = 145(20) = 2,900 lb = 2.90 kips.
Use the appropriate fresh or hardened density for the requested stage, and count self-weight once.
Hardened Mechanical Properties
Concrete is strong in compression and comparatively weak and variable in direct tension. Flexural cracking can occur well before compressive crushing. Elastic modulus depends on concrete density and compressive strength under the relationship specified by the current reference; Poisson behavior, tensile strength, and fracture response are separate properties. Unreinforced concrete still carries compression and limited tension before cracking, but it lacks reinforcement to bridge structural tensile cracks.
Durability depends on exposure and transport mechanisms as well as strength. Low permeability, suitable air entrainment for freezing exposure, appropriate materials, consolidation, cover, curing, and crack control can limit water, chloride, sulfate, and carbonation damage. A high cylinder strength alone does not guarantee durability in every exposure.
For a 2026 PE Civil: Structural exam, use ACI 318-14 and the PE Civil Reference Handbook active for the test date. Do not apply an acceptance rule or property equation from the April 2027 reference set.
Specified, Average, and Tested Strength
Suppose design documents specify f'c = 5.0 ksi. Structural resistance calculations use that specified value unless the governing problem states another basis. Mixture proportioning must aim higher to account for production variability. If the problem states f'cr = 6.2 ksi, the target margin is
f'cr - f'c = 6.2 - 5.0 = 1.2 ksi.
That margin is not extra design strength. A designer cannot replace 5.0 ksi with 6.2 ksi merely because the producer targets the higher average.
Now two companion cylinders from one sample test at 5.2 and 5.6 ksi. If the applicable definition calls for their average, the batch strength-test result is
(5.2 + 5.6)/2 = 5.4 ksi.
The individual 5.2-ksi cylinder is a specimen result; 5.4 ksi is the defined strength-test result in this illustrative pair. ACI 318-14 acceptance considers defined strength tests and the required series/individual criteria. One low cylinder, one result below f'c, or the average of all project tests is not by itself the entire acceptance decision. Follow the supplied code's sampling, test definition, and investigation provisions.
Creep and Shrinkage
Creep is time-dependent strain under sustained stress. Shrinkage is load-independent volume reduction associated with moisture loss and other material processes. A freely shrinking member shortens without stress; restraint by reinforcement, supports, or adjacent placements can create tensile stress and cracking. Creep can increase beam deflection, shorten columns, redistribute force in indeterminate structures, and relax some restraint stresses.
A 15-ft concrete member carries sustained compressive stress f = 1.2 ksi. Let problem-given E_c = 4,000 ksi, creep coefficient φ = 1.8, and shrinkage strain magnitude ε_sh = 400 × 10⁻⁶. Elastic strain is
ε_e = f/E_c = 1.2/4,000 = 300 × 10⁻⁶.
Creep strain is
ε_cr = φε_e = 1.8(300) = 540 × 10⁻⁶.
If elastic, creep, and shrinkage shortening are additive under the problem's simplified free-shortening model,
ε_total = 300 + 540 + 400 = 1,240 × 10⁻⁶.
For L = 15 ft = 180 in,
Δ = 180(0.001240) = 0.223 in shortening.
Actual prediction depends on age at loading, humidity, member size, curing, concrete composition, stress history, reinforcement, and restraint. Do not add a shrinkage strain as though it were stress, and do not count instantaneous elastic strain again if a supplied long-term factor already includes it.
Concrete-Property Workflow
- Identify whether the question concerns fresh, early-age, specified-age, or long-term concrete.
- Separate material proportions from placement and curing conditions.
- Label every strength value as
f'c,f'cr, specimen result, or defined strength-test result. - Use ACI 318-14 acceptance logic only after identifying the required sample and result set.
- For deformation, separate elastic, creep, shrinkage, and thermal components and determine restraint.
- Check durability exposure, density, cracking, and unreinforced behavior independently of compressive strength.
The correct number is not enough; its role in design, production, testing, or long-term response must also be correct.
A project specifies f′c = 5.0 ksi and the concrete mixture has a required average target f′cr = 6.2 ksi. Which value normally forms the stated design-strength basis?
What total free shortening is calculated for the worked 15-ft member when elastic, creep, and shrinkage strains are added under the stated simplified assumptions?
One concrete cylinder result is lower than f′c. What is the appropriate next interpretation?