Concrete — properties, testing & mix design

Concrete

Concrete is a composite material of cement (binder), fine aggregate, coarse aggregate and water (with admixtures as needed) that hardens by hydration of cement. Plain cement concrete (PCC) is strong in compression but weak in tension; reinforced cement concrete (RCC) combines it with steel; prestressed concrete applies pre-compression.

Ingredients

• Cement — OPC, PPC, PSC or other types as required.
• Aggregates — fine and coarse, clean, strong and well graded (see Aggregates & Mortars).
• Water — for hydration and workability.
• Admixtures — chemical and mineral (see Concrete Mix Design & Admixtures).

Quality of mixing water

Water fit for drinking is generally suitable. IS 456 requires water to be free from injurious amounts of oils, acids, alkalis, salts, sugar and organic materials, with a pH of not less than 6, and limits on solids such as organic matter (about 200 mg/L), inorganic solids (about 3000 mg/L), sulphates (about 400 mg/L as SO₃) and chlorides (about 2000 mg/L for plain concrete and 500 mg/L for reinforced concrete). Sea water should not be used for reinforced or prestressed concrete (chlorides cause corrosion).

Grades of concrete (IS 456)

Concrete is designated by M followed by its characteristic compressive strength (N/mm²) of 150 mm cubes at 28 days.

• Minimum grade for reinforced concrete is M20 (IS 456).

Nominal mixes

Nominal mixes may be used for concrete up to M20; design mixes are preferred and required for higher grades.

Exposure conditions (IS 456 — reinforced concrete)

The maximum cement content (OPC, excluding fly ash and slag) is generally limited to 450 kg/m³ to reduce shrinkage and thermal cracking.

Production of concrete

1. Batching — measuring ingredients: weigh batching (accurate, preferred) or volume batching (for small works; allowance for bulking of sand; cement always in whole bags).
2. Mixing — machine mixing (tilting or non-tilting drum mixers, pan mixers, batching plants) until uniform; hand mixing on a watertight platform is allowed only for small works with an extra quantity of cement (commonly 10%).
3. Transporting — pans, wheelbarrows, buckets and cranes, chutes, belt conveyors, pumps, transit mixers (ready-mixed concrete) — avoiding segregation and loss of workability.
4. Placing — in clean, oiled, rigid formwork; in layers; avoiding large free fall (which causes segregation); using tremie for underwater placement.
5. Compaction — removal of entrapped air (about 5% air voids can reduce strength by about 30%): needle (immersion) vibrators, surface vibrators (slabs), formwork (external) vibrators, vibrating tables (precast); hand rodding for small works. Over-vibration causes segregation and bleeding.
6. Finishing — screeding, floating, trowelling, texturing.
7. Curing — maintaining moisture and temperature for hydration.

Curing

Minimum curing period (IS 456): at least 7 days for concrete with OPC; at least 10 days where mineral admixtures or blended cements are used; longer periods (e.g. 10 and 14 days respectively) in hot, dry weather.

Methods: ponding (slabs), wet coverings (hessian, sand), sprinkling, membrane curing (curing compounds, polythene sheets), steam curing (precast — accelerated strength), autoclave curing, infra-red curing.

Fresh concrete

Workability

Workability is the ease with which concrete can be mixed, transported, placed, compacted and finished without segregation.

Factors: water content (most important), aggregate–cement ratio, aggregate size, shape, texture and grading, use of admixtures (plasticisers, air entrainment, fly ash), temperature and time.

Segregation and bleeding

• Segregation — separation of coarse aggregate from mortar (or grout from aggregates) — due to excess water, poor grading, large free fall, over-vibration, long transport.
• Bleeding — water rising to the surface after placing (a form of segregation) — due to high w/c ratio, lean mixes, poorly graded or coarse sand; causes weak surface layer (laitance), porous channels and reduced bond with reinforcement.
• Harshness — difficulty in finishing due to lack of fines/paste.

Workability tests

Degree of workability (typical correlations):

IS 456 recommends slump ranges by placing condition — e.g. low slumps for mass concrete and pavements, about 50–100 mm for heavily reinforced sections, higher for pumped concrete or congested reinforcement (commonly using superplasticisers).

Hardened concrete

Compressive strength

• Measured on 150 mm cubes (IS 516) cured in water and tested at 7 and 28 days (cylinders 150 × 300 mm are used in some codes; cylinder strength is roughly 0.8 of cube strength).
• Characteristic strength ($f_{ck}$) — the strength below which not more than 5% of test results are expected to fall.
• Target mean strength for mix design: $f_{ck}^{\prime }=f_{ck}+1.65s$ ($s$ = standard deviation).
• Typical strength gain with OPC: about 65–70% of 28-day strength at 7 days.

Water–cement ratio law and gel–space ratio

Other factors: degree of compaction, curing and age, cement type, aggregate properties and bond, temperature, admixtures, specimen size and shape, loading rate.

Tensile and flexural strength

• Concrete's tensile strength is only about 8–12% of its compressive strength.
• Flexural strength (modulus of rupture) — tested on beams (e.g. 150 × 150 × 700 mm) under two-point loading; IS 456 gives

• Split tensile strength — cylinder loaded along its length (Brazilian test): $f_t=\frac{2P}{\pi DL}$.

Modulus of elasticity

Poisson's ratio of concrete ≈ 0.15–0.20.

Shrinkage

• Plastic shrinkage — rapid evaporation from fresh concrete (hot, windy weather) → surface cracks; prevented by early curing, windbreaks, fog spraying.
• Drying shrinkage — loss of water from hardened concrete; IS 456 suggests a total shrinkage strain of about 0.0003 in the absence of data.
• Autogenous shrinkage (self-desiccation in low w/c concretes), carbonation shrinkage.

Creep

Creep is the gradual increase in strain under sustained load. It increases with higher stress, earlier age at loading, higher w/c, lower humidity and more paste. IS 456 gives creep coefficients of about 2.2 (loading at 7 days), 1.6 (28 days) and 1.1 (1 year). Creep causes long-term deflections and loss of prestress but relieves stress concentrations.

Durability

Durability — ability to resist weathering, chemical attack and abrasion while retaining its properties. Key threats:

Sampling and acceptance

IS 456 specifies sampling frequency based on quantity of concrete (e.g. 1 sample for 1–5 m³, 2 for 6–15 m³, 3 for 16–30 m³, 4 for 31–50 m³, and 4 plus one for each additional 50 m³ beyond that); each sample consists of 3 cubes whose average is the test result. Acceptance criteria compare the mean of consecutive results and individual results with the characteristic strength as specified in the code.

Non-destructive tests

Maturity of concrete

Strength development depends on time and temperature:

(Nurse–Saul function with a datum temperature of about −10 °C.) Concretes of the same mix with equal maturity have approximately equal strength — useful for estimating strength under different curing temperatures and for steam curing.

Worked examples

Frequently tested points

• Potable water generally suitable; pH ≥ 6; chlorides ≤ 500 mg/L for RCC; no sea water for RCC.
• Ordinary M10–M20; standard M25–M55; high strength ≥ M60; RCC minimum M20.
• Nominal mixes: M10 1:3:6, M15 1:2:4, M20 1:1.5:3.
• Exposure: mild 300 kg/m³, 0.55, M20 … extreme 360 kg/m³, 0.40, M40; max OPC content 450 kg/m³.
• Weigh batching preferred; hand mixing needs extra cement; compaction removes air (5% air ≈ 30% strength loss).
• Curing: 7 days (OPC), 10 days (blended/mineral admixtures).
• Slump mould 300 mm high, 100/200 mm diameters; true, shear, collapse slumps; compaction factor for low workability; Vee-Bee for very stiff mixes.
• Bleeding → laitance; segregation → honeycombing.
• 150 mm cubes; $f_{ck}$ = 5% fractile; $f_{ck}^{\prime }=f_{ck}+1.65s$.
• Abrams' w/c law; gel–space ratio $240x^3$.
• $f_{cr}=0.7\sqrt{f_{ck}}$; $E_c=5000\sqrt{f_{ck}}$; shrinkage strain ≈ 0.0003; creep coefficients 2.2/1.6/1.1.
• Durability: carbonation, chlorides, sulphates, ASR, freeze–thaw.
• NDT: rebound hammer (surface hardness), UPV (quality, cracks), cores.
• Maturity $\sum (T+10)\Delta t$.

Concrete mix design

Mix design (mix proportioning) is the process of selecting suitable ingredients and determining their relative proportions to produce concrete of required strength, workability and durability as economically as possible.

Requirements

• Fresh concrete: workability suited to placing and compaction; no segregation or excessive bleeding.
• Hardened concrete: specified characteristic compressive strength; durability for the exposure condition (maximum w/c, minimum cement content, cover).
• Economy: minimum cement content consistent with the above (lower cost, less heat and shrinkage).

Nominal versus design mixes

Factors governing mix design

Grade (characteristic strength) and degree of quality control (standard deviation); type and grade of cement; exposure conditions; maximum nominal size, shape and grading of aggregates; workability (slump); type of mixing and placing; use of admixtures; maximum and minimum cement content.

IS 10262:2019 method

Step 1 — Target strength

Step 2 — Water–cement ratio

• Selected from established relationships (curves in the code or the producer's experience) between free w/c ratio and 28-day compressive strength for the cement used.
• Checked against durability — must not exceed the maximum w/c ratio for the exposure condition in IS 456; the lower value governs.

Step 3 — Water content

• For 20 mm nominal maximum size (angular coarse aggregate) and 50 mm slump, the base free water content is about 186 kg/m³ (about 208 kg/m³ for 10 mm and 165 kg/m³ for 40 mm aggregate).
• Increase about 3% for every additional 25 mm slump above 50 mm.
• Reduce for rounded/sub-angular aggregates and for water-reducing admixtures (plasticisers about 5–10%, superplasticisers about 20% or more, as established by trials).

Step 4 — Cement (binder) content

Check against the minimum cement content for exposure (IS 456) and the maximum (commonly 450 kg/m³ OPC); if below the minimum, increase cement and recompute w/c. With fly ash or slag, the total cementitious content and replacement limits are considered.

Step 5 — Proportion of coarse aggregate

The volume of coarse aggregate per unit volume of total aggregate is taken from the code table for the maximum size of coarse aggregate and the grading zone of fine aggregate, for a w/c ratio of 0.50 — e.g. for 20 mm aggregate: about 0.60 (Zone I), 0.62 (Zone II), 0.64 (Zone III) and 0.66 (Zone IV).

• Adjust by ±0.01 for every ∓0.05 change in w/c ratio (lower w/c → more coarse aggregate).
• Reduce (e.g. by about 10%) for pumpable concrete.

Step 6 — Mix calculations (absolute volume method)

Per cubic metre of concrete:

Step 7 — Trial mixes and adjustments

• Trial mix 1 checked for workability (slump) and absence of segregation; water/admixture adjusted keeping w/c constant.
• Further trials at w/c ratios varying by about ±10% of the chosen value; cubes tested at 7 and 28 days.
• The final mix is selected from the strength–w/c relationship.
• Field adjustments for moisture content and absorption of aggregates (aggregate masses in design are in SSD condition).