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Topics Covered
Eight advanced concrete types — each with mix design method, key parameters, IS standards and worked proportions
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Self-Compacting Concrete (SCC)
EFNARC 2005 · IS 9103:1999 (admixtures) · No vibration required — flows and fills by gravity alone
Self-Compacting Concrete (SCC) is a highly workable concrete that flows under its own weight, passes through congested reinforcement and fills formwork without any mechanical consolidation. It combines a very low w/c ratio, high paste volume, VMA (viscosity-modifying admixture), and high-dose PCE SP to achieve controlled flow without segregation.
Slump Flow
550–850 mm
EFNARC SF1–SF3
V-Funnel Time
6–25 sec
VS1–VS2 class
J-Ring Gap
≤ 10 mm
Passing ability
Typical w/c
0.30–0.40
Very low
Paste Volume
38–42%
High paste
PCE SP Dose
0.5–1.2%
% cement mass
MSA
≤ 20 mm
Typically 10–16mm
SCC Mix Design Approach
SCC mix design differs fundamentally from IS 10262 normal concrete design. The starting point is paste volume and powder (cement + SCM + filler) content, not water content from a table. The EFNARC method (European Federation of Specialist Construction Chemicals) is the most widely adopted approach in India for SCC.
SCC MIX DESIGN — EFNARC APPROACH:
Step 1: Select target slump flow class (SF1: 550–650mm, SF2: 660–750mm, SF3: 760–850mm)
Step 2: Select w/p ratio (w/powder ratio by volume): typically 0.85–1.05
Step 3: Choose powder content: 400–600 kg/m³ (cement + FA + limestone filler)
Step 4: Choose CA content: 28–35% of concrete volume (lower than normal concrete)
Step 5: Calculate FA (sand) as balance volume
Step 6: Dose PCE SP to achieve target flow; VMA if segregation risk
TYPICAL SCC PROPORTIONS (M35 equivalent, SF2):
Cement (OPC 53): 380 kg/m³
Fly Ash: 120 kg/m³
Fine Agg (Zone II): 900 kg/m³
Coarse Agg (10mm): 750 kg/m³
Water: 170 L/m³ (w/c = 0.45, w/powder = 0.34)
PCE SP (Type F): 7.5 L/m³ (0.8% of total powder)
VMA (cellulose): 1.5 L/m³
Total powder: 500 kg/m³
Fresh tests required: Slump flow (EN 12350-8), V-funnel (EN 12350-9),
L-box (EN 12350-10), J-ring (EN 12350-12)
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SCC Fresh Property Classes — EFNARC 2005
Used for specification and acceptance in Indian SCC projects
| Test | Class 1 | Class 2 | Class 3 | Method |
|---|
| Slump Flow (mm) | SF1: 550–650 | SF2: 660–750 | SF3: 760–850 | EN 12350-8 |
| V-Funnel (sec) | VS1: 8–25 | VS2: ≤ 8 | — | EN 12350-9 |
| L-Box (H2/H1) | PA1: ≥ 0.80 (2 bars) | PA2: ≥ 0.80 (3 bars) | — | EN 12350-10 |
| J-Ring (mm) | PJ1: ≤ 10 (12 bars) | PJ2: ≤ 10 (16 bars) | — | EN 12350-12 |
| Sieve Segregation (%) | SR1: ≤ 20 | SR2: ≤ 15 | — | EN 12350-11 |
SCC Applications in India: Pre-stressed concrete box girders (bridges), drilled shafts and bored piles (where vibration access is impossible), architectural concrete (exposed surfaces where vibration marks are unacceptable), heavily congested high-rise core walls, precast façade panels, and repair of congested existing structures. SCC is increasingly specified in IS-compliant projects for elements with clear bar spacing < 40mm where standard vibrated concrete cannot ensure adequate compaction.
⚠️ SCC Is Not Simply "Wet Concrete": Adding water to normal concrete does not produce SCC — it produces weak, segregated concrete. SCC requires carefully balanced PCE SP for flow, VMA for cohesion, controlled powder content for paste viscosity, and lower CA content for deformability. The slump flow test (≥550mm), not the slump cone test (used for normal concrete), is the correct acceptance test for SCC. A design achieving 200mm slump is still not SCC.
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High-Performance Concrete (HPC)
ACI 363R · IS 10262:2019 Annex C · fck ≥ 60 MPa · Very low w/c · Enhanced durability
High-Performance Concrete (HPC) is defined by ACI 363R as concrete with w/c ≤ 0.40 and 28-day compressive strength ≥ 60 MPa (cylinder), typically accompanied by enhanced durability — low permeability, high resistance to chloride ingress, carbonation and chemical attack. It requires very low w/c, silica fume, high-WR PCE SP, and carefully selected premium-quality aggregates.
Cylinder Strength
≥ 60 MPa
ACI 363R / 28d cyl
Cube Strength
≥ 75 MPa
Equivalent IS 516
w/c Maximum
≤ 0.35
Very low
Silica Fume
7–15%
% OPC mass
PCE SP Dose
0.8–1.5%
% cement mass
Permeability
< 1000 C
RCPT, ASTM C1202
Cement (OPC)
400–500 kg/m³
+ SCMs
MSA
10–20 mm
Granite/basalt
HPC MIX DESIGN — KEY DIFFERENCES FROM IS 10262 M55:
1. STRENGTH CRITERION:
Target IS 10262: fcr = fck + 1.65 × S
For M75 (cube): fcr = 75 + 1.65 × 6.0 = 84.9 MPa
Use actual S (not Table 1) — production variability higher for HPC
2. w/c SELECTION:
Figure 1 extrapolation unreliable above M55.
Use actual lab-established strength-w/c curve for:
• Specific OPC 53 lot + SF combination
• Cured at project site temperature
Target w/c: typically 0.25–0.35 for M70–M100
3. AGGREGATE QUALITY (Critical):
LA abrasion: ≤ 20% (aggregate IS the strength limit above M70)
ACV: ≤ 20%
SG: ≥ 2.70 (basalt preferred)
MSA: 10mm recommended (better compaction around dense SF matrix)
4. SILICA FUME (Mandatory for HPC):
7–12% of OPC mass: fills capillary voids (particle size 0.1–0.5µm)
Pozzolanic reaction: SiO₂ + Ca(OH)₂ → C-S-H (secondary)
Effective w/c contribution: k = 2.50
5. CURING (Critical — more critical than normal concrete):
Sealed curing immediately after placing (autogenous shrinkage risk)
7+ days moist curing minimum
No early stripping — extended formwork retention mandatory
TYPICAL HPC PROPORTIONS (M80 cube / M65 cylinder):
OPC 53: 440 kg/m³
Silica Fume: 44 kg/m³ (10% of OPC)
Fly Ash: 66 kg/m³ (15% of OPC)
Water: 140 L/m³ (w/c = 0.318)
CA (10mm basalt): 920 kg/m³
FA (Zone II): 760 kg/m³
PCE SP: 10.5 L/m³ (≈1.2% total binder)
eff. w/c = 140/(440+2.5×44+0.25×66) = 0.255
📌 HPC Durability Properties
- RCPT (ASTM C1202): Chloride permeability <1000 coulombs (very low) vs 4000–8000 for normal M30 concrete. Achieved through dense SF microstructure filling capillary pores.
- Carbonation depth: <1 mm in 50 years for well-designed HPC vs 20–50mm for M25 normal concrete in urban environments.
- Water absorption: <2% by mass (IS 3085) vs 4–8% for normal concrete.
- Abrasion resistance: 3–5× normal concrete — preferred for industrial floors, bridge decks, spillways.
- Service life prediction: HPC with w/c ≤ 0.35 + SF typically achieves 100-year design life in moderate chloride environments vs 30–50 years for conventional M30.
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Reactive Powder Concrete (RPC) / UHPC
fck 150–800 MPa · Ultra-High Performance Concrete · Steel fibres mandatory · No coarse aggregate
Reactive Powder Concrete (RPC), also called Ultra-High Performance Concrete (UHPC), achieves compressive strengths of 150–800 MPa through extreme optimisation: elimination of coarse aggregate (all particles <600µm), very high silica fume content, very low w/c (0.16–0.25), steel fibres for ductility, heat-curing, and controlled pressure during setting. It is primarily used for specialty precast structures, bridge deck panels, blast-resistant elements and nuclear applications.
Compressive Str.
150–800 MPa
28d cylinder
Tensile Str.
10–50 MPa
With steel fibres
w/c Ratio
0.16–0.25
Extremely low
Max Particle
≤ 600 µm
No coarse agg
Silica Fume
25–35%
% OPC mass
Steel Fibres
2–4% (vol)
φ0.2mm, L13mm
PCE SP Dose
2–4%
% cement mass
Heat Curing
90°C / 48h
Often mandatory
TYPICAL DUCTAL®-TYPE RPC PROPORTIONS (200 MPa):
Material Qty (kg/m³) Vol (m³/m³)
OPC 53 Grade: 710 0.225
Silica Fume (25%): 178 0.081
Silica Sand (600µm): 1010 0.380
Ground Quartz: 213 0.097
Water: 126 0.126 w/c = 0.178
PCE SP (25% WR): 25 0.024
Steel Fibres (2%): 157 0.020 φ0.2mm × 13mm
Total: 2419 1.000 m³
28d compressive (without heat curing): ~150 MPa
28d compressive (90°C / 48h curing): ~200 MPa
Flexural strength: 30–50 MPa (vs 3–5 MPa for normal concrete)
Fracture energy: 20,000–40,000 J/m² (vs 100 J/m² normal concrete)
RPC / UHPC in India: Commercial RPC production in India is limited to specialty precast plants (primarily in Pune, Hyderabad, and Chennai regions). Applications include pedestrian bridge deck panels (20–40% lighter than conventional RCC), blast/impact-resistant barrier elements, nuclear waste containment panels, and decorative ultra-thin architectural cladding (8–15mm thick). Material cost is typically ₹25,000–60,000/m³ — 5–10× normal M35 concrete — but the structural efficiency (very thin sections, no rebar in some applications) can make it economically competitive for specific applications.
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Fibre-Reinforced Concrete (FRC)
IS 17452:2020 · Steel, polypropylene, glass, basalt fibres — crack resistance, toughness and post-crack ductility
Fibre-Reinforced Concrete (FRC) incorporates discrete fibres distributed randomly throughout the concrete matrix to improve post-crack behaviour, reduce shrinkage cracking, enhance toughness and impact resistance. IS 17452:2020 (Steel Fibre Reinforced Concrete) is the primary Indian standard. Fibres do not typically increase first-crack strength but dramatically change post-crack behaviour — from brittle to ductile failure.
| Fibre Type | Material | Typical Dose (kg/m³) | l/d Ratio | Primary Benefit | IS Standard | 2026 Cost (₹/kg) |
|---|
| Steel (hooked end) | High carbon steel wire | 25–80 kg/m³ | 50–80 | Flexural toughness, post-crack ductility, impact resistance | IS 17452:2020 | ₹75–120/kg |
| Polypropylene (PP) | Synthetic polymer | 0.5–3 kg/m³ | 50–200 | Plastic/early-age shrinkage crack control; fire resistance (spalling) | IS 9103 (admixtures) | ₹150–250/kg |
| Glass (AR-glass) | Alkali-resistant glass | 15–30 kg/m³ | 100–200 | Thin sections; shotcrete; architectural panels | ASTM C1666 | ₹120–200/kg |
| Basalt | Volcanic basalt rock | 5–20 kg/m³ | 50–150 | Corrosion-free; thermal stability; marine structures | GOST standards | ₹180–300/kg |
| Micro-PP (0.2–0.5mm) | Synthetic polymer | 0.9–1.8 kg/m³ | — | Fire spalling resistance in tunnel linings (M50+) | — | ₹350–500/kg |
FRC MIX DESIGN ADJUSTMENTS (Steel Fibres, 40 kg/m³):
Base mix: M30 IS 10262 design (C=331, W=149, CA=928, FA_sand=1019 kg/m³)
Step 1: Add steel fibre volume to absolute volume balance
V_fibre = 40 / (7850 × 1) = 0.0051 m³/m³
New FA_sand = (V_FA_sand − V_fibre) × SG_FA × 1000
= (0.3846 − 0.0051) × 2.65 × 1000 = 1005 kg/m³
Step 2: Increase water/SP to compensate for fibre-induced viscosity
Water increase: +8–15 L/m³ per 40 kg steel fibres (typical)
Or: Increase PCE SP dose by 0.15–0.25% to maintain slump
Step 3: Verify workability — slump typically drops 25–50mm per 40 kg/m³ fibres
Target slump for FRC: increase by 25mm over equivalent non-FRC
Use slump ≥ 100mm base (fibres reduce effective slump)
Step 4: Check fibre-induced bleeding
High steel fibre content → increased bleeding tendency
Use FA or GGBS to reduce bleed water
Workability check: Vebe time or flow table (fibres make slump less reliable)
SFRC hardened property tests: IS 17452 — flexural toughness (EN 14651)
FRC Applications in India (2026): Industrial floors and warehouses (steel fibres 40–60 kg/m³ replacing mesh reinforcement, ₹50–80 lakh annual savings per 10,000m² floor), tunnel linings (steel + PP fibre combination), bridge deck overlays, precast drainage channels, shotcrete for slope stabilisation (steel fibres 25–35 kg/m³), and high-rise transfer slabs. IS 17452:2020 now provides design guidance making SFRC mainstream for industrial floor design per TR 34 / ACI 360.
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Lightweight Concrete (LWC)
IS 2185 Part 1–4 · Density 300–1850 kg/m³ · Structural, semi-structural and non-structural types
Lightweight concrete has a fresh density below 1850 kg/m³, achieved by using lightweight aggregates (expanded clay, LECA, pumice), foam/air entrainment (foam concrete, aerated autoclaved concrete), or omitting fine aggregate (no-fines concrete). The reduced self-weight can significantly reduce structural loads in high-rise buildings and long-span bridges.
| LWC Type | Density (kg/m³) | Compressive Str (MPa) | Thermal Conductivity | IS Standard | Primary Use |
|---|
| Structural LWC (LWAC) | 1400–1850 | 17–60 MPa | 0.5–1.0 W/mK | IS 9142:1979 | Structural slabs, bridge decks |
| Aerated Autoclaved (AAC) | 400–800 | 2–8 MPa | 0.10–0.25 W/mK | IS 2185 Part 3 | Non-load bearing walls, insulation panels |
| Foam Concrete | 300–1600 | 1–25 MPa | 0.08–0.6 W/mK | IS 13086:1991 | Void filling, insulating screeds, lightweight fills |
| LECA / Expanded Clay | 1000–1600 | 8–30 MPa | 0.3–0.8 W/mK | IS 9142:1979 | Structural LWC, roof screeds |
| No-Fines Concrete | 1600–1900 | 2–15 MPa | 0.3–0.7 W/mK | IS 13086 | Drainage walls, drainage layers |
| Pumice Concrete | 700–1400 | 4–20 MPa | 0.2–0.5 W/mK | IS 2185 Part 4 | Insulating structural elements |
STRUCTURAL LWC (LECA) MIX DESIGN:
Target: Density 1600–1800 kg/m³, fck ≥ 25 MPa
Key differences from normal concrete IS 10262 mix design:
1. AGGREGATE: Replace crushed stone CA with pre-wetted LECA
LECA SG = 0.50–0.90 (varies by grade)
LECA water absorption = 5–25% (pre-soak 24h before use)
LECA DRBD = 400–700 kg/m³ (IS 9142 tested)
2. WATER CONTENT: Must account for LECA pre-absorption
Total water = mixing water + LECA absorption water
Design on net free water (mixing water only) for w/c ratio
3. CEMENT CONTENT: Higher than equivalent normal concrete
Minimum 350–420 kg/m³ for structural grade
4. WORKABILITY: Lower than normal concrete for same slump
Target slump 75–100mm (higher slump aids LECA segregation)
Add SP or air entrainer to improve workability
TYPICAL PROPORTIONS (Structural LWC, fck=25 MPa):
OPC 53: 380 kg/m³
LECA (pre-wet): 450 kg/m³ (dry basis)
River Sand (FA): 600 kg/m³
Free water: 190 L/m³ (w/c = 0.50)
LECA absorption: 90 L/m³ (absorbed — not part of w/c)
Air (entrained): 5%
Fresh density: ≈ 1720 kg/m³
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Early-Strength Concrete
Achieve specified strength in 24–72 hours — precast demoulding, fast-track construction, cold weather
Early-strength concrete is designed to achieve a target minimum strength within 24, 48 or 72 hours rather than the standard 28 days. This is required for precast demoulding (minimum strength typically 20–25 MPa at 16–24 hours), fast-track road repair (opening to traffic in 6–24 hours), and cold weather concreting where early strength prevents freeze damage.
| Strategy | Early Strength Gain | Effect on 28d Str | IS Standard | Cost Premium | Application |
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| OPC 53 + low w/c (<0.40) + PCE SP | +++ | Neutral / positive | IS 12269 | SP only | Standard method for M35+ early strength |
| Rapid Hardening Cement (RHC) | +++ | Slight reduction | IS 8041:1990 | ₹200–400/t premium | Precast, cold weather |
| Type C Accelerating Admixture | ++ | Slight reduction at 28d | IS 9103 Type C | ₹50–150/m³ | Cold weather (≠CaCl₂ for RCC) |
| Steam curing (65–85°C) | +++ | May reduce slightly at 28d | IS 9012 | Infrastructure cost | Precast plants |
| Low w/c + SRPC (Type V) | ++ | Neutral | IS 12330 | ₹300–500/t premium | Cold/sulphate exposure |
| Calcium Silicate Hydrate (CSH) seeds | ++++ | Positive | — | ₹200–500/m³ | Specialty precast; emerging technology |
EARLY-STRENGTH MIX DESIGN — TARGET 25 MPa AT 24 HOURS:
Method: OPC 53 + Low w/c + PCE SP + Type C accelerator
Step 1: Determine required 24h strength and equivalent 28d strength
Target 24h: 25 MPa (precast demoulding)
OPC 53 typical 24h/28d ratio: ≈ 0.55
Required 28d: 25/0.55 = 45.5 MPa → Design for M40 grade
Step 2: Select very low w/c to achieve high early strength
w/c = 0.35 (strength criterion governs for M40)
Lower w/c increases early C3S hydration rate
Step 3: Use Type C accelerator to advance early hydration
Triethanolamine (TEA) or calcium formate
Dose: 0.03–0.06% cement mass
Effect: +3–6 MPa additional strength at 24h
Step 4: Specify steam or warm water curing if applicable
Steam at 65°C for 12h: equivalent to 3–5 days ambient curing
Warm water (40°C): significant early strength boost
TYPICAL EARLY STRENGTH MIX (M40, target 25 MPa at 24h):
OPC 53: 400 kg/m³
Water: 140 L/m³ (w/c = 0.35)
PCE SP: 2.0 L/m³
Accelerator: 0.2 L/m³ (calcium formate or TIPA-based)
CA (20mm): 900 kg/m³
FA (Zone II): 880 kg/m³
Expected 24h strength (ambient 25°C): ~26–30 MPa
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Underwater / Tremie Concrete
IS 456 Cl. 14.5 · CIRIA C544 · Tremie pipe method · Anti-washout admixtures
Underwater concrete must flow from the tremie pipe tip and spread without segregation or cement washout, even while submerged. The fresh concrete displaces water upward rather than mixing with it, relying on a cohesive, high-slump mix design and continuous tremie pipe immersion to prevent water intrusion.
Slump
150–220 mm
Very high — self-levelling
Cement Min
≥ 400 kg/m³
IS 456 Cl. 14.5
MSA
≤ 20 mm
Flow requirement
AWA Admixture
Required
Anti-washout agent
Slump Flow (SCC)
600–750 mm
If SCC-based
Tremie Immersion
≥ 1.5 m
At all times
Overdesign Str
+20–30%
Water dilution factor
UNDERWATER / TREMIE CONCRETE MIX DESIGN:
Key requirement: Concrete must maintain cohesion and NOT mix with
water during placement. The tremie pipe is kept immersed ≥1.5m
in concrete — concrete flows radially from the tip, pushing water up.
MIX DESIGN MODIFICATIONS FROM NORMAL M30:
1. INCREASE CEMENT: Minimum 400 kg/m³ (IS 456 Cl. 14.5)
Reason: Cement washout during placing reduces effective content;
design mix compensates by 20–30% additional cement.
Target 28d DESIGN strength: fcr_UW = fcr × 1.25 (25% strength loss factor)
2. ADD ANTI-WASHOUT ADMIXTURE (AWA):
Welan gum, cellulose ether, or polyethylene oxide
Dose: 0.1–0.3% cement mass
Effect: increases viscosity → resists water washout
Note: AWA reduces strength ~5–10% at equivalent w/c
3. HIGH SLUMP (not SCC):
Target 180–220mm slump (not slump flow)
High slump needed for tremie flow without pump pressure
Achieved by PCE SP Type G (retarding — avoids set during placing)
4. REDUCE COARSE AGGREGATE:
CA ≤ 850 kg/m³ (vs 900–950 for normal concrete)
Lower CA improves flow and reduces blockage risk in tremie pipe
5. INCREASE FINE AGGREGATE / FINES:
Higher paste volume and fine content improves cohesion
Add FA or limestone filler to increase powder content to 450–550 kg/m³
TYPICAL TREMIE CONCRETE (M30 equivalent underwater):
Design for 28d cube strength = 30 × 1.25 = 37.5 MPa (= M35 design)
OPC 53: 420 kg/m³
Fly Ash (20%): 84 kg/m³
Water: 189 L/m³ (w/c = 0.45)
CA (20mm): 820 kg/m³
FA (Zone II): 920 kg/m³
PCE SP Type G: 5.0 L/m³
AWA (Welan): 1.2 kg/m³
⚠️ Tremie Pipe Rules (IS 456 Cl. 14.5 + CIRIA C544): (1) Tremie pipe must be immersed minimum 1.5m in concrete at all times — never lifted above concrete surface; (2) Concrete must be placed continuously without interruption; (3) Never add water to improve slump — adjust SP before placing; (4) Overdesign strength by 25% minimum to account for dilution and non-uniform compaction; (5) Test concrete at surface after displacing initial "contaminated" batch — first 0.5m³ per tremie is typically discarded; (6) Do NOT use vibrators — vibration causes segregation and breaks the cohesive plug.
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Shrinkage-Compensating Concrete
ACI 223R · Expansive cement or shrinkage-reducing admixtures (SRA) — controlling drying shrinkage cracking
Shrinkage-compensating concrete either expands during early curing (to pre-compress the reinforcement) or uses shrinkage-reducing admixtures (SRA) to chemically reduce drying shrinkage. Both approaches address the fundamental problem that conventional concrete shrinks 400–800 microstrains on drying — typically producing visible surface cracks in slabs and walls.
| Method | Mechanism | Shrinkage Reduction | IS / ASTM Standard | Cost Premium | Application |
|---|
| Expansive Cement (Type K) | Ettringite expansion pre-stresses rebar; net result = lower drying shrinkage | Eliminates shrinkage; controlled expansion 0.05–0.30% | ACI 223R; ASTM C845 | ₹500–1500/m³ | Water-retaining structures, slabs without joints |
| SRA — Shrinkage Reducing Admixture | Reduces surface tension of pore water → less capillary tension → less shrinkage | 25–50% reduction in drying shrinkage strain | ASTM C494 Type S; IS 9103 | ₹200–500/m³ | Large floor slabs, parking decks, bridge decks |
| Internal Curing (pre-wetted LWA) | Lightweight aggregates release water during self-desiccation, reducing autogenous shrinkage | 40–80% autogenous shrinkage reduction | ASTM C1761; ACI 308R | ₹150–400/m³ | Low w/c HPC, bridge decks, white-topping |
| Polypropylene Fibres | Physically arrest plastic shrinkage cracks before they propagate | Eliminates plastic shrinkage cracking; 20–30% drying shrinkage reduction | IS 9103; IS 17452 | ₹150–300/m³ | Industrial floors, roof slabs, pavements |
| Pozzolanic SCMs (FA, GGBS) | Reduce heat and capillary porosity; secondary reduction in drying shrinkage | 10–20% shrinkage reduction | IS 3812 / IS 16714 | Neutral or saving | All structural concrete — indirect benefit |
SHRINKAGE REDUCTION — DESIGN APPROACH:
STEP 1: QUANTIFY SHRINKAGE RISK
Thin slabs (≤150mm): High risk — large surface area / volume ratio
Long elements (≥15m without joints): High risk
Low w/c (<0.40): High autogenous shrinkage risk
High ambient temperature: Accelerates drying shrinkage
STEP 2: SELECT STRATEGY
Plastic shrinkage (first 4h): Polypropylene micro-fibres (0.9 kg/m³)
Autogenous shrinkage (low w/c HPC): Internal curing (pre-wetted LWA)
Drying shrinkage (long-term): SRA + polypropylene fibres
Elimination (water-retaining): Expansive cement + restrained expansion
STEP 3: MIX DESIGN MODIFICATION (SRA Example):
Base M30 mix: C=331, W=149, w/c=0.45
Add SRA at 1.5% cement mass:
SRA dose = 331 × 0.015 = 4.97 kg/m³ ≈ 5 L/m³
SRA replaces part of mix water (contributes to fluidity):
Adjust water: W_new = 149 − 5 = 144 L/m³ (approx)
Adjusted w/c = 144/331 = 0.435 ← slight improvement
Expected shrinkage reduction: 30–45% at 90 days
Crack width reduction: from ~0.3mm to ~0.15mm (indicative)
Cost addition: 5 L × ₹800/L ≈ ₹4,000/m³ (SRA is expensive)
→ Polypropylene fibres (₹900/m³) may be more economical alternative
📌 Joint Spacing vs Shrinkage-Compensating Concrete
- Conventional RCC slabs: Contraction joints at 3–5m spacing absorb shrinkage movement by controlled cracking at joint grooves.
- SRA concrete: May permit joint spacing of 6–9m for the same crack-width performance — reducing joint maintenance cost.
- Expansive cement concrete: With sufficient rebar (0.15–0.5% area ratio), joint spacing can extend to 30–60m or joints can be eliminated entirely in some floor applications (per ACI 223R).
- Post-tensioned slabs: The combination of PT compression and SRA essentially eliminates shrinkage cracking — a common choice for large commercial podium slabs in India.
