Mix design of concrete and property of green concrete

ACI method (ACI 211.1) is the absolute volume method: the quantities of water, cement, coarse aggregate and air are chosen from tables, and the fine aggregate is the remainder of the 1 m$^3$ volume.

Key concepts

• Strength depends on the w/c ratio (Abrams' law); water content depends on slump, maximum size and air.
• The coarse aggregate quantity depends on its size and the fineness modulus of sand (bulk volume of dry-rodded aggregate per unit volume of concrete).
• Fine aggregate is found from the absolute volume (volume of 1 m$^3$ minus others).

Steps

1. Target strength: $f'_{cr} = f'_c + 1.34 s$ or $f'_c + 2.33 s - 3.45$ (use the larger). If $s$ is unknown, $f'_{cr} = f'_c + 8.3$ MPa (for 21-35 MPa).
2. Maximum aggregate size from section, bar spacing (1/5 of narrowest dimension, 3/4 of clear spacing).
3. Slump from the type of work (Table: 25-75 mm for footings, 75-100 mm for beams/columns).
4. Water content and air from Table (ACI 211.1 Table 6.3.3), for the slump and size; with entrapped air % (3% for 9.5 mm down to 1% for 37.5 mm in non-air-entrained concrete; 2% for 20 mm).
5. w/c ratio from strength (Table 6.3.4) and check for exposure/durability; take the lower.
6. Cement content = water / (w/c); check the minimum.
7. Coarse aggregate: bulk volume per unit volume from Table (by size and sand FM) $\times$ dry-rodded bulk density.
8. Fine aggregate by absolute volumes:

$$V_{FA} = 1 - \left(\frac{W}{1000} + \frac{C}{\rho_c \cdot 1000} + \frac{CA}{\rho_{ca} \cdot 1000} + \text{air}\right)$$

Mass of FA = $V_{FA} \times$ SG $\times 1000$. 9. Adjust for moisture (absorption, surface moisture) and trial batch; check slump and strength; modify.

Example (f'c = 25 MPa, slump 75-100 mm, 20 mm, FM = 2.8, no air entrainment)

• $f'_{cr} = 25 + 8.3 = 33.3$ MPa; w/c = 0.49 (interpolated).
• Water = 205 kg/m$^3$, air = 2%; cement = 205/0.49 = 418 kg.
• CA = 0.62 $\times$ 1600 = 992 kg (BD 1600).
• Volumes: water 0.205, cement 418/3150 = 0.133, CA 992/2700 = 0.367, air 0.02, FA = 1 - 0.7252 = 0.2748 m$^3$, FA = 0.2748 $\times$ 2600 = 714 kg.
• Mix per m$^3$: C 418, W 205, FA 714, CA 992 kg; ratio 1 : 1.71 : 2.37.

DoE (British) method (Department of Environment, Road Note 4 / BRE 1988) selects the mix from charts and tables. It is based on the w/c ratio, free-water content and the proportion of fine aggregate.

Key concepts

• Target mean strength = characteristic strength + margin, where margin = $k \times s$.
• w/c ratio from the strength vs w/c chart for the type of cement and aggregate; compare with durability limit.
• Free-water content depends on slump/Vebe, maximum size and aggregate type (crushed/uncrushed).
• Wet density of fresh concrete from chart (depends on free water and aggregate density).
• Fine aggregate proportion from chart (maximum size, slump, w/c, grading zone).

 f_ck -> target f_m -> w/c (chart) -+
 slump,size,type -> water (table) --+-> cement
 free water + SG -> density (chart) -> total aggregate
 w/c,slump,zone -> %fines (chart)  -> FA, CA

Steps

1. Target mean strength: $f_m = f_{ck} + 1.64 s$ (margin $= k s$, $k = 1.64$ for 5% defectives) or IS value $1.65 s$. Standard deviation $s$ from the site data or table (e.g. 4-5 MPa).
2. w/c ratio: Table 2 gives 28-day strength at w/c 0.5 for the cement/aggregate type; read the w/c on the curve of Fig 1 for the target strength. Check the maximum w/c limit for exposure.
3. Free-water content from Table 3 (kg/m$^3$) for slump (0-10, 10-30, 30-60, 60-180 mm), size (10, 20, 40 mm), crushed/uncrushed.
4. Cement content = water/(w/c); check the minimum and maximum limits (e.g. 300 and 550 kg/m$^3$).
5. Wet density of concrete from Fig 3 (free water, aggregate SG).
6. Total aggregate content = wet density - cement - water.
7. Proportion of fine aggregate from Fig 4 (% passing 600 micron).
8. Fine and coarse aggregate = proportion $\times$ total aggregate; coarse aggregate split into sizes (for 10/20/40 mm in 1:2:3 ratio etc.).
9. Trial mix, check workability and strength; adjust.

Sample table (illustration)

Workability is the property of freshly mixed concrete that determines the ease and homogeneity with which it can be mixed, placed, compacted and finished (IS 6461 Part 7). It is the combination of mobility, stability (resistance to segregation and bleeding) and compactability. IS 456 cl. 7.1 gives the workability required for various placing conditions.

Methods of measurement (IS 1199)

1. Slump test (IS 1199; field and laboratory): A cone 300 mm high, 100 mm top and 200 mm bottom diameter is filled in four layers of equal height, each tamped with 25 strokes of a 16 mm rod. The cone is lifted vertically; the slump is the settlement of the concrete from the top of the cone. For a true slump, shear slump or collapse, the test is repeated. Range: 25-125 mm. Not suitable for very dry or very wet mixes. IS 456: slump 25-75 mm for lightly reinforced sections, 50-100 for heavily reinforced.

2. Compaction factor test (laboratory; useful for low workability, CF 0.7-0.95): concrete falls through two hoppers into a cylinder; weigh partially compacted concrete ($W_1$), then fully compacted ($W_2$):

$$\text{CF} = \frac{W_1}{W_2}$$

3. Vee-Bee test (very dry mixes): the time in seconds (Vebe seconds) to change the shape of the slump cone to a cylinder under vibration; 5-30 s.

4. Flow table test (IS 9103 / 1199): a 760 mm table lifted and dropped 15 times; the average spread diameter gives the flow %; for flowing concrete.

5. Kelly ball test: penetration of a 15 cm hemisphere into fresh concrete (field).

6. Others: remoulding test, Ve-Be, K-slump, L-box, V-funnel, J-ring (for SCC).

Slump and Kelly ball are used at site, while compaction factor, Vee-Bee and flow table are laboratory tests.

Nominal mix is a concrete mix with fixed proportions of cement, sand and aggregate by volume (e.g. 1:1.5:3 for M20, 1:2:4 for M15) which gives the strength expected, without laboratory design, from experience. IS 456 cl. 9.3 permits it for M5, M7.5, M10, M15 and M20 grades on small or unimportant jobs.

Differences

Points to consider when using nominal mix in the field

1. Use at most M20 grade (IS 456); keep within the proportions of Table 9 (cement, total aggregate per 50 kg cement, sand proportions).
2. Batching by weight is preferable; if by volume, a gauge box of 50 kg bag (35 L) is used and bulking of sand is allowed for.
3. Use sand and aggregate that meet IS 383, with proper grading, and clean water.
4. Keep w/c as low as workability permits (not above 0.5-0.6 for M20).
5. Mix thoroughly, compact properly, cure for 7 days minimum.
6. Do cube tests; if cubes fail, shift to a design mix.

The water-cement ratio (w/c) is the ratio of the mass of water to the mass of cement in the mix. It is the most important factor controlling the strength of workable, fully compacted concrete.

Abrams' law

Strength is inversely related to the w/c ratio:

$$f_c = \frac{A}{B^{\,w/c}}$$

where $A$ and $B$ are constants (Abrams: $A = 96.5$ MPa, $B = 7$ in Abrams' original work) depending on cement, age and curing.

 Strength
  |\
  | \
  |  `.
  |    `-.
  |        `--.___
  +---------------------> w/c
  0.3  0.4  0.5  0.6  0.7

Why strength falls as w/c rises

• Water needed for hydration is only about 0.23-0.38 of the cement mass; the extra water remains in the paste and leaves capillary pores when it evaporates. Strength depends on the gel/space ratio (Powers); more pores mean lower strength.
• A higher w/c gives a weaker bond between the paste and the aggregate and a weaker transition zone, and more bleeding.
• A lower w/c increases strength, density, durability and impermeability, but reduces workability, and below about 0.35 full compaction is difficult (with partly hydrated cement, so use superplasticisers).

Typical: w/c 0.4 gives about 45-50 MPa, 0.5 about 35 MPa, 0.6 about 28 MPa, and 0.7 about 20 MPa at 28 days (OPC, well compacted).

Segregation is the separation of the constituents of fresh concrete so that it is no longer uniform: coarse aggregate separates from mortar, or water separates from the mix. It causes honeycombing, weak and non-uniform concrete, poor durability and surface defects.

Forms of segregation

1. Coarse aggregate separating out: heavy coarse particles settle or roll away from the mortar (common in dry, harsh and gap-graded mixes).
2. Paste (grout) separating from the coarse aggregate: in wet, high-slump mixes, the cement paste flows away from the aggregate.
3. Water separating (bleeding): water rises to the surface of a wet mix.

Causes

Poor grading, a too-wet or too-dry mix, excess vibration, dropping from height, long chutes, rough handling, differences in density of the materials.

Control during discharge from hopper

• Keep the discharge gate close to the receiving surface (not more than 1-1.5 m free fall), with a funnel or baffle.
• Open the gate fully and discharge in the centre, let the concrete fall vertically to the middle of the receiving vehicle, not against its side.
• Use a hopper with a sloping bottom of 60 degrees and a central outlet.

Control during placing

• Place concrete as close as possible to its final position, in layers 300-450 mm thick; do not move it with a vibrator.
• Limit free fall to 1.5 m; use chutes (with baffles), tremie or drop pipes (elephant trunk) for deeper sections.
• Use well-graded aggregates, a suitable w/c ratio and fine content, or admixtures (air entrainment, viscosity modifier).
• Avoid over-vibration; use right vibrators; clean, tight formwork.
• Keep chutes at a suitable slope (about 1 vertical to 2-3 horizontal) so that concrete neither stops nor shoots; avoid long transport over rough roads, and re-mix if segregation has already occurred.

IS method (IS 10262:2009) designs a mix for a required grade from the target strength, w/c ratio, water content and aggregate proportions by absolute volume, and is used with IS 456.

Steps

1. Design data: grade, maximum aggregate size, workability, exposure, cement type, specific gravities, absorption.
2. Target mean strength: $f'_{ck} = f_{ck} + 1.65 s$ (s from Table 1 of IS 10262: M20 4.0, M25 4.0, M30 5.0, M35 5.0 N/mm$^2$), or $f_{ck} + X$ (X = 5.0, 5.5, 6.5 for M10-15, M20-25, M30 and above), whichever is higher.
3. w/c ratio from the strength vs w/c curves (Fig 1, by cement 33/43/53 grade) for the target strength; check against IS 456 Table 5 limits of exposure; adopt the lower.
4. Water content from Table 2 (186 kg/m$^3$ for 20 mm angular aggregate at 50 mm slump); adjust +3% for every 25 mm of extra slump; reduce by superplasticiser (20% or more).
5. Cement content = water / (w/c); check the minimum from IS 456 Table 5 and maximum 450 kg/m$^3$.
6. Volume of coarse aggregate per unit volume of total aggregate from Table 3 (maximum size and zone of fine aggregate for w/c 0.5); adjust by $\pm$ 0.01 for each $\mp$ 0.05 change in w/c.
7. Mix calculations per m$^3$ using absolute volumes:

$$V_{agg} = 1 - \left[\frac{C}{\rho_c 1000} + \frac{W}{1000} + \frac{\text{admix}}{\rho_a 1000} + \text{air}\right]$$

Mass of CA = $V_{agg} \times V_{CA} \times SG_{CA} \times 1000$ ; FA = $V_{agg} (1 - V_{CA}) \times SG_{FA} \times 1000$ (air 2% for 20 mm entrapped). 8. Trial mixes: three trials with w/c varied; check the strength and slump; select the best. 9. Moisture and absorption corrections.

Sample table

Graph

 f_target (MPa)
  50|\
  40| `.\
  38|---`-`.   <- read w/c = 0.43
  30|       `-.
    +-----------> w/c
     0.35  0.5

Bleeding (water gain) is the rise of mixing water to the surface of freshly placed concrete, owing to the settlement of the heavier solid particles. It is a form of segregation.

Why and how it occurs

1. The solid particles (cement, aggregate) are heavier than water and cannot hold all the mixing water in suspension after placing and compaction.
2. Solids settle under gravity, and displaced water moves upward through channels (and along aggregate and steel surfaces).
3. Water collects at the top surface (or underneath aggregate and reinforcement as pockets). It is more with a high w/c, coarse cement, poorly graded mix, deficiency of fines, thick sections and over-vibration.

Influence of bleeding rate

High bleeding

• Weak, porous, dusty top layer (laitance) with poor wear resistance, and bad bond to the next lift.
• Water gaps below aggregate and bars: weak bond, low strength, permeable paths.
• Increases w/c at the top, plastic shrinkage and settlement cracking.
• Scaling and low durability.

Low bleeding

• Good, but if the surface dries too fast (hot, windy), plastic shrinkage cracks occur because the bleed water cannot replace evaporated water.
• Concrete with very low bleeding is hard to finish if finishing is delayed.

Control

Use lower w/c, finer cement, more fines, air entrainment, well-graded aggregates, proper compaction, pozzolanic admixtures, and delay finishing until bleed water is gone.

Hot weather (ambient above 40 °C, or high temperature with low humidity and wind; IS 7861 Part 1, IS 456 cl. 14.1.1 & ACI 305) speeds up setting, reduces slump, increases water demand, and causes plastic shrinkage cracking. The concrete temperature at placing should be kept below about 35 °C (IS 7861: not above 40 °C).

Precautions

1. Materials: keep aggregates in shade and sprinkle with water; use chilled water or crushed ice as a part of mixing water; shade and paint water tanks and pipes white; use cement at low temperature (not warm from silo).
2. Mix: use a low-heat cement or PPC, lower cement content, and a retarder or water-reducing retarder admixture; keep w/c fixed and avoid adding water to regain slump.
3. Transport and placing: shorten the time between mixing and placing; cover mixers, ready-mix trucks and pump lines; place at night or early morning; avoid delay and cold joints.
4. Forms and subgrade: wet the formwork, reinforcement and subgrade before placing (no standing water); shade the work with sunshades.
5. Protection and curing: start curing immediately; use fog sprays, evaporation retarders, wet hessian, plastic sheeting, curing compound; continuous curing for at least 7 days; protect from wind to prevent plastic shrinkage cracks.
6. Provide additional joints and thermal control in thick sections.

Method: ACI 211.1 (absolute volume).

Data: $f'_c = 35$ MPa; fresh-water exposure; $G_{FA} = 2.6$, $G_{CA} = 2.7$; dry-rodded bulk density of CA = 1600 kg/m$^3$; FM of sand = 2.8; nominal size 20 mm (taken as 19 mm in ACI tables); slump 50 mm; non-air-entrained; cement SG assumed 3.15.

Step 1: Target strength. Standard deviation is not given, so $f'_{cr} = f'_c + 8.3$ (ACI 318 for $f'_c$ up to 35 MPa; when $s$ is unknown):

$$f'_{cr} = 35 + 8.3 = 43.3\ \text{MPa}$$

Step 2: Water and air (ACI 211.1 Table 6.3.3, 25-50 mm slump, 19 mm size, non-air-entrained): water = 190 kg/m$^3$, entrapped air = 2%.

Step 3: w/c ratio (Table 6.3.4(a), non-air-entrained): 40 MPa to 0.42 and 45 MPa to 0.38; interpolating for 43.3 MPa:

$$w/c = 0.42 - \frac{(43.3 - 40)}{5}(0.04) = 0.394 \approx 0.39$$

Durability check: for fresh water exposure the maximum w/c is 0.50 (ACI 318 Table 4.2.2), so 0.39 governs.

Step 4: Cement = $190/0.39 = 487$ kg/m$^3$.

Step 5: Coarse aggregate. Bulk volume per unit volume (Table 6.3.6, 19 mm, FM 2.8) = 0.62:

$$CA = 0.62 \times 1600 = 992\ \text{kg/m}^3$$

Step 6: Fine aggregate by absolute volume.

$V_{FA} = 1 - 0.7321 = 0.2679$ m$^3$, so $FA = 0.2679 \times 2.6 \times 1000 = 697$ kg/m$^3$.

Answer (per m$^3$): water 190 kg, cement 487 kg, fine aggregate 697 kg, coarse aggregate 992 kg; w/c = 0.39; ratio 1 : 1.43 : 2.04 (total mass about 2366 kg/m$^3$; surface and absorbed moisture corrections and trial mixes follow).

Method: ACI 211.1 (absolute volume).

Data: $f'_c = 25$ MPa; fresh-water exposure; $G_{FA} = 2.6$, $G_{CA} = 2.9$; dry-rodded bulk density of CA = 1550 kg/m$^3$; FM = 2.9; maximum size 20 mm (19 mm table); slump 50 mm; non-air-entrained; cement SG 3.15 (assumed).

Step 1: Target strength ($s$ not given): $f'_{cr} = 25 + 8.3 = 33.3$ MPa.

Step 2: Water and air (Table 6.3.3: 25-50 mm slump, 19 mm): water = 190 kg/m$^3$, air = 2%.

Step 3: w/c (Table 6.3.4(a)): 30 MPa to 0.54 and 35 MPa to 0.47; interpolating for 33.3 MPa:

$$w/c = 0.54 - \frac{3.3}{5}(0.07) = 0.494 \approx 0.49$$

Fresh-water exposure limit w/c $\leq 0.50$; satisfied, so adopt 0.49.

Step 4: Cement = $190/0.49 = 388$ kg/m$^3$.

Step 5: Coarse aggregate. From Table 6.3.6 (19 mm): FM 2.8 gives 0.62 and FM 3.0 gives 0.60; for FM 2.9 the bulk volume = 0.61:

$$CA = 0.61 \times 1550 = 945.5 \approx 946\ \text{kg/m}^3$$

Step 6: Fine aggregate (absolute volume)

$V_{FA} = 1 - 0.6594 = 0.3406$ m$^3$, so $FA = 0.3406 \times 2600 = 886$ kg/m$^3$.

Answer (per m$^3$): water 190 kg, cement 388 kg, fine aggregate 886 kg, coarse aggregate 946 kg; w/c = 0.49; ratio 1 : 2.29 : 2.44 (total about 2410 kg/m$^3$). Adjust for moisture and verify by trial mix.

Workability is the property of freshly mixed concrete that decides how easily it can be mixed, transported, placed, compacted and finished without segregation (IS 6461).

Factors affecting workability

1. Water content: more water gives higher workability (main factor).
2. Water-cement ratio and cement content / paste volume.
3. Aggregate properties: shape (rounded better), texture (smooth better), maximum size (larger gives higher), grading (well graded better), and porosity/absorption.
4. Aggregate-cement ratio and fine-to-coarse aggregate ratio.
5. Fineness and type of cement.
6. Admixtures: plasticisers, superplasticisers and air-entrainers raise it; accelerators lower it.
7. Temperature: high temperature reduces workability (faster hydration and evaporation).
8. Time: slump loss with elapsed time after mixing.
9. Method of mixing and compaction: mixer type, mixing time; humidity and wind.

Workability is the ease with which a concrete mix can be mixed, transported, placed, compacted and finished with the minimum loss of homogeneity (IS 6461 Part 7). It includes three components: mobility (flow), stability (resistance to segregation and bleeding) and compactability (ability to be fully compacted). It depends on water, cement paste, aggregate grading, shape and size, and admixtures; it is measured by slump, compaction factor or Vee-Bee tests (IS 1199).

Impact on the quality of concrete

• Low workability (too stiff): the mix is difficult to compact, entrapped air and voids remain, causing honeycombing, poor bond to the steel and low strength (1% of voids reduces strength by about 5-6%). Congested reinforcement is not filled.
• Too high workability (excess water): causes segregation and bleeding, laitance, a high w/c ratio with lower strength, higher permeability, more shrinkage and cracks, and poor durability.
• Suitable workability gives full compaction, uniform density and homogeneity, a good surface finish, good bond with reinforcement and the strength designed for, with the lowest labour and energy cost.

 Quality
   |        ____
   |      /      \
   |     /        \   <- optimum workability
   |    /          \
   +----------------> workability
  honeycombs   segregation

Therefore the right workability for the type of structure (IS 456 cl. 7.1; e.g. slump 25-75 mm for lightly reinforced sections, 50-100 mm for heavily reinforced sections) should be provided, using admixtures instead of extra water.

Measures of workability (IS 1199)

• Slump test: slump in mm of a cone-moulded sample (25-125 mm).
• Compaction factor test: ratio of the weight of partially compacted concrete to the fully compacted (0.7-0.95).
• Vee-Bee test: time in seconds for a slump cone to remould into a cylinder under vibration (for dry mixes).
• Flow table test: spread diameter of concrete on a table after 15 drops (flowing concrete).
• Kelly ball penetration test, remoulding test, and for SCC: slump flow, L-box, V-funnel.

Role of the water-cement ratio

• Workability: a higher w/c gives a more fluid, workable mix; it affects the slump directly (about 10-25 mm per 1-2% change in water content, practically for a fixed cement content).
• Strength: strength falls as w/c rises (Abrams' law, $f = A/B^{w/c}$); excess water leaves capillary pores.
• Durability: high w/c gives high permeability, so ingress of chlorides, sulphates and water causes corrosion and frost damage (IS 456 Table 5 limits w/c to 0.55-0.45 for mild to extreme exposure). Too low w/c (below 0.35) gives poor workability and incomplete compaction unless a superplasticiser is used. So w/c should be as low as compaction permits.

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

Methods to control workability

1. Water content: increase/decrease the water, keeping w/c fixed by changing cement as well.
2. Cement paste content: more paste (cement + water) improves workability.
3. Aggregate grading, shape and size: well-graded, rounded and larger aggregates improve it.
4. Ratio of fine to coarse aggregate: adjust the sand percentage.
5. Admixtures: plasticisers, superplasticisers, air-entraining agents (increase), accelerators (reduce slump), mineral admixtures (fly ash increases).
6. Temperature control of the mix (cool ingredients).
7. Mixing time and method, and delivery time.

Explained: use of admixtures (plasticiser / superplasticiser)

The initial mix has a given w/c for the required strength, e.g. 0.45. A plasticiser is added (0.5-2% of cement mass) to raise the slump from 50 mm to 150 mm without adding water. The admixture molecules adsorb on cement particles, giving them the same charge, so they repel each other and the flocs break up, releasing trapped water. Thus the mix flows easily at the same w/c, strength and durability are kept, and cement is not wasted. The slump is checked at the site, and the dose is adjusted (the effect lasts 30-60 min). The alternative of adding water would only raise w/c and lower strength.

Workability is the ease with which concrete can be mixed, placed, compacted and finished without segregation (IS 6461 Part 7).

Slump test procedure (IS 1199; IS 7320)

Apparatus: slump cone (frustum) of 300 mm height, 200 mm bottom and 100 mm top internal diameter; tamping rod 16 mm dia, 600 mm long with bullet end; base plate (non-absorbent, smooth); scale.

Procedure

1. Clean the mould and place it on a smooth, horizontal, non-absorbent base plate; hold it down with the foot pieces.
2. Fill the cone in four layers, each about one-quarter of the height; tamp each layer with 25 strokes of the rod, uniformly distributed, penetrating into the layer below.
3. After the top layer, strike off the surface level with the top of the cone.
4. Lift the cone vertically slowly (in 5-10 seconds) without any twisting.
5. Measure the slump, the difference between the height of the mould and the height of the highest point of the subsided concrete, to the nearest 5 mm.
6. Complete the test within 2 minutes of sampling; if the concrete shears off or collapses, the test is repeated.

Recommended slump (IS 456 cl. 7.1): 25-75 mm for lightly reinforced sections in slabs, beams, walls and columns; 50-100 mm for heavily reinforced sections; 75-100 mm for trench fill, etc.

Types of slump

A slump test gives one of the following forms:

 True slump   Shear slump   Collapse slump
  ,----.        ,---.          ______
 /      \      /     \_        /      \
 uniform      one half slides  completely
 subsidence   sideways         flows out

1. True slump: the concrete subsides evenly all round, keeping its shape; this is the only valid reading.
2. Shear slump: one half of the cone slips sideways along an inclined plane; indicates lack of cohesion; the test is repeated; if it persists, the mix is unsuitable (harsh, lean).
3. Collapse slump: the concrete collapses fully and spreads, showing a very wet, high-slump mix (high water or a plastic mix, requiring another method such as the flow table). (Zero slump: no subsidence, very dry mix.)

Measurement

The slump cone (300 mm high) is filled in 4 layers, each tamped with 25 strokes, struck off level, then lifted vertically. The slump is the vertical distance between the top of the mould and the top of the subsided concrete (nearest 5 mm). For a true slump this is the reading; for a shear slump, the test is repeated; for a collapse the mix is reported as collapsed. The measure is not suitable for mixes with slump below 10 mm or above about 125 mm.

Quality control of concrete at site (IS 456 Clause 15, 16)

1. Materials: test cement (IS 4031), aggregates (IS 2386, grading, bulking, moisture), water and admixtures before use; store properly (cement off the ground, aggregate separated).
2. Mix proportioning: use design mix with weigh batching; correct for moisture, bulking and absorption; keep the w/c fixed.
3. Mixing and transport: use mechanical mixers (mixing at least 2 min); avoid segregation; place within the time allowed.
4. Workability checks: slump test on every batch/truck.
5. Placing, compaction and curing: proper compaction (vibrators), formwork cleanliness, cover blocks, curing for 7 days minimum (10 days for PPC, IS 456 cl. 13.5).
6. Sampling and strength tests: cubes (150 mm) taken as per IS 456 Table 11 (1 sample for 1-5 m$^3$, 2 for 6-15, and so on); tested at 7 and 28 days; acceptance by cl. 16.1 (mean of 4 cubes $\geq f_{ck} + 0.825 s$, individual $\geq f_{ck} - 4$ N/mm$^2$).
7. Records, supervision, and calibration of equipment; core tests or NDT (rebound hammer, UPV) in case of doubt.

Slump test in detail (IS 1199)

Apparatus: slump cone (bottom dia 200 mm, top 100 mm, height 300 mm), 16 mm tamping rod 600 mm long, base plate, scale.

Procedure: place the cleaned, moist cone on the base plate; fill in four layers of about one-quarter the height each; tamp each layer with 25 strokes of the rod; strike off the top; lift the cone vertically within 5-10 s; measure the subsidence (the slump) from the top of the cone to the average height of the concrete.

Results: true slump (valid), shear slump (repeat) and collapse slump (too wet). Typical slump values: 25-75 mm for lightly reinforced sections, 50-100 mm for heavily reinforced sections. The test is quick, but not suitable for very dry or very wet mixes. Variations of more than ± 25 mm from the design indicate a change in water content or grading.

Concrete mix design is the selection of the proportions of cement, water, fine and coarse aggregate (and admixtures) to produce concrete with the required workability when fresh, the required strength and durability when hardened, and the lowest cost. The following fundamental concepts are commonly adopted:

1. Abrams' water-cement ratio law: strength is governed by the w/c ratio ($f = A/B^{w/c}$); a lower w/c ratio gives higher strength and durability. It is the basis of all methods.
2. Water content for workability: the water required depends on the workability (slump), maximum size, shape and grading of aggregate and air content (tables or charts); cement content = water / (w/c ratio).
3. Target mean strength is higher than the characteristic strength: $f_{target} = f_{ck} + k\,s$ ($k = 1.65$ in IS 10262), allowing for variation (standard deviation $s$) so that not more than 5% of results fall below $f_{ck}$.
4. Durability requirements: limits on maximum w/c ratio and minimum cement content for the exposure condition (IS 456 Table 5), which may override the strength requirement.
5. Aggregate grading and packing: the volume of coarse aggregate and the proportion of fine aggregate are chosen for a well-packed, workable and economical mix (by fineness modulus or grading zone).
6. Absolute volume method: the total volume of all ingredients (each as mass / specific gravity) plus air equals 1 m$^3$.
7. Trial mixes and adjustments: laboratory trial batches are made to check slump and strength; corrections for moisture, absorption and bulking are applied at site.
8. Economy: use the minimum cement that meets the requirements, the maximum practicable aggregate size, and admixtures.

Factors influencing concrete mix design

1. Characteristic strength required (grade) and the target mean strength; level of quality control (standard deviation).
2. Workability required (slump, compaction factor) for the type of placing and compaction.
3. Durability / exposure condition: max w/c ratio and min cement content (IS 456 Table 5).
4. Maximum size, shape and grading of aggregate and zone of fine aggregate.
5. Type and grade of cement, and specific gravity.
6. Water-cement ratio and water content.
7. Admixtures and mineral additives.
8. Properties of materials: specific gravity, absorption, moisture and bulking.
9. Placing conditions: reinforcement density, section size, temperature, and method of transport.
10. Economy and available materials.

Indian method (IS 10262:2009)

The mix is designed in these steps:

1. Collect the data: grade, size, workability, exposure, cement, SG and so on.
2. Target strength $f'_{ck} = f_{ck} + 1.65 s$ (Table 1: s = 4 for M20/M25, 5 for M30 and above) or $f_{ck} + X$, whichever is higher.
3. w/c ratio from Fig 1 (28-day strength vs w/c for cement 33/43/53 grade) and check IS 456 Table 5; adopt the lower.
4. Water content from Table 2 (186 kg/m$^3$ for 20 mm at 50 mm slump; $\pm 3\%$ for each 25 mm change in slump; less for rounded aggregates (by 10 kg); superplasticiser 20% reduction).
5. Cement content = water / (w/c); check the minimum (IS 456 Table 5) and the maximum limit of 450 kg/m$^3$.
6. Volume of coarse aggregate from Table 3 (size, fine aggregate zone, w/c 0.5; change of 0.01 for each 0.05 change in w/c).
7. Absolute volume calculation: volume of concrete minus water, cement, admixture and air gives the aggregate volume, split into FA and CA by the CA fraction.
8. Trial mixes to check workability and strength, then adjust.

Basic principles of mix design

1. Strength-w/c relation (Abrams' law): the w/c ratio governs strength and durability.
2. Workability is governed by the water content, which depends on the aggregate size, shape, grading and the slump required.
3. Target mean strength = specified strength + margin for variability ($k s$), based on the degree of quality control.
4. Durability limits (maximum w/c, minimum cement) for the exposure.
5. Aggregate grading and proportion of fine to coarse aggregate to get a dense, cohesive mix.
6. Absolute volume concept: sum of volumes of all ingredients = 1 m$^3$ (or wet density method in DoE).
7. Economy with trial mixes and field adjustments.

Factors not considered in the DoE method

• The DoE method does not explicitly include the effect of air entrainment (it is for non-air entrained concrete), the use of admixtures such as superplasticisers, nor mineral additives (fly ash/slag), without modification.
• The bulking of sand, absorption and moisture content are applied only afterwards as site corrections.
• It does not consider the specific gravity of cement (taken as 3.15) or durability directly except through a check of w/c limits; maximum size of aggregate is limited to 10, 20, 40 mm; fineness modulus of sand is replaced by grading zones.
• Pumped concrete, high-strength mix (above 50-60 MPa) and lightweight concretes are outside its range.
• It does not consider the placing conditions (congestion of reinforcement, temperature) directly.

The British (DoE) method (BRE/DoE 1988, "Design of Normal Concrete Mixes") selects the mix from charts and tables; it is a wet-density method.

Steps

1. Target mean strength: $f_m = f_{ck} + k\,s$, with $k = 1.64$ (5% defectives) and $s$ = standard deviation (about 5-8 MPa, from site data).
2. w/c ratio: from Table 2 find the strength at w/c 0.5 for the cement type and aggregate (crushed/uncrushed) and age; plot on Fig 1 the curve and read the w/c for the target strength; check the maximum from durability requirements and use the lower.
3. Free-water content: from Table 3 for slump (0-10, 10-30, 30-60, 60-180 mm) or Vebe time, maximum size (10, 20, 40 mm) and aggregate type; for mixed types: $W = \frac{2}{3} W_f + \frac{1}{3} W_c$.
4. Cement content = free water / (w/c); check the minimum and maximum limits.
5. Total aggregate content: from Fig 3, find the wet density of concrete for the free water content and aggregate SG; total aggregate = wet density - cement - water.
6. Fine aggregate proportion: from Fig 4 using the grading zone (% passing 600 micron), size, slump and w/c.
7. Fine and coarse aggregate masses: FA = proportion $\times$ total aggregate; CA = remainder; coarse aggregate may be split into sizes.
8. Trial mix and adjust for slump and strength; correct for moisture content at site.

 f_ck+ks -> w/c -> + water -> cement
                       |
   density chart -> total agg -> FA / CA

Method: ACI 211.1.

Data: $f'_c = 25$ MPa (28 days); $s = 4$ MPa; $G_{FA} = 2.6$, $G_{CA} = 2.7$; dry-rodded density of CA = 1650 kg/m$^3$; FM = 2.8; air = 2%.

Assumptions: maximum size 20 mm (19 mm in ACI table), slump 75-100 mm (beams and columns), non-air-entrained, cement SG 3.15.

Step 1: Required average strength (ACI 318, $f'_c \leq 35$ MPa), larger of:

$$\begin{aligned} f'_{cr} &= f'_c + 1.34 s = 25 + 1.34 \times 4 = 30.36\ \text{MPa} \\ f'_{cr} &= f'_c + 2.33 s - 3.45 = 25 + 9.32 - 3.45 = 30.87\ \text{MPa} \end{aligned}$$

So $f'_{cr} = 30.87$ MPa.

Step 2: Water (Table 6.3.3, 75-100 mm, 19 mm) = 205 kg/m$^3$; air = 2% as given.

Step 3: w/c (Table 6.3.4(a)): 30 MPa to 0.54, 35 MPa to 0.47; for 30.87 MPa:

$$w/c = 0.54 - \frac{0.87}{5}(0.07) = 0.528 \approx 0.53$$

Step 4: Cement = $205/0.53 = 387$ kg/m$^3$.

Step 5: CA = $0.62 \times 1650 = 1023$ kg/m$^3$ (Table 6.3.6, 19 mm, FM 2.8).

Step 6: FA (absolute volume)

$V_{FA} = 0.2732$ m$^3$, $FA = 0.2732 \times 2600 = 710$ kg/m$^3$.

Answer (per m$^3$): cement 387 kg, water 205 kg, FA 710 kg, CA 1023 kg; w/c = 0.53; ratio 1 : 1.83 : 2.64. Moisture corrections and trial batches are required.

Method: ACI 211.1 with w/c given as 0.48.

Assumed data: maximum size 20 mm (19 mm in ACI tables); slump 75-100 mm; non-air-entrained, entrapped air 2%; $G_{FA} = 2.65$, $G_{CA} = 2.70$, cement 3.15; dry-rodded density of CA = 1600 kg/m$^3$; FM of sand = 2.8.

Step 1: Water (ACI Table 6.3.3, 75-100 mm, 19 mm) = 205 kg/m$^3$, air = 2%.

Step 2: w/c = 0.48 (given; from the ACI w/c table it corresponds to an average strength of about 36 MPa, which gives a margin of about 6 MPa over $f_{ck}$ = 30 MPa).

Step 3: Cement = $205/0.48 = 427$ kg/m$^3$.

Step 4: Coarse aggregate = $0.62 \times 1600 = 992$ kg/m$^3$ (Table 6.3.6, 19 mm, FM 2.8).

Step 5: Fine aggregate by absolute volume

$V_{FA} = 1 - 0.7280 = 0.2720$ m$^3$, $FA = 0.2720 \times 2650 = 721$ kg/m$^3$.

Answer (per m$^3$): cement 427 kg, water 205 kg, FA 721 kg, CA 992 kg; ratio 1 : 1.69 : 2.32 with w/c = 0.48 (total about 2345 kg/m$^3$). Correct for moisture and absorption of the aggregates at site, then confirm by trial batches.

Method: DoE (BRE 1988); w/c is given as 0.46.

Assumptions (state clearly since the paper's tables are only partly reproduced): OPC; crushed aggregate of maximum size 20 mm; workability medium (slump 30-60 mm); combined aggregate SG = 2.7; sand in grading zone 2 (about 60% passing 600 micron).

Step 1: Target mean strength (check): the standard deviation for M35 with good control $s \approx 5$ MPa, so $f_m = 35 + 1.64 \times 5 = 43.2$ MPa; the w/c of 0.46 given from the strength curve is accepted.

Step 2: Free-water content (DoE Table 3, 20 mm crushed, medium workability) = 230 kg/m$^3$.

Step 3: Cement = $230/0.46 = 500$ kg/m$^3$ (within the 300-550 kg/m$^3$ limits).

Step 4: Wet density (Fig 3, free water 230 and SG 2.7) $\approx$ 2350 kg/m$^3$.

Step 5: Total aggregate = $2350 - 230 - 500 = 1620$ kg/m$^3$.

Step 6: Fine aggregate proportion (Fig 4: 20 mm, slump 30-60 mm, w/c 0.46, zone 2) $\approx$ 38%:

$$\begin{aligned} FA &= 0.38 \times 1620 = 616\ \text{kg/m}^3 \\ CA &= 1620 - 616 = 1004\ \text{kg/m}^3 \end{aligned}$$

Check by absolute volume: $230/1000 + 500/3150 + 616/2700 + 1004/2700 = 0.989$ m$^3$, so about 1.1% air - acceptable.

Answer (per m$^3$): cement 500 kg, water 230 kg, FA 616 kg, CA 1004 kg; w/c 0.46; ratio 1 : 1.23 : 2.01. If the table values supplied with the paper differ, replace the water content in Step 2 with the value read from them and repeat Steps 3-6.

Method: DoE (BRE 1988).

Data: M25; maximum size 25 mm; $G_{CA} = 2.7$, $G_{FA} = 2.6$; moderate exposure; FM of FA = 3.0 (Zone 2/1 sand).

Assumptions: OPC 43, crushed aggregate, medium workability (slump 30-60 mm), $s = 4$ MPa (IS 456 Table 8 for M25), cement SG 3.15, no admixture.

Step 1: Target mean strength

$$f_m = f_{ck} + 1.64 s = 25 + 1.64 \times 4 = 31.6\ \text{MPa}$$

Step 2: w/c ratio. From the strength (49 MPa at w/c 0.5 for crushed OPC at 28 days) and Fig 1, 31.6 MPa corresponds to w/c $\approx$ 0.7. Durability (IS 456 Table 5, moderate exposure, RCC): max w/c = 0.50, min cement 300 kg/m$^3$. So adopt w/c = 0.50.

Step 3: Free water (DoE Table 3, crushed, 30-60 mm): 20 mm = 230, 40 mm = 250; interpolating for 25 mm = 235 kg/m$^3$.

Step 4: Cement = $235/0.50 = 470$ kg/m$^3$ (above 300, below 550 kg/m$^3$).

Step 5: Wet density (Fig 3, water 235, SG about 2.65) $\approx$ 2350 kg/m$^3$; total aggregate = $2350 - 235 - 470 = 1645$ kg/m$^3$.

Step 6: Fine aggregate proportion (Fig 4, size 25 mm, 30-60 mm slump, w/c 0.5) $\approx$ 40%:

$$FA = 0.40 \times 1645 = 658\ \text{kg},\quad CA = 1645 - 658 = 987\ \text{kg}$$

Volume check: $0.235 + 470/3150 + 658/2600 + 987/2700 = 1.003$ m$^3$ - satisfactory.

Mix per m$^3$: cement 470 kg : water 235 kg : FA 658 kg : CA 987 kg, ratio 1 : 1.40 : 2.10, w/c = 0.50.

Quantities for 2 m$^3$

Answer: for 2 m$^3$, cement 940 kg (about 19 bags of 50 kg), water 470 L, FA 1316 kg, CA 1974 kg. Correct for moisture/absorption at site and verify by trial mix.

Method: IS 10262:2009 with IS 456:2000.

Data: M30; 20 mm angular aggregate; compaction factor 0.85 (medium workability, about 50 mm slump); fair quality control; severe exposure.

Assumptions: OPC 43 grade ($G = 3.15$); sand Zone II, $G_{FA} = 2.65$; $G_{CA} = 2.74$; no admixture; aggregates in SSD condition.

Step 1: Target strength. Fair control: $s = 5.0$ N/mm$^2$ for M30 (IS 10262 Table 1).

$$f'_{ck} = 30 + 1.65 \times 5 = 38.25\ \text{N/mm}^2$$

(this is higher than $f_{ck} + X = 30 + 6.5 = 36.5$).

Step 2: w/c ratio. Fig 1 (OPC 43) gives about 0.45 for 38.25 N/mm$^2$. For severe exposure (IS 456 Table 5, RCC): max w/c = 0.45, min cement 320 kg/m$^3$. Adopt w/c = 0.45.

Step 3: Water (IS 10262 Table 2): 20 mm = 186 kg/m$^3$ at 50 mm slump (angular, no correction). Water = 186 kg/m$^3$.

Step 4: Cement = $186/0.45 = 413$ kg/m$^3$; $> 320$ (min) and $< 450$ (max).

Step 5: Coarse aggregate volume (Table 3: 20 mm, Zone II, w/c 0.50 gives 0.62). For w/c 0.45 (0.05 lower): $+0.01$, so volume of CA = 0.63, FA = 0.37. Entrapped air for 20 mm = 2%.

Step 6: Mix calculation per m$^3$ (absolute volume)

$$\begin{aligned} CA &= 0.6628 \times 0.63 \times 2.74 \times 1000 = 1144\ \text{kg} \\ FA &= 0.6628 \times 0.37 \times 2.65 \times 1000 = 650\ \text{kg} \end{aligned}$$

Answer (per m$^3$): cement 413 kg, water 186 kg, FA 650 kg, CA 1144 kg; w/c = 0.45; ratio 1 : 1.57 : 2.77 (about 2393 kg/m$^3$). Corrections for moisture and absorption, and trial mixes, follow.

Abrams' law (D. A. Abrams, 1918) states that for a given set of materials and age, the strength of fully compacted concrete is inversely related to the water-cement ratio: the lower the w/c ratio, the higher the strength.

$$f_c = \frac{A}{B^{\,w/c}}$$

where $f_c$ is the compressive strength, $w/c$ is the water-cement ratio by mass, and $A$, $B$ are empirical constants depending on the cement, age, curing and testing (Abrams: $A = 96.5$ MPa, $B = 7$ for 28 days).

 f_c
  |\
  | `.
  |   `-.
  |      `--.____
  +----------------> w/c

Explanation: only about 0.23-0.38 of the cement mass of water is needed for hydration; the extra water evaporates leaving capillary pores, which weaken the paste, so strength falls rapidly as w/c increases. The law is valid only for workable mixes that can be fully compacted.

Abrams' rule

Abrams (1918) stated that for a given concrete made with given materials, the strength depends only on the water-cement ratio; a lower w/c ratio gives higher strength:

$$f_c = \frac{A}{B^{\,w/c}}$$

($A$, $B$ are constants of the materials and age; e.g. $A = 96.5$ MPa, $B = 7$). It holds for plastic, fully compacted mixes.

Additional factors not covered by the rule

1. Degree of compaction (air voids) Abrams' law assumes complete compaction. Voids from poor compaction reduce strength severely: 1% air voids reduces strength by about 5-6%, and 5% voids by up to 30%. The relative strength depends on the density ratio (compacting factor); Low w/c, stiff mixes need powerful vibration, and with too high a w/c the mix segregates.

2. Age and curing (temperature and moisture) Abrams' law applies at a stated age. Strength increases with age as hydration proceeds (about 60% at 7 days, 100% at 28 days, 120-130% at 1 year) only if the concrete is kept moist (curing) and at a suitable temperature; lack of curing leaves capillary pores and reduces strength by 30-50%. Higher temperature gives faster early strength but lower later strength.

Other factors: aggregate properties (size, shape, texture, strength, cleanliness), cement type and content, admixtures, aggregate-cement ratio, and the transition zone.

Segregation is the separation of the constituents of a fresh concrete mix, so that the mix is no longer homogeneous: the coarse aggregate separates from the mortar, or the paste separates from the aggregate. It results in honeycombing, weak and non-uniform concrete.

Bleeding is a form of segregation in which part of the mixing water rises to the surface of freshly placed concrete as the heavier solids settle. It forms a water layer (and laitance) on the surface, and water pockets under aggregate and bars, weakening the concrete.

Controlling segregation during placing

1. Limit the free fall of concrete to 1.5 m; use chutes, drop pipes (elephant trunk) or a tremie for deeper placing.
2. Place concrete as near to its final position as possible and do not use a vibrator to move it over long distances.
3. Place in layers of 300-450 mm and compact each layer promptly; avoid over-vibration.
4. Use a well-graded aggregate, a suitable cohesive mix with enough fines, and not too high a slump (use superplasticiser instead of water); consider air entrainment.
5. Keep chutes at proper slope (and use baffles at the end) and avoid discharging against formwork or reinforcement.
6. Use tight formwork that does not leak the paste; make a barrier or hopper at the discharge end to receive concrete.
7. Re-mix if segregation has occurred before placing.

Segregation is the separation of the constituents of fresh concrete so that the mix is no longer uniform: coarse aggregate separates from the mortar (or, in a wet mix, cement paste separates from aggregate). It gives honeycombing, weak and porous patches and a loss of strength and durability.

Causes of segregation

• Too wet or too dry (harsh) mix, high slump with poor cohesion.
• Poorly graded aggregate: excess of coarse or very large size, lack of fines.
• Dropping concrete from more than about 1.0 to 1.5 m, or letting it slide down a chute.
• Over-vibration, or vibrating a very wet mix; striking against reinforcement or form.
• Long transport on rough ground, or discharging a heap that rolls to the edges.
• Large difference between density of aggregate and mortar (e.g. light-weight aggregate).

Control: well-graded aggregate, adequate cement paste, correct W/C, short free fall, trunk or tremie pipes, proper vibration.

Bleeding (water gain) is the upward movement of mixing water to the top surface of freshly placed concrete as the heavier solids settle. The water collects as a layer on the surface or under coarse aggregate and reinforcement bars.

Causes of bleeding

• High water content / high W/C ratio and high slump.
• Lack of fines in the aggregate, or coarse and poorly graded sand.
• Low fineness of cement or low cement content (less surface area to hold water).
• Prolonged vibration, or deep placing in one lift.
• Low temperature (slower setting gives more time for settlement).

Effects: a weak, porous, dusty surface (laitance) with poor wear resistance; weak bond with the next lift; water pockets under bars and aggregate, which lower bond and make the concrete more permeable. Control: lower W/C, more fines, finer cement, air entrainment, correct vibration, and removal of laitance before the next lift.

Shrinkage

Shrinkage is the time-dependent reduction in volume of concrete, without any applied load, caused by loss of moisture and by chemical reaction (hydration and carbonation). Typical long-term values are 0.0003 to 0.0008 (300 to 800 microstrain).

Creep

Creep is the gradual increase in strain of concrete with time under a constant sustained stress, over and above the instantaneous elastic strain. It is caused mainly by seepage of gel water out of the C-S-H layers and by slip of gel particles. Creep continues for years, with about 50% occurring in the first 3 months and 75 to 80% in the first year.

Segregation

Separation of the ingredients of fresh concrete (coarse aggregate from mortar, or paste from aggregate) so that the mass is non-uniform. Causes: wet or harsh mix, poor grading, free fall over 1.5 m, over-vibration and rough handling. It produces honeycombing and weak spots.

Bleeding

Rise of mixing water to the surface of freshly placed concrete as solids settle. It happens with high W/C, lack of fines and coarse cement. It leaves a weak, porous layer (laitance), reduces bond with steel and aggregate, and lowers durability. Control by lowering W/C, adding fines and using air-entraining agents.

Effects of hot weather (above about 30 to 35 °C, or low humidity and wind) on concrete

• Faster setting and loss of slump, so more water tends to be added (higher W/C).
• Rapid evaporation causing plastic shrinkage cracks.
• Higher early strength but lower 28-day and long-term strength.
• Difficulty in placing, finishing and forming cold joints.
• Higher thermal cracking and drying shrinkage, more permeability, difficulty in air entrainment.

Precautions (IS 456, Cl. 14.2; ACI 305)

1. Keep the concrete temperature at placing below 40 °C (IS 456, Cl. 14.2 limit). Shade aggregate stockpiles, paint tanks and pipes white, sprinkle water on aggregates, and use chilled water or crushed ice as part of the mixing water.
2. Use low-heat or sulphate-resistant cement, or fly ash blends; use retarding or water-reducing admixtures (not extra water).
3. Reduce transit time; mix, transport and place quickly; schedule casting in the early morning, evening or night.
4. Dampen the subgrade and formwork before casting; keep forms shaded.
5. Use windbreaks, sunshades, fog sprays or evaporation retardants to stop plastic shrinkage.
6. Start curing immediately: wet hessian, ponding, continuous spraying, or membrane curing compound; cure for at least 10 days (IS 456, Cl. 13.5.1).
7. Place smaller lifts and provide enough joints.

Hot-weather concreting (ambient above about 30 to 35 °C, low humidity or wind) is done so that the concrete temperature at placing stays below 40 °C (IS 456, Cl. 14.2) and moisture loss is stopped.

Appropriate methods before and during concreting

• Select low-heat or sulphate-resisting cement, or Portland pozzolana cement; use a retarding water-reducing admixture so the W/C does not need to be raised.
• Cool the ingredients: shade and sprinkle aggregate stockpiles, keep water tanks and pipes shaded or white-painted, use chilled water or crushed ice in the mix.
• Keep the haul short, use covered transit mixers, mix just before placing and avoid delay.
• Cast in the early morning, evening or night.
• Wet the subgrade, forms and reinforcement before casting (no standing water); use shades and windbreaks.
• Place in thin layers; vibrate properly; plan joints to avoid cold joints.

Specific measures during finishing and curing

1. Prevent plastic shrinkage: use fog spray, evaporation retardant film or cover with polythene between finishing operations.
2. Begin curing as soon as the surface can take it, without marring it.
3. Cure continuously by ponding, wet hessian/sand, or sprinkling; do not let the surface dry even for a short time (wet-dry cycles cause cracking).
4. Alternatively, apply a curing compound or cover with white polythene sheet to reflect heat.
5. Cure for at least 10 days in hot and dry weather (IS 456, Cl. 13.5.1), 14 days with blended cement.
6. Delay stripping of forms and protect the concrete from heat and wind; keep forms wet.
7. Control joint spacing and keep the temperature difference between concrete and the air small, to avoid thermal cracking.

Effects of cold weather (below about 5 °C)

• Slower hydration, so setting and strength gain are delayed and formwork must stay longer.
• If fresh concrete freezes before reaching about 3.5 MPa, the water turns to ice and expands about 9%, giving permanent loss of strength (up to 50% or more) and poor durability.
• Freeze-thaw damage of young concrete, scaling of the surface.
• Thermal stress cracking from the temperature difference between the core and the cold surface.

Precautions (IS 456 Cl. 14.1; ACI 306)

1. Heat the mixing water (up to about 60 °C) and, if needed, the aggregates; do not let cement touch water hotter than about 60 °C (flash set). Concrete at placing should be not less than 5 °C (IS 456: 5 °C min, and 10 to 20 °C is recommended).
2. Use rapid-hardening cement, higher cement content, lower W/C and accelerating admixtures (non-chloride for RCC), and air-entraining agents.
3. Remove snow, ice and frost from forms, reinforcement and subgrade; never place concrete on frozen ground.
4. Cover with insulating blankets, polythene and straw, or use heated enclosures.
5. Keep the concrete above 5 °C for at least 48 hours (about 3 days) and protect it for 7 days; delay form stripping and test cubes cured under site conditions.
6. Avoid sudden cooling when protection is removed.

Curing is the process of keeping freshly placed concrete moist and at a suitable temperature for a period, so that cement hydration continues and the concrete gains strength and durability. Without curing the surface dries, hydration stops, and strength can be 30 to 50% lower with more shrinkage cracks and permeability.

Methods: ponding or immersion; covering with wet hessian, sand or straw and sprinkling; spraying water; membrane curing compounds and polythene sheets; steam curing for precast units.

Period (IS 456, Cl. 13.5.1): keep exposed surfaces continuously damp for at least 7 days for ordinary Portland cement and at least 10 days where mineral admixtures or blended cements are used. In dry and hot weather the period must not be less than 10 days; for blended cements it is recommended to extend it to 14 days. Formwork is removed only after the minimum striking period (IS 456, Cl. 11.3).