Concrete Technology and Mix Design

What to remember

• Concrete = cement + water + fine aggregate + coarse aggregate (+ admixtures). Strength is mainly governed by the water-cement ratio (Abrams' law): lower ratio, higher strength.
• Grade M25 means characteristic cube strength of 25 N/mm² at 28 days on 150 mm cubes. Target mean strength = fck + 1.65 s.
• Workability is measured by slump, compaction factor, Vee-Bee and flow tests. Curing keeps concrete moist so hydration continues.

Cement

Main compounds (Bogue):

Gypsum is added (about 2–3%) to control the setting of C₃A. Hydration gives calcium silicate hydrate (C-S-H gel) which gives strength, and calcium hydroxide.

Types of cement: Ordinary Portland cement (OPC) grades 33, 43 and 53 (grade = 28-day mortar strength in N/mm²). Portland pozzolana cement (PPC, with fly ash or calcined clay: lower heat, better durability, slower early strength), Portland slag cement (PSC, good for marine and mass work), rapid hardening cement (more C₃S, finer), low heat cement (more C₂S, for dams), sulphate-resisting cement (low C₃A), white cement (low iron), high alumina cement (rapid strength, not for structural use in hot humid conditions, not mixed with Portland cement).

Standard tests on cement:

• Fineness: sieve method or Blaine air permeability; finer cement hydrates faster.
• Normal (standard) consistency: Vicat apparatus, plunger 10 mm × 50 mm, 5–7 mm from the bottom of the mould; typically near 30% water.
• Setting time: initial not less than 30 minutes and final not more than 600 minutes (10 hours) for OPC; Vicat needle.
• Soundness: Le Chatelier (expansion not more than 10 mm) and autoclave test; unsound cement has free lime or magnesia.
• Compressive strength: mortar cubes, 70.6 mm (about 50 cm² face) by standard sand. For 43 grade, minimum 23, 33 and 43 N/mm² at 3, 7 and 28 days.
• Specific gravity of cement is about 3.15. Bulk density about 1440 kg/m³. A standard bag of cement is 50 kg.

Aggregates

• Fine aggregate: passing 4.75 mm sieve (sand). Coarse aggregate: retained on 4.75 mm. Maximum size is commonly 20 mm for RCC and 40 mm or more for mass concrete.
• Grading: fineness modulus = sum of cumulative percentages retained on the standard sieves/100. Sand 2.2–3.2 is common; coarse aggregate gives 5.5–8. Sand is classed in grading zones I to IV (zone I coarse, zone IV fine). Well-graded aggregate gives dense concrete with less cement.
• Shape: cubical, angular aggregate is best; flaky and elongated pieces reduce workability and strength. Tests: flakiness index, elongation index, crushing value, impact value and Los Angeles abrasion value (lower value means tougher aggregate); specific gravity, water absorption, soundness.
• Bulking of sand: surface moisture film pushes grains apart, increasing the volume; it is greatest at about 4–6% moisture and drops to zero for fully saturated or dry sand. Allow for it when batching by volume.
• Water for mixing must be clean and free of harmful salts; potable water is generally fit. pH not less than 6.

Fresh concrete

Workability is the ease of mixing, placing, compacting and finishing without segregation. It rises with water, with rounded aggregate and with admixtures.

Defects: segregation (separation of coarse aggregate), bleeding (water rising to the surface, giving laitance), honeycombing (poor compaction). Vibration compacts concrete, but over-vibration causes segregation. Bleeding is reduced by finer cement and air entrainment.

Hardened concrete and durability

• Strength tests: cube 150 mm (compressive strength = load/area); standard cylinder 150 × 300 mm gives about 0.8 of cube strength; flexural (modulus of rupture) fcr = 0.7√fck; split tensile test. Example: failure load of 675 kN on 150 × 150 cube gives 675,000/22,500 = 30 N/mm².
• Modulus of elasticity (code formula): Ec = 5000√fck. For fck = 25: Ec = 25,000 N/mm². Flexural strength 0.7√25 = 3.5 N/mm².
• Density: plain concrete about 2400 kg/m³, reinforced about 2500 kg/m³.
• Creep: slow increase of strain under sustained load. Shrinkage: volume reduction on drying (and plastic shrinkage in the first hours). Both rise with water content.
• Curing: keep moist and warm for hydration. Minimum 7 days for OPC and 10 days where mineral admixtures are used or weather is hot and dry. Methods: ponding, wet gunny bags, curing compound, steam curing.
• Non-destructive tests: rebound hammer (surface hardness), ultrasonic pulse velocity (quality, voids and cracks), and also cover meter and half-cell for corrosion.
• Durability: controlled by low permeability, adequate cover, minimum cement content and maximum w/c ratio fixed by exposure class (mild to extreme). Sulphate attack, chloride-induced corrosion and carbonation are the main threats.

Mixing, placing and special concretes

• Batching: weigh batching is more accurate than volume batching. Cement is best measured by whole bags.
• Mixing: machine mixing time is usually not less than about two minutes after all materials are in the drum. Concrete should be placed before initial setting, so retarders help when haul is long. Free fall height should be limited to avoid segregation.
• Formwork: must be rigid and tight and is removed only after the concrete gains enough strength. Side forms of beams and columns come out early; slab soffits and beam soffits stay longest.
• Special concretes: ready-mixed concrete (made in a plant and carried in transit mixers), pumped concrete, self-compacting concrete (high flow without vibration), fibre-reinforced concrete (steel or polymer fibres reduce cracking), light-weight concrete, high-performance concrete (low w/c with superplasticiser and silica fume), and prestressed concrete (needs high grade, usually M30 or above).
• Cold and hot weather: in hot weather use cool water, retarders and prompt curing; in cold weather protect from freezing since frozen fresh concrete loses strength.

Admixtures

• Plasticisers and superplasticisers: reduce water for the same workability, raise strength.
• Retarders: delay setting (hot weather, long haul). Accelerators: speed setting (cold weather).
• Air-entraining agents: small bubbles, frost resistance and workability, but some loss in strength.
• Mineral admixtures: fly ash, silica fume and ground granulated slag improve durability and reduce heat.

Grades and mix design

Grades: M10 to M20 for plain and light work, M20 and above for reinforced concrete. Nominal mixes (volume): M5 1:5:10, M7.5 1:4:8, M10 1:3:6, M15 1:2:4, M20 1:1.5:3. Nominal mixes are used for small, ordinary work; grades above M20 should be design mixes.

Statistics: Characteristic strength is the value below which not more than 5% of test results are expected to fall. Target mean strength fₜ = fck + 1.65 s, where s is the standard deviation. Example: M25 with s = 4 N/mm²: fₜ = 25 + 6.6 = 31.6 N/mm².

Design mix (IS 10262 steps):

• 1. Find target strength.
• 2. Choose the w/c ratio from strength curves (and check the limit for exposure).
• 3. Choose water content from the aggregate size and slump.
• 4. Cement = water/(w/c), subject to the code minimum cement content.
• 5. Find coarse aggregate proportion from the sand zone and w/c.
• 6. Absolute volume method: aggregate volume = 1 − (cement/(Gc × 1000) + water/1000 + admixture + air).
• 7. Run trial mixes and adjust.

Example: cement 300 kg (G = 3.0), water 180 kg. Cement volume = 300/3000 = 0.10 m³; water 0.18 m³; aggregate (all) = 1 − 0.28 = 0.72 m³ (ignoring air). w/c = 180/300 = 0.6.

Nominal mix quantities (dry volume method): dry volume = 1.54 × wet volume. For M20 (1:1.5:3, total 5.5 parts) per m³: cement = 1.54/5.5 = 0.28 m³ = 0.28 × 1440 = about 403 kg, or about 8 bags.

Water-cement ratio: about 0.38 is the ratio for complete hydration (0.23 chemically bound + 0.15 gel water). Extra water gives capillary pores and weaker, more permeable concrete. Abrams' law: strength = A/Bʷ/ᶜ.

Exam traps

• M25 means 25 N/mm² characteristic cube strength, not mean strength and not cylinder strength.
• Slump measures workability, not strength.
• Initial setting time is at least 30 minutes; final setting at most 600 minutes.
• C₃A reacts fastest and needs gypsum; C₂S is slow and gives later strength.
• Higher w/c ratio means lower strength and lower durability.
• Bulking of sand is greatest at about 4–6% moisture, not at full saturation.
• Flaky aggregate lowers workability; low crushing value means a stronger aggregate.
• Low heat cement has more C₂S and less C₃S; rapid hardening cement has more C₃S.

One-liners

• 1. Abrams' law: strength falls as w/c rises.
• 2. Target mean strength = fck + 1.65 s.
• 3. Ec = 5000√fck (N/mm²).
• 4. Flexural strength fcr = 0.7√fck.
• 5. M20 nominal mix is 1:1.5:3.
• 6. Dry volume of concrete = 1.54 × wet volume.
• 7. Standard cube size is 150 mm.
• 8. Le Chatelier test measures soundness.
• 9. Gypsum controls setting of C₃A.
• 10. Fineness modulus = cumulative % retained/100.
• 11. Minimum curing period for OPC is 7 days.
• 12. Rebound hammer gives surface hardness.

Practice questions

1. The target mean strength for M25 concrete with standard deviation 4 N/mm2 (fck + 1.65 s) is

   1. 25 N/mm2
   2. 31.6 N/mm2
   3. 35 N/mm2
   4. 29 N/mm2
   Answer

   B. 31.6 N/mm2

   25 + 1.65 x 4 = 31.6 N/mm2.

2. The target mean strength for M20 concrete with s = 4 N/mm2 is

   1. 26.6 N/mm2
   2. 24 N/mm2
   3. 20 N/mm2
   4. 30 N/mm2
   Answer

   A. 26.6 N/mm2

   20 + 1.65 x 4 = 26.6 N/mm2.

3. A 150 mm cube fails at 675 kN. The compressive strength is

   1. 4.5 N/mm2
   2. 45 N/mm2
   3. 20 N/mm2
   4. 30 N/mm2
   Answer

   D. 30 N/mm2

   675,000/22,500 = 30 N/mm2.

4. The modulus of elasticity of concrete of grade M25 by the code formula 5000 root fck is

   1. 25,000 N/mm2
   2. 125,000 N/mm2
   3. 50,000 N/mm2
   4. 5,000 N/mm2
   Answer

   A. 25,000 N/mm2

   5000 x 5 = 25,000 N/mm2.

5. The flexural strength of M25 concrete by the formula 0.7 root fck is

   1. 1.75 N/mm2
   2. 5 N/mm2
   3. 7 N/mm2
   4. 3.5 N/mm2
   Answer

   D. 3.5 N/mm2

   0.7 x 5 = 3.5 N/mm2.

6. A mix has 300 kg cement and 180 kg water per cubic metre. The water-cement ratio is

   1. 1.67
   2. 0.6
   3. 0.5
   4. 0.4
   Answer

   B. 0.6

   180/300 = 0.6.

7. A mix has 300 kg cement of specific gravity 3.0 and 180 kg water per cubic metre (ignore air). The absolute volume of all aggregate is

   1. 0.72 m3
   2. 0.62 m3
   3. 0.82 m3
   4. 0.28 m3
   Answer

   A. 0.72 m3

   Cement 0.10 + water 0.18 = 0.28 m3; aggregate = 1 - 0.28 = 0.72 m3.

8. For an M20 nominal mix 1:1.5:3 with dry volume factor 1.54 and cement bulk density 1440 kg/m3, the cement per m3 of concrete is nearly

   1. 520 kg
   2. 250 kg
   3. 300 kg
   4. 403 kg
   Answer

   D. 403 kg

   Cement volume = 1.54/5.5 = 0.28 m3; 0.28 x 1440 = 403 kg.

9. The nominal mix of M15 concrete by volume is

   1. 1:4:8
   2. 1:1.5:3
   3. 1:2:4
   4. 1:3:6
   Answer

   C. 1:2:4

   M15 is 1:2:4; M10 is 1:3:6; M20 is 1:1.5:3.

10. The initial setting time of ordinary Portland cement should be not less than

    1. 10 minutes
    2. 30 minutes
    3. 60 minutes
    4. 600 minutes
    Answer

    B. 30 minutes

    The final setting time is not more than 600 minutes.

11. The final setting time of ordinary Portland cement should be not more than

    1. 600 minutes
    2. 300 minutes
    3. 60 minutes
    4. 30 minutes
    Answer

    A. 600 minutes

    The limit is 10 hours.

12. Soundness of cement is tested using

    1. Vicat needle
    2. Blaine apparatus
    3. Slump cone
    4. Le Chatelier apparatus
    Answer

    D. Le Chatelier apparatus

    Expansion should be not more than 10 mm.

13. Gypsum is added to cement to control the setting of

    1. C4AF
    2. C2S
    3. C3A
    4. C3S
    Answer

    C. C3A

    C3A reacts rapidly and causes flash set; gypsum retards it.

14. The compound mainly responsible for early strength of cement is

    1. C3A
    2. C3S
    3. C4AF
    4. C2S
    Answer

    B. C3S

    Tricalcium silicate hydrates fast and gives early strength.

15. Low heat cement is rich in

    1. C3A
    2. free lime
    3. C2S
    4. C3S
    Answer

    C. C2S

    C2S hydrates slowly and gives less heat.

16. Fine aggregate is aggregate passing the sieve of size

    1. 4.75 mm
    2. 20 mm
    3. 2.36 mm
    4. 10 mm
    Answer

    A. 4.75 mm

    Coarse aggregate is retained on the 4.75 mm sieve.

17. The cumulative percentages retained on the standard sieves are 2, 10, 25, 45, 68 and 90. The fineness modulus is

    1. 3.4
    2. 2.0
    3. 1.4
    4. 2.4
    Answer

    D. 2.4

    Sum = 240; FM = 240/100 = 2.4.

18. Bulking of sand is greatest at a moisture content of about

    1. 15 percent
    2. 4 to 6 percent
    3. fully saturated
    4. 0 percent
    Answer

    B. 4 to 6 percent

    Bulking falls to zero for dry and fully saturated sand.

19. The slump test is used to measure the

    1. setting time
    2. density of hardened concrete
    3. workability of fresh concrete
    4. compressive strength
    Answer

    C. workability of fresh concrete

    The slump cone gives the consistency of concrete.

20. The compaction factor test is best suited for concrete of

    1. very high workability
    2. very dry mixes only
    3. low to medium workability
    4. flowing concrete only
    Answer

    C. low to medium workability

    It is used where slump is too low to measure accurately.

21. Which test is used for very dry concrete mixes?

    1. Slump test
    2. Vee-Bee test
    3. Le Chatelier test
    4. Flow table test
    Answer

    B. Vee-Bee test

    Vee-Bee time measures the remoulding effort of stiff mixes.

22. Rising of water to the surface of freshly placed concrete is called

    1. bleeding
    2. honeycombing
    3. segregation
    4. creep
    Answer

    A. bleeding

    Bleeding leaves a weak laitance layer.

23. Separation of coarse aggregate from the mortar in fresh concrete is called

    1. bleeding
    2. shrinkage
    3. hydration
    4. segregation
    Answer

    D. segregation

    Over-vibration, high fall or poor grading causes it.

24. The minimum curing period for ordinary Portland cement concrete is

    1. 7 days
    2. 3 days
    3. 14 days
    4. 28 days
    Answer

    A. 7 days

    It is 10 days where mineral admixtures are used or the weather is hot and dry.

25. The rebound hammer test gives an indication of

    1. internal voids only
    2. surface hardness of concrete
    3. steel corrosion
    4. cement content
    Answer

    B. surface hardness of concrete

    The rebound number correlates with surface strength.

26. The ultrasonic pulse velocity test is used to assess

    1. workability
    2. bleeding
    3. cement fineness
    4. quality, voids and cracks in concrete
    Answer

    D. quality, voids and cracks in concrete

    A higher pulse velocity shows denser concrete.

27. Superplasticisers are used to

    1. reduce water for the same workability
    2. delay the setting only
    3. entrain air only
    4. increase shrinkage
    Answer

    A. reduce water for the same workability

    They disperse cement grains, so water can be cut and strength raised.

28. The strength of a standard cylinder (150 x 300 mm) compared with a 150 mm cube is

    1. about 2 times
    2. about 0.8 times
    3. equal
    4. about 1.25 times
    Answer

    B. about 0.8 times

    Cylinders show lower strength because of shape effect.

29. Reinforced concrete has a unit weight of about

    1. 15 kN/m3
    2. 40 kN/m3
    3. 10 kN/m3
    4. 25 kN/m3
    Answer

    D. 25 kN/m3

    RCC density is about 2500 kg/m3 (plain concrete 2400 kg/m3).

30. The characteristic strength of concrete is the strength below which not more than this proportion of test results is expected to fall:

    1. 10 percent
    2. 1 percent
    3. 5 percent
    4. 50 percent
    Answer

    C. 5 percent

    This is the 95 percent confidence value.

31. The water-cement ratio needed for complete hydration of cement is nearly

    1. 0.38
    2. 0.2
    3. 0.8
    4. 0.6
    Answer

    A. 0.38

    About 0.23 chemically bound water and 0.15 gel water.

32. Portland slag cement is especially suited for

    1. marine and mass concrete work
    2. rapid repair work
    3. fireproof lining only
    4. white finishes
    Answer

    A. marine and mass concrete work

    Low heat and good sulphate and chloride resistance help in such work.

33. High alumina cement should not be mixed with Portland cement because

    1. it gives a white colour
    2. it removes gypsum
    3. the strength becomes too high
    4. the mixture sets flash and weakens
    Answer

    D. the mixture sets flash and weakens

    Mixing causes quick, weak setting.

34. The standard cube size for the compressive strength test of concrete is

    1. 70.6 mm
    2. 150 mm
    3. 300 mm
    4. 100 mm
    Answer

    B. 150 mm

    The standard cube is 150 mm; the mortar cube for cement is 70.6 mm.

35. Abrams' law states that the strength of concrete

    1. decreases as the water-cement ratio increases
    2. is independent of w/c ratio
    3. increases with water content
    4. depends only on aggregate size
    Answer

    A. decreases as the water-cement ratio increases

    Strength = A/B^(w/c).

36. Cement mortar cubes for the cement strength test have a face area of about

    1. 225 cm2
    2. 25 cm2
    3. 50 cm2 (70.6 mm side)
    4. 100 cm2
    Answer

    C. 50 cm2 (70.6 mm side)

    70.6 x 70.6 = 4984 mm2, about 50 cm2.

37. Consider the statements. 1. Segregation is reduced by using well-graded aggregate. 2. Excess vibration can cause segregation. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

38. Consider the statements on aggregates. 1. Flaky aggregates reduce the workability of concrete. 2. A higher aggregate impact value means a tougher aggregate. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A higher impact value indicates weaker aggregate.

39. Consider the statements on cement. 1. Rapid hardening cement has more C3S. 2. Low heat cement has more C3S. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Low heat cement has more C2S and less C3S.

40. Consider the statements on w/c ratio. 1. A higher ratio gives higher strength. 2. A higher ratio gives higher permeability. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    More water leaves more capillary pores, lowering strength.

41. Consider the statements. 1. M25 concrete has a characteristic cube strength of 25 N/mm2. 2. The strength is tested at 7 days. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The grade is based on the 28-day strength.

42. Consider the statements on curing. 1. Curing assists continued hydration. 2. It is needed only for the first hour. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Curing must continue for several days.

43. Consider the statements. 1. Gypsum controls the setting of C3A in cement. 2. The bulk density of cement is about 1440 kg/m3. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are standard facts.

44. Consider the statements. 1. The initial setting time of OPC may not exceed 30 minutes. 2. Soundness of cement is tested with the slump cone. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Initial setting time must be at least 30 minutes; soundness uses the Le Chatelier apparatus.

45. Match the test with its purpose. P) Slump Q) Le Chatelier R) Vicat S) Rebound hammer

    1. P-cement soundness, Q-concrete workability, R-surface hardness, S-setting time
    2. P-concrete workability, Q-cement soundness, R-setting time and consistency, S-surface hardness
    3. P-setting time, Q-surface hardness, R-concrete workability, S-cement soundness
    4. P-surface hardness, Q-setting time, R-cement soundness, S-concrete workability
    Answer

    B. P-concrete workability, Q-cement soundness, R-setting time and consistency, S-surface hardness

    Standard uses.