Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for same is still unknown. The rate of gain of concrete compressive strength in higher during the first 28 days of casting and then it slows down. In 28 days, concrete gains almost 99% of its strength. So, we test the concrete compressive strength on 28 days.

For more details: https://theconstructor.org/concrete/why-we-test-concrete-strength-after-28-days/6060/

It is found that cylinder specimen gives lower strength compared to cube specimen. Also, cylinder test gives more uniform strength than cube test. It may be because,

• The cylinder strength is less affected by the friction at loading surface.
• The slenderness ratio for cylinder will be more than that of cube (Approximately double, For Cylinder = 300/150 = 2 and for cube 150/150). So, the cylinder has less compressive strength.

Honeycomb is the space or void inside the concrete.

Causes

• Improper compaction as well as mixing of ingredients.
• Due to too stiff (Hard) concrete.
• When large size aggregate is used.
• Due to Segregation of ingredients of concrete
• Caused if there is not adequate gap between steel rods to enter concrete mix into it.

Effects

• Reduce Strength of Concrete
• Generally, occur in beam, slab, column etc. which may cause collapse of whole structure.
• Dangerous for earthquake structure
• Easy corrosion of Rebar.

Remedies

• Mix concrete materials properly.
• Use standard size aggregate
• Use vibrator for compaction of concrete. (Too much compaction of concrete with vibrator leads to segregation in concrete)

For more details:

Top 5 Cause of Honeycomb in Concrete – Remedies of Honeycomb

Effect of Mixing Time in drum research paper by TekRaj Gyanwali

Effect-of-change-in-mixing-time-of-mixer-on-wet-density-dry-density-workability-and-compressive-strength-of-m-20-grade-concrete

Main Effects – Variation in Slump value, Compaction Factor, Compressive Strength

Slump Value

• For initial three minutes of mixing slump value is higher due to inadequate mixing time.
• From 4 to 7 minutes of mixing the slump value is better than that of other durations of mixing.
• From 8 to 10 minutes of mixing the slump value again increases due to segregation of concrete ingredients caused by prolonged mixing

Compressive strength

• For initial 2-3 minutes of mixing, results show low values of compressive strength is due to adequate mixing
• For mixing time within 4 to 7 minutes the compressive strength was maximum
• Further mixing may result in lower strength probably due to segregation of concrete ingredients.

Compaction Factor

• For initial two minutes of mixing results shows higher values compaction factor. But due inadequate mixing, the concrete cannot be properly compacted.
• For final 2 minutes also, the results are higher due to inadequate compaction but due to segregation of material

Plasticizer (Water Reducer)

Anionic surfactants like lignosulphonates and their derivatives, salts of hydrocarbons, etc.,

Nonionic surfactants like polyglycol esters, acids of hydroxylated carboxylic acids and their derivatives,

Other compounds like carbohydrates.

Superplasticizer

Sulphonated Melamine- Formaldehyde condensates (SMF)

Sulphonated Naphthalene-Formaldehyde condensates (SNF)

Modified lignosulphonates

Carboxylated acrylic easters copolymers

Retarder

Calcium sulphate (gypsum)

starch / sugar and carbohydrate derivatives

cellulose

lignosulphonic-acids

Soluble zinc salts, borates, etc.

Accelerator

Calcium chloride

Calcium nitrite/ calcium nitrate/ calcium formate

Sodium chloride/ sodium fromate

Barium chloride

Triethanolamine

Air Entraining Admixtures

Natural wood resins, animal or vegetable fats and oils, fatty acids, aluminium powder, etc. are used in manufacture of air-entraining

For construction of Tunnel, the best concrete is shotcrete.

It is concrete with small sized coarse aggregate that is conveyed through hose and pneumatically projected at a high velocity. The momentum of the concrete itself is sufficient to support and compact itself.

Shotcrete can be produce by dry or wet process.

In heavily congested reinforcement area, self-compaction concrete must be used.

It is a special type of concrete that has ability to flow under its own weight through heavily congested reinforcements and thin sections retaining the homogeneity of mix without segregation.

No vibration is needed.

• Good Workmanship and maintain quality on each steps of work.
• Use of Quality Material (Cement, Sand, Aggregate, Water and Admixtures)

In Entrained air, the air is introduced intentionally using air entraining admixtures. But in entrapped air, the air is introduced unintentionally due to insufficient compaction of concrete.