Initial & Final setting Time of cement & concrete

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Initial & Final setting Time of cement & concrete

Minimum thickness of slab is 125 mm.

Water absorption should not be more than 15 %.

Dimension tolerance for cubes + – 2 mm.

Compressive strength of Bricks is 3.5 N /mm²

Maximum Free fall of concrete allowed is 1.50 m.

In soil filling as per IS code for every 100 sqm 3 sample for core cutting test should be taken.

Electrical conduits shall not run in column

Earth work excavation for basement above 3 m should be stepped form

Any back filling shall be compacted 95% of dry density at the optimum moisture content and in layers not more than 200mm for filling above structure and 300 mm for no structure

F soling is specified the soling stones shall be laid at 45° to 60° inclination (and not vertical) with interstices filled with sand or moorum.

A set of cube tests shall be carried out for each 30 cum of concrete / each levels of casting / each batch of cement.

Water cement ratio for different grades of concrete shall not exceed 0.45 for M20 and above and 0.50 For M10 / M15 concrete

For concrete grades M20 and above approved admixture shall be used as per mix design requirements.

Cement shall be stored in dry places on a raised platform about 200mm above floor level and 300mm away from walls. Bags to be stacked not more than 10 bags high in such a manner that it is adequately protected from moisture and contamination.

Samples from fresh concrete shall be taken and at least a set of 6 cubes of 150mm shall be prepared and cured. 3 Cubes each at 7 days and 28 days shall be tested for compressive strength. The test results should be submitted to engineer for approval. If results are unsatisfactory necessary action/rectification/remedial measures has to be exercised.

Water used for both mixing and curing shall be clean and free from injurious amounts of oils, acids, alkalis, salts, sugar and organic materials or other substances that may be deleterious to concrete or steel. The ph. shall be generally between 6 and 8.

Cement shall be tested for its setting.
1. The initial setting time shall not be less than 30 minutes.
2. The final setting time shall not be more than 10 hours or 600 minutes

Slump IS 456
Lightly reinforced 25 – 75 mm
heavily reinforced 75 – 100 mm
Trench fill (insitu & Tremie) 100 – 150 mm (For Tremie no need of vibrator)

Curing Days Required
Super Sulphate cement: 7 days
Ordinary Portland cement OPC: 10 days
Minerals and Admixture added cement: 14 days

Cube Samples as per IS-456-2000 Pg. No:29
1 – 5 M³ : 1 No.
6 – 15 M³ : 2 No’s
16 – 30 M³ : 3 No’s
31 – 50 M³ : 4 No’s
Above 50 M³ : 4 + 1 No of addition sample for each 50 M³.

SETTING PROCESS OF CEMENT

When water is mixed with cement to form a paste, reaction starts. In its pure form, the finely ground cement is extremely sensitive to water. Out of the three main compounds, viz. C3A, C3S and C2S, reacts quickly with water to produce a jelly-like compound which starts solidifying. The action of changing from a fluid state to a solid state is called ‘setting’ and should not be confused with ‘hardening’.

During the next stage of hydration, cement paste starts hardening owing to the reaction of C3S and C2S with water and the paste gains strength. The first few minutes, the setting action is more predominant and later on the hardening action becomes dominant. In practice, such solidifying action or loss of plasticity is required to be delayed, because some time is needed for mixing, transporting and placing of concrete into final position before the mix loses its plasticity due to the setting action.

It is usually specified that the plastic concrete should be placed and consolidated before initial set has occurred and it should not then be disturbed until concrete has hardened. This initial setting time should not be too small and therefore, the standard specifies minimum initial setting time.Once initial stiffening of concrete has taken place, it is desirable that it should harden or gain strength as rapidly as possible, so that there is a minimum of delay before shuttering can be removed and the risk of frost damage in cold climate is minimized. The standard, therefore, specifies the maximum value of final setting time.

It is not however, possible in practice to exactly locate the initial setting time and final setting time. The Indian Standards have selected two arbitrary points which relate setting of cement to the time measured from the moment the water is added.

‘Initial setting time’ is defined as the period elapsing between the time when water is added to the cement and the time at which the needle of 1 mm square section fails to pierce the test block to a depth of about 5 mm from the bottom of the mould. A period of 30 minutes is the minimum initial setting time, specified by ISI for ordinary and rapid hardening Portland cements and 60 minutes for low heat cement.

The ‘final setting time’ is defined as the period elapsing between the time when water is added to cement and the time at which the needle of 1 mm square section with 5 mm diameter attachment makes an impression on the test block . 600 minutes is the maximum time specified for the final set for all the above mentioned Portland cements. IS: 269-1976 specifies the strengths in compression on the standard mortar-cube.

Fig: Vicat Apparatus for Cement Setting Time Test

Compressive strength test has two functions to fulfill. Firstly, it is a final check on the quality of cement. Secondly, in case of doubt, it also helps us to classify the cement as ordinary Portland cement, rapid hardening Portland cement or low heat Portland cement, according to the strength it gives after 3 days and 7 days curing.

It is important to note the difference between setting and hardening of cement at this stage.These compounds contribute very little to the mechanical strength of concrete, but cement starts losing its plasticity because of loss of water due to reaction and formation of gel. This loss of plasticity without development of strength is called setting action.

Cement is said to harden when the cement paste further reacts with water bringing C2S and C3S into action. These compounds contribute to the mechanical strength. Hardening, therefore, is associated with the development of strength.