| penetration resistance appratus |
Why we conduct the Concrete Setting Time Test?
- It is essential that cement set neither too rapidly nor too slowly. In the first case there might be insufficient time to transport and place the concrete before it becomes too rigid. In the second case too long a setting period tends to slow up the work unduly, also it might postpone the actual use of the structure because of inadequate strength at the desired age.
- Setting time is the time required for stiffening of cement paste to a defined consistency.
- Initial setting time test is important for transportation, placing and compaction of cement concrete.
- Initial setting time duration is required to delay the process of hydration or hardening.
- It is the time taken for the cement paste or cement concrete to harden sufficiently and attain the shape of the mould in which it is cast.
- Determination of final setting time period facilitates safe removal of scaffolding or form.
- During this period of time primary chemical reaction of cement with water is almost completed.
Objective:
To determine the initial and final setting time of concrete by penetration resistance method.
Apparatus:
- IS Sieve 4.75mm,
- penetration resistance apparatus,
- Cube mould of size 150x150x150mm,
- Tamping rod of 16mm diameter and 600mm long rounded at one end.
- Non absorptive metal container having minimum lateral dimension of 150 mm and minimum depth of 150 mm.
- Penetration resistance apparatus having a pressure gauge of capacity 700 to 900 N. Removable needles of 645, 323, 161, 65, 32 & 16 mm2.
- Pipette
Preparation of test Sample
Prepare a batch of concrete of sufficient volume to provide enough mortar to fill the test container. Remove all mortar from the sample of concrete by sieving it through 4.75mm IS Sieve on a non-absorptive surface. Thoroughly mix the mortar by hand on the non-absorptive surface and place it in a cube mould of size 150x150x150mm in layers of 50mm each and compact each layer by rodding with tamping rod so as to penetrate the mortar with round end. Rod the mortar once for each 6.5cm2of the top surface of the specimen and distribute the strokes uniformly over the cross-section of the specimen.
After the completion of the rodding, tamp the sides of the mould lightly with tamping rod to close the voids left by tamping rod and to further level the surface of the specimen. Upon the completion of specimen preparation, the mortar surface shall be atleast 13mm below the top edge of the container to provide space for collection and removal of bleed water. Atleast three separate batches shall be made for each test condition and one rate of hardening test shall be conducted for each batch.
Procedure:
Procedure:
Remove bleeding water from the surface of the concrete just prior to making penetration test measurement. To facilitate collection of bleeding water tilt the specimen carefully at an angle of 12 degrees from the horizontal by placing a block under one side two minute prior to removal of bleeding water. Insert a needle of appropriate size depending upon the state of hardening of the mortar, in the penetration resistance apparatus and bring the bearing surface of the needle into contact with the mortar surface. Gradually and uniformly apply a vertical force downward on the apparatus until the needle penetrates the mortar to a depth of 25mm as indicated by the scribe mark.
The time required to penetrate to a depth of 25mm shall be approximately 10 seconds. Record the force required and the time of application, measured as the elapsed time after initial contact of cement and water. The clear distance between the two impression needles shall be atleast two diameters of the needles being used but not less than 13mm. The clear distance between any needle impressions and the side of the mould shall not be less than 25mm. Make penetration tests at hourly test intervals for normal mix and normal temperature, the initial test being made at an elapsed time of 3-4 hours. Not less than six penetration resistance determinations shall be made in each rate of hardening test and the time interval between penetration resistance determinations shall be so as to give a satisfactory rate of hardening curve, as indicated by equally spaced points. Continue the test until penetration resistance of atleast 275Kgf/cm2is reached.Calculate the penetration resistance as force required, N/mm2 or Kgf/cm2as the force required to cause a 25mm depth of penetration of the needle divided by area of the bearing face of the needle.
For each variable and condition of concrete under test, the results from each of the three hardening tests shall be plotted separately, showing penetration in Kgf/cm2as the ordinate and elapsed time in hours and minutes in abscissa, where 35Kgf/cm2and one hour are represented by not less than 13mm.
Plotting of Graph
- Plot a graph of penetration resistance as ordinate and elapsed time a abscissa
- Connect the various points by a smooth curve.
Finding out Initial and Final Setting Time from the Graph
- From the penetration resistance equal to 3.43 MPa, draw a horizontal line. The point of intersection of this with the smooth curve is read on the x-axis which gives initial setting time.
- Time of initial and final setting shall be calculated by averaging the elapsed times, determined from the curves plotted at which penetration resistance of 35Kgf/cm2 and 275Kgf/cm2 respectively are reached.
- Times of setting shall be reported in hours and minutes to the nearest minute.
| Initial and Final Setting Time from the Graph |
Founder & Admin of civilengineeringsolution.com, I am a civil engineer working as a Engineer (QA/QC).