The penetration resistance test used for the determination of initial and final setting time of concrete mixture in laboratory and project site, and the effects of variables, such as water content; type and amount of cementitious material; or admixtures, upon the time of setting of concrete. The test is conducted on a sample of sieved mortar.
The time at which cement begins to harden and lose its plasticity is called the initial setting time of cement. However, the final setting time begins when concrete loses its plasticity completely and hardens. According to ASTM C403M-16, the initial and final time of concrete can be determined when the penetration resistance equals 3.5 MPa, 27.6 MPa, respectively.
There are certain factors that may influence setting times of concrete like the fineness of cement, the presence of salts in sand, atmospheric conditions. For instance, cement needs a temperature of 27°c to complete its hydration, but during winters temperature is low and consequently the hydration takes a longer time.
Apparatus
1. Container
The lateral dimension shall be at least 150 mm and the height at least 150 mm.
2. Penetration Needles
Penetration needles, which commonly attached to the loading apparatus, have the following bearing areas 645, 323, 161, 65, 32, and 16 mm2.
3. Loading Apparatus
It is used to measure the force required to cause penetration of the needles. The device shall be capable of measuring the penetration force with an accuracy of 10 N and shall have a capacity of at least 600 N.
4. Tamping Rod
It is round and straight steel rod with 16 mm in diameter and approximately 600 mm in length.
5. Pipet
It is used for drawing off bleed water from the surface of the test specimen.
6. Thermometer
The thermometer shall be capable of measuring the temperature of the fresh mortar to 60.5°C.
7. Stop Watch
Sample Preparations
- For tests under field conditions, obtain a representative sample of the fresh concrete from which three specimens shall be prepared for the test.
- For tests under laboratory conditions, make the concrete and specify its slump and air content.
- The concrete passed through a 4.75-mm sieve shall be used for preparation of specimens.
- For testing to prove compliance of a material with performance requirements, make at least three separate concrete batches for each variable under investigation. Perform one time of setting test on each batch.
- For other tests, prepare three test specimens from one batch of concrete for each test variable
- Record the time at which initial contact was made between cement and mixing water.
- Thoroughly mix the mortar by hand methods on the non-absorptive surface. Measure and record the temperature of the mortar.
- Place the mortar in the container, or containers, in a single layer.
- Remove air pockets in the specimen by rocking the container, tapping the sides of the container with the tamping rod, rodding the mortar, or placing the container on a vibrating table
- Finally, level the top surface.
Procedures of Concrete Setting Time Test
- Eliminate bleed water from specimen surface using pipet.
- Based on the degree of mortar setting time, insert suitable needle size in the penetration resistance apparatus.
- Bring the bearing surface of the needle into contact with the mortar surface.
- Apply a vertical force gradually and uniformly downward on the apparatus until the needle penetrates the mortar to a depth of 25±2 mm.
- Write down the force needed to penetrate the needle to a depth of 25±2 mm.
- Record the time of load application, measured as elapsed time after initial contact of cement and water.
- Calculate the penetration resistance by dividing the recorded force by the bearing area of the needle.
Precautions For Subsequent Tests
In subsequent penetration tests take care to avoid areas where the mortar has been disturbed by previous tests. The following precautions shall be considered before the next test conducted on the sample on which first penetration test is conducted.
- The minimum distance between needle impressions is 15 mm.
- The maximum and minimum clear distance between any needle impression and the side of the container 25 mm and 50 mm, respectively.
- For conventional concrete mixtures at laboratory temperatures of 20 to 25 °C, make the initial test after an elapsed time of 3 to 4 h after initial contact between cement and water. Subsequent tests should be made at 1?2- to 1-h intervals.
- For concrete mixtures containing accelerators, it is advisable to make the initial test after an elapsed time of 1 to 2 h and subsequent tests at 1?2-h intervals.
- For concrete mixtures containing retarders, the initial test may be postponed until an elapsed time of 4 to 6 h.
- Make at least six penetrations for each time-of-setting test, with time intervals of such duration as to provide a satisfactory curve of penetration resistance versus elapsed time.
- Continue testing until one at least penetration resistance reading equals or exceeds 27.6 MPa.
Graphing Test Results
The plot of penetration resistance versus elapsed time provides information on the rate of setting. Create the graph of penetration resistance on y-axis versus elapsed time x-axis, using a scale such that 3.5 MPa and 1 h are each represented by a distance of at least 15 mm. Visually, determine the times of initial and final setting when the penetration resistance equals 3.5 MPa, 27.6 MPa, respectively.
Sources of Errors
There are certain factors that may lead to provide results which is not representative of the concrete sample under considerations. Judgement of the operator is required to identify those points that should not be included in the data analysis.
- Interference of the larger particles in the mortar
- Presence of large voids within the penetration zone.
- Interference from the impressions generated by adjacent penetrations.
- Failure to maintain the instrument perpendicular to the test surface during penetration
- Errors in reading the load variations in rate of loading.