Setting Time of Concrete: The penetration resistance test is used to determine the initial and final setting times of concrete mixtures in the laboratory and on-site, as well as the effect of variables e.g. water content; form and quantity of cementitious material; and admixtures on the setting time of concrete. The setting time test is performed on a sieved mortar sample.
The initial setting time of cement is the point at which it starts to harden and lose its plastic state. The final setting time, on the other hand, starts when the concrete loses all of its plasticity and hardens fully. ASTM C403 specifies that the initial and final times of concrete can be calculated when the resistance to penetration reaches 3.5 MPa and 27.6 MPa, respectively.
Certain factors, such as the cement fineness, the existence of salts in the sand, and the atmospheric conditions, can affect the setting time of concrete. For example, cement requires a temperature of 27°c to fully hydrate, but the temperature is low during the winter, delaying hydration.
Instruments for Testing Setting Time of Concrete
The lateral dimension must be > 150 mm in length and the height must be > 150 mm in height.
2. Penetration Needles
Penetration needles have the following bearing areas: 645, 323, 161, 65, 32, and 16 mm2.
3. Loading Device
It is used to determine the amount of force needed to penetrate the needles. The system shall be capable of measuring penetration force to a precision of ten Newtons and have a power of at least 600 Newtons.
It is a round and straight steel rod with a diameter of 16 mm and a length of around 600 mm.
It is used to remove bleed water from the test specimen’s surface.
6. Temperature Measuring Device
The thermometer must be able to measure temperatures up to 60.5°C in the fresh mortar.
7. Stop Watch
Preparation of Samples
- For field experiments, collect a representative sample of fresh concrete from which three specimens will be prepared.
- Make the concrete and specify its slump and air content for laboratory testing.
- To prepare specimens, concrete shall be passed through a 4.75-mm sieve.
- To demonstrate a material’s compliance with performance criteria, prepare at least three distinct concrete batches for each variable being investigated. Conduct a single setting test on each batch.
- Prepare three research specimens from one batch of concrete for each other test.
- Keep track of the time when the cement and mixing water first came into contact.
- Mix the mortar thoroughly by hand on a non-absorbent surface. Take the temperature of the mortar and record it.
- Arrange the mortar in a single layer in the tub or containers.
- By rocking the jar, tapping the container’s sides with the tamping rod, rodding the mortar, or putting the container on a vibrating surface, air pockets in the specimen may be removed.
- Finally, ensure that the top surface is level.
Procedures of Test
- Using a pipet, remove bleed water from the specimen surface.
- Insert the appropriate needle size into the penetration resistance apparatus based on the degree of mortar setting time.
- Bring the needle’s bearing surface into contact with the mortar’s surface.
- Vertically push the apparatus downward steadily and evenly until the needle penetrates the mortar to a depth of 252 mm.
- Calculate the force needed to penetrate the needle 252 mm deep.
- Keep track of the time the load was applied, as determined by the elapsed time between the initial contact of cement and water.
- Divide the recorded force by the needle’s bearing area to determine the penetration resistance.
Recommendations for Subsequent Tests
Proceed with caution in subsequent penetration tests, avoiding places where the mortar has been disturbed by previous tests. The next steps must be taken prior to conducting the subsequent test on the sample from which the initial penetration test was performed.
- 15 mm is the minimum distance between needle penetration points.
- The direct distance between any needle penetration points and the mould’s side is 25 and 50 mm, respectively.
- Conduct the initial test on standard concrete mixes at lab temperatures of 20 to 25 °C, 3 to 4 hours after first contact of cement with water. Subsequent experiments should be performed at intervals of 12 to 1 hour.
- It is recommended to conduct the initial test on concrete mixtures containing accelerators after 1 to 2 hours and subsequent tests at 12-hour intervals.
- In the case of concrete mixtures made with setting time retarders, the initial test may be delayed until 4 to 6 hours have passed.
- At least 6 penetrations should be performed for each time-of-setting test, with time intervals long enough to give an acceptable plot of penetration resistance vs elapsed time.
- Continue with the testing process until a penetration resistance reading matches or larger than 27.6 MPa on at least one occasion.
Making Graphs of Test Results
Graphing the penetration resistance against elapsed time reveals the setting pace. Create a plot of penetration resistance on the y-axis against elapsed time on the x-axis. Determine the initial and final setting times visually when the penetration resistance is equal to 3.5 MPa and 27.6 MPa, respectively.
Certain factors which contribute to the production of results that are not representative of the concrete specimens under consideration. The operator’s judgment is needed to determine which points should be excluded from the data analysis.
- Interference of the mortar’s larger particles
- Wide voids exist inside the penetration region.
- Interference caused by adjacent penetrations’ impressions.
- During penetration, the instrument is not held perpendicular to the test surface.
- Errors in the reading of load variations in loading rate.