Determining the optimum dosage of a superplasticizer is essential for achieving the desired workability, strength, and durability of concrete while avoiding unnecessary admixture consumption. Two of the most widely used laboratory methods for this purpose are the Marsh Cone Test and the Mini Slump Cone Test. In this article, you will learn both methods step by step, understand how to interpret the test results, and identify the optimum (saturation) dosage of a superplasticizer for your cementitious system.
Optimum Dosage of Superplasticizer by Marsh Cone Test
Purpose
The Marsh Cone Test is a simple and reliable method used to evaluate the fluidity of cement paste containing different types and dosages of superplasticizers. It helps determine the optimum dosage of a superplasticizer and assess its compatibility with cement.
Why is this Test Important?
- Determines the relative fluidity of cement paste by measuring the time required for a fixed quantity of paste to flow through the Marsh cone.
- Helps identify the optimum superplasticizer dosage for maximum dispersion.
- Assesses the compatibility between cement and different superplasticizers.
Limitations
- The optimum dosage obtained for cement paste cannot be directly applied to concrete, as aggregates influence the required dosage.
- Field conditions involving high shear mixing may require dosage adjustments.
Apparatus Required
- Marsh cone
- Stopwatch
- Graduated measuring cylinder
- Hobart mixer
- Beaker
Precautions
- Perform the test under controlled temperature and humidity to obtain consistent results.
- Maintain the same mixing procedure, mixing time, and operator throughout all trials.
- For liquid superplasticizers, correct the mixing water based on the admixture’s solid content.
Test Procedure
Step 1
Mix cement with 70% of the total mixing water in a Hobart mixer at low speed until a uniform paste is obtained.
Step 2
Take the remaining 30% of water in a separate beaker and thoroughly mix the measured quantity of superplasticizer into it.
Step 3
Add this superplasticizer solution to the mixer and continue mixing for 120 seconds.
Step 4
Stop the mixer and quickly scrape the paste sticking to the bowl sides. Resume mixing for another 120 seconds to obtain a homogeneous paste. Follow the same procedure for every superplasticizer dosage tested.
Step 5
Close the outlet of the Marsh cone with your finger and pour approximately 1000 ml of the prepared paste into the cone.
Step 6
Place a graduated cylinder beneath the outlet and release the finger to allow the paste to flow freely.
Step 7
Start the stopwatch immediately and record the time required for the paste to discharge through the cone.
Step 8
Repeat the test for all selected superplasticizer dosages, keeping the water-to-binder ratio and mineral admixture replacement constant.
Step 9
Record the flow time for each mix immediately after mixing.
Step 10
Compare the recorded flow times. A lower flow time indicates higher paste fluidity and better dispersion of cement particles.
Step 11
Identify the optimum dosage as the dosage at which the flow time shows a sharp reduction. Beyond this point, further addition of superplasticizer produces little or no improvement. This point is known as the saturation dosage.
Step 12
Plot a semi-log graph with:
- X-axis: Superplasticizer dosage (%)
- Y-axis (log scale): Flow time (seconds)
The curve gradually levels off after the optimum dosage, indicating that adding more superplasticizer does not significantly improve fluidity.
Step 13
To evaluate cement–superplasticizer compatibility, measure the flow time at 5 minutes after mixing. Collect the paste, cover it with a plastic sheet to prevent moisture loss, and repeat the test after 60 minutes. Plot a second semi-log graph for the 60-minute readings. Similar trends in both graphs indicate good compatibility between the cement and the superplasticizer.
Observations
- Cementitious material used: _______________________
- Water-to-binder ratio (w/b): _______________________
- Superplasticizer type: _______________________
Record the flow time for each dosage in the observation table.
Report
Optimum Superplasticizer Dosage: _______________________
Note: The saturation dosage is the dosage at which the flow time reaches its minimum and remains nearly constant. Increasing the superplasticizer beyond this point does not produce any significant improvement in paste fluidity
Example Data
Assume a cement paste with a water-to-binder ratio (w/b) = 0.35 was tested using different dosages of a polycarboxylate ether (PCE) based superplasticizer.
| Superplasticizer Dosage (% by weight of binder) | Flow Time (seconds) |
|---|---|
| 0.00 | 42.5 |
| 0.20 | 30.8 |
| 0.40 | 21.5 |
| 0.60 | 16.4 |
| 0.80 | 15.1 |
| 1.00 | 14.9 |
| 1.20 | 14.8 |
Optimum Dosage of Superplasticizer by Mini Slump Cone Test
The mini slump cone test is performed to evaluate the flow characteristics of cementitious paste and determine the optimum dosage of a superplasticizer.
The test is useful for the following purposes:
- To compare the relative fluidity of different cementitious pastes by measuring their spread after lifting the mini slump cone.
- To determine the optimum dosage of a superplasticizer for a given cementitious system.
- To compare the effectiveness of different superplasticizers under identical test conditions.
- To identify the dosage at which the cement paste begins to exhibit bleeding, indicating excessive superplasticizer content.
Limitations
- The optimum dosage determined using cement paste cannot be directly adopted for concrete, even if the same binder composition is used. Concrete contains aggregates that significantly influence the required dosage.
- The results should be interpreted carefully for applications involving high shear mixing, as the performance of the superplasticizer may differ under such conditions.
Apparatus Required
- Mini slump cone
- Non-absorbent square base plate or glass plate (minimum size 400 mm × 400 mm) with center markings or measurement graduations
- Vernier caliper
- Wooden mallet
Precautions
Before conducting the test, ensure the following:
- Perform the test in a temperature- and humidity-controlled environment. Variations in environmental conditions can significantly affect the test results.
- Follow exactly the same mixing and testing procedure for every trial to obtain consistent and comparable results.
- If a liquid superplasticizer is used, determine its solid content and apply the necessary water correction while preparing the paste.
Test Procedure
Step 1
Apply a thin layer of grease to the inside surface of the mini slump cone. Place the cone at the center of the non-absorbent base plate or glass plate.
Step 2
Prepare the cementitious paste according to the required mix proportions. Pour approximately 40 mL of the freshly prepared paste into the mini slump cone.
Step 3
Ensure that the mini slump cone is placed vertically and remains stable on the base plate before proceeding with the test.
Step 4
Gently tap the sides of the mini slump cone using a wooden mallet. This helps eliminate any entrapped air bubbles within the paste.
Step 5
Carefully lift the mini slump cone vertically upward in one smooth motion. Avoid any twisting, vibration, or lateral movement, as these may disturb the spread of the paste.
Step 6
Allow the paste to spread freely for 1 minute.
Measure the diameter of the spread in two mutually perpendicular directions using a Vernier caliper. For improved accuracy, measurements may also be taken along additional directions and averaged.
Step 7
The measured spread represents the fluidity of the cement paste. A larger spread indicates better dispersion of cement particles and improved performance of the superplasticizer.
Step 8
Conduct the test for mixes containing different dosages of the superplasticizer while maintaining:
- Constant water-to-binder (w/b) ratio
- Fixed replacement level of mineral admixtures (by weight or volume), wherever applicable
Step 9
Measure and record the spread diameter for each superplasticizer dosage.
Step 10
Identify the dosage at which further increases in superplasticizer content produce only a marginal increase in spread. This dosage is considered the optimum dosage.
Also observe the paste for any signs of bleeding. If bleeding occurs at higher dosages, those dosages should be considered excessive and beyond the optimum level.
Step 11:
Prepare a semi-logarithmic graph with:
- X-axis: Superplasticizer dosage (% by weight of binder)
- Y-axis (log scale): Measured spread diameter
The graph helps visualize the relationship between dosage and fluidity and assists in identifying the optimum dosage.
Observations and Calculations
Record the following details before performing the calculations:
- Cementitious materials used: ____________________________
- Water-to-binder ratio (w/b): ____________________________
Record all measured spread values for the different superplasticizer dosages in the observation table for further analysis.
