Test methods for the optimum dosage in concrete and the effect of water-reducing agents in concrete
Test methods for determining the optimum dosage of water-reducing agents in concrete:
1. Slump test
1. Test procedure:
Fix the concrete mix ratio and prepare multiple concrete mixtures by changing only the amount of water reducer added. Fill the mixture into the slump cone in three layers, tamping each layer with a tamping rod 25 times, leveling it off, then lifting the cone vertically to measure the height of the mixture slump.
2. Result analysis:
Use the slump as an indicator to plot the relationship curve between the slump and the amount of water reducer added. Determine the appropriate range of water reducer added according to the slump range required for concrete construction. Generally, the appropriate amount of water reducer added is when the slump meets the construction requirements and there is no segregation or bleeding.
2. Spread test
1. Test procedure:
For concrete with high fluidity, the spread test is often used. The concrete mixture is placed in a slump cone, and after lifting the cone, the diameter of the spread of the concrete mixture on a flat surface is measured.
2. Result analysis:
Similar to the slump test, a curve is plotted showing the relationship between the spread and the water-reducing agent dosage. Based on the engineering requirements for concrete fluidity, the optimum dosage of the water-reducing agent is determined, so that the spread of the concrete meets the construction requirements and has stable performance.
3. Strength test
1. Test procedure:
Prepare concrete specimens with different water reducer dosages, and maintain them under standard curing conditions until the specified age (e.g., 7 days, 28 days), and then perform a compressive strength test.
2. Result analysis:
Draw a curve showing the relationship between strength and water reducer dosage. Generally, when the water reducer dosage is low, the strength increases with the increase of the dosage; however, if the dosage is too high, the strength may decrease. The optimum dosage is the one that gives the concrete the desired strength and the highest strength.
4. Workability evaluation test
1. Test procedure:
The workability of the concrete is evaluated based on the slump, spread, water retention and cohesiveness. The overall workability of the concrete mixture is observed by adjusting the dosage of the water reducer.
2. Result analysis:
When the workability of the concrete is good, that is, the slump or spread meets the construction requirements, and the water retention and cohesiveness are also good, without phenomena such as bleeding, segregation, or bottom grabbing, the corresponding water reducer dosage can be considered the optimal dosage range.
The following equipment is required for the test of the optimal dosage of water reducer in concrete:
1. Mixing equipment
Concrete mixer: used to mix the concrete mixture to evenly blend all the raw materials.
2. Forming and compaction equipment
1. Slump cone: used to determine the slump of the concrete mixture, and is the main instrument for the slump test.
2. Compaction rod: used to insert and compact the concrete mixture during the slump test and the forming of concrete test pieces.
3. Test mould: Different sizes of test moulds are selected according to the test requirements, such as the 150mm×150mm×150mm cube test mould for making compressive strength test pieces.
4. Vibrating table: It is used for vibrating and compacting the concrete test pieces during the forming process to make the concrete mixture more compact and uniform.
3. Measurement and testing equipment
1. Electronic scale: It is used to accurately weigh various raw materials and water reducers.
2. Measuring cylinder: used to measure liquids, such as the amount of water used in concrete mixtures.
3. Ruler: used to measure the slump height and spread diameter of the concrete mixture after the slump cone is lifted during the slump test and the spread test.
4. Compression testing machine: used to test the compressive strength of concrete specimens that have been cured to a specified age and to determine their compressive strength.
4. Other equipment
1. Trowel: used to level and trim the surface of the concrete specimen.
2. Curing box or curing chamber: provides a standard curing environment for the concrete specimen to ensure that the curing temperature and humidity meet the requirements.
The specific steps for determining the optimum dosage of concrete water reducer through the slump test are as follows:
1. Test preparation
1. Prepare the raw materials such as cement, sand, stone, water and water reducer according to the designed concrete mix proportion, and ensure that the quality of the raw materials meets the relevant standard requirements.
2. Check whether the test equipment such as the slump cone, tamping rod, electronic scale and measuring cylinder is in good condition and can be used normally.
2. Preparation of concrete mixture
1. Fix the amount of cement, sand and stone in the concrete mix ratio, and weigh different quantities of water reducer according to the initially set range of water reducer dosage. For example, the dosage can be set at 0.2%, 0.4%, 0.6%, 0.8% of the amount of cementitious material, and the corresponding quantities of water reducer weighed.
2. Pour the weighed cement, sand and stone into the concrete mixer and mix them evenly.
3. Measure a certain amount of water according to the designed water content, mix a portion of the water with the water reducer to make a water reducer solution.
4. Pour the remaining water and the water reducer solution into the mixer together, and mix with the dry materials to form a uniform concrete mixture. The mixing time should meet the requirements of the relevant standards.
3. Slump test operation
1. Wash the slump cone, place it on a rigid, horizontal, non-absorbent base plate, and moisten the inside and outside walls and base plate of the cone with a damp cloth.
2. Load the concrete mixture into the slump cone in three layers, each layer being approximately the same height. Use a tamping rod to tamp each layer 25 times in a spiral direction from the outside to the center. When tamping the bottom layer, the rod should be inserted to the bottom. When tamping the upper layer, the rod should be inserted through the upper layer and then inserted into the lower layer by about 20-30mm.
3. After tamping the top layer, use a spatula to scrape off the excess concrete mixture from the barrel opening and smooth it.
4. Lift the slump cone vertically and smoothly, and complete the lifting within
5-10s. The entire process, from the start of loading to the lifting of the slump cone, should be completed within 150s.
5. Immediately after lifting the slump cone, measure the height difference between the height of the cone and the highest point of the concrete specimen after slump, which is the slump value of the concrete mixture, accurate to 1mm.
4. Record and analyze the results
1. Record the slump values of concrete mixtures with different water reducer dosages, and at the same time observe and record the cohesiveness and water retention of the concrete mixture. The cohesiveness can be judged by tapping the side of the concrete with a tamping rod to see if there is any spalling or scattering of the gravel; the water retention can be judged by observing the surface of the concrete for any bleeding.
2. Plot the relationship between the slump and the water reducer dosage by taking the water reducer dosage as the abscissa and the slump value as the ordinate.
3. According to the slump range required for concrete construction, find the corresponding range of water reducer dosage on the curve. At the same time, comprehensively consider the cohesiveness and water retention of the concrete, and select the water reducer dosage that makes the concrete work well, that is, when the slump meets the construction requirements and the cohesiveness and water retention are both good, as the reference value for the optimal dosage. If necessary, a refinement test can be carried out near the reference value of the optimal dosage to further determine a more precise optimal dosage.
Water-reducing agents work in concrete in the following ways:
First, dispersion:
Water-reducing agent molecules have hydrophilic and hydrophobic groups. After being added to the concrete, the hydrophobic groups adsorb on the surface of the cement particles in a certain direction, while the hydrophilic groups face outward. This causes the cement particles to carry the same electrical charge, creating electrostatic repulsion, which disperses the cement particles from each other, breaks up the floc structure, releases the trapped water, and increases the fluidity of the concrete.
2. Lubrication:
The hydrophilic groups of the water-reducing agent adsorb a large number of water molecules, forming a stable water film on the surface of the cement particles, which acts as a lubricant, reducing the frictional resistance between the cement particles and making the concrete mix more fluid.
3. Steric hindrance:
After the water-reducing agent molecules are adsorbed on the surface of cement particles, a polymer adsorption layer with a certain thickness is formed, which produces a steric hindrance effect, preventing cement particles from approaching each other and agglomerating, so that the cement particles can be more evenly dispersed in the concrete, improving the homogeneity and stability of the concrete.
4. Delaying cement hydration:
Some water-reducing agents can adsorb on the surface of cement particles, delaying contact between the particles and water and thus the hydration rate of the cement. This allows the concrete to maintain good workability for a longer period of time, facilitating construction operations.
