1 Determination method of water reduction rate in the current standard “Concrete Admixtures” (GB807621997)
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1.1 Introduction to the standard method
The water reduction rate is basically the same as the slump of the benchmark concrete and the mixed concrete. The ratio of the difference between the unit water consumption of admixture concrete and the unit water consumption of benchmark concrete.
WR = (W0-W1/ W0)×100
Where W0 — Baseline Unit water consumption of concrete;
W1 — Unit water consumption of concrete mixed with admixtures;
WR — Water reduction rate.
WR is calculated as the arithmetic mean of three batches of tests, accurate to one decimal place. If the difference between the maximum value or minimum value of three batches of tests and the average value exceeds 15%, the middle value shall be taken as the water reduction rate of the admixture. If the difference between the maximum value and the minimum value and the average value exceeds 15%, the test should be repeated.
1.2 Advantages of the method
(1) More accurately measure the water reduction of admixtures rate, the error of general test results is < 5%, and can better reflect the cohesion and water retention of concrete.
(2) When the admixture has adaptability problems to cement, it can accurately reflect the plasticizing effect of the admixture in cement and measure slump more accurately. degree loss rate.
Disadvantages of the 1.3 method
(1) Heavy workload. Because only by estimating the water reduction rate of the admixture can the slump of the concrete containing admixtures and the benchmark concrete be basically the same. However, the water reduction rate given in the instructions of general admixtures fluctuates widely, which will inevitably increase the difficulty of estimation. Sometimes, in order to determine the water reduction effect, it often takes several trial mixings to get the result.
(2) Although “Test Method for Homogeneity of Concrete Admixtures”
(GB807721987 ), there is a method to measure the workability of mortar to calculate the mortar water reduction rate, but the water consumption still needs to be determined through estimation. (3) The design of the mix ratio and the measurement of slump may affect the accuracy of the results. For example, improper sand rate causes inaccurate measurement of concrete slump, which affects the calculation results of water reduction rate.
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2 The rapid determination method introduced in this article
2.1 Basic principles
This method is passed The variable water volume method is used to determine the water consumption of the standard consistency of cement paste and the water consumption of the standard consistency of cement paste mixed with admixtures to calculate the water reduction rate of admixtures. The adjusted water volume method is used to determine the water consumption of the standard consistency of cement paste mixed with admixtures. Calibration is carried out to achieve the purpose of quickly measuring the water reducing rate of admixtures, and provide a simple and accurate estimation basis for measuring the water reducing rate of admixtures according to GB807621997.
2.2 Instruments and Equipment
(1) Cement slurry mixer.
(2) Cement standard consistency meter.
2.3 Operation steps
(1) Weigh 500g of cement and pour it into the slurry mixer In the pot, accurately weigh the added water amount W1, and mix according to the method of “Water Amount for Standard Consistency of Cement, Determination of Setting Time Stability” (GB1346289). The water amount should be able to control the standard consistency of cement within the range of 28 ±2mm.
(2) After mixing is completed, measure the test cone sinking depth S1, and calculate the water consumption P1 for the standard consistency of cement according to P = 33.4-0.185s.
(3) Accurately weigh 500g of admixture and cement according to the recommended dosage of admixture and the amount of water added in 2.3(1), according to GB1346-89 Method for mixing. If the first mixing method is used, the admixture should be added together with the cement; if the simultaneous mixing method is used, the admixture should be dissolved into a solution in advance and added together with the water. If the water retention method is used, the admixture should be added 1 to 3 minutes after the water. Stir slowly for 2 minutes, then stir quickly for 2 minutes.
(4) After mixing is completed, measure the test cone sinking depth S2, and calculate the standard consistency of cement slurry mixed with admixtures according to the formula in [2.3 (2)] Water consumption P2.
(5) Water reduction rate of admixture WR1 = (P1 -P2 /P1) × 100 •The test results are accurate to one decimal place.
(6) Calculate the actual water consumption of 500g cement mixed with admixture according to the measured water reduction rate of admixture. Repeat steps (3) (4) and record Calculate the water consumption W2 at this time, and the result should be able to control the water consumption of the standard consistency within the range of 28 ±2m. If the water consumption for standard consistency is less than 28 ±2mm, appropriate amount of water should be added.
(7) Actual water reduction rate of admixture
WR = (W1-W2 / W1)×100
2.4 Advantages of this method
(1) Test intuitiveness Strong, small workload, rapid measurement results, when the admixture is not suitable for cement, the application of this method can greatly reduce labor intensity.
(2) This method is particularly suitable for the selection test of admixtures, admixture dosage test, and admixture mixing method test.
(3) This method can also be applied to determine the compressive strength ratio of admixtures.Poor setting time. 2.5 Precautions for applying this method
(1) Since the W/C for measuring the standard consistency of cement is generally between 0.26 and 0.28, it is different from that of concrete in actual production. There is a certain difference between W/C between 0.4 and 0.6, and there is a large error in the measurement results of some cements mixed with admixtures. Because the inactive components in cement can be sensitively displayed when W/C= 0.5, and within this water-cement ratio range, it is also consistent with the actual use of cement in concrete.
(2) Since this method does not consider the water absorption of sand and gravel, the water reduction rate should be reduced by 1 to 2% during the trial mixing of concrete.
(3) If the water reduction rate of the admixture exceeds 15%, 15% of the water addition amount can be deducted in advance, and then the test is carried out according to step 2.3(3). Add 15% to the measured WR, which is the water reduction rate of the admixture. This can reduce the test error. </p
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