Then, the quality of unknown sand and gravel is detected through two embodiments of mud content and a fluidity test.
5.1. Implementation Ⅰ
- (1)
Mud content test
The new materials shall be tested in accordance with the requirements of T 0333-2000 fine aggregate mud content test (sieve washing method) in JTG E42-2005 Test Rules for Aggregates of Highway Engineering [
27]. The mud content of the samples shall be calculated to 0.1% according to the formula.
where:
is the mud content of the sample (%);
is the weight of the dried sample before the test (g);
is the weight of the dried sample after the test (g).
The arithmetic mean of the test results of the two samples was used as the measurement value. When the difference between the two results exceeds 0.5%, a new sample should be taken for testing.
During the mud content test, samples of about 400 g (m) per mass are weighed according to the test procedures. The final mud content shall meet the following requirements: 3.0 or less. If the mud content of the incoming material meets the requirements of this standard, proceed to the next step of detection; Otherwise, if the batch of material is unqualified, the yard will not receive it.
For the three new batches of materials, 400 g is taken, respectively, for the mud content detection. The test results are shown in
Table 5:
Under the same test condition, the mud contents of sample 1 and sample 3 are 3.8% and 3.3%, respectively, which is not satisfied (mud content) ≤ 3.0 requirements. Sample 2 meets the mud content standard and can be tested for fluidity in the next step after preliminary screening. Sample 2 is selected as the next step of the test material, hereafter referred to as the unknown material.
- (2)
Fluidity test
The screened sample mortar is prepared from the sample whose mud content is not greater than the set threshold of mud content. The material ratio and mixing of the sample mortar, standard sand mortar, and river sand mortar are based on the requirements specified in GBT 17671-1999 Test Method for Strength of Cement Mortar (ISO Method) [
28] and JC/T681-2005 Planetary Cement Mortar Mixer [
29].
According to the requirements of GBT 17671-1999 Test Method for Strength of Cement Mortar (ISO Method), the quality mix proportion of cement mortar shall be one part of cement, three parts of sand, and half water, that is, 1:3:0.5 for cement: standard sand: water. In order to better simulate the actual effect of concrete, a 1% water-reducing agent shall be added. The final design mix ratio of cement: sand: water is 2.5:5:1. The specific preparation method is as follows: take water according to the set mass ratio scale and add cement into the pot. The water reducer makes its state select the appropriate adding time. If it is a powder water reducer, add it into the pot together with cement and other powders, and the liquid water reducer is added into the pot together with water. The cement used is ordinary Portland cement.
After using the cement mortar mixer to mix at a low speed for the set time, evenly add the sample into the mixture. The speed of mixing at low speed is: the rotation of the mixer shaft is 140 ± 5 r/min, and the revolution is 62 ± 5 r/min. According to the specification requirements, completely mix the mixture evenly and complete the preparation of the sample mortar. The setting time is 30 s.
The preparation method of standard sand mortar and river sand mortar is the same as that of sample mortar, except that the standard sand mortar is obtained by replacing the sample with ISO standard sand of the same quality, and the river sand mortar is obtained by replacing the sample with river sand of the same quality.
The standard sand group is set as control group 1, river sand 1, river sand 2, and river sand 3 corresponding to control group 2, control group 3, and control group 4, respectively, and the samples to be judged are test groups.
Control group 1: weigh 400 g cement, 800 g standard sand, 160 g water, and 4 g water reducer, respectively, and add water and cement into the pot in turn. The water reducer shall be added at the appropriate time according to its state: powder water reducer shall be added together with cement and other powders, and liquid water reducer shall be added together with water. After 30 s of low-speed mixing with a cement mortar mixer, add standard sand evenly at the beginning of the second 30 s. Mix the materials completely and evenly according to the specification requirements to complete the preparation of standard sand mortar.
In the same way as the control group 1, the control groups 2, 3, 4, and the test group (the standard sand is replaced with the corresponding river sand or sample of the same gram) are mixed with cement mortar to complete the preparation of two groups of river sand mortar and sample mortar.
According to the test procedures of GBT-2419-2005 Method for Determining the Fluidity of Cement Mortar [
30], under the condition that the standard temperature of the laboratory is 20 ± 2 °C and the relative humidity is >50%, take the materials separately, and carry out the fluidity test on five groups of newly mixed materials to determine their fluidity values. At this time, their fluidity values are recorded as d. At the same time, stand the mixed mortar for 30 min and measure its fluidity value, which is recorded as d’.
It shall be completed within 6 min from the time of adding water to the mortar to the time of measuring the diffusion diameter. After jumping the table, use a caliper to measure the diameters of the two directions perpendicular to each other on the bottom surface of the mortar, calculate the average value, and take an integer (mm). The average value is the cement mortar fluidity of the water volume.
Calculate the fluidity loss s of each group, and according to the test results of the four control groups, define the fluidity range of qualified sand materials, that is, the loss threshold range: if the fluidity value of the experimental group conforms to the range, the material is available river sand. Otherwise, the material does not meet the material requirements of the stockyard and is unqualified, which is weathered sand or chowder sand (river sand mixed with weathered sand).
Loss of fluidity s = d − d′
The fluidity test results are shown in
Table 6.
Under this test scheme, the performance of control group 1 was good. When it jumped the table 23 times, it exceeded the 300 mm range of the disc table, and the first fluidity value was recorded as 300 mm. In control groups 2, 3, and 4, the initial mobility values were all between 280 mm–300 mm, and there was almost no change in the mobility values after standing for 30 min.
After standing for 30 min, compared with the control test, the material fluidity loss of the experimental group was serious, up to 76 mm, which could be determined that the material of the experimental group was blown sand or mixed sand.
According to the test results, the fluidity loss of problem materials is obviously larger, and the fluidity loss of qualified materials is less than 10 mm. The results of multiple tests can be combined to determine the range of qualified material mobility.
5.2. Implementation II
For the four new batches of materials, 400 g is taken, respectively, for the mud content detection. The test results are shown in
Table 7:
Among them, the mud content of sample 2 is 3.6%, which does not meet Qn ≤ 3. The clay content of samples 1 and 3 met the requirements, and they were selected for the next step of the fluidity test.
Standard sand was taken and set as control group 1, river sand 1 and river sand 2 corresponding to control group 2 and control group 3, and sample 1 and sample 3 were set as unknown material 1 and unknown material 2 corresponding to test group 1 and test group 2.
Weigh 400 g cement, 800 g standard sand/river sand 1/river sand 2/unknown material 1/unknown material 2, 160 g water, and 4 g water reducing agent, respectively, and add water and cement into the pot in turn. The appropriate adding time of water reducing agent is selected according to its state: a powder water-reducing agent is added together with cement and other powder, and a liquid water-reducing agent is added together with water. After mixing with the cement mortar mixer at a low speed for 30 s, add the sand evenly at the beginning of the second 30 s, and mix the material completely and evenly according to the time required by the specification.
Five groups of fresh mix materials were tested for fluidity, and the fluidity values of fresh mix and material standing for 30 min were measured, respectively, to calculate the fluidity loss. The fluidity test results are shown in
Table 8.
Under the same test conditions, the fluidity of test group 1 is good, and the fluidity loss is 0, which is qualified river sand. The fluidity loss of test group 2 is serious, and its value is up to 52 mm. It can be judged as weathered sand or mixed sand, and the material is unqualified, so it will not be accepted.
The flow of qualified river sand can be determined from the above tests as shown in
Figure 7.
This flow chart provides a method to judge the quality of river sand, including the following steps:
- (1)
Determine the mud content of sand and gravel with a fineness modulus of 2.3–3.0. If the measured mud content is greater than the set threshold of mud content (mud content ≤ 3.0), the sample is unqualified. When measuring the mud content, carry out at least two mud content measurement tests, and take the arithmetic mean value as the mud content of the sample to be judged.
- (2)
The sample mortar is prepared from the sample whose mud content is not greater than the set threshold of mud content, and the fluidity of the sample mortar is tested to obtain the fluidity loss of the sample mortar between two test moments. The sample mortar is a mixture of cement, sample, and water with a set mass ratio, and the preparation method of the sample mortar is as follows: weigh the cement and water with a set mass ratio, add the sample after mixing for a set time, continue mixing for a set time, and complete the preparation of the sample mortar. When preparing the sample mortar, add a water reducer, which is mixed with cement and water. Water reducing agent is added in the preparation process of the standard sand mortar and river sand mortar.
- (3)
Set up multiple groups of control experiments to determine the range of loss threshold, and compare the obtained mobility loss with the range of loss threshold (mobility loss value > 15 mm). When the mobility loss is within the range of the loss threshold, the sample is qualified as river sand. Otherwise, it is unqualified as weathered sand or mixed sand.
The mortar used for the fluidity test in the control experiment is a standard sand mortar and several groups of river sand mortar. The standard sand mortar is a mixture of cement, standard sand, and water with a set proportion, and the river sand mortar is a mixture of cement, river sand, and water with a set proportion.
The fluidity of sample mortar, standard sand mortar, and several groups of river sand mortar are tested by the mortar fluidity tester. The time interval between two test moments is 25 min–35 min.
In addition, the fluidity test is conducted at a temperature of 20 ± 2 °C and relative humidity of >50%.