Method for determining critical adding amount of coal ash in sulfate corrosion resisting concrete for cast-in-place pile

[0024] The following examples are used to illustrate the present invention.

[0025] A method for determining the critical content of fly ash in sulfate-resistant concrete for cast-in-place piles includes the following steps:

[0026] (1) Raw materials

[0027] The cementing material used is Tianshan brand P?O42.5 cement, grade Ⅱ fly ash, selected local aggregate in Xinjiang, coarse aggregate uses continuous graded pebbles with a particle size of 5~31.5mm, fine aggregate uses medium-coarse sand; Beijing Institute of Construction Engineering AN1000 superplasticizer.

[0028] (2) Determination of the critical content of fly ash

[0029] The chemical composition of the selected fly ash and cement is shown in Table 2, and the mineral content of the cement measured by XRD is shown in Table 3. Combining the chemical composition of cement and fly ash in Table 2, we can see that the model is a=47%, b=22%, c=57.9%, with formula (1), the critical content x of fly ash is 34%, which is a preliminary estimate of the maximum proportion of fly ash replacing cement in this experiment is 34 %.

[0030] Table 2 Chemical composition of cement and fly ash

[0031]

[0032] Table 3 Mineral composition content of cement

[0033]

[0034] (3) Experimental verification of sulfate corrosion resistance of concrete for cast-in-place piles

[0035] (3-1) Mix ratio

[0036] The concrete strength grade of the designed cast-in-place pile is C30, the water-binder ratio of the benchmark concrete is 0.45, and the number is F0; the proportion of fly ash instead of cement is 20%, 30% and 40%, which are numbered F20, F30 and F40 in sequence. See the specific mix ratio Table 4 shows.

[0037] Table 4C30 concrete mix ratio

[0038]

[0039] (3-2) Sulfate corrosion resistance test of cast-in-place pile concrete

[0040] According to the requirements of the test piles and site conditions, the applicant’s engineering project in the northeast area of ​​the mixing station in Wujiaqu City East Industrial Park, Urumqi, Xinjiang, is equipped with test piles, bored piles, pile length 10m, pile diameter 800mm, pile center distance 3m, The concrete pouring on site works well.

[0041] After reaching the age of 28 days, perform the sulfate corrosion test in accordance with the national standard GB/T50082-2009, and cut the core sample into 10cm high specimens. To ensure the sameness of other test variables, take the same depth of each test pile The test is carried out on the drill core samples at the level. Each pile is divided into two groups of deep and shallow samples. The compressive strength of each sample before and after 150 sulphate dry-wet cycles and the durability test results of anti-sulphate corrosion are measured. See Table 5.

[0042] Table 5 Test results of resistance to sulfate attack

[0043]

[0044] Analyzing the ratio of strength degradation before and after sulfate corrosion, it was found that the two sets of concrete samples showed similar laws: as the amount of fly ash in the concrete increased, the sulfate resistance of the concrete increased to a certain extent; but when fly ash was added When the amount is increased from 30% to 40%, the percentage of strength degradation of concrete decreases greatly, and the ability of concrete to resist sulfate becomes worse. The strength percentages of the two groups of samples before and after sulfate corrosion were reduced from 75.64% to 61.24%, and 77.05% to 63.64%, and the strength degradation degree of F40 exceeded F0; with 40% fly ash content, 150 times sulfate The strength degradation percentage of concrete after dry-wet cycles is less than 75%, which does not meet the requirements of the national standard GB/T50082-2009; in addition, the strength design grade of this experiment is C30, and the compressive strength of the benchmark group F0 is 48MPa. The 28d strength of concrete is reduced to a certain extent. When the mixing amount reaches 40%, the strength of concrete drops to 31MPa, which is a relatively low level. It also shows that the quality of concrete deteriorates when the mixing amount reaches 40%. It is not conducive to the resistance of concrete to sulfate corrosion.

[0045] A comprehensive analysis of the test data shows that for concrete to resist sulfate corrosion, the maximum amount of fly ash should be maintained at 30%, which is more consistent with the 34% estimated by the present invention based on the chemical composition of fly ash and cement. The field data is a good proof for this model and illustrates the rationality of the method of the present invention. At the same time, the experimental verification results show that the method of the present invention is simple and effective.