Methods for predicting the air content of hardened cement mortar

Abstract

This invention discloses a method for predicting the air content of hardened cement mortar, comprising: obtaining the mix proportion of cement mortar, wherein the raw materials of the cement mortar include cement-based cementitious materials, water, sand, and admixtures, and the mix proportion is 1:w:x:y, where w is the amount of water, x is the amount of sand, and y is the amount of admixtures; obtaining the density of the raw materials of the cement mortar; obtaining the trend of the cumulative water absorption of sand over time; testing the apparent density of the cement mortar mixture; testing the setting time of the cement mortar; calculating the first water absorption rate and the second water absorption rate of sand; calculating the hysteretic water absorption of sand; and calculating the air content of the hardened cement mortar. This invention solves the problem of inaccurate or untimely detection of the air content of cement mortar by introducing the hysteretic water absorption of sand.

Description

Technical Field

[0001] This invention relates to the field of civil engineering construction technology, specifically to a method for predicting the air content of hardened cement mortar. Background Technology

[0002] Cement mortar is a civil engineering material made by binding loose sand together with cement-based binders and water. It is widely used in the construction of brick and stone masonry, filling gaps in walls and floors, and plastering walls and ceilings. Mortar not only serves to bond and fix materials but also enhances the overall stability and waterproofing of the structure.

[0003] The production of cement mortar relies on mixing cementitious materials, water, sand, and admixtures. Due to mechanical mixing and the action of certain air-entraining admixtures, the mixing process inevitably incorporates both large and small air bubbles. An appropriate amount of air bubbles contributes to the workability of the mortar mixture and the freeze-thaw resistance of the hardened mortar; excessive air bubbles negatively impact strength and density. Therefore, the air content of mortar is an important indicator affecting the performance of hardened mortar, commonly expressed as the ratio of the volume of air bubbles in a given volume of mortar to the total volume of the mortar.

[0004] According to JGJ / T 70–2009, "Standard for Test Methods of Basic Performance of Building Mortar," the air content of mortar is commonly tested using instrumental methods and density methods. Both methods reflect the air content of the mortar mixture, not the air content after hardening. Extensive data shows that the air content of the mortar mixture and the air content of hardened cement mortar are not equal. A significant source of this difference is the delayed water absorption phenomenon of sand. After the cement mortar mixture content is tested, the open pores of the sand continue to absorb water, reducing the volume of air bubbles (the sum of enclosed, introduced, and unabsorbed air in the open pores). The total volume of the cement mortar decreases accordingly, but the reduction ratios are not consistent. Therefore, the air content of cement mortar changes before and after hardening. The delayed water absorption phenomenon of sand is not considered in existing test method standards. In particular, with the depletion of river sand resources, manufactured sand made from crushed rocks and recycled fine aggregate made from crushed construction waste have been widely used. The hysteresis water absorption of these two types of sand is much greater than that of traditional natural sand. The difference in air content before and after hardening of mortar prepared from them is particularly obvious. If this is not taken into account and the air content of the mixture is mistakenly equated with the air content after hardening, it will cause certain misjudgments in the evaluation of the strength and frost resistance of the mortar.

[0005] Chinese patents CN103728196A, "A Method for Determining the Air Content of Hardened Concrete", and CN103728205A, "Method for Determining the Air Content of Hardened Concrete", provide methods for testing the air content of hardened concrete (note: cement mortar can be considered a special type of concrete). However, the test samples are concrete that has been hardened for 28 days, which is too long for projects that need to obtain the air content in a timely manner. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, a method for predicting the air content of hardened cement mortar is provided. The air content of hardened cement mortar is calculated using the properties of the cement mortar mixture and the properties of the mortar raw materials, thereby solving the problem that existing methods are inaccurate or untimely in detecting the air content of cement mortar.

[0007] To achieve the above objective, a method for predicting the air content of hardened cement mortar is provided, comprising the following steps:

[0008] To obtain the mix proportion of cement mortar, the raw materials of the cement mortar include cement-based cementitious materials, water, sand and admixtures, and the mix proportion is 1:w:x:y, where w is the amount of water, x is the amount of sand, and y is the amount of admixtures.

[0009] The density of the raw materials for cement mortar is obtained, wherein the density of the sand is the apparent density of the sand.

[0010] Establish a trend model for the cumulative water absorption of sand over time. The trend model is k = k(t), where k is the cumulative water absorption per 100g of sand and t is time.

[0011] The actual apparent density of the cement mortar was tested, and the first time interval was recorded. The first time interval is the time from the addition of water and mixing to the completion of the apparent density test. The formula for calculating the actual apparent density is:

[0012]

[0013] Where Γ is the actual apparent density, m is the mass of the cement mortar mixture, and V is the volume of the cement mortar mixture;

[0014] The setting time of the cement mortar was tested, and a second time of the cement mortar was recorded, which is the time interval from the addition of water and mixing to the setting of the cement mortar.

[0015] Based on the relationship between the cumulative water absorption and time, the first time, and the second time, the first water absorption rate and the second water absorption rate of the sand are obtained by linear interpolation. The first water absorption rate is the volume of water absorbed by a unit mass of sand up to the first time, and the second water absorption rate is the volume of water absorbed by a unit mass of sand up to the second time.

[0016] k1=10k(t1), k2=10k(t2),

[0017] Where k1 is the first water absorption rate and k2 is the second water absorption rate;

[0018] Based on the first and second water absorption rates, the hysteretic water absorption of the sand is calculated using the following formula:

[0019] Δ = 10 -6 (k2-k1)x, where Δ is the volume of water absorbed by the sand with a hysteresis absorption of x parts by mass during the interval between the first time and the second time;

[0020] The air content of the hardened cement mortar was calculated as follows:

[0021]

[0022] Where, ρ c ρ is the density of the cementitious material. w ρ is the density of water. s ρ is the apparent density of sand. p This refers to the density of the admixture.

[0023] Furthermore, the cement-based cementitious material is cement.

[0024] Furthermore, the cement-based cementitious material is a mixture of cement and mineral admixtures.

[0025] Furthermore, the mineral admixture is at least one of fly ash, granulated blast furnace slag powder, and silica fume.

[0026] Furthermore, the sand is at least one of natural sand, manufactured sand, and recycled fine aggregate, and the sand is dried sand.

[0027] Furthermore, the apparent density of the sand was obtained based on the test in GB / T 14684–2022 "Sand for Construction".

[0028] Furthermore, when obtaining the apparent density of the cement mortar, the apparent density of the cement mortar is obtained by testing according to JGJ / T 70–2009 "Standard for Test Methods of Basic Properties of Mortar".

[0029] Furthermore, when obtaining the setting time of the cement mortar, the setting time of the cement mortar in real time is obtained by testing according to JGJ / T 70–2009 "Standard for Test Methods of Basic Performance of Mortar".

[0030] The beneficial effect of this invention is that the method for accurately calculating the air content of mortar, without changing the testing principle specified in industry standards, only adds the testing of two material performance indicators (setting time of cement mortar and water absorption curve of sand) to obtain the air content after hardening before cement mortar sets and hardens, which greatly improves the accuracy. It can help engineering managers to quickly and accurately assess the quality of cement mortar and play a role in quality control of structural safety. Attached Figure Description

[0031] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0032] Figure 1 This is a graph showing the relationship between the water absorption rate of sand and the air content of mortar in Examples 1 to 6 of the present invention. Detailed Implementation

[0033] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0035] This invention provides a method for predicting the air content of hardened cement mortar, comprising the following steps:

[0036] S1. Obtain the mix proportion of cement mortar. The raw materials of cement mortar include cement-based cementitious materials, water, sand and admixtures. The mix proportion is 1:w:x:y, where w is the amount of water, x is the amount of sand, and y is the amount of admixtures.

[0037] In this invention, the mix proportion of cement mortar is first consulted, and the following ratio of materials to cement-based cementitious materials is obtained from the mix proportion of mortar (mass ratio of various raw materials). Specifically, the raw materials of the mortar include cement-based cementitious materials, water, sand, and admixtures. The cement-based cementitious materials are cement or a mixture of cement and mineral admixtures, and the mineral admixtures are at least one of fly ash, granulated blast furnace slag powder, and silica fume.

[0038] Table 1. Proportion of Raw Materials and Cementitious Materials in Cement Mortar

[0039] raw materials The ratio of raw material usage to cementitious material usage Cement-based cementitious materials 1 water w sand x admixtures y

[0040] For example, if the mix proportion of cement mortar shows "cement-based cementitious material: water: sand: admixture = 100:40:300:2", then w = 0.4, x = 3, y = 0.02.

[0041] S2. Test the density of the raw materials for the mortar (as shown in Table 2 below). Note that for sand, its apparent density should be tested according to GB / T 14684–2022 "Construction Sand". The apparent density of sand is the ratio of its mass to its apparent volume. Apparent volume is the sum of the solid volume and the volume of closed pores, excluding the volume of open pores, which can absorb water when the sand comes into contact with water.

[0042] Table 2. Density of mortar raw materials

[0043] Material <![CDATA[Density (kg / m 3 )]]> Cement-based cementitious materials <![CDATA[ρ c ]]> sand <![CDATA[ρ s ]]> admixtures <![CDATA[ρ p ]]> water <![CDATA[ρ w ]]>

[0044] S3. Test the trend of cumulative water absorption of sand over time and obtain the relationship between the cumulative water absorption of 100g of sand and time.

[0045] Specifically, according to the method in Chinese patent CN111650092A "A Method and Device for Testing the Water Absorption Rate of Mechanized Sand", the cumulative water absorption of 100g from the time of water addition to different time points t is tested, and the relationship between the cumulative water absorption k(t) and time t is obtained, which is expressed as k = k(t).

[0046] S4. Test the actual apparent density of the mortar Γ (kg / m³) 3 The time interval from adding water and mixing to the completion of the apparent density test is recorded as time t1. The formula for calculating the actual apparent density of the mortar is:

[0047]

[0048] Where m is the mass of the mortar mixture (kg), and V is the volume of the mortar mixture (m³). 3 ).

[0049] Specifically, step S4 includes:

[0050] ① Weigh the mortar mixture sample and determine its mass m (kg);

[0051] ② Weigh the sample volume V (m³) of the mortar mixture. 3 );

[0052] ③ Calculate the actual apparent density ρ (kg / m³) of the mortar. 3 ).

[0053] The apparent density of mortar refers to the ratio of the mass of the mortar to its actual volume (including the volume of air bubbles).

[0054] S5. Test the setting time of the cement mortar and record the second time t2, where the second time refers to the time interval from adding water and mixing to the setting of the cement mortar.

[0055] S6. Based on the relationship between the cumulative water absorption and time obtained in step S3, the first time obtained in step S4, and the second time obtained in step S5, interpolate to obtain the first water absorption rate k1 (mL / kg) and the second water absorption rate k2 (mL / kg) of the sand. The first water absorption rate is the cumulative water absorption volume per unit mass of sand up to the first time, and the second water absorption rate is the cumulative water absorption volume per unit mass of sand up to the second time.

[0056] k1=10k(t1) (2),

[0057] k2=10k(t2) (3).

[0058] S7. Based on the first water absorption rate and the second water absorption rate, calculate the hysteretic water absorption Δ(m) of the sand. 3 / kg), where the delayed water absorption is the volume of water absorbed by x mass parts of sand within the interval between the first and second times, and the calculation formula is:

[0059] Δ = 10 -6 (k2-k1)x (4).

[0060] S8. Calculate the air content of the cement mortar:

[0061]

[0062] The derivation of this expression is as follows:

[0063] Let M be the mass of the cement mortar mixture prepared by mixing 1 kg of cement-based binder with other raw materials according to the mix proportion. Then:

[0064] M = 1 + w + x + y (6).

[0065] According to the definition of apparent density, the mass M of cement mortar is the apparent density Γ (kg / m³). 3 The volume V1 (m³) during the apparent density test 3 ),Right now:

[0066] M=ΓV1 (7).

[0067] Combining equations (6) and (7), we get:

[0068]

[0069] In this cement mortar mixture, the volume of sand includes its apparent volume (solid volume plus closed pore volume, excluding open pores) and open pore volume. The open pore volume of the sand can be further divided into three parts:

[0070] a) The volume of water absorbed during the time period 0 to t1, v a (m 3 Since the water absorption of 100g of sand at t=t1 is k (mL), that is, the water absorption of 1kg of sand at t=t1 is 10. –5 k(m 3 Therefore, there is

[0071] v a =10 -5 x·k(t1) (9).

[0072] b) The volume of water absorbed during the time period t1 to t2, v b ,

[0073] v b =10 -5 x(k(t2)-k(t1)) (10).

[0074] c) The volume of water absorbed after the mortar has set (t2) v c ,

[0075] v c =10 -5 x(k max -k(t2)) (11).

[0076] Where, k max The volume (mL) of water absorbed by 100g of sand when it is fully saturated.

[0077] When t = t1, the total volume of the mortar mixture is:

[0078]

[0079] Where u is the volume of voids other than sand particles in the mortar mixture (the sum of enclosed air and introduced air);

[0080] When t = t2, v b When filled with water, the volume of voids outside the sand particles remains almost unchanged. The total volume of this mortar mixture is:

[0081]

[0082] At this point, the total gas volume a2 in the mortar is equal to the void volume u outside the sand particles and the open pores v that are not filled when the mortar sets. c ,Right now:

[0083] a2 = v c +u (14).

[0084] According to the definition of air content, the air content A of hardened cement mortar is the ratio of the total amount of gas a2 in the hardened cement mortar to the volume V2 of the hardened mortar, that is:

[0085]

[0086] Combining equations (8), (12), and (13), we get:

[0087]

[0088] Substituting equation (16) into equation (13), we get:

[0089]

[0090] Combining equations (14) to (17), we get:

[0091]

[0092] Equation (18) is the final expression for calculating the air content of hardened cement mortar, which is equivalent to Equation (5).

[0093] To specifically demonstrate the calculation effect of the method for predicting the air content of cement mortar according to the present invention, the following Examples 1 to 6 and Comparative Examples 1 to 2 are provided for air content calculation. The comparative examples are calculated using conventional techniques (without considering delayed water absorption), and the results are the air content of the cement mortar mixture; the examples are calculated according to the present invention (considering the delayed water absorption phenomenon of sand), and the results are the air content of the hardened cement mortar.

[0094] In Examples 1 to 3 and Comparative Example 1, the cement mortar mix ratio was known to be cementitious material: water: sand: admixture = 100:45:200:1. The densities of the aforementioned raw materials were also known to be 3100, 1000, 2600, and 1020 (unit: kg / m³). 3 (where sand is the apparent density). After the mortar was mixed, the apparent density was measured to be 1970 kg / m³ according to the "13.2 Density Method" of JGJ / T 70–2009 "Standard for Test Methods of Basic Performance of Building Mortar". 3 The test was completed 6 minutes after water was added and mixed; according to the "8 Setting Time Test" of JGJ / T70–2009 "Standard for Test Methods of Basic Performance of Mortar", the setting time was measured to be 270 minutes.

[0095] Example 1

[0096] (1) From the above, w = 0.45, x = 2, y = 0.01; ρ c=3100, ρ w =1000, ρ s =2600, ρ p =1020; Γ=1970, first time t1=6min; second time t2=270min.

[0097] (2) Test the relationship between the cumulative water absorption of 100g of sand and time.

[0098] Example 1 uses river sand (a type of natural sand) to test the cumulative water absorption of river sand at different time points. The relationship between the cumulative water absorption k (mL) of 100g sand and time t (min) is shown in Table 3.

[0099] Table 3. Relationship between cumulative water absorption of 100g river sand and time

[0100] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 0.2 0.4 0.5 0.6 0.7 0.8 0.9 0.9 0.9

[0101] (3) Calculate the first water absorption rate and the second water absorption rate.

[0102] k1=10k(6)=10×0.08=0.8mL / kg;

[0103] k2=10k(270)=10×0.9=9mL / kg.

[0104] (4) Calculate the hysteresis water absorption of sand.

[0105] Δ = 10 -6 ×(9-0.8)×2=1.64×10 -5 m 3 / kg.

[0106] (5) Calculate the air content of hardened cement mortar

[0107]

[0108] Example 2

[0109] (1) From the above, w = 0.45, x = 2, y = 0.01; ρ c =3100, ρ w =1000, ρ s =2600, ρ p =1020; Γ=1970, first time t1=6min; second time t2=270min;

[0110] (2) Test the relationship between the cumulative absorption of 100g of sand and time.

[0111] Example 2 uses manufactured sand. The cumulative water absorption of the manufactured sand at different time points was tested. The relationship between the cumulative water absorption k (mL) of 100g sand and time t (min) is shown in Table 4.

[0112] Table 4. Relationship between cumulative water absorption of 100g of manufactured sand and time

[0113] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 0.6 1.1 1.5 1.8 2 2.2 2.3 2.3 2.3

[0114] (3) Calculate the first water absorption rate and the second water absorption rate.

[0115] k1=10k(6)=10×0.24=2.4mL / kg,

[0116] k2=10k(270)=10×2.3=23mL / kg.

[0117] (4) Calculate the hysteresis water absorption of sand.

[0118] Δ = 10 -6 ×(23-2.4)×2=4.12×10 -5 m 3 / kg.

[0119] (5) Calculate the air content of hardened cement mortar

[0120]

[0121] Example 3

[0122] 1) From the above, w = 0.45, x = 2, y = 0.01; ρ c =3100, ρ w =1000, ρ s =2600, ρ p =1020; Γ=1970, first time t1=6min; second time t2=270min.

[0123] (2) Test the relationship between the cumulative water absorption of 100g of sand and time.

[0124] Example 3 uses recycled sand from waste bricks (a type of recycled fine aggregate) to test the cumulative water absorption of the recycled sand at different time points. The relationship between the cumulative water absorption k (mL) of 100g sand and time t (min) is shown in Table 5.

[0125] Table 5. Relationship between cumulative water absorption of 100g of recycled sand from waste bricks and time.

[0126] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 2.0 3.3 4.4 5.2 6.5 7.2 8.0 8.4 8.8

[0127] (3) Calculate the first water absorption rate and the second water absorption rate.

[0128] k1=10k(6)=10×0.8=8mL / kg,

[0129] k2=10k(270)=10×8.6=86mL / kg.

[0130] (4) Calculate the hysteresis water absorption of sand.

[0131] Δ = 10 -6 ×(86-8)×2=1.56×10 -4 m 3 / kg.

[0132] (5) Calculate the air content of hardened cement mortar

[0133]

[0134] Comparative Example 1

[0135] For Comparative Example 1, neglecting the delayed water absorption of sand, the formula for calculating its air content (according to JGJ / T70–2009 "Standard for Test Methods of Basic Performance of Building Mortar" "13.2 Density Method") is as follows:

[0136]

[0137] The calculation process is as follows:

[0138]

[0139] The air content of cement mortar calculated using conventional techniques is 11.7%, which means that the air content of the cement mortar mixture is 11.7%; it can also be understood as: assuming that the sand no longer absorbs water after the mortar mixture content test is completed, the air content after hardening is 11.7%.

[0140] In some cases, for the same type of sand, the cumulative water absorption has the same relationship with time, but different amounts of sand result in different delayed water absorption, and the air content difference of the prepared mortar before and after hardening is also different. Therefore, Examples 4 to 6 and Comparative Example 2 are provided below for specific illustration.

[0141] In Examples 4 to 6 and Comparative Example 2, the cement mortar mix ratio was known to be cementitious material: water: sand: admixture = 100:45:300:1. The densities of the aforementioned raw materials were also known to be 3100, 1000, 2600, and 1020 (unit: kg / m³). 3 (where sand is the apparent density). After the mortar was mixed, the apparent density was measured to be 2030 kg / m³ according to the "13.2 Density Method" of JGJ / T 70–2009 "Standard for Test Methods of Basic Performance of Building Mortar". 3The test was completed 6 minutes after water was added and mixed; according to the "8 Setting Time Test" of JGJ / T70–2009 "Standard for Test Methods of Basic Performance of Mortar", the setting time was measured to be 270 minutes.

[0142] Example 4

[0143] Examples four through six all take into account the delayed water absorption of sand and use the method of the present invention to calculate the air content.

[0144] (1) Calculate the theoretical apparent density of the mortar.

[0145] From the above, we get w = 0.45, x = 3, y = 0.01; ρ c =3100, ρ w =1000, ρ s =2600, ρ p =1020;;Γ=2030, first time t1=6min; second time t2=270min.

[0146] (2) Test the relationship between the cumulative water absorption of 100g of sand and time.

[0147] Example 4 uses river sand (a type of natural sand) to test the cumulative water absorption of river sand at different time points. The relationship between the cumulative water absorption k (mL) of 100g sand and time t (min) is shown in Table 6.

[0148] Table 6. Relationship between cumulative water absorption of 100g river sand and time

[0149] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 0.2 0.4 0.5 0.6 0.7 0.8 0.9 0.9 0.9

[0150] (3) Calculate the first water absorption rate and the second water absorption rate.

[0151] k1=10k(6)=10×0.08=0.8mL / kg,

[0152] k2=10k(270)=10×0.9=9mL / kg.

[0153] (4) Calculate the hysteresis water absorption of sand.

[0154] Δ = 10 -6 ×(9-0.8)×3=2.46×10 -5 m 3 / kg.

[0155] (5) Calculate the air content of hardened cement mortar

[0156]

[0157] Example 5

[0158] (1) From the above, we get w = 0.45, x = 3, y = 0.01; ρ c =3100, ρ w =1000, ρ s =2600; ρ p =1020; Γ=2030, first time t1=6min; second time t2=270min.

[0159] (2) Test the relationship between the cumulative water absorption of 100g of sand and time.

[0160] Example 5 uses manufactured sand. The cumulative water absorption of the manufactured sand at different time points was tested. The relationship between the cumulative water absorption k (mL) of 100g sand and time t (min) is shown in Table 7.

[0161] Table 7. Relationship between cumulative water absorption of 100g of manufactured sand and time.

[0162] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 0.6 1.1 1.5 1.8 2 2.2 2.3 2.3 2.3

[0163] (3) Calculate the first water absorption rate and the second water absorption rate.

[0164] k1=10k(6)=10×0.24=2.4mL / kg,

[0165] k2=10k(270)=10×2.3=23mL / kg.

[0166] (4) Calculate the hysteresis water absorption of sand.

[0167] Δ = 10 -6 ×(23-2.4)×3=6.18×10 -5 m 3 / kg.

[0168] (5) Calculate the air content of hardened cement mortar

[0169]

[0170] Example 6

[0171] (1) From the above, we get w = 0.45, x = 3, y = 0.01; ρ c =3100, ρ w =1000, ρ s =2600, ρ p =1020; Γ=2030, first time t1=6min; second time t2=270min.

[0172] (2) Test the relationship between the cumulative water absorption of 100g of sand and time.

[0173] Example 6 uses recycled sand from waste bricks (a type of recycled fine aggregate) to test the cumulative water absorption of the recycled sand at different time points. The relationship between the cumulative water absorption k (mL) of 100g of sand and time t (min) is shown in Table 8.

[0174] Table 8. Relationship between cumulative water absorption of 100g of recycled sand from waste bricks and time.

[0175] t / min 0 15 30 45 60 90 120 180 240 300 k(t) / mL 0 2.0 3.3 4.4 5.2 6.5 7.2 8.0 8.4 8.8

[0176] (3) Calculate the first water absorption rate and the second water absorption rate.

[0177] k1=10k(6)=10×0.8=8mL / kg,

[0178] k2=10k(270)=10×8.6=86mL / kg.

[0179] (4) Calculate the hysteresis water absorption of sand.

[0180] Δ = 10 -6 ×(86-8)×3=2.34×10 -4 m 3 / kg.

[0181] (5) Calculate the air content of hardened cement mortar

[0182]

[0183] Comparative Example 2

[0184] Comparative Example 2, without considering the delayed water absorption of sand, uses the following formula for calculating its air content (according to JGJ / T70–2009 "Standard for Test Methods of Basic Performance of Building Mortar" "13.2 Density Method"):

[0185]

[0186] The calculation process is as follows:

[0187]

[0188] The air content of cement mortar calculated using conventional techniques is 11.9%, which means that the air content of the cement mortar mixture is 11.9%; it can also be understood as: assuming that the sand no longer absorbs water after the content test of the mortar mixture is completed, then the air content after hardening is 11.9%.

[0189] See Figure 1 The effect of sand performance parameters on the air content of hardened mortar is shown when the air content of the mortar mixture is constant. Figure 1It can be seen that the relative amount of sand (x) and the water absorption rate (k2–k1) per unit mass of sand during the period from the completion of the apparent density test of the mortar mixture to the setting of the mortar have a decisive influence on the accuracy of the calculated air content of the hardened mortar. When the relative amount of sand is constant, the more water absorbed per unit mass of sand during the t1–t2 period, the greater the decrease in air content of the mortar during this period (Examples 1 to 3, or Examples 4 to 6); when the water absorption per unit mass of sand remains constant during the t1–t2 period, the greater the relative amount of sand, the greater the decrease in air content of the mortar during this period (see Examples 1 and 4, or Examples 2 and 5, or Examples 3 and 6). From Figure 1 It can also be seen that ignoring the physical quantity (k2–k1) can lead to the mistaken assumption that the air content of cement mortar is equal before and after hardening. Using conventional techniques can result in an overestimation of the air content of hardened cement mortar, with the largest miscalculation reaching nearly 11 percentage points. Generally, for every 1 percentage point increase in the air content of cementitious base materials, the strength decreases by 5 percentage points. An 11 percentage point deviation in air content is sufficient to cause a significant misjudgment of the mortar's strength and durability.

[0190] The key takeaway of this invention is that recycled fine aggregates and certain manufactured sands exhibit significant water absorption lag. For these sands, in addition to testing the apparent density of the cement mortar mixture, sand samples should be taken to measure the cumulative water absorption of the sand over time, thereby obtaining reliable air content of the hardened mortar.

[0191] Based on the "13.2 Density Method" of JGJ / T 70–2009 "Standard for Test Methods of Basic Properties of Mortar", this invention introduces the water absorption rate test of sand and the setting time test of cement mortar, and modifies and improves the density method. It realizes the scientific calculation of the content of hardened cement mortar, explains the reason for the inconsistency of air content before and after hardening, and solves the problem of inaccurate evaluation of the air content after hardening by using the air content of the mixture. It helps to more accurately judge the strength and frost resistance of hardened cement mortar.

[0192] The method for predicting the air content of hardened cement mortar in this invention, without changing the testing principles stipulated in industry standards, solves the problem of overestimation of the air content of hardened cement mortar by simply adding two material performance indicators (the trend of cumulative water absorption of sand over time and the setting time of cement mortar). This significantly improves efficiency and accuracy, helps engineering managers to quickly and accurately assess the quality of cement mortar, and plays a positive role in improving the safety and durability of building structures.

[0193] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A method for predicting the air content of hardened cement mortar, characterized in that, Includes the following steps: To obtain the mix proportion of cement mortar, the raw materials of the cement mortar include cement-based cementitious materials, water, sand and admixtures, and the mix proportion is 1:w:x:y, where w is the amount of water, x is the amount of sand, and y is the amount of admixtures. The density of the raw materials for cement mortar is obtained, wherein the density of the sand is the apparent density of the sand. Establish a trend model for the cumulative water absorption of sand over time. The trend model is k = k(t), where k is the cumulative water absorption per 100g of sand and t is time. The actual apparent density of the cement mortar was tested, and the first time interval was recorded. The first time interval is the time from the addition of water and mixing to the completion of the apparent density test. The formula for calculating the actual apparent density is: Where Γ is the actual apparent density, m is the mass of the cement mortar mixture, and V is the volume of the cement mortar mixture; The setting time of the cement mortar was tested, and a second time of the cement mortar was recorded, which is the time interval from the addition of water and mixing to the setting of the cement mortar. Based on the relationship between the cumulative water absorption and time, the first time, and the second time, the first water absorption rate and the second water absorption rate of the sand are obtained by linear interpolation. The first water absorption rate is the volume of water absorbed by a unit mass of sand up to the first time, and the second water absorption rate is the volume of water absorbed by a unit mass of sand up to the second time. k1=10k(t1), k2=10k(t2), Where k1 is the first water absorption rate and k2 is the second water absorption rate; Based on the first and second water absorption rates, the hysteretic water absorption of the sand is calculated using the following formula: Δ = 10 -6 (k2-k1)x, where Δ is the volume of water absorbed by the sand with a hysteresis absorption of x parts by mass during the interval between the first time and the second time; The air content of the hardened cement mortar was calculated as follows: Where, ρ c ρ is the density of the cementitious material. w ρ is the density of water. s ρ is the apparent density of sand. p This refers to the density of the admixture.

2. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, The cement-based binder is cement.

3. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, The cement-based binder is a mixture of cement and mineral admixtures.

4. The method for predicting the air content of hardened cement mortar according to claim 3, characterized in that, The mineral admixture is at least one of fly ash, granulated blast furnace slag powder, and silica fume.

5. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, The sand is at least one of natural sand, manufactured sand, and recycled fine aggregate, and the sand is dried sand.

6. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, The apparent density of the sand was obtained based on the test in GB / T 14684–2022 "Sand for Construction".

7. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, When obtaining the apparent density of the cement mortar, the apparent density of the cement mortar was obtained by testing according to JGJ / T 70–2009 "Standard for Test Methods of Basic Properties of Mortar".

8. The method for predicting the air content of hardened cement mortar according to claim 1, characterized in that, When obtaining the setting time of the cement mortar, the setting time of the cement mortar in real time is obtained by testing according to JGJ / T 70–2009 "Standard for Test Methods of Basic Performance of Mortar".

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