Effect of Curing Temperature on Crack Resistance of Low-Heat Portland Cement Hydraulic Lining Concrete

1. Experiment

1.1. Raw Materials and Mix Proportion

1.1.1. Raw Material

The raw material used in this test is 42.5 low heat Portland cement (hereinafter referred to as low heat cement), number is LHC. Class F Grade I fly ash. Secondary natural sand, small stone, medium stone. Standard superplasticizer, air entraining agent. The solid content of water reducer is 15.48 %, The solid content of air-entraining agent is 58.5 %. Cement testing is as follows Table 1、Table 2. The coarse aggregate is broken pebbles，The fine aggregate is natural sand. Aggregate test results are shown in Table 3.

1.1.2. Mix Proportion

This test is based on the absolute volume method in SL/T352-2020《 Hydraulic Concrete Test Procedures》. The indoor laboratory optimizes the allocation of C₉₀35 secondary distribution pumping concrete, Control the slump at 180~220mm. The gas content is controlled at 4.0%~5.0%. Fluidity and cohesion are better. Low heat Portland cement concrete is numbered DRH. As shown in Table 4.

1.1.3. Test Scheme

This test passed standard curing (20°C), The quasi-environmental temperature box maintains the specimen according to the actual temperature on site. The standard curing specimen number is DRH-I. The specimen number of quasi-environmental maintenance is DRH-II. The first 30 days of age were cured at different temperatures, The same standard maintenance is carried out in the later 60 days. The curing temperature is shown in Figure 1. Seven age tests of compressive strength, elastic modulus, splitting tensile strength and axial tensile strength were carried out by DRH. Through N-S maturity calculation formula, F-P equivalent age calculation formula and D-L equivalent age calculation formula, three maturity calculation functions are obtained. Three fitting functions of exponential function, logarithmic function and hyperbolic function are fitted. Determine the most accurate fitting relationship between maturity and strength, Verify the relationship between strength and maturity. The macroscopic failure mechanism was further explained by XRD, SEM and other microscopic means.

2. Experiment Results and Analysis

2.1. Test Result

2.1.1. Compressive Strength

Two maintenance methods, The compressive strength of concrete meets the design requirements. Figure 2 shows the compressive strength of concrete with two curing methods on the left side. The simulated temperature is lower than the standard temperature before 1 day of curing temperature. So DRH-II is 1.7 MPa lower than DRH-I. As the temperature of the quasi-environmental maintenance box rises. At the age of 3d and 7d, DRH-II was 1.3MPa and 2.1MPa higher than DRH-I, respectively. With the decrease of temperature, The intensity value of DRH-II is lower than that of DRH-I. Later curing at the same temperature, The difference between the two strength values is small. The right side of Fig.2 is the growth rate of compressive strength. Based on 28d, DRH-II was 3.6% lower than DRH-I at 1d age. At 3d and 7d ages, DRH-II was 5.7% and 7.1% higher than DRH-I, respectively. The growth rate is roughly the same when curing at the same temperature in the later period. At 90 days of age, DRH-II was 11% lower than DRH-I. On the whole, the strength development and strength growth rate change with the increase and decrease of temperature. It can also be concluded that the early strength growth of DRH is better than the experimental value. It is beneficial to crack resistance of concrete at early age.

2.1.2. Elastic Modulus

Elastic modulus is also an important index in the mechanical properties of concrete. This test is maintained by two curing temperature methods. The corresponding elastic modulus of concrete and its growth rate are obtained Figure 3. The left side is the elastic modulus of concrete. Basically the same as the development of compressive strength. Among them, 3d、7d age，DRH-II is 2.5GPa and 1GPa higher than DRH-I, respectively. On the right side is the growth rate of elastic modulus of concrete. Taking 28d age as the reference value. Among them, 3d、7d age,The growth rate of DRH-II was 11.4% and 6.6% higher than that of DRH-I, respectively. As the temperature decreases, the growth rate also decreases. The growth rate of DRH-II is still lower than that of DRH-I when cured at the same temperature. At the age of 90 days, the difference between the two is 2.3%. Combined with the compressive strength,It is found that the temperature change has an effect on the compressive index of concrete mechanical properties.

2.1.3. Splitting Tension

Splitting tensile strength is also one of the crack resistance indexes of concrete. The left side of Figure 4 is the splitting tensile data value. The simulated temperature is high at 3d and 7d ages. The strength of DRH-II is 0.3MPa and 0.3MPa higher than that of DRH-I. The overall strength increases and decreases with the rise and fall of temperature. The right side of Fig.4 is the growth rate of splitting tensile strength. Taking 28d age as the reference value. The early growth rate is basically the same as the compressive growth rate. The growth rate of DRH-II was 11.4%, 12% and 3.5% higher than that of DRH-I at 3d, 7d and 14 d. But although the later stage is the same temperature maintenance, But the growth rate of DRH-II is still higher than that of DRH-I. This is because relative to the simulated temperature of 28d. The temperature rose relatively in the later period. This reflects that the splitting tensile is more sensitive to temperature changes.

2.1.4. Axial Tensile Strength

The axial tensile strength of concrete is very important to the crack resistance of concrete. The left side of Figure 5 is the axial tensile strength of concrete. The strength of DRH-II was 0.1MPa and 0.21MPa higher than that of DRH-I at 3d and 7d ages. On the right side is the growth rate of axial tensile strength of concrete. The age of 28d is the reference value. The growth rate of DRH-II was 2.8% and 9.7% higher than that of DRH-I at 3d and 7d. In the later period, the growth rate decreased due to the decrease of temperature. But at the age of 90d, The growth rate of DRH-II was 1.3% higher than that of DRH-I. Combined with splitting tensile strength, It shows that the temperature change is more sensitive to the tensile strength of concrete.

2.2. Maturity Theoretical Analysis

Combined with the above, we can conclude that the increase of curing temperature will increase the strength of concrete. To further analyze the relationship between curing temperature and strength.Next, we will use three maturity calculation formulas to calculate the maturity index. And through the exponential function, logarithmic function, hyperbolic function three kinds of function fitting. The highest precision is selected to represent the relationship between maturity and strength development.

2.2.1. Maturity Indicator

Maturity means that the strength growth of concrete is the result of the combined influence of temperature and time. And the strength is a function of the product of temperature and time.

N-S maturity calculation formula [17]:

$$\mathrm{M}=\sum _0^{\mathrm{t}}(\mathrm{T}-{\mathrm{T}}_0)∆\mathrm{t}$$

(1)

M——Maturity,℃ ·ｄ;T——Actual temperature of concrete，℃;Ｔ₀——Reference temperature(-10℃);$∆\mathrm{t}$——Curing age,d;

P equivalent age [18]:

$$T_e=\sum _0^{t}exp\left[{\displaystyle \frac{E}{R}}\right({\displaystyle \frac{1}{273+T_c}}-{\displaystyle \frac{1}{273+T}}\left)\right]∆t$$

(2)

T_e——Equivalent age,d;E——Activation energy,J/ｍol(Ｔ≥20℃,Ｅ＝33500J/mol;Ｔ＜20℃,Ｅ＝33500+1470(20-T)J/mol));R——Gas constant(8.3144j/(mol·K));T_c——Reference temperature(20℃);T——Actual temperature of concrete,℃;$∆t$——Time interval,d.

L equivalent age [19]:

$$\mathrm{t}=\sum {\mathsf{\alpha }}_{\mathrm{T}}{\mathrm{t}}_{\mathrm{T}}$$

(3)

T——Equivalent age,d;${\mathsf{\alpha }}_{\mathrm{T}}$——The temperature is the equivalent coefficient of T;${\mathrm{t}}_{\mathrm{T}}$——Duration of temperature T,h.

Check the specification can be obtained at 20 °C, The D-L equivalent coefficient is 1.0. The rest also has the specification to know.（1）-（3）, The maturity index at different curing temperatures can be calculated. The calculation results of maturity index are as follows: Table 5. The F-P equivalent age and D-L equivalent age are the same in standard curing. Curing according to on-site temperature, The equivalent age calculated by the F-P equivalent age calculation model and the D-L equivalent age calculation model is relatively close.

2.2.2. The Relationship Between Maturity and Strength

After the maturity value is obtained, the maturity-strength relationship fitting equation can be determined. The exponential, logarithmic and hyperbolic curves were used to fit the compressive strength, splitting tensile strength, axial tensile strength and maturity respectively.

Exponential function, Freiesleben and Pedersen [20], It is found that there is a relationship between hydration heat and maturity between maturity and strength:

$$\mathrm{S}={\mathrm{S}}_{\mathrm{\infty }}{e}^{{-\left({\displaystyle \frac{\mathsf{\tau }}{\mathrm{M}}}\right)}^{\mathrm{a}}}$$

(4)

S——Concrete strength,MPa;M——Maturity,(℃.h) or h;${\mathrm{S}}_{\mathrm{\infty }}$——Final strength of concrete,MPa;$\mathsf{\tau }$——Temporal characteristic parameters,(℃.h) or h;a——Shape factor.

Logarithmic function, Plowman [21], It is proposed based on the linear relationship between strength and maturity:

$$\mathrm{S}=\mathrm{a}+\mathrm{b}\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{M}\right)$$

(5)

S——Concrete strength,MPa;M——Maturity,(℃.h) or h;a, b are obtained by function fitting.

Hyperbolic function,Kee [22],It is found that the relationship between maturity and strength can also be expressed by hyperbola:

$$\mathrm{S}={\displaystyle \frac{\mathrm{M}}{\mathrm{m}\mathrm{M}+\mathrm{n}}}$$

(6)

S——Concrete strength,MPa;M——Maturity,(℃.h) or h;m,n are obtained by function fitting.

2.2.3. The Establishment and Analysis of Function Model

Using fitting software, （4）-（6）, The relationship curve between compressive strength and maturity of low heat Portland cement concrete can be obtained, Figure 6. Relationship curve between splitting tensile strength and maturity, Figure 7. Relationship curve between axial tensile strength and maturity, Figure 8. Relationship curve between elastic modulus and maturity, Figure 9. Overall, The compressive strength, splitting tensile strength, axial tensile strength and elastic modulus of DRH-I and DRH-II increase with the increase of maturity index. The fitting curves obtained by the three fitting methods of F-P equivalent age and D-L equivalent age are roughly consistent. It shows that both can accurately predict the development law of concrete strength.

The hyperbolic function and logarithmic function in the fitting relationship between maturity and strength development can be well fitted. The correlation coefficients of hyperbolic function in the relationship between compressive strength and maturity index are 0.93, 0.95 and 0.94. The correlation coefficients of logarithmic function are 0.90, 0.92 and 0.91 respectively. The fitting correlation coefficients of splitting tensile strength and maturity hyperbolic function are 0.96, 0.99 and 0.98 respectively. The correlation coefficients of logarithmic function are 0.93, 0.98 and 0.97. The correlation coefficients of axial tensile and maturity hyperbolic curves are 0.92, 0.97 and 0.95, respectively. The correlation coefficients of logarithmic function are 0.92, 0.95 and 0.95. The fitting coefficients of elastic modulus and maturity hyperbolic curve are 0.92, 0.95 and 0.94 respectively. The correlation coefficient of logarithmic function is 0.9, 0.92, 0.91. The correlation coefficient of exponential function fitting is below 0.90. Effect is not good.The fitting effect of hyperbola is the best,Logarithmic function fitting followed. Therefore, the hyperbolic function fitting model is the most accurate. It can be seen from the relationship between the four indexes of concrete mechanical properties and maturity, The fitting relationship between splitting tensile strength and maturity is the best. Combined with the variable temperature curing test can be concluded that, The splitting tensile strength is most affected by temperature change.Therefore. In practice, the influence of splitting tensile on concrete should be considered more.

2.3. Microscopic Test

This microscopic test selected 0.36 water-binder ratio. The curing age is 7d, 28d and 90d. X-ray diffraction ( XRD ), scanning electron microscopy ( SEM ) and energy spectrum analysis methods were used to analyze the cementitious system of low heat cement with 25% fly ash. The microstructure, hydration products and long-term Ca(OH)₂ content of low-heat cement were analyzed.

2.3.1. X-Ray Diffraction Analysis

The phase analysis of low heat cement at different curing ages was carried out by XRD. The Ca(OH)₂ in hydration products and C₂S in low heat cement were investigated. The XRD pattern was drawn in the range of 2θ angle 10-80°. As shown in Figure 10. On the left side is the XRD spectrum of 7d, 28d, 90d low heat cement. The right side is the main Ca(OH)₂ and C₂S characterization diagram of the characteristic peaks in the left XRD spectrum. The C₂S diffraction peaks were found near 25°and 45°by long-age hydration. The diffraction peaks of Ca(OH)₂ were found near 20°and 32°. C₂S is the main support point for the high strength of low heat cement in the later stage. Ca(OH)₂ also plays a role in filling pores in cement. To make the low heat concrete structure more compact,Ensure the later strength development, It is beneficial to the long-term crack resistance of lining structure.

2.3.2. Scanning Electron Microscope

The microstructure characteristics and distribution of hydration products of low-heat cement at different curing ages were analyzed by scanning electron microscopy. The results are shown in Figure 11, Figure 12 and Figure 13. It is found that Ca(OH)₂ in low heat cement is mostly stacked in flakes. Good compactness,From the early stage to the later stage, It is surrounded by C-S-H gel. With low heat cement high content of C₂S,Later with the hydration reaction, The more the C-S-H gel develops, the more it develops. Can fill the pores, It can make the concrete structure more stable. A considerable amount of Ca(OH)₂ is retained. Ensure the later strength development, It plays an important role in crack resistance of concrete.

2.3.3. Energy Spectrum Analysis

The cementitious system of low-heat cement with 25% fly ash at different curing ages was tested by energy spectrum analysis(EDS). As shown in Figure 14, Figure 15 and Figure 16. It was found that the mass percentage of Ca element was 37.95%——50.45%. The atomic percentage is 20.66%——29.9%. The mass percentage of Si element is 7.05%——11.34%. The atomic percentage is 5.96%——8.81%. C-S-H gel can be formed in the long-term hydration process. The concrete structure is more compact, Ensure late strength. There is still a certain amount of Mg element in 90d, Generating MgO, It has a slight expansion effect on concrete. Inhibitory contraction,Improve the durability of concrete.

3. Conclusions

• This experiment found that temperature has an effect on the mechanical properties of concrete. The compressive strength, splitting tensile strength, axial tensile strength, elastic modulus and its growth rate of standard curing and quasi-environmental curing increase with the increase of temperature. The temperature decreases and decreases. The temperature change has the greatest influence on the splitting tensile strength of concrete.
• This test found that the difference of concrete performance change is within the acceptable range. It shows that the research done by the laboratory has a strong reference significance for the actual situation on the spot.
• The hyperbolic fitting function based on F-P equivalent age and D-L equivalent age can well predict the development law between maturity and strength. Strength increases with the increase of maturity.
• Microscopic experiments show that there is a large amount of C-S-H gel in the later stage of low heat cement. Compact structure, And has the effect of micro expansion, It is beneficial to the development of mechanical properties of low heat cement concrete in the later stage.