Effect of super absorbent polymer mixing method on shrinkage performance of cast-in-place concrete confined by existing concrete

To study the effect of different super absorbent polymer (SAP) mixing methods on mechanical and shrinkage properties of internal curing concrete, SAP mixing mode was used as the study variable, and simulated the bridge leveling structure. The flow performance test, mechanical performance test, free shrinkage test, restrained shrinkage test and scanning electron microscope observation test were carried out on the internal curing concrete. The influence of different mixing methods on the flow properties, mechanical properties, shrinkage properties and microstructure of internal curing concrete were analyzed. The results showed that the amount of mixing water had a great influence on the fluidity when adding pre-absorbent SAP; The effect of SAP on the mechanical properties is minimal when the dosage is 0.2% of the mass of cementing material; Pre-absorbent SAP has the best effect on improving shrinkage cracks of concrete; SAP stored water at the initial stage of hydration, promoted the generation of hydrolysis products, prolong the hydration process, filled the gap inside the concrete, and improved the anti-cracking performance. It is recommended that SAP should be partially pre-absorbed.


Introduction
With the continuous advancement of transportation engineering, bridges, as fundamental components of transportation infrastructure, have played a crucial role in supporting social and economic development [1][2][3].Nevertheless, due to daily traffic loads and environmental conditions [4,5], the issue of shrinkage and cracking in the concrete leveling layer of bridges is increasingly severe [6], significantly impacting the service life of bridges.During bridge usage, shrinkage cracks frequently develop in the concrete leveling layer adjacent to the prefabricated box girder [7,8], resulting in issues such as debonding and bulging between the bridge deck pavement and the structural layer.Due to the complexities involved in reinforcing and repairing bridge structures, these issues significantly compromise bridge safety [9].Therefore, effectively mitigating the likelihood of concrete shrinkage cracks [10], reducing debonding between the bridge deck pavement layer and the structural layer, and extending the service life of concrete bridges have emerged as pressing technical challenges requiring immediate attention.
Aiming at the shrinkage cracking problem of bridge concrete, most researchers have adopted the method of adding admixture into concrete to alleviate the early cracking of concrete.Internal curing concrete with SAP as internal curing agent has become a new trend of structural self-curing [11,12].Jensen 's research [13] on the selfshrinking property of SAP cement mortar showed that SAP could effectively improve the self-shrinking deformation of the cement paste, and even slightly expand in the initial hardening stage.Igarashi et al [14] studied the shrinkage performance of SAP internal curing concrete, found that SAP could SAP could significantly relieve contraction, and some concrete would not shrink or slightly expand when SAP was excessive.Wang et al [15][16][17][18] tested the shrinkage of internal curing concrete in different environments, the results showed that the self-shrinkage and drying shrinkage of concrete were significantly improved after adding SAP, but the shrinkage reduction effect was weakened in strong alkali environment, resulting in early cracking of cement base.Qin et al [19] studied the particle size and dosage of SAP, found that when the particle size of SAP is 100 ∼ 250 μm and the dosage is 0.3% ∼ 2.0% of the mass of cementing material, the internal self-drying and shrinkage of cement base can be improved significantly.
At present, scholars in various countries have conducted a lot of research on the mechanical properties of internal curing concrete.Assmann [20] studied the hydration degree, internal porosity and compressive strength of internal curing concrete.It is proved that SAP not only improves the hydration degree of cement, but also produces pores inside the concrete, and the strength improvement caused by the increase of hydration degree is weaker than the strength reduction caused by SAP producing pores inside the cement base.SAP pre-absorption or direct addition of water has an adverse effect on the 28d strength of concrete.Beushausen et al [21] learned the effect of environmental relative humidity on the compressive strength of internal curing concrete.It is found that when the environmental relative humidity was 65%, the compressive strength increased by 19%, and the permeability coefficient of concrete decreased by 30%.Huang et al [22] studied the effect of SAP on mechanical properties and shrinkage properties under different mixing methods, and found that partial pre-absorption method of incorporation had the least influence.
In summary, the addition of internal curing agents can decrease concrete shrinkage [23].However, the majority of existing research focuses solely on measuring concrete shrinkage performance under unconstrained conditions, neglecting the shrinkage reduction effects under the constrained conditions typical of aged concrete.Furthermore, the current understanding of the internal curing mechanism of SAP in concrete mixtures remains inadequate, and the calculation of water requirement for SAP pre-absorption lacks accuracy, consequently contributing to a general decline in the strength of internally cured concrete [11,12,24].
In summary, the effectiveness of SAP in improving the shrinkage cracking performance of leveling layer concrete under various incorporation methods remains unclear.Furthermore, further research is needed to investigate the influence of the amount of pre-absorbed water in SAP on mortar fluidity and concrete strength.Therefore, this study designed various SAP mixing methods based on the SAP absorption characteristic curve and investigated their effects on the flow properties, mechanical properties, and shrinkage properties of internal curing concrete.Simultaneously, the study simulated a bridge leveling layer structure to evaluate the restrained shrinkage performance of internal curing concrete under the constraints of double-layer concrete and circular forms.The restrained shrinkage cracking stress of internal curing concrete was calculated, and the likelihood of shrinkage cracking was assessed.The curing mechanism of SAP under different mixing methods was elucidated by scanning the microstructure of concrete.

Aggregate
The size of the coarse aggregate ranges from 4.75 mm to 19.5 mm.The aggregate mixing ratio between 4.75-9.5 mm and 9.5-19.5 mm aggregates is 4:6.The crushing value is 17.5%, the needle-like particle content is 4.3%, and the mud content is 0.12%.The fine aggregate consists of machine-made sand with a fineness modulus of 2.5, classified as medium sand.

Internal curing agent
A superabsorbent polymer (SAP) was utilized as an internal curing agent.This particular SAP consists of highly cross-linked sodium acrylate, produced through aqueous solution polymerization with a 0.06% cross-linking agent.The average particle size of the SAP is 106 μm.
Researchers [25,26] have proposed measuring the water absorption and release characteristic curves of SAP in a cement centrifuge.The findings indicate that the adsorption and release processes of SAP in cement centrifugal solutions can be broadly categorized into three stages.In the first stage, the rapid water absorption phase occurs, with the absorption rate quickly escalating to its maximum.During the second stage, the slow water release phase is observed, where the liquid absorption rate gradually decreases to a stable level.The final stage is characterized by stability, where the liquid absorption rate stabilizes and ceases to change.Concurrently, studies [27] have demonstrated that the pore solution concentration in cement slurry experiences minimal fluctuation before the final setting, maintaining at approximately 0.7 mol l −1 .
Drawing on the aforementioned research theories, a 0.7 mol l −1 sodium chloride solution was employed to simulate the cement centrifugal solution in this experiment, and the tea-bag method [25,28,29] was utilized to determine the water absorption characteristic curve of SAP.Initially, SAP is pre-saturated in water to ascertain the maximum water absorption rate.Subsequently, the pre-saturated SAP is immersed in a sodium chloride solution to measure and determine the water storage and release rates upon reaching equilibrium within the solution.By integrating the water absorption curve in water (ascending phase) with the water release curve in a cement-based centrifuge (descending phase), the suction characteristic curve of SAP was derived.
The SAP absorption characteristic curve, as measured, is depicted in figure 1.The SAP absorption ratio ultimately stabilized at approximately 143 times its own mass.Following the release of water by the saturated SAP-equivalent to 108 times its own mass-in the cement-based centrifuge, only 35 times its own mass in water remained stored.

Mixing water and admixture
Deionized water was utilized as the test water.
The superplasticizer used was of the polycarboxylate type, exhibiting a water reduction rate of 30% and a slump retention value (after 1 h) of at least 140 mm.

Mix design
Set cement mass of concrete unit cubic meter as C, water-cement ratio as w, maximum water absorption rate of polymer as a w and maximum water storage rate as , d w then the unit curing water quantity w i of internal curing concrete and the unit polymer content m SAP can be calculated according to the following formula: The calculation results show that the theoretical content of super absorbent polymer is about 0.2% of the cementitious material.Considering the deviation between the theoretical calculation and the actual test, the test content of the polymer was adjusted to 0.1%, 0.2% and 0.3%.
Based on the liquid absorption characteristic curve of SAP, five mixing methods and three dosages are set for SAP.The five incorporation methods are shown in table 1.In the table Based on different incorporation methods, the concrete mix ratio of each group is designed, as shown in table 2.

Preparation and curing method of internal curing concrete
The balance, concrete mixer, concrete test mold, shaking table, and other equipment used in the preparation of concrete, in accordance with the relevant regulations.The internal curing concrete is prepared according to the following steps: (1) The dry SAP is calculated and weighed according to the cement content in the concrete mix ratio for the leveling layer.For the Y0, Y1, and Y2 groups, which were mixed with pre-absorbed SAP, the corresponding quantities of pre-absorption water were weighed.The SAP was then allowed to pre-absorb for one hour in a pre-wetted beaker.
(2) For the G0 and G1 groups, which were mixed with dried SAP, the aggregate, cement, and dried SAP were dry-mixed for one minute.This was followed by mixing with mixing water and a water-reducing agent for four minutes to ensure even mixing.For the Y0, Y1, and Y2 groups, mixed with pre-absorbed SAP, the aggregate, sand, and cement were initially dry-mixed for one minute to ensure even mixing.Subsequently, water was added and stirred for three minutes.Finally, the fully pre-absorbed SAP was added and stirred for an additional three minutes to achieve uniform mixing.
(3) The evenly mixed concrete mixture is transferred into the plastic test mold in a single action.A spatula is inserted along the inner wall of the mold to ensure that the concrete mixture extends above the top of the (4) Upon completing the vibration process, the specimen was placed in a room maintained at a temperature of 20 ± 5 °C and a relative humidity exceeding 50%.Following hardening, the concrete was demolded and numbered before being transferred to the curing room, which is maintained at a temperature of 20 °C ± 2 °C and a relative humidity exceeding 95%.
2.4.Experimental method 2.4.1.Flow performance The maximum aggregate size utilized in the test was less than 20 mm, with the visually observed slump significantly exceeding 10 mm.The fluidity of the concrete mixture was assessed based on the slump measurement method outlined in the consistency test of the mixture specified in GB/T 50080-2002 [31].
The impact of five SAP mixing methods on the working performance of concrete was examined.SAP content was set at 0.1%, 0.2%, and 0.3% of the cementitious material mass.

Mechanical property
(1) Compressive strength test Following the internal curing concrete preparation method outlined in section 2.3, the mix proportion depicted in table 2, and using cubic samples measuring 150 × 150 × 150 mm with three samples per group, the study focused on the 28-day cube compressive strength of internal curing concrete, in accordance with GB/T 50081-2002 [32].The study examined the variation in compressive strength with SAP contents of 0.1%, 0.2%, and 0.3% of the cementitious material mass.
(2) Splitting tensile strength test Following the preparation method of internal curing concrete outlined in section 2.3, the waterabsorbing resin content was adjusted to 0.2%, and cubic specimens measuring 150 × 150 × 150 mm were prepared, with three specimens in each group.The splitting tensile strength test was conducted in accordance with GB/T 50081-2002 [32].
The splitting tensile strength of standard concrete cubes at various curing ages was assessed.The impact of five SAP mixing methods on the splitting tensile strength of concrete cubes at identical curing ages was investigated.
(3) Elastic modulus test Elastic modulus tests were conducted on the J0, G1, Y1, and Y2 groups.Following the internal curing concrete preparation method outlined in section 2.3 and utilizing 0.2% SAP content, a cube specimen measuring 150 mm × 150 mm × 300 mm was prepared.Each set comprised six specimens, with three designated for measuring the axial compressive strength and the remaining three for assessing the static compressive elastic modulus of the concrete.The test equipment adopts the RMT-301 rock and concrete mechanics test system, and employs a micro-deformation measuring instrument (displacement sensor with a system) to measure specimen deformation.

Plastic shrinkage test of cement mortar
Employing the research method for early plastic shrinkage of thin-layer mortar, shrinkage is directly measured rather than relying on crack parameters.Vertical shrinkage per unit length of mortar within a glass tube serves as the evaluation index for assessing plastic shrinkage performance.
Adopting the research method for early plastic shrinkage in thin-layer mortar, shrinkage is measured directly rather than through crack parameters.The vertical shrinkage per unit length of mortar within a glass tube serves as the evaluation index for assessing plastic shrinkage performance.Plastic shrinkage in internal curing concrete primarily arises from water evaporation and mortar shrinkage, hence, cement mortar plastic shrinkage is utilized for concrete shrinkage assessment.The measurement device is a vertical glass tube, depicted in figure 2. The glass tube has an inner diameter of 30 mm and a length of 1100 mm.Its bottom is sealed with a plastic cap, and its inner wall is coated with oil to minimize friction.The raw material for cement mortar consists of the concrete mix's remainder, excluding the coarse aggregate.Once the cement mortar is loaded, the test tube's top is sealed with a rubber stopper, and a plastic tube is inserted through the center of this stopper.This approach reduces both the contact area with air and the rate of evaporation.Conversely, the shrinkage of the mortar within the straight pipe is indirectly determined by monitoring the liquid level decrease in the plastic tube.The plastic tube is filled with water to create a liquid seal, and a blank reference group without mortar is established to account for the influence of water evaporation.Throughout the experiment, the glass tube remained upright.The decrease in the liquid level inside the tube was measured at consistent intervals, and the shrinkage per unit length of the mortar column was calculated.This process evaluates the impact of adding SAP on the plastic shrinkage properties of cement mortar.

Free shrinkage test of strip concrete
To investigate the free shrinkage of concrete, a specimen measuring 150 × 150 × 550 mm was fabricated, and its free shrinkage properties were analyzed using an integrated temperature and shrinkage test system.The equipment used to measure concrete's free shrinkage includes a FS-NM vibrating wire strain gauge and a vibrating wire data acquisition instrument.During the pouring process, the strain gauge was embedded within the concrete.The acquisition instrument collected the strain gauge's vibration frequency and the concrete's internal temperature changes.The process for measuring free shrinkage is depicted in figure 3.
Prior to pouring concrete, the mold is coated with a layer of release agent.The strain gauge's position is manually secured before the concrete is poured.For the free shrinkage specimen, the strain gauge is positioned at the center, with the wire oriented along the length.Measurement begins once pouring is complete, with the concrete being demolded after a day of curing.After demolding, the free shrinkage concrete specimen is positioned upright to prevent bottom restraint, facilitating the measurement of concrete's free shrinkage.
Following the analysis of collected vibration frequency and temperature data, the free shrinkage of concrete using various water-absorbing resin mixing methods is calculated.

Constrained shrinkage test of strip concrete
The restrained shrinkage measurements are conducted using the strain gauge and data acquisition instrument described in section 2.4.4.Double-layer restrained shrinkage specimens were fabricated in distinct layers.The lower layer consists of an ordinary concrete specimen (150 mm × 150 mm × 550 mm) aged six months, with a cutting machine used to create a groove at the old and new contact interfaces, measuring 2 mm in interval, width, and depth.One day prior to pouring, the old concrete specimen was soaked and moistened, with surface water being wiped off one hour before pouring.A 2 mm layer of interfacial binder is uniformly applied at the contact interface to prevent detachment between the old and new concrete interfaces.The interfacial binder consists of cement and water mixed in a 3:1 ratio.The prepared interface is depicted in figure 4. For comparison with the ring restrained shrinkage test (where the ring specimen's height is 100 mm), the cast-in-place concrete layer was sized at 150 mm × 100 mm × 550 mm.Prior to pouring, the strain gauge was positioned at the center of the interface on the old concrete specimen, with measurements commencing post-pouring.Following a day of curing, the shrinkage of the upper cast-in-place concrete, constrained by the lower aged concrete, was assessed.

Restraint shrinkage and cracking test of ring concrete
Following ASTM C 1581-04 [33], the ring restraint shrinkage test for concrete was conducted.The ring concrete specimen was fabricated using a concrete shrinkage cracking mold.To investigate the shrinkage of annular concrete, a strain gauge and a stress-strain acquisition instrument were utilized to monitor changes in steel ring strain over curing age.The shrinkage behavior of J0, G0, Y1, and Y2 annular concrete under annular constraint was examined.
Annular concrete was prepared with the concrete crack resistance mold depicted in figure 5.The mold's inner ring consists of an integral forming steel ring, while the outer ring comprises two semi-steel rings tightly joined by bolts, and the bottom features a grooved circular steel plate.To assess the stress induced by concrete shrinkage in the inner steel ring, a resistance strain gauge is affixed to its inner surface.The annular shrinkage stress in concrete was indirectly calculated by measuring the deformation captured by the strain gauge [34].
The strain gauge employed in the test was a BE120-3AA (featuring built-in temperature self-compensation), and the instrument for measuring the resistance strain gauge was a static resistance strain gauge.Prior to pouring concrete, a layer of release agent was uniformly applied to both the surface of the bottom plate and the inner surface of the outer ring.Subsequently, the strain gauge was affixed, and the strain acquisition instrument connected to the gauge to measure its initial deformation before concrete pouring, aiming to minimize error.
Following the concrete pour, the moment the concrete enters the mold is documented as the onset of concrete shrinkage.Resistance readings from the strain gauge were collected at 6-hour intervals on the first day and at 12-hour intervals starting from the second day.Each measurement point was assessed thrice during each measurement session.The test continued until the annular concrete exhibited cracking, a significant change in the strain gauge's resistance value was observed, or the resistance value stabilized with the aging of the concrete.

Electron microscope scanning test
After the shrinkage test, each group of concrete specimens were randomly sampled.After crushing, the cement matrix fragments with relatively flat surface were screened out, and the hydration process of internal cement was terminated by soaking in alcohol solution.After drying, the samples were fixed on the sample table with conductive tape to complete the gold spraying.The samples were observed under scanning electron microscope (SEM) at perspectives of 500, 2000, and 10000 times.The equipment used is the SU8020 type of field emission scanning electron microscope.The morphology of the pores left in the matrix, the morphology of the interface between the aggregate and the cement paste, and the structural compactness at the micro-cracks were observed.

Flow performance analysis
Due to the high absorption of SAP, either dry or pre-water absorption will have a significant impact on the flow performance of cement mortar.Based on this premise, the flow performance of SAP internal curing concrete is studied.The concrete slump was measured, as shown in figure 6.
As depicted in figure 6, when the content of SAP constitutes 1% of the cementitious material, the slump of concrete remains consistent.When SAP constitutes 2% of the cementitious material, dry SAP (G0, G1) decreases the slump, while pre-absorbed SAP (Y1, Y2, Y0) increases it, especially Y0.When the SAP content reaches 3% of the cementitious material, compared with the control group, dry SAP significantly reduces the slump, saturated SAP markedly increases it, while partially pre-absorbed SAP has minimal impact.With the increase in SAP content, the moisture content of SAP has a more pronounced influence on fluidity, whereas some pre-absorbed SAP has minimal effect.
Studies have shown that the addition of SAP improves the workability of concrete [35,36].Dry SAP absorbs mixing water from the mortar and decreases the water-binder ratio [37], leading to an increase in yield stress and plastic viscosity, and a decrease in slump and fluidity [38][39][40].The G1 group added additional mixing water.However, due to the complex ion composition of the cement mortar, SAP could not absorb all the additional water, leading to a change in the actual water-binder ratio of the mortar.Additionally, SAP particles adhere to aggregates and cement particles, increasing the internal friction of the mortar, resulting in a lower slump for G1 compared to J0.
In the Y0 group with pre-saturated water-absorbing SAP, the internal ion concentration of SAP is lower than that of the mortar, resulting in water release during the mixing stage.This increases the water-binder ratio of the mortar and subsequently increases the slump.In the group Y1 with reduced mixing water, SAP releases water to compensate for the reduced mixing water, resulting in a slump similar to that of the benchmark group.In the group Y2 with partially pre-absorbed SAP, the internal ion concentration of SAP is similar to that of the external mortar.Therefore, it does not absorb or release water during the mixing process, and consequently does not alter the water-binder ratio of the mortar.Thus, its slump is close to that of the reference group.

Mechanical properties analysis 3.2.1. Mechanical properties
The cube compressive strength of internal curing concrete was investigated.The influence of SAP content on the compressive strength and splitting tensile strength of internal curing concrete with 0.2% SAP content (this content has the least influence on compressive strength) was studied respectively.The results are depicted in figures 7 and 8.
As observed in figure 7, with the increase in SAP content, the compressive strength curves of each experimental group exhibited a trend of initially increasing and then decreasing.At an SAP content of 0.2%, the compressive strength of each material group reached its maximum.This suggests that this type of SAP has minimal impact on the strength of concrete at a dosage of 0.2%, aligning with the theoretical SAP dosage calculated in section 2.2.
The effect of SAP on internal curing concrete is more pronounced at lower content.While the volume shrinkage of SAP produces pores, it releases water to increase the degree of hydration, and the generated hydrate fills the pores.The decrease in strength caused by the increase in porosity is smaller than the increase caused by the increase in hydration degree, resulting in an overall increase in compressive strength.Excessive SAP results in uneven water distribution in the matrix [41], while an increase in air within the concrete decreases its real density [42].Therefore, an increase in porosity during the hardening stage leads to a decrease in compressive strength [14,43,44].The decrease in compressive strength of the Y0 group is the largest, which may be due to the increase in mixing water or the additional release of saturated SAP, reducing the water-binder ratio simultaneously with the increase in concrete porosity, thereby reducing strength.
From figure 8, it can be observed that SAP reduces the splitting tensile strength of concrete under different mixing methods.The addition of extra mixing water (G1, Y0) can significantly decrease the strength.Other methods of incorporation (G0, Y1, Y2) have a lesser effect.The splitting tensile strength of each group was compared at 90 days: The general decline in strength may be attributed to the close relationship between pore size and distribution with tensile properties.Studies have shown that SAP achieves internal curing by facilitating hydration [45][46][47].After the initial setting of cement, the stored water in SAP is gradually released, helping to form a denser, more uniform cementing matrix [48], r thereby reducing porosity, increasing density, and improving the tensile strength of concrete.However, when the SAP content is too high, the internal porosity of concrete increases [49], and the negative effect caused by excessive porosity outweighs the curing effect.Therefore, an SAP content of 0.2% allows SAP itself to be regarded as a defect within the concrete [50].
Under normal circumstances, the compressive strength of concrete is generally proportional to the tensile strength, although this relationship is not constant.This proportional relationship can vary depending on the specific concrete mix ratio, materials used, curing conditions, and the age of the concrete.With SAP as the sole variable, the reason why the relatively low compressive strength of group J0 and group Y1 is not proportional to the tensile strength may be attributed to differences in microstructure.Due to more favorable internal hydration conditions, Y1 produces more hydrolysis products than G1.These fibrous hydrolysis products are more brittle, contributing to a greater role in compressive strength.However, they play a minor role in tensile strength, resulting in a lack of proportionality in the tensile strength ratio of Y1 and Y2.Both dry SAP and saturated SAP lead to a decrease in mechanical properties, but reducing water consumption or partial pre-water absorption can mitigate the negative impact of saturated SAP.In other words, incorporating SAP through unsaturated pre-water absorption is more beneficial for internal concrete curing.

Elastic modulus
The SAP content is 0.2%, and the elastic modulus of concrete with various SAP mixing methods is measured.The test results are depicted in figure 9.
From figure 9, it is evident that the elastic modulus decreases with various SAP incorporation methods.In the early stages of concrete curing, there is a significant decrease in the elastic modulus, with the concrete mixed with pre-absorbent SAP (Y1, Y2) exhibiting the most pronounced effect.After 7 days of curing, the elastic modulus of concrete mixed with pre-absorbent SAP exceeds that of the dry mixing group G0 and approaches that of the reference group J0.
With increasing curing age, the growth trend of elastic modulus varies among each group.The development trend of elastic modulus in the G0 group is characterized by rapid growth in the early stages followed by slower growth in the later stages.Specifically, after 7 days, the growth rate of concrete elastic modulus is only 4.07%.The development trend of elastic modulus in the Y1 and Y2 groups is largely consistent, with a slow development in the early stages and a noticeable increase in the later stages.This indicates that pre-absorbent SAP delays the hydration process of concrete and has a better curing effect than dry SAP.
Analyze the reasons for the above phenomena.When SAP is pre-absorbed, it stores part of the water at the initial stage of hydration, which slows down the hydration rate of cement and results in a low elastic modulus in the early stage.However, the rate of water evaporation is effectively reduced.The SAP water released later increases the hydration degree of cement, and the generated calcium silicate hydrate and ettringite fill the pores left after SAP shrinkage, thus the elastic modulus in the later stage is reduced less.Dry SAP absorbs the water of the mixture during the mixing stage and decreases the water-binder ratio of the mixture, thus led to a small decrease in the early elastic modulus.However, SAP absorbs the moisture of the mixture during the mixing stage, which aggravates the early evaporation of the concrete, leading to insufficient water in the later stage of curing, resulting in a rapid increase in the elastic modulus of the concrete in the early stage and a small increase in the later stage.
In summary, when SAP adopts the pre-water absorption mixing method, it has a lesser effect on the elastic modulus, while the partial pre-water absorption mixing method has the best curing effect on concrete.

Plastic shrinkage property
The liquid level drop caused by the plastic shrinkage of mortar in a straight glass tube is measured, and the shrinkage per unit length of the mortar column is calculated as shown in figure 10.
As can be seen from figure 10, with the increase in age, the plastic shrinkage of cement mortar increases, and the shrinkage rate increases rapidly before slowing down.Up to 100 min, the plastic shrinkage of mortar in each group is consistent, at about 300 microstrain.After 100 min, the shrinkage of mortar increases sharply.By 300 min, the plastic shrinkage of mortar reaches the peak and remains stable.
The development trend of plastic shrinkage of cement mortar in the test group and reference group is basically the same, both of which begin to increase sharply at 100 min, reach the maximum at 300 min, and then remain stable.The final plastic shrinkage of cement mortar in group J0 is relatively large, at about 3500 microstrain.The plastic shrinkage in the experimental group with SAP was significantly lower than that in the J0 group, especially in Y1 and Y2 group, and the shrinkage at 300 min was about 25% lower than that in the J0 group.This indicates that SAP can effectively reduce the plastic shrinkage of cement mortar.
The reasons for the change in mortar plastic shrinkage were analyzed.During the first 100 min after the mortar was loaded into the glass tube, the mixture did not react, and the decrease in the liquid level in the glass tube mainly resulted from the evaporation of water, with minimal change.At 100 min, the hydration rate increases, leading to intensified volume shrinkage of the mortar mixture caused by cement hydration.By 200 min, the hydration rate of cement began to slow down, and the cement mortar began to harden.At 300 min, the plastic shrinkage of cement mortar basically ends.The hydration rate of the Y1 and Y2 groups was slowed down when pre-absorbent SAP was added, resulting in a significant improvement in the plastic shrinkage of cement mortar, and the final plastic shrinkage was small.When G0 is mixed with dry SAP, the improvement effect on the plastic shrinkage of mortar is limited, and the final plastic shrinkage is slightly less than that of the J0 group.

Free shrinkage performance of strip concrete
The temperature and vibration frequency data collected by the strain gauge and the acquisition instrument are derived, and the free shrinkage performance of concrete under different water-absorbing resin mixing methods is calculated.These results are shown in figures 11-13.
From figure 11, it can be seen that the internal temperature of concrete increases rapidly at first and then decreases rapidly, finally stabilizing at the initial temperature.By analyzing the peak temperature inside the concrete, the reference group J0 reached the highest temperature of 37.04 °C in 575 min at the earliest.The dry doping group G0 reached 36.49°C at 690 min, ranking second.The pre-saturated water-reducing group Y1 and the partially pre-absorbed group Y2 reached the temperature peak at the latest, with peak temperatures of 33.67 °C (885 min) and 33.40 °C (845 min), respectively.In conclusion, the hydration reaction in J0 and G0 was more intense, resulting in a higher heating rate and peak temperature compared to the pre-absorption group Y1 and Y2.The hydration process of Y1 and Y2 is longer, and the duration of the temperature peak is longer than that of J0.
From figure 12, it can be seen that the vibration frequency of each group has the same changing trend, remaining stable after the initial vibration frequency rises to the maximum vibration frequency.The period of rapid frequency change is during the first 7 days, with subsequent frequencies remaining basically unchanged.The maximum frequency variation amplitude for the J0 group was 58.21 Hz, followed by 44.19 Hz for the G0 group.The variation in strain gauge frequency for both Y1 and Y2 groups is basically the same, at 38.98 Hz and 37.88 Hz respectively.
The internal changes of free shrinkage specimens were analyzed, and the addition of SAP changed the hydration rate.When dry SAP is added, SAP absorbs water during the mixing period and releases water prematurely, resulting in a short hydration process and a relatively poor internal curing effect.When preabsorbent SAP is added, water is stored inside the SAP, so the initial hydration rate is reduced.SAP releases water in the late curing period, slowing down the evaporation rate of water in the pores and prolonging the hydration process.Therefore, the peak temperature reached inside the concrete decreases, and the arrival time of the peak temperature is delayed accordingly.
It can be seen from figure 13 that the concrete in each group shrinks rapidly in the early stage of curing and tends to become stable after reaching the peak.The growth trend of shrinkage is roughly the same as that of vibration frequency, which increases rapidly in the first 4 days and then slows down to stabilize.After 28 days of curing, the shrinkage of the reference group J0 reached 266.04 microstrain.The G0 of the dry group was 211.99 microstrain, which is about 20% lower than that of the reference group.The shrinkage of Y1 and Y2 was basically the same, measuring 179.25 microstrain and 177.26 microstrain, respectively, which is about 35% lower than that of the reference group.
The reason for the above phenomenon may be that different SAP incorporation methods affect the hydration process of concrete.In the Y1 and Y2 groups, pre-absorbent SAP was added, which stored water for internal curing, thus weakening the shrinkage due to the loss of water in capillary pores, resulting in reduced shrinkage.In the G0 group, dry SAP absorbs the mixed water in the cement slurry during the initial stage of curing, which reduces the water-binder ratio of the concrete and causes premature water release.Therefore, the hydration in the later stage of curing is not sufficient, weakening the curing effect.The turning point of short-   As can be seen from the temperature variation diagrams in figures 14 and 11, the internal temperature variation trend of constrained concrete is consistent with that of free-shrinkage concrete.Compared with freeshrinkage concrete, the maximum temperature reached inside the restrained concrete specimen is significantly reduced, and the time required to reach the peak is shortened.The peak temperature of the reference group J0 reached 27.77 °C in 525 min, and that of the dry mixing group G0 reached 26.91 °C in 655 min.The peak temperature and peak time of concrete in the pre-saturated water-reducing group Y1 and the partial pre-water absorption group Y2 were similar, reaching 26.51 °C and 26.19 °C at 760 min and 720 min, respectively.
From the vibration frequency change diagrams of figures 12 and 15, it can be seen that the vibration frequency change pattern of the constrained specimen is consistent with that of the free shrinkage specimen.The vibration frequency increased rapidly in the first 4 days and remained stable after reaching the maximum vibration frequency.The internal temperature of the restrained shrinkage specimen is lower than that of the free shrinkage specimen.This may be due to the smaller size of the restrained concrete shrinkage specimen, resulting in relatively less heat released by hydration, thus reducing the temperature peak reached inside the concrete.When SAP mixing methods are the same, the change in the hydration rate and hydration degree of cement in free shrinkage and constrained shrinkage specimens follows the same pattern, and the change trend of internal temperature of concrete is consistent.
As can be seen from figure 16, the change rules of constrained shrinkage and free shrinkage specimens in all experimental groups are similar, exhibiting rapid growth of shrinkage in the first 4 days, and then slowing down before becoming stable.At the same time, the shrinkage of cast-in-place concrete is obviously lower than that of free shrinkage under the constraint of old concrete.At 28 days, the constrained shrinkage of the reference group J0 reached 133.15 microstrain.The constrained shrinkage of the dry mixing group G0 was 101.69 microstrain, slightly lower than that of J0.The restrained shrinkage of the pre-saturated water-reducing group Y1 and the partially pre-absorbed group Y2 was significantly smaller, reaching 93.2 microstrain and 89.54 microstrain, respectively, representing a decrease of 30.1% and 32.8%.At the same time, a phenomenon of concrete retraction similar to that observed in the free shrinkage specimen also occurred in the constrained specimen.
It can be seen from the shrinkage change of restrained concrete specimens that the addition of SAP can effectively reduce the shrinkage of restrained concrete.The improvement effect of dry SAP is limited.The effect of pre-saturated absorption with reduced mixing water and partial pre-water absorption is similar, which can  more effectively reduce the shrinkage.The variation law of shrinkage of restrained and free shrinkage specimens is consistent, indicating that the hydration mechanism of the two is consistent.

Constrained shrinkage performance of annular concrete
Based on the measured strain values, the strain change diagram of the annular concrete is depicted, as shown in figure 17.
As can be seen from figure 17, with the increase of curing age, the compression strain, that is, the shrinkage strain of annular concrete, firstly rapidly increases and then tends to stabilize.Compared to the standard group J0, the shrinkage strain of the dry mixing group G0, the pre-saturated water-reduction group Y1, and the partial pre-absorption group Y2 decreased significantly, indicating that the incorporation of SAP can effectively reduce the shrinkage strain of the annular concrete.However, a sudden change in strain of the annular concrete mixed with dry SAP occurred later, and the concrete specimens showed shrinkage cracking.
The shrinkage process of cement-based materials can be divided into three stages [51]: liquid stage, skeleton formation stage, and final hardening stage.The volume shrinkage caused by initial hardening is the most significant.According to the analysis of shrinkage strain, it can be seen that in the first 4 days, concrete gradually formed a solid skeleton from the liquid state, and the concrete shrinkage strain increased rapidly during this stage.After 7 days, the concrete was initially solidified, the skeleton was basically formed, and the stress entered a stable development period.At around 21 days, J0 group ordinary concrete reached the shrinkage limit.The strain of the annular concrete decreased abruptly, causing the concrete ring to crack from micro-cracks to final cracks, and the concrete specimens developed perforating cracks.At about 28 days, the concrete ring of G0 group reached the shrinkage limit, leading to an abrupt decrease in concrete strain.Simultaneously, through cracks appeared in the annular concrete specimen.The stress in Y1 and Y2 groups increased rapidly before 7 days, and the shrinkage strain increased steadily after 7 days until no cracking was observed in the 28-day concrete ring.
The reasons for the change in shrinkage strain of ring concrete are analyzed.SAP reduces the shrinkage of concrete, thereby reducing the strain on the steel ring.Some studies have shown that adding SAP can alter the texture of concrete, improving workability and plasticity [35,36].Gel materials provide a buffer and help improve the stability of fresh concrete [52], while aiding in sealing the cracks inside the concrete [53].Drying SAP absorbs cement slurry in the liquid phase, leading to a decrease in the water-binder ratio of the cement mixture and a decrease in fluidity.At the same time, the evaporation rate of water in the capillary pores increases, causing SAP to complete water release in the early and middle stages of curing, leading to insufficient hydration in the later stage of curing, weakening the internal curing effect.Therefore, the shrinkage strain of G0 group concrete decreases slightly, and the concrete specimens still crack.Pre-saturated water-absorption SAP has little effect on concrete in the liquid phase and releases water into mixed water, or partially pre-absorbed water, effectively storing water.Upon entering the skeleton formation stage, SAP provides sufficient internal curing moisture, thereby delaying the internal hydration process of concrete.This results in the generation of more hydrolysis products to fill the internal voids of concrete, consequently delaying the development of internal shrinkage stress in the concrete ring, which does not manifest until the end of the test.After cement hardening, the micro-structure is mainly composed of the following components: calcium silicate hydrate (C-S-H), calcium hydroxide (CH), unhydrated binder, and aggregate [54,55].Analysis of the micro-morphology of specimens in each group at a magnification of 500 times reveals significant cracks on the matrix surface of both the free shrinkage specimens and the constrained specimens in the reference group J0, along with a large number of holes with large diameters, indicating a loose structure and relatively poor compactness.The pores and holes on the surface of the matrix of G0 are relatively fewer than those in J0, although the structure remains loose.The matrix surface of the pre-saturated water-reduction group Y1 appears relatively flat, with smaller crack width and hole diameter.Analysis of the micro-morphology of specimens in each group at a magnification of 2000 times reveals that the cement matrix surface of the reference group J0 exhibits obvious micro-cracks, with the width of cracks on the constrained matrix surface being significantly larger than that of the unconstrained group, and no obvious traces of hydration are observed.On the surface of the cement matrix of the dry-mixed group G0, shorter needle-like hydrolysis products (e.g., ettringite) can be observed.A large amount of acicular or fibrous cement hydrate is observed on the surface and voids of the cement matrix in the pre-saturated water-reduction group Y1, with the length and thickness of the hydrolysis products being significantly larger than those in the G0 group.
Analysis of the micro-morphology of specimens in each group at magnifications of 10000 times reveals obvious micro-cracks in the cement matrix of the reference group J0, with the surface width of cracks in the constrained matrix being significantly larger than that of the unconstrained group, and only a few traces of hydrate are observed on the matrix.Dense needle-like hydrolysis products can be observed on the surface of the cement matrix in the dry-mixed group G0.A large amount of fibrous ettringite grows on the surface and in the interstitial space of the Y1 cement matrix.Analysis of the microstructure of each group of specimens reveals that the longer the cement hydration time, the longer the existence of hydrates, resulting in stronger morphology, often appearing rod-like or fibrous.Conversely, with shorter hydration times of cement, the existence time of the product is shorter, resulting in shorter shapes, usually needle-like morphology.As SAP gradually releases water in the cement matrix, it hydrates with nearby un-hydrated cement to produce hydrates that can fill and heal micro-cracks [56][57][58].Simultaneously, when water penetrates into the crack, the entry of water causes SAP to expand, resulting in SAP gel filling macro cracks and limiting water flow.
Coarse fibrous cement hydrolysis products are more prevalent in the matrix of the Y1 group, indicating a longer cement hydration process and sufficient ettringite and other hydrolysis products to fill the voids generated by cement solidification.Consequently, SAP cement-based materials are often regarded as possessing self-healing properties [56][57][58].The number of hydrolysis products on the cement matrix of the G0 group was significantly lower than that of the Y1 group, with morphology being short needle-like, indicating a weaker internal curing effect of dry SAP compared to pre-absorbent SAP.As indicated above, pre-absorbent SAP can release the water stored inside in the later period, effectively prolonging the hydration process, enhancing the hydration degree of cement, and increasing the crack resistance of concrete.Consequently, pre-absorbent SAP demonstrates a superior internal curing effect on concrete, effectively enhancing the microstructure and compactness of concrete.

Microscopic mechanism analysis
Combining the experimental results of each group, we analyzed the microscopic mechanism of SAP.The microscopic reaction mechanism is illustrated in figure 21.
When SAP is dry mixed, it immediately starts absorbing water upon addition to the cement slurry, thereby reducing the water-cement ratio of the cement mortar and necessitating an increase in mixing water.Consequently, concrete mixed with dry SAP exhibits higher early strength but poorer fluidity.As SAP absorbs the cement slurry during the mixing stage, early evaporation of the concrete is intensified, leading to premature water release from the SAP.SAP promotes cement hydration [45] while simultaneously inducing volume shrinkage and void formation in cement-based materials [59], which adversely affects strength [60].The impact of SAP addition on concrete strength and shrinkage cracking depends on the relative influence of these two factors.The hydrolysis products (hydrated calcium silicate and ettringite) formed during later stages of curing are insufficient to fully compensate for the decrease in concrete strength caused by the pores resulting from SAP volume shrinkage, thereby limiting the internal curing effect of SAP incorporated in its dry form.
When SAP is mixed using the pre-saturated water method, the internal solution concentration of SAP is lower than that of the mortar solution, resulting in water release beginning during the mixing stage after the addition of mortar.If the mixing water is reduced during the mixing stage, SAP releases water to maintain the water-cement ratio of the mortar, thereby minimally affecting the early strength of concrete.If the water consumption is not reduced, the higher water-cement ratio of the mortar has a more significant negative impact on the early strength of concrete.As SAP stores water, the hydration rate of cement slows down, resulting in the gradual development of early strength in concrete.However, simultaneously, it effectively reduces water evaporation.Water is typically released after concrete solidification due to absorption by SAP [61].Thus, during the concrete skeleton formation stage, capillary pores are dried, and SAP supplies pre-absorbed water to them, thereby reducing capillary forces [37].SAP can release sufficient water to enhance the degree of cement hydration reaction, resulting in the formation of hydrated calcium silicate and ettringite, which fill the voids caused by SAP shrinkage.
When SAP is partially pre-absorbed, it stores water at its maximum capacity.During the mixing stage, the internal ion concentration of SAP is similar to that of cement mortar.At this point, SAP neither absorbs nor releases water, resulting in minimal impact on the water-cement ratio of mortar and the strength of concrete.In the later stage of curing, SAP releases water and shrinks, creating pores.However, concurrently, the hydration reaction generates hydrated calcium silicate and ettringite, which fill these pores.The internal curing effect of the partially pre-absorbed method is comparable to that of pre-saturated water absorption and mixing SAP with reduced mixing water.

Conclusion
SAP internal curing concrete is selected as the focus of investigation, and a mix ratio design is conducted to analyze the liquid absorption characteristic curve of SAP.Slump tests, mechanical property tests, and shrinkage tests are conducted, and the microstructure is examined using an electron microscope.By investigating the mix ratio, mechanical properties, and shrinkage performance of internal curing concrete under various SAP incorporation methods, the following conclusions are drawn: (1) Both dried SAP and pre-saturated water-absorbing SAP significantly impact the fluidity of concrete, which can be enhanced by respectively adjusting the amount of mixing water.The impact of partially pre-absorbed water SAP is the least pronounced.
(2) The optimal dosage of SAP is 0.2% of the cementitious material.Regardless of the amount and method of SAP addition, it almost invariably has a detrimental effect on concrete strength.Notably, adding presaturated water SAP and reducing the compressive strength of the mixed water have the least adverse effects.Interestingly, the compressive strength even increases when the addition amount is 0.2%.
(3) The shrinkage reduction effect of dried SAP on concrete is limited, as evidenced by significant shrinkage strain in strip concrete and subsequent cracking in ring concrete.Partial pre-absorbent SAP and presaturated water-absorbent SAP yield superior improvement effects, significantly reducing concrete shrinkage strain and preventing later-stage cracking.
(4) The mixing methods of SAP have a significant influence on the early hydration rate, internal temperature, and initial setting time of concrete.During the early stage of hydration, SAP stores water, thereby reducing the hydration rate, lowering the maximum temperature within the concrete, delaying the time of peak temperature, effectively minimizing water loss, postponing the initial setting time, and achieving internal curing effects.
(5) SAP can effectively extend the hydration process of concrete and facilitate the formation of hydrolysis products.Pre-absorbent SAP demonstrates a superior internal curing effect, leading to the formation of denser and stronger hydrolysis products.This effectively fills the internal voids of concrete, enhances overall compactness, and improves anti-cracking performance.

2. 1 .
Raw materials 2.1.1.Cement P•O 32.5 ordinary Portland cement is utilized.The basic physical properties, components, and indices of the cement conform to the test specification requirements.

Figure 5 .
Figure 5. Constrained shrinkage test of ring concrete.

Figure 11 .
Figure 11.Internal temperature of free shrinkage specimen.

Figure 12 .
Figure 12.Vibration frequency of free shrinkage specimen.

3. 5 .
Constrained shrinkage performance of strip concrete he shrinkage of the cast-in-place concrete layer under the constraint of old concrete under different SAP mixing methods is calculated.The results are shown in figures14-16.

Figure 13 .
Figure 13.Free shrinkage strain of internal curing concrete.

3. 7 .
Micro experimental analysis 3.7.1.Results analysis of electron microscope scanning experiment The scanning electron microscope observation results of the microscopic test are shown in figures 18-20.In these figures, Z represents the sample taken from the free shrinkage specimen, while Y represents the sample taken from the restrained shrinkage specimen.

Table 2 .
Mix proportion of internal curing concrete.