Review on Multifactorial Coupling Effects and the Time-Dependent Behavior of Lateral Pressure on Concrete Formworks

Abstract

This critical review synthesizes evidence on the multifactorial coupling mechanisms and time-dependent evolution of lateral pressure in concrete formworks, addressing significant limitations in current design standards (GB50666, CIRIA 108, ACI 347). Through a structured analysis of 60+ experimental and theoretical studies, we establish that lateral pressure is governed by nonlinear interactions between concrete rheology, casting dynamics, thermal conditions, and formwork geometry. The key findings reveal that (1) casting rate increments >5 m/h amplify peak pressure by 15–27%, while SCC thixotropy (Athix > 0.5) reduces it by 15–27% at <5 m/h; (2) secondary vibration induces 52–61% pressure surges through liquefaction; and (3) sections with a width >2 m exhibit 40% faster pressure decay due to arching effects. (4) Temporal evolution follows three distinct phases—rapid rise (0–2 h), slow decay (2–10 h), and sharp decline (>10 h)—with the temperature critically modulating transition kinetics. Crucially, the existing codes inadequately model temperature dependencies, SCC/HPC rheology, and high-speed casting (>10 m/h). This work proposes a parameter-specific framework integrating rheological thresholds (Athix, Rstr), casting protocols, and real-time monitoring to enhance standard accuracy, enabling an optimized formwork design and risk mitigation in complex scenarios, such as water conveyance construction and slipforming.

1. Introduction

Concrete is the most commonly used building material in civil engineering, with applications in residential construction, municipal bridges, subway tunnels, and other projects. As the construction industry continues to expand, concrete technology has advanced toward achieving greater fluidity, workability, self-compaction [1], anti-seepage, and anti-cracking properties, as well as the ability to support large volumes, provide high strength, good performance, and ecological sustainability. Among these, the casting process is a key component in the construction process, and the formwork, as an important support structure in the concrete casting process, directly impacts the molding effect of concrete and structural safety in terms of its design and construction quality [2]. The core challenge in formwork design is accurately calculating the lateral pressure of freshly poured concrete on the formwork. Specifically, the lateral pressure of concrete formwork refers to the lateral pressure exerted by concrete on the formwork during the casting process. The level of this pressure and the change law have a significant impact on construction safety, the quality of the project, and construction efficiency. If the lateral pressure is greater than the ultimate capacity of the formwork section or connecting members, it may cause excessive formwork deformation, disengagement, or even collapse. In particular, a too-high formwork design will increase the cost, self-weight, and construction difficulty [3,4]. Hurd [5] revealed that concrete formwork can account for up to 60% of the total cost of a site-cast concrete project, and that existing codes for calculating the lateral pressure of a concrete formwork vary widely, especially when considering different factors. And some researchers emphasize that formwork safety risk prediction in complex construction environments (e.g., subway tunnels) needs to be combined with intelligent algorithms [6].

The main factors affecting the lateral pressure of concrete on the formwork include the concrete casting rate, vibration and secondary vibration, formwork size, concrete mixture, and rheology [7,8,9,10,11,12]. In addition, the current methods of concrete vibration rely entirely on mechanical systems that are powerful and have a high intensity. These improvements in concrete performance indicators and construction techniques can affect the pressure exerted by the cast-in-place concrete on the formwork.

In summary, the factors influencing the lateral pressure of the concrete formwork are multifaceted and exhibit clear time-dependent characteristics. Therefore, an in-depth study of these influencing factors and their interrelationships would help optimize formwork design and construction plan, as well as improve the safety and economics of concrete casting. In this work, the authors systematically analyzed the main influencing factors of the lateral pressure of concrete formwork and its time-dependent behavior, summarized and analyzed the existing specifications, and provided theoretical support and practical guidance for the successful implementation of construction projects.

2. Test Setups and Measurement Transducers

2.1. Test Setup

Measuring the lateral pressure test of concrete formwork can be categorized into on-site and laboratory tests. Wei et al. [13] carried out a 120 m ultra-deep diaphragm wall cast at the construction site, and placed pressure sensors at different locations for measurement. Arslan et al. [12] conducted indoor tests using regular formwork on 7 wall formworks, each with dimensions of 100 × 200 × 15 cm. In these tests, one surface of each formwork was secured by welding to the supporting structure to stabilize it during concrete placement, while the other surface was mounted with pins on its upper part to allow for rotation.

Constructions often require extensive formwork for fresh concrete casting. However, indoor tests are not available; therefore, as shown in Figure 1, Zhang et al. [14] used a new system to simulate the concrete casting process. This setup consisted of a concrete clamping mechanism, a hauling system, several steel strip tubes simulating reinforcing bars, and a jack, which was filled with concrete by a cubic container. The top of the setup was loaded with a jack to simulate the weight of concrete at different levels and time. The cube container and the jack were subsequently screwed to the equipment, with four channels on the side waist allowing for movement of the steel belt. The traction device consisted of four traction belts and a manual crank, secured at specific angles to align flush with the channels and belts. The end of each belt was connected to a dynamometer, which was used to record the tension, and considered as friction on the belt during sliding. This measuring device was similar to that of Ghoddousi et al. [15], Chen et al. [16], and Kim et al. [17], and could reasonably simulate the lateral pressure exerted on the formwork by the vertical pressure of the concrete mixture, thus facilitating indoor testing.

2.2. Measurement Sensors

Understanding the differences between pressure sensor types and calibration processes remains critical, as these factors can significantly affect the accuracy of pressure readings. Various types of pressure sensors, such as thin/thick film and deflected versus non-deflected, can be utilized. These sensors exhibit excellent stability and frequency response performances, and their compact size makes them easy to calibrate, making them ideal for most building construction applications. When a lateral force is applied to the sensor during formwork casting [18], the applied pressure is converted into an electrical signal.

Numerous studies have used this type of sensor [19,20], and despite their advantages, the precision sensing components of these sensors remain susceptible to physical damage. These hazards mainly include the sharp edges of aggregates that can result in mechanical impacts on the sensor probes during operation, which is especially evident if the mounting location is not properly selected or if no protective devices are added. These sensors and concrete have high contact surface maintenance requirements. When the end of the lubrication treatment is not appropriate, the hardened concrete is prone to adhesion, resulting in future cleaning difficulties and possibly affecting the measurement accuracy. Notably, the price and maintenance costs of these high-precision sensors remain relatively high, making it necessary to establish a sound management mechanism in the formulation of operational specifications and equipment protection to reduce non-essential losses and extend the service life of equipment.

Talesnick et al. [21] considered the effect of the sensor’s size relative to the particle/aggregate size of the concrete mixture and the effect of membrane deflection on the reliability of concrete pressure measurements. The test object consisted of a standard concrete mixture with a maximum aggregate size of 10 mm, with deflecting and non-deflecting sensor configurations 23 and 80 mm in diameter. The test results indicated that the response of the deflecting membrane sensor depended on the maximum aggregate size. Conversely, the response of the non-deflecting membrane sensor was independent of the maximum aggregate size. The deflecting membrane pressure sensor indicated residual lateral pressure long after the concrete had cured, which matched with the results obtained by Gregori et al. [22]. In contrast, the pressure of the non-deflecting pressure sensor decreased and reached zero when the concrete was cured.

McCarthy et al. [23] compared three measurement methods: (1) deflecting membrane pressure sensors, (2) tensile loads in the template ties, and (3) strain in the template frame. The results showed that all methods provided consistent results in measuring the fluid concrete pressure and that the change in lateral pressure with time was not significant during the hardening of the concrete.

Amziane [24] and Andriamanantsilavo et al. [25]. used two-cell devices in which the center of the thin latex membrane was maintained in the proper position by regulating the air pressure. This regulation was based on the feedback of the linear variable differential transformer (LVDT) located at the center of the flexible latex membrane. This method used air pressure to balance the lateral pressure exerted by the cement paste on the formwork. This enabled the precise control of the membrane position, leading to enhanced measurement accuracy and the effective monitoring and analysis of lateral concrete pressure changes during casting. Assaad et al. [26,27,28,29] used conventional deflecting membrane pressure sensors, which were mounted on the inner surface of a 200 mm diameter plastic pipe, flush with the pipe wall, indicating a gradual decrease in the measured lateral pressure from 400 to 700 min, followed by a rapid decrease in the lateral pressure to zero within the range of 50–100 min. This phenomenon was similar to the results obtained for cementitious materials (i.e., cement paste, mortar, concrete) by Amziane [24].

3. Results and Discussion

3.1. Factors Affecting the Lateral Pressure of Concrete Forms

The lateral pressure of concrete on the formwork was found to be influenced by several factors, with the physical properties of the fresh concrete (rheology, slump), vibration process (depth, frequency), temperature, hardening time, and the size of the formwork serving as important influencing factors [8,9,10,11,12]. The physical properties of the concrete, such as fluidity, compactness, and strength, were found to directly affect the lateral pressure exerted on the formwork during its set. Concrete with better flowability can fill the formwork more uniformly during casting, reducing localized pressures, whereas poorly flowing concrete may lead to pressure concentration and the deformation of the formwork [15,26]. Construction parameters, such as the casting method, casting rate, vibration method, and casting height, can change in the lateral pressure. In the case of rapid casting, the lateral pressure of concrete remains higher [30], leading to uneven forces in the formwork. In addition, vibration and secondary vibration can affect the distribution of lateral pressure. Environmental factors, such as elevated temperatures, can accelerate water evaporation from concrete [31], preventing proper strength development under unsuitable conditions and potentially increasing unstable lateral pressure on the formwork.

The lateral pressure of the formwork of concrete has been shown to be mainly affected by the casting rate, vibration and secondary vibration, the size of the formwork, temperature, and the ratio and flow characteristics of the concrete [8,32,33]. These factors interact with each other, jointly determining the distribution of the lateral pressure of the formwork during the concrete setting process.

3.1.1. Effect of the Casting Rate on Formwork Lateral Pressure

The casting rate of concrete into the formwork, defined as the rate at which fresh concrete is poured, can lead to a greater difference in hydration times when the rate is faster. During rapid continuous casting, the weight of the fresh concrete can affect its initial setting time, causing cohesion from the hydration reaction in the concrete layer to yield and disintegrate. This action and the increasing height of the poured concrete will raise the lateral pressure. Furthermore, reducing the casting rate during the process will allow time for the concrete to develop shear strength and friction, thus limiting the increase in the lateral formwork pressure [34].

To determine the effect of the casting rate on the lateral pressure on the concrete formwork, Gardner [34] reduced the casting rate from 20 m/h to 3.5 and 1 m/h, and the maximum lateral pressure on the formwork was reduced from 38 kPa to 24 and 10 kPa. Khayat [35] observed a slight increase in the formwork lateral pressure when the concrete casting speed was increased from 10 to 25 m/h. Assaad et al. [36] tested SCC mixtures at casting rates ranging from 5 to 25 m/h and showed that a reduction in the casting rate from 25 to 5 m/h reduced the maximum formwork lateral pressure by 15%, and a linear correlation was obtained with the formwork lateral pressure at lower casting rates [37]. Omran et al. [38] investigated the lateral pressure of concrete formwork with different thixotropies under the conditions of a concrete casting height of 7 m, slump of 700 ± 20 mm, temperature of 20 °C, and casting speed in the range of 2~30 m/h. The researchers found that regardless of whether the thixotropy of the concrete was high or low, the lateral pressure on the formwork increased with the increased casting speed. The lateral pressure values of the concrete mixtures with different thixotropies increased by about 27% and 15% when the casting speed was increased from 2 to 5 m/h. This also reflected that low thixotropy could not withstand the vertical load generated during the subsequent concrete casting due to its inability to withstand the subsequent vertical load. Therefore, it had a higher lateral pressure.

Summarizing the experimental data of the above studies, as shown in Figure 2 [35,36,38], when the concrete is cast at a small rate, the formwork lateral pressure is slight, but as the casting rate increases, the rate of increase in the formwork lateral pressure accelerates. When the casting rate is increased to a certain level (e.g., 10 m/h), the value of the lateral pressure of the formwork increases significantly, but its increase is slight. This is due to the concrete’s insufficient fluidity when the casting rate is lower, resulting in non-uniform filling in the formwork. However, when the casting rate was increased, the fluidity of the concrete was enhanced; this caused the lateral pressure to increase rapidly. However, when the casting rate reached a certain degree, the setting time and hardening characteristics of the concrete began to play a role, and the viscosity increased, which slowed down the growth of the lateral pressure. However, the higher casting rate produced a higher formwork lateral pressure. Therefore, to reasonably design the formwork for concrete casting, it was necessary to fully recognize and consider the casting rate and accurately calculate the lateral pressure of the formwork under its influence.

3.1.2. Effect of Vibration and Secondary Vibration on the Lateral Pressure of the Formwork

In concrete casting, vibration can be used to minimize air bubbles and pores, improve compactness, enhance the reinforcing bar bonding, and improve strength and stability. However, vibration and secondary vibration parameters heavily rely on the designed lateral formwork pressure. The dynamics of the formwork’s lateral pressure were found to be mainly influenced by the combined effect of vibration and the rheological properties of the concrete.

In the aforementioned tests, the specimen was divided into n sections along the height in layered casting and vibration. When the eighth layer of concrete was compacted, its upper surface was subjected to vertical pressure, increasing the lateral pressure of the eighth layer of concrete [39,40]. With the cessation of vibration, the i-th layer of concrete experienced thixotropic recovery. This resulted in fluidity decay and the increase in the concrete’s strength due to hydration forming a coupling effect. Therefore, unlike the hydrostatic pressure state, which produced a linear increase in the lateral pressure with height, the lateral pressure of the formwork produced by the concrete no longer continued to grow with increasing the height of the casting to a specific value [9], as shown in Figure 3a.

As shown in

Table 1. Comparison table of vibration and secondary vibration data.

Data From the Literature	Formwork Size (cm)	Layered Vibration Depth (cm)	Average Growth Rate of Lateral Pressure After Secondary Vibration (%)	Calculation Formula
Zhang et al. [40]	60 × 60 × 300	50, 100	61.6	$F_{hv}=F_v+{\displaystyle \frac{h_{v}A{D}_c-\sum \mu U△h_i{F}_{ui}}{A}}$	(1)
Zhang et al. [41]	30 × 120 × 300	50	52.89
Puhe et al. [42]	30 × 120 × 300	50	53.62
	15 × 120 × 300	50	28.24

, Zhang et al. [40,41] showed that with 50 cm of layered vibration, with an increase in the concrete casting height of the newly cast concrete on the formwork, the generated lateral pressure gently increased. However, with 100 cm of layered vibration, increasing the casting height of the freshly placed concrete led to a fluctuating but overall increasing trend in the lateral pressure on the formwork. After the template lateral pressure increased significantly due to vibration, a vibration rod inserted too deep caused it to significantly increase, and all specimens from top to bottom underwent a secondary vibration, thus demonstrating the effect of the second vibration on the template lateral pressure. A comparison of the formwork lateral pressure tests of concrete walls under ultra-deep vibration conditions studied by Puhe et al. [42] revealed that the formwork lateral pressures all showed a significant increase following the second bottom-up re-vibration, as shown in Table 1.

In the case of ultra-deep vibration or secondary vibration, the concrete begins to liquefy under the action of the high-frequency vibration load and becomes fluid. At this point, the lateral pressure model of the formwork at the vibration position is shown in Figure 3b, where F₀ is the lateral pressure of the concrete on the formwork above the vibrator rod position, kPa; F_hv is the lateral pressure of the formwork at the vibration liquefaction position, kPa; h_v is the insertion depth of the vibrator rod, m; and τ is the shear stress, kPa.

Within the vibration-induced fluidization zone adjacent to the formwork, lateral pressure comprises two distinct components governed by transient fluid mechanics: (1) Static Hydrostatic Pressure arising from the gravitational loading of the overlying concrete column and (2) Dynamic Vibration Pressure (the additional stress) generated by stress waves radiating from the poker vibrator. These components superpose linearly at the formwork–concrete interface during active vibration. The superposition of the two comprises the total lateral pressure. The magnitude of vertical compressive stress on liquefied concrete by concrete above the vibrating rod position is considered not only related to the height of concrete above the vibrating rod position, but also related to the lateral pressure and friction factor of concrete on the formwork above the vibrating rod position. Therefore, for the vibration formwork lateral pressure calculation, two researchers [40,42] used Formula (1) for the calculation (Table 1), where F_hv is the vibration liquefaction position template side pressure (kPa); F_v denotes the vibration pressure (kPa); h_v is the depth of insertion of the vibrating rod (m); A is the area of the concrete vibration liquefaction area (m²); Dc is the concrete gravity (kN/m³); μ is the friction factor between the freshly cast concrete and the formwork, the steel formwork is usually taken as being in the range of 0.15−0.35, and timber formwork is generally taken as being in the range of 0.30−0.60; U is the circumference of the concrete vibration liquefaction (m); ∆hi is the thickness of the i-th layer of concrete; and Fui is the lateral pressure of the formwork of the i-th layer of concrete (kPa).

For wall thickness (B) that does not exceed twice the radius of action of the vibrating rod (R_v) wall structure, the concrete vibration liquefaction area a and perimeter U can be determined based on Figure 4:

A = 2R_vB, U = 2(2R_v + B),

where B is the thickness of the wall, L is the length of the wall, and R_v is the effective radius of action of the vibrating rod, generally 8–10 times the diameter of the vibrating rod. For the calculation of the lateral pressure of the short-edge template, an imaginary template must be assumed from the edge of the template. During the calculation of the lateral pressure of the long-edge template, 2R_v can be selected as the calculation length, with the imaginary template boundaries set at both ends of the calculation section [42].

The data in Table 1 clearly show that the implementation of the secondary vibration process leads to a significant increase in the lateral pressure exerted on the concrete forms, which ranges from 28.24% to 61.6%. This has essential cost implications in engineering practice, as the amount of material and cost of a formwork system (especially the supporting members) tend to show an approximately linear proportionality to the amount of lateral pressure it needs to resist. Based on this linear relationship, it is reflected that an effective reduction in the lateral pressure of the formwork, e.g., up to 50%, usually means that the cross-sectional size of the support member can be reduced accordingly by about 20% to 30%. This optimization of structural size directly reduces material consumption, which is ultimately reflected in the project cost. It is expected to achieve cost savings approximately ranging from 15% to 20% [5].

3.1.3. Influence of Formwork Size on Formwork Lateral Pressure

Concrete formwork lateral pressure varies to different degrees depending on the size of the formwork. When casting small-sized cross-sectional structures, the formwork will receive vibration energy from each layer of the concrete due to an internal vibration, resulting in a large liquified concrete depth, thus increasing the formwork lateral pressure. The small-size cross-section body casting rate involves continuous casting. As indicated by the above analysis, the casting rate will increase the formwork lateral pressure, causing it to turn into hydraulic pressure.

A large cross-section casting body exhibits spatial conditions for the formation of the arch effect, while stability analyses of small cross-sections (e.g., coal pillar support structures) similarly validate the critical role of size effects on stress distribution [43]. According to the particle flow theory analysis, freshly mixed concrete serves as a granular body with internal friction and cohesion. During the casting process, aggregates will produce uneven displacement between the occurrence of the grain silo phenomenon, resulting in the redistribution of stress between the concrete aggregates to form the arch line. Due to the arch effect, the formwork lateral pressure will be lower for formwork with large cross-sectional sizes than for small cross-sectional sizes [44].

For vertical formwork, CIRIA Report 108 [10] and ACI Committee Report 347 [11] separate the test models used for walls and columns. Cross-section lengths and widths of up to 2 m each (pier and column bodies) were categorized as small section sizes, while cross-section lengths or widths of more than 2 m (walls and foundations) were categorized as large.

Khayat et al. [34] conducted formwork lateral pressure tests on SCC in two different column sizes: 2100 mm height and 200 mm width, and 3600 mm height and 920 mm width. The samples were subsequently tested at the same casting speed of 10 m/h. Initially, the formwork lateral pressure of concrete cast in the large columns presented 99% of the hydrostatic pressure, slightly higher than that of the small columns at 96%. However, over time, the formwork lateral pressure was reduced to 5% of the hydrostatic pressure for the large columns after 20 min and the small columns after 38 min. It shows that the expansion effect of small columns is more significant, and the SCC, due to the higher proportion of coarse aggregate, enhances aggregate friction, reflecting a more obvious grain silo phenomenon, which affects the rate of decrease in the lateral pressure of the formwork. It can be seen that the formwork size did not significantly affect the lateral pressure magnitude, but the grain silo phenomenon in the larger formwork resulted in a significantly higher rate of lateral pressure reduction after the casting was completed, while the smaller formwork showed a slower deceleration of lateral pressure.

3.1.4. Effect of Concrete Temperature on the Lateral Pressure of Formwork

Bulk concrete hydration heat cannot be ignored. The temperature of the concrete formwork lateral pressure is noticeable. Specifically, by increasing the concrete temperature, the rate of the cement hydration reaction will accelerate, and the yield strength of the freshly mixed concrete will accelerate. Therefore, the self-stabilizing ability of the concrete is promoted in a relatively short period, which contributes to the decline in the lateral pressure of the concrete formwork [39]. Concrete cast at higher initial temperatures can form a gel structure that exhibits stronger cohesion, endowing the plastic concrete with greater shear strength to carry greater vertical loads and causing the rate of decrease in the formwork lateral pressure to gradually increase over time. Notably, higher initial temperatures may also lead to a more pronounced loss of fluidity in the slump, reducing the lateral pressure of the concrete on the formwork [31].

Omran et al. [38] studied the effect of concrete temperature on the rate of lateral pressure drop in the formwork at a concrete temperature in the range of 10–32 °C and a casting rate of 5 m/h. It was found that increasing the concrete temperature reduced the lateral pressure and that the concrete temperature had a significant effect on decreasing the rate of lateral pressure decay and shortening the pressure relief time. When the concrete temperature was increased from 12 °C to 22 °C and from 22 °C to 30 °C, the pressure elimination time was reduced by 85 and 155 min, respectively. Assaad et al. [36] investigated the effect of initial concrete temperature in the range of 10–30 °C at a casting rate of 10 m/h on the rate of decrease in the lateral pressure in the formwork. They found that the change in temperature of freshly cast concrete had a limited effect on the maximum lateral pressure generated by the concrete at the time of casting. Figure 5 contains the experimental data from Omran et al. [38] and Assaad et al. [36], with the right axis ∆K(t)(0 − t_c) representing the ratio of the initial maximum lateral pressure (k₀) to the pressure elimination time (t_c). The results show that ∆K(t)(0 − t_c) increased significantly when the temperature increased, while the lateral pressure elimination time decreased to varying degrees. When the temperature was increased from 10 °C to 30 °C, the time required for pressure elimination (t_c) was drastically reduced by 122.4 min on average.

Zhang [3] measured the lateral pressure of concrete on formwork at different temperatures, and based on the data analyzed in the literature, the effect of concrete temperature on the lateral pressure of formwork showed a clear negative correlation trend. When the temperature increases, the formwork lateral pressure decreases significantly. The data show that formwork lateral pressure decreases by 26.21% when the temperature rises from 8 °C to 18 °C. This decrease stems mainly from the accelerating effect of temperature on the initial setting time of the concrete, which leads to a more rapid development of internal shear strength, thus reducing the efficiency of the hydrostatic pressure transfer from the fluid state to the formwork. It is shown that in cold climates (around 5 °C), low temperatures significantly prolong the initial setting time of concrete and retard the development of strength, which results in a significant increase in the lateral pressure on the formwork. Therefore, the lateral pressure exerted by the concrete on the formwork must be carefully evaluated and calculated when producing concrete formwork in low-temperature conditions.

This phenomenon clearly illustrates that the process of dissipating formwork lateral pressure after the completion of casting is highly sensitive to temperature and that an increase in temperature significantly accelerates the rate of decrease in formwork lateral pressure by concrete. For the actual formwork project, this means that when working in a hot environment, the concrete reaches a steady state more quickly, which allows for the safe removal of the formwork earlier, significantly speeding up the turnover of the formwork and shortening the occupation time of the same set of formworks on a single construction site. Considering that formwork rental costs account for a significant proportion of the overall formwork costs (according to ACI [11], the proportion can be as high as 35%), if high-temperature construction conditions can bring about, for example, a 40% increase in formwork turnover, then it can be deduced that, in a large-scale project, cost savings of around 10% to 15% can be realized on formwork rental costs alone. Effective utilization of temperature conditions to improve turnover efficiency is a crucial way to significantly reduce the overall cost of formwork.

3.1.5. Effect of Mix Ratio and Rheological Properties of Concrete on the Lateral Pressure of Formwork

Specific factors for concrete proportioning and rheological properties, such as the cement type and amount, aggregate properties, maximum aggregate size, aggregate–binder ratio, water–binder ratio, and chemical admixtures, constantly affect the lateral pressure of concrete forms. The thixotropy of concrete is characterized by dynamic changes in its rheological properties over time and with varying shear histories. At rest, the formation of flocculent structures between cement particles through van der Waals forces, electrostatic interactions, and early nucleation of C-S-H gel [45] leads to an increase in the apparent viscosity (i.e., “shear thickening”). When external shear is applied (e.g., by casting or vibration), the floc structure breaks down and the viscosity decreases (i.e., “shear thinning”) [34]. This reversible structural reconstruction ability has a dual regulatory effect on the lateral pressure of the formwork.

Thixotropy measurements of concrete can be quantified by static yield stress tests and thixotropic ring tests, among others. The static yield stress test primarily measures the initial yield stress, τ₀, of concrete after it has been left to stand for varying periods. Ghoddousi et al. [15] defined the thixotropic index, $A_{thix}={\displaystyle \frac{{\tau }_0(t)-{\tau }_0,\min }{{\tau }_0,\min }}$ (where τ₀ min is the minimum yield stress), by recording the τ₀ value added by a rotational rheometer after 10 min and 30 min of concrete rest. In contrast, thixotropic ring experiments are usually performed by applying a linearly increasing–decreasing shear rate in a rheometer, with the hysteresis ring area characterizing the thixotropic strength. And the hysteresis area of highly thixotropic concrete (e.g., SCC with metakaolin) can be up to more than twice that of low thixotropic concrete [38].

Controlling the mix ratio and rheological properties of concrete is essential to study the effect of its workability on the lateral pressure of the formwork. The core advantage of highly thixotropic concrete is its ability to rapidly rebuild the flocculent structure between castings, which enhances the shear strength and reduces the effective liquid concrete height [37]. The significant effect of thixotropy is demonstrated in the study by Omran et al. The lateral pressure of high thixotropic concrete only increases by about 15% when the casting rate is increased, whereas the increase for low thixotropic concrete is as high as 27%. Notably, thixotropy also accelerates the recovery of concrete viscosity after vibration and effectively inhibits pressure redistribution within the formwork. Quantitative evidence for this is provided by Zhang et al. [14], who found that concrete with a thixotropic index (A_thix) greater than 0.5 exhibited a 40% increase in the lateral pressure decay rate, significantly reducing the support time required for the formwork. Therefore, optimizing the thixotropy of concrete by carefully regulating the mix ratio is a key strategy for managing the lateral pressure of the formwork and improving construction efficiency and safety.

Gregori et al. [22] reduced the maximum lateral pressure by decreasing the water–cement ratio at a constant casting rate. The formwork lateral pressure reduced even more significantly when fly ash was used. Chen et al. [16] carried out formwork lateral pressure tests at different casting heights for different kaolin dosages and found that a small amount of kaolin doped at a casting height of 4 m significantly reduced the formwork lateral pressure, and the formwork lateral pressure was minimized when the kaolin dosage was 0.5%. Through thixotropy testing, Parviz [15] proposed a correlation equation for predicting the lateral pressure of concrete formwork through thixotropy tests with a correlation of 0.93, and that less cement decreased the formwork lateral pressure and more cement decreased the rate of decrease in the formwork lateral pressure. Assaad et al. [26] designed different concrete ratios and tested them in PVC pipes with a 2800 mm height and 200 mm diameter, and a sand ratio from 0.1 to 0.3. These results show that a coarser aggregate has a faster pressure drop rate.

The flocculation rate of the concrete mixes was affected not only by its material composition, but also by some rheology-modulating admixtures, effectively improving the thixotropic properties of the concrete. Kim et al. [17] measured the effect of silica fume, calcined kaolin, and wet clay on the lateral pressure of the SCC formwork. The researchers concluded that the inclusion of 20% silica fume, 1% calcined kaolin, and 0.66% wet clay achieved a minimum lateral pressure of the formwork and that the inclusion of even small amounts of these ingredients improved the lateral pressure of the formwork. Zilong et al. [46] investigated the effects of lateral formwork pressure by mixing fly ash, slag powder, as well as limestone, and binary composites. The results showed that mixing fly ash could reduce the yield shear stress and plastic viscosity of self-compacting concrete, and mixing slag could cause the thixotropic properties of the self-compacting concrete to decrease dramatically, while mixing limestone powder increased the SCC yield shear stress and plastic viscosity, thus significantly reducing the lateral pressure of the concrete formwork. Chong et al. [47] showed that when raw sludge was used as a thickener, it reduced the formwork lateral pressure. Dried sludge had little effect on the formwork lateral pressure; however, both effectively increased its rate of decline.

3.2. Time-Dependent Law of Lateral Pressure on Concrete Forms

Concrete, in its initial state after casting, is considered a non-homogeneous mixture, similar to a cohesionless soil in its resting state [39]. At this point, the interaction between the concrete’s solid particles and the water will cause it to exhibit some lateral pressure, similar to the lateral pressure experienced by cohesionless soils under static conditions. As the hydration process continues, the lateral pressure of the concrete will gradually decrease over time. The rate of this process has been shown to be affected by the hydration degree. At a faster hydration reaction, the lateral pressure will decrease faster, and vice versa. Meanwhile, at a slower hydration process, the rate of decrease in the lateral pressure will be delayed. At this stage, the strength and stability of the concrete are not yet fully developed; thus, its lateral pressure reflects the redistribution of its internal structure and the flow characteristics of the water. With prolonged time and the evaporation of water, concrete will gradually harden to form a stable solid structure, thus causing its lateral pressure to change [21].

To study the time-dependent pattern of concrete, the authors summarized and analyzed some of the literature data, as shown in Figure 6. Talesnick et al. [21] installed pressure sensors in 100 and 150 mm PVC pipes to determine the variation in formwork lateral pressure over time from casting to the end of concrete curing. Among these, vibration was carried out during the casting of the 100 mm pipe, and no vibration was carried out for the 150 mm pipe. The researchers found that when vibration was present, the lateral pressure of the formwork rose sharply to peak shortly after the concrete was cast and then continued to fall. Within the first 100 min, the formwork lateral pressure dropped relatively sharply, followed by a gentle decline. However, 400–500 min after casting, the lateral pressure dropped to zero.

In the absence of a vibration process, the formwork’s lateral pressure shows two peaks, with the second peak approximately 10 kPa higher than the first, and both lower than the peak in the presence of a vibration. In the absence of vibration, the concrete is fluid at the beginning of the casting, and the lateral pressure on the formwork increases rapidly, forming the first peak. This peak is typically related to the height and properties of the concrete, as well as the casting rate. During the concrete hydration heating process after casting, the reaction between cement and water releases a large amount of heat, which increases the internal temperature of the concrete and causes thermal expansion. When the expansion deformation is restricted by the formwork, the internal compressive stress increases. As the temperature peaks and then gradually decreases, the thermal expansion effect diminishes, but the stresses accumulated during this process may still be transmitted through the formwork, creating a second pressure peak.

Omran et al. [38] represented the development of formwork lateral pressure over time for SCC in three casting forms: 1. Continuous casting; 2. Introduction of a WP (wait period, which means continuous casting is paused for a scheduled time and then resumed); and 3. Introduction of two WPs. As shown in Figure 6, the time-dependent pattern of the formwork lateral pressure during continuous casting is typical, peaking after a continuous rise during casting followed by a decline. Near the 40 min introduction of a WP, the formwork lateral pressure continued to drop, and re-casting caused the lateral pressure to increase. Then, the peak and the maximum lateral pressure decreased by 11% compared to continuous casting. When the first WP was introduced at 20 min, the formwork lateral pressure decreased slightly and increased when the formwork was cast again. A second WP was introduced at approximately 60 min, when the rate of decline was slightly faster than the first WP, and the maximum lateral pressure was reduced by 15% compared to continuous casting. This WP caused the concrete to be stationary for a given time without being disturbed by continuous casting, allowing the inter-particle shear strength to develop and reducing lateral pressure. Billberg et al. [20] validated the model by casting SCC on eight walls. Several walls were selected, with the measurement point determined as 0.5 m from the bottom of the formwork lateral pressure time-dependent pattern data. The data indicate that the formwork lateral pressures continuously increased during casting, peaking at 1–2 h, and then decreasing.

As shown in Figure 6, different test conditions resulted in different curves. In the early stage of casting, the concrete flowed and was capable of flowing freely and filling in the formwork. At this time, the self-weight and mobility of the concrete acted together on the formwork, generating lateral pressure and continuing to increase. As the concrete set and hardened, the internal structure gradually developed. The strength gradually increased, leading to low initial formwork lateral pressures. Later, lateral pressure changes diminished with time. However, in the presence of external interruptions, such as vibration during casting, pressure redistribution can occur within the concrete, affecting the formwork’s lateral pressure. The long-term creep effect of the concrete at the time of casting also affects the concrete restraint stress. Hence, the prediction model needs to be optimized with the in situ monitoring data in the actual project [48].

3.3. Comparative Analysis of Concrete Formwork Lateral Pressure Codes

3.3.1. Concrete Formwork Lateral Pressure Distribution Diagram and Related Calculation Equation

Figure 7a presents a typical concrete pressure envelope, with the lateral pressure exerted by the concrete on the formwork. The lateral pressure exerted by fresh concrete is lower than the hydrostatic pressure. Among other factors, concrete hardening, the casting method, and friction between the concrete particles and formwork surfaces all influenced the amount of lateral pressure. Most formwork design methods characterize pressure as hydrostatic pressure at a certain distance below the free surface of the concrete to simplify several problems. In this context, the pressure can be maintained at a constant until the bottom of the formwork reaches the maximum value predicted by the calculation criteria. When calculating the concrete formwork’s lateral pressure, some specifications can be obtained, according to Figure 7b, where P_max represents the maximum lateral pressure.

(1)

According to the Chinese standard Code Construction of Concrete Structures (GB50666-2011), when the insertion vibrator and casting rate are not greater than 10 m/h, the concrete slump should not be greater than 180 mm. The standard value of freshly cast concrete on the side of the formwork pressure can be calculated through the following two formulas, taking the lesser of these values:

$$\begin{array}{l}F=0.28{\gamma }_c{t}_0\beta V^{{\displaystyle \frac{1}{2}}},\\ F={\gamma }_{c}H\end{array}$$

(2)

where P_max is the maximum lateral pressure of the formwork (kPa), γ_c is the unit weight of concrete (kN/m³), t₀ is the initial setting time of fresh concrete (h), which can be taken as t₀ = 200/(T + 15) (T is the temperature of concrete), and β is the correction coefficient for the effect of a concrete slump. When the slump is between 50 and 90 mm, a value of 0.85 should be used. For slump values between 90 and 130 mm, 0.90 should be used. With a slump value between 130 and 180 mm, 1.0 should be used. In this work, V represents the concrete casting rate (m/h) and H is the vertical distance from the point where lateral pressure is being calculated to the top surface of the fresh concrete (m).

(2)

The British Code calculates the lateral pressure of the formwork based on the provisions in the CIRIA Report No. 108. Concrete Pressure on Formwork (CIRIA Report 108) [10], which proposes a pressure curve based on Figure 7b. This can be calculated according to the following equations, taking the minimum of these values:

$$\begin{array}{l}{P}_{\max }={\gamma }_c(C_1\sqrt{V}+C_{2}K\sqrt{H-C_1\sqrt{V}}),\\ P_{\max }={\gamma }_{c}h\end{array}$$

(3)

where P_max is the maximum lateral pressure of the formwork (kPa); γ_c is the unit weight of concrete (kN/m³); V is the casting rate (m/h); K is the temperature correction factor, K = (36/T + 16)²; T is the concrete casting temperature (°C); H is the vertical height of the formwork (m); h is the concrete casting height (m); C₁ is the influence coefficient of formwork shape and size, taking 1.0 for wall structures and 1.5 for column structures; and C₂ is the influence coefficient of concrete constituent materials. When C₁$\sqrt{V}$ > H, it was calculated by $P_{\max }={\gamma }_{c}h$.

(3)

The U.S. code ACI 347-2004 “Guide to Formwork for Concrete” [11] proposes a pressure curve based on Figure 7b for columns and walls with a casting rate of <2.1 m/h and casting height of ≤4.2 m. The maximum lateral pressure can be calculated as:

$$P_{\max }=C_w{C}_c\left[7.2+{\displaystyle \frac{785V}{T+17.8}}\right]$$

(4)

where P_max satisfies 30C_wkpa < P_max < ρgh; ρ is the density of concrete (kg/m³); g is the gravitational constant and acceleration of gravity (9.81 N/kg); h is the depth of flowing or plastic concrete in the formwork from the casting point to the analysis point (m); P_max is the maximum lateral pressure (kPa); V is the concrete casting rate (m/h); T is the concrete molding temperature (°C); C_w is the concrete specific gravity coefficient, as shown in

Table 2. Concrete specific gravity coefficient.

Concrete Density	Cw
Less than 2240 kg/m3	Cw = 0.5[1 + (ω/2320 kg/m3)] not less than 0.80
2240~2400 kg/m3	1.0
More than 2400 kg/m3	Cw = ω/2320 kg/m3

; and C_c is the chemical coefficient, as shown in

Table 3. Chemical coefficient.

Cement Type or Mixture	Cc
Types I, II, and III without concrete retarder	1.0
Types I, II, and III mixed with concrete retarder	1.2
Other types or mixtures without retarder, less than 70% slag or 40% fly ash	1.2
Other types or mixtures with retarder, less than 70% slag or 40% fly ash	1.4
Mixtures containing more than 70% slag or 40% fly ash	1.4

.

For walls with a casting rate of <2.1 m/h and casting height of >4.2 m or walls with a casting rate in the range of 2.1–4.5 m/h, the formula for calculating the maximum lateral pressure is given by

$$P_{\max }=C_w{C}_c\left[7.2+{\displaystyle \frac{1156}{T+17.8}}+{\displaystyle \frac{244V}{T+17.8}}\right]$$

(5)

where P_max satisfies 30C_w < P_max < ρgh, ρ is the density of concrete (kg/m³), g is the gravitational constant (9.81 N/kg), and h signifies the depth of flowing or plastic concrete in the formwork from the casting point to the analysis point (m). P_max is the maximum lateral pressure (kPa), V is the concrete casting rate (m/h), T is the concrete molding temperature (°C), C_w is the concrete specific gravity coefficient, as shown in Table 2, and C_c is the chemical coefficient, as shown in Table 3.

Yu [49] utilized multiple linear regression to develop a model equation for formwork lateral pressure applicable to conventional shallow vibration conditions, as shown in Figure 7b. As indicated in this equation, the maximum lateral pressure on the formwork will never exceed the hydrostatic pressure of a fluid with the same density as the concrete. The maximum lateral pressure can be calculated as

$$P_{\max }=C_m{C}_f\left[31.1+7.8H-0.5(T+17.8)+0.8{(\alpha )}^{\frac{1}{2}}-14.8\log (t)\right]$$

(6)

where P_max is the maximum lateral pressure of the formwork (kPa), C_m is the material coefficient of the concrete, C_f is the shape and size coefficient of the formwork, H signifies the depth of the concrete (m), T is the temperature of the concrete (°C), α is the slump of the concrete (mm), and t is the time of concrete casting (h).

3.3.2. Comparative Analysis of Formulae Related to the Lateral Pressure of Concrete Forms

Many researchers have used different test conditions for correction to compare formulas applicable to each factor when selecting a formula for formwork lateral pressure.

Figure 8, Figure 9 and Figure 10 show the comparison between the experimental and calculated values for different casting rates by Santilli [50,51] et al. and Zhang et al. [3]. A line as a function of y = x will appear in the figure, and data points located below the line indicate measured values that are lower than the predicted value, which means that the canonical formula for that value is conservative. Since there are safety issues at points beyond the function line, consider choosing a model that fits better and is less likely to have safety issues.

Santilli et al. [50,51] analyzed the experimental models for lateral pressure of freshly mixed concrete in wall–column bodies, comparing five models in different ranges, specifically the ACI Committee 347 [11], Yu [49], Rodin [44], hydrostatic, and CIRIA Report 108 [10]. This paper takes data from the ACI Committee 347 [11] and CIRIA Report 108 [10]. As shown in Figure 8, the predicted values of CIRIA are all larger than the measured values, but close to the measured values, and the quartile of 95% of the ratio distribution (M/C95%) is less than 1 in the columns at casting rates less than 3 m/h. In the walls, the predicted values of ACI were all greater than the measured values, resulting in a better match between the two. As shown in Figure 9, the predicted values of ACI are high in the columns at casting rates between 3 m/h and 10 m/h, while the values of CIRIA are relatively small compared to ACI. In walls, the predicted values of CIRIA are partially lower than the measured values, indicating that they cannot be considered in the actual project and would be a safety risk. As shown in Figure 10, the measured and predicted values of CIRIA are more in line with the expected values in the wall when the casting rate exceeds 10 m/h.

Zhang et al. [3] conducted an under-equation comparison by correcting the casting rate, concrete slump, initial setting time, and ambient temperature to compare four specifications: GB50666-2011, TZ210-2005, ACI Committee 347R-04 [11], and CIRIA Report No. 108 [10]. In this paper, three specifications, GB50666-2011, ACI Committee 347R-04 [11] and CIRIA Report No. 108 [10], are selected for comparison with Santilli [50,51]. From Figure 8 and Figure 9, it can be seen that the CIRIA.108 specification has the highest calculated value when the casting rate is less than 3 m/h, and the ACI 347 specification has the highest value when it is greater than 3 m/h. Zhang [3] also mentioned that, regardless of the casting rate, the TZ210 specification demonstrated the smallest calculated value, which may pose a safety risk in the actual project, in agreement with the results of Jia et al. [52]. In contrast, the concrete slump and initial setting time were only considered by GB50666. For the same concrete slump, the results show that the calculated value of CIRIA.108 is three-times higher than the calculated value of TZ 210, while the lateral pressure of the formwork calculated by GB50666 increases with an increase in the initial setting time of the concrete with a linear relationship. The values calculated using the standard TZ 210 specification were significantly lower than the other three under the same casting rate conditions, possibly leading to safety risks. Only ACI 347 and CIRIA.108 accounted for ambient temperature, and at low ambient temperatures, the differences between the calculation specifications were sufficiently significant to cause accidents. When the temperature increased from 5 °C to 30 °C, the calculated values for ACI 347 dropped by 42%, and for CIRIA.108 they dropped by 61%.

From the above analysis, it is clear that in walls, ACI is recommended for casting rates less than 3 m/h and CIRIA is recommended for casting rates greater than 10 m/h. In columns, CIRIA is recommended for casting rates less than 3 m/h, and ACI is recommended for casting rates between 3 m/h and 10 m/h. Summarizing the analysis of the above data, the characteristics of the calculation formulae and the applicable conditions of each code are shown in

Table 4. Comparison of calculation formulas in different codes.

Standard	Advantage	Disadvantage/Limitations	Applicable Conditions
GB50666-2011 (China)	• Explicitly considers slump and initial setting time. • Simple calculation for low casting rates (V ≤ 10 m/h).	Ignores ambient temperature corrections; underpredicts pressure at low temperatures (deviation up to 30%); inadequate for SCC/HPC rheology or high-speed casting (>10 m/h).	Casting rate ≤ 10 m/h; slump ≤ 180 mm; wall/column structures.
ACI 347-2004 (USA)	Incorporates concrete temperature (T) and admixtures (Cc); valid for diverse mix designs (slag, fly ash).	Lacks guidance for thixotropic SCC/HPC; underestimates pressure at high casting rates (>10 m/h); limited validation for large sections (>2 m).	When V < 2 m/h in wall casting and 3 m/h < V < 10 m/h in column casting; H ≤ 4.2 m.
CIRIA 108 (UK)	Accounts for temperature (K) and section size (C1); best for large sections (>2 m).	Conservative for high-speed casting (>10 m/h) (underpredicts by 15–27%); complex calibration of C2 (material dependency); overpredicts pressure for small sections at low V.	Walls with V > 10 m/h; columns with V < 3 m/h; large cross-sections (walls/foundations).
EN 1992-1-1 (Europe)	Formwork flexibility: reduces pressure for deformable formwork; explicit arching-effect formulas for flexible molds.	Limited guidance for SCC rheology; ignores vibration effects.	All formwork types; requires rigidity classification (rigid/flexible).
JASS 5 (Japan)	Fiber-reinforced concrete (FRC): higher C values for steel/polymer fibers (increased viscosity); vibration depth limits for FRC.	Underdeveloped for SCC; empirical C values.	Conventional concrete and FRC; low-to-moderate casting rates (<7 m/h).

.

According to the above analysis, although existing design codes and standards have improved the safety of formworks, the research on complex construction environments and new materials remains insufficient. Therefore, future research should focus on the innovative design of the formwork system and the development of intelligent monitoring technology to further reduce the occurrence of construction accidents.

4. Conclusions

A comprehensive literature summary of a range of issues related to lateral formwork pressures generated by freshly poured concrete and time-dependent patterns led to the following conclusions.

(1)

Lateral pressure of concrete formwork was influenced by multifactor coupling

Concrete formwork lateral pressure is subjected to the coupling of casting rate, vibration, formwork size, temperature, and rheological properties. An increased casting rate (>10 m/h) was found to increase the lateral pressure by up to a range of 15–27% (Omran) by enhancing the kinetic energy and viscosity of the concrete. The additional compressive stresses in the liquefied zone resulted in a 52–61% increase in lateral pressure fluctuations when the vibration depth exceeded 50 cm (Zhang et al.). Small sections (<2 m) exhibited lateral pressures close to 96–99% of hydrostatic pressure due to the absence of an arch effect (Khayat). By contrast, large sections (>2 m) had a 40% faster pressure drop due to the aggregate grain bin phenomenon (CIRIA).

(2)

Time-dependent evolution of concrete formwork lateral pressure

The evolution of lateral pressure on the concrete formwork demonstrated a three-phase characterization. This included a rapid rise period (0–1 h), where mobility dominated, the pressure increased linearly with the concrete height level (typically reaching peak values in the 30–120 kPa range depending on mix and casting conditions), and vibration triggered an instantaneous peak.

This was followed by a slow decay period (>2 h), where the hydrated gel formed and the pressure index decreased (e.g., decaying from the peak at a rate reducing pressure by 10–30 kPa over this period). This was significantly affected by temperature, with the decay rate increasing by 15–30% for every 10 °C increase (e.g., pressure elimination time t_c reduced by 85–155 min for a 12–30 °C temperature rise; Omran/Assaad).

The last stage involved the plunge period (>10 h), where the strength development-driven pressure decreased to zero after the initial setting, and the interruption of casting triggered earlier decay, with a maximum drop of 15%.

(3)

Limitations and optimization paths of the canonical models

Existing codes exhibit significant differences in parameters. Specifically, China’s GB50666-2011 ignores temperature corrections, with low-temperature predictions deviating by up to 30%. Meanwhile, UK CIRIA 108 can be applied for large cross-section walls; however, predictions for high flow velocities (>10 m/h) remain conservative (Santilli). In addition, the ACI 347-2004 (USA) code improves the accuracy of the admixture system through the chemical coefficient (Cc), however it remains insufficient for thixotropic concrete. Therefore, code collaboration (e.g., integrating the slump correction of GB50666 with chemical coefficients of ACI347) serves as the key to improving the prediction performances of the lateral pressure in concrete forms.

5. Discussion

The lateral pressure exerted by fresh concrete on formwork serves as a critical factor that influences the safety and efficiency of slipform construction and high-strength concrete (HSC) applications. Although existing studies and design codes offer foundational insights, the unique challenges posed by modern construction techniques and advanced materials demand a refined discussion of unresolved issues and future directions [53].

(1)

Slipform construction and dynamic pressure management

Slipform techniques, characterized by continuous casting and formwork movement, amplify dynamic coupling between lateral pressure evolution and formwork stability. A few key challenges are mentioned below.

High casting rates: slipform systems often require casting speeds exceeding 10 m/h; however, current codes (e.g., CIRIA 108) underestimate pressure increments at these rates. For example, data obtained by Omran indicated a 27% lateral pressure increase when the casting speed increased from 2 to 5 m/h in thixotropic concrete. This relation necessitated velocity-dependent correction factors for slipform-specific models.

Real-time pressure redistribution occurs as the sliding formwork changes boundary conditions, disrupting static pressure assumptions. Embedded fiber-optic sensors (Gamil et al.) or wireless piezoelectric arrays can enable real-time monitoring, allowing for adaptive adjustments to casting rates or vibration protocols.

Thixotropy–formwork interaction: self-compacting concrete (SCC) used in slipform systems has demonstrated rapid thixotropic recovery, potentially reducing lateral pressure during pauses. However, prolonged stoppages risk localized pressure spikes after resumption. A hybrid model integrating thixotropy kinetics (Assaad) and formwork kinematics has been proposed to address this.

The rheological properties of concrete, especially the thixotropic annular area (Ath) and the rate of structural recovery (R_str), are key parameters for predicting and regulating the lateral pressure of the formwork in slipform construction. When A_th > 15 Pas, it indicates that the concrete has weak thixotropic recovery (low thixotropy) and its rheological structure is slow to rebuild after shear. In this state, if the casting rate exceeds 5 m/h, the lateral pressure of the formwork will increase significantly by 15–27% (Orman). A more stringent design of formwork support systems or proactive reduction in casting rates are required to address this risk. Conversely, when Ath < 10 Pas, the concrete exhibits high thixotropy and its internal structure can be rapidly reconstructed after shear thinning. At casting rates of less than 10 m/h, this feature helps to reduce peak formwork lateral pressures by up to 15–27%, thus permitting faster casting rates. However, when casting continues after a lengthy construction stoppage, it is essential to be alert to the possibility of pressure surges due to thixotropic structural damage, and real-time monitoring is required. On the other hand, the rate of structural recovery, R_str, reflects the rate at which the concrete’s structural strength develops as it rests. With R_str > 0.3 Pas, the concrete flocculates quickly during the stoppage of the slipform, effectively limiting the pressure rise to less than 10% when casting continues (Assaad), guaranteeing the continuity of the work. When R_str < 0.1 Pas, the structure formation is too slow, significantly increasing the risk of lateral pressure build-up in the formwork. Consider adjusting the vibration scheme or optimizing the dope system to accelerate the initial structure formation.

Given the inherent limitations of the current specifications in accurately predicting the behavior of complex rheological systems, such as Self-Compacting Concrete (SCC) and High-Performance Concrete (HPC), quantitative thresholds based on the above rheological parameters (Ath, R_str) provide the construction site with a scientific basis for real-time evaluation and dynamic adjustment of the workability of the concrete and the construction process (e.g., casting rate, vibration strategy) to achieve more proactive formwork side stress management. In addition, the seismic performance of steel–PEC combined beam nodes [54,55] shows that the design of structural connectors needs to consider the dynamic load transfer mechanism simultaneously, which is informative for optimizing the boundary conditions of the slipform system.

(2)

HPC or UHPC and rethinking rheology–pressure relationships

The low water-to-binder ratio and high viscosity of HPC fundamentally change its rheological behavior, challenging conventional lateral pressure models.

Delayed vs. accelerated pressure decay: HPC’s accelerated hydration (due to supplementary cementitious materials) shortens the slow-decay phase (2–10 h); however, its high yield stress may prolong the rapid-rise phase. Therefore, experimental validation is needed to quantify these competing effects, especially for mixtures with silica fume (>15%) or nanoscale additives.

Shear-thinning behavior: HPC’s non-Newtonian flow under vibration complicates pressure predictions. Discrete element modeling (DEM) coupled with CFD simulations may elucidate how shear-thinning rheology mitigates or exacerbates localized pressure concentrations near reinforcement.

Code adaptability: current codes (e.g., ACI 347) lack explicit guidelines for HPC’s unique binder systems. A revised chemical coefficient (Cc) table, incorporating HPC-specific admixtures (e.g., polycarboxylate superplasticizers), is needed to improve prediction accuracy. In the future, deep learning prediction methods [56] and 3D fine-grained models [57,58,59] should be fused to establish a multifactor coupled real-time monitoring system for lateral pressure.