Prestressed High-strength Concrete Research Articles

Durable life assessment of centrifugal stratified PHC piles in marine environment

This study proposes an analytical approach to assess the durable life of pre-stressed high-strength concrete (PHC) pipe piles exposed to the marine environment with consideration of centrifugal stratification and different end types. The durable life of the PHC pile is divided into two stages: the initiation stage for chloride penetration and the propagation stage for rust accumulation. To simulate the two stages, a two-layer composite cylinder diffusion model and a thick-walled ring mechanical model are developed based on the governing mechanisms. The proposed diffusion model is verified via comparisons with the numerical models with respect to chloride concentration under different stages. Then, extensive parametric studies based on the proposed method are performed to investigate the effects of end type, cover thickness, and steel bar diameter on the durable life of a PHC pipe pile. The results indicate that both increasing the cover thickness and using the closed-ended pile can significantly extend the durable life of a PHC pipe pile. Furthermore, the open-ended pile with a thinner mortar layer has a longer durable life, while the durable life of a closed-ended pile only slightly increases with a larger thickness of the mortar layer.

Analysis of Influence Factors of Negative Skin Friction on PHC Pile in High-Backfill Sites

It is recognized that NSF on a pile affects the structural and geotechnical design. The formation of NSF on a pre-stressed high-strength concrete (PHC) pile under a control building in a Vietnamese project was investigated using a verified numerical model in FLAC3D. The height of backfills, the consolidation degree of soil, working loads on the PHC pile, and the pile diameter were analyzed in sensitivity analyses to investigate the influence on NSF. It was found that higher backfills and higher consolidation degree were dominant in the formation of NSF, causing an increase of the dragload and an upward movement of the neutral plane. The working load and the pile diameter affected the dragload and the position of the neutral plane less. A limit decrease of the dragload induced by NSF was observed when the working load increased. Accelerating the soil consolidation process would be suggested to avoid great displacement due to NSF.

Time Dependence of Neutral Plane Position in a PHC-Supported Soft Subgrade

This case study focuses on an International Circuit project in China where unmanned automobiles are to be tested in the future. The subgrade undergoes extensive settlement because of the widely distributed lacustrine deposits with high void ratio and compressibility. Prestressed high-strength concrete (PHC) pile-supported earth platforms are used for ground improvement in this area, but their effectiveness has not yet been confirmed. Associated with this adoption, the negative skin friction (NSF) of the piles and the related neutral plane (NP) position is of importance. Although NSF has been examined in detail, the NP position has not, especially with regard to its time dependence. This paper reports the settlement of the pile-supported soft soil subgrade as obtained from long-term observations of full-scale tests. The NP position is determined according to the layered settlement of the soil and the pile settlement, both of which are obtained from long-term field observations. The results reveal that the NP position has a clear time dependence that develops in stages. The NP is initially elevated with time, and then it gradually approaches a constant value. The existing models fail to predict this time dependence of the NP position in the studied sites. A exponential model is used, the fitting parameters of which have practical implications. The adopted model works well with the studied sites, especially in terms of reflecting the time dependence of the NP position and distinguishing the different stages of its development. This paper enhances the understanding of the NP and provides a reference data set for related engineering practices.

Static Load Test and Numerical Analysis of Influencing Factors of the Ultimate Bearing Capacity of PHC Pipe Piles in Multilayer Soil

Prestressed high-strength concrete (PHC) pipe piles have been widely used in engineering fields in recent years; however, the influencing factors of their ultimate bearing capacity (UBC) in multilayer soil need to be further studied. In this paper, a static load test (SLT) and numerical analysis are performed to obtain the load transfer and key UBC factors of pipe piles. The results show that the UBC of the test pile is mainly provided by the pile shaft resistance (PSR), but the pile tip resistance (PTR) cannot be ignored. Many factors can change the UBC of pipe piles, but their effects are different. The UBC of the pipe pile is linearly related to the friction coefficient and the outer-to-inner diameter ratio. Changes in the pile length make the UBC increase sharply. Low temperatures can produce freezing stress at the pile–soil interface. The effect of changing the Young modulus of pile tip soil is relatively small.

Experimental Study of Thermal Response of Vertically Loaded Energy Pipe Pile

Energy piles are a relatively new technology that have dual function as heat transferring and load bearing. Due to the influence of temperature cycles, additional thermal stress and relative displacement of the pile will be generated; this is different from the load transferring mechanism of the conventional pile. In order to study the thermodynamic characteristics of the energy pipe pile under dual working conditions and temperature cycles, field tests were carried out on the PHC (prestressed high-strength concrete) energy pipe pile without constraining on the top of the piles. Displacement gauges were arranged on the top of the pile, and concrete strain gauges (temperature, strain) were embedded in the pile. The variation laws of temperature, thermal strain, thermal stress, side friction resistance, and displacement of the pile top during the temperature cycling were analyzed. The test results show that the heat exchange system reached a stable state after being heated for 5 days in summer. The average temperature of the pile increased by 15.17 °C, to 34.68 °C; it was low at both ends and high in the middle part. After 5 days in the winter environment, the average temperature of the pile decreased by 10.09 °C, to 9.54 °C, which was high at both ends and low in the middle. The thermal stress was generated inside the pile, and the maximum compressive stress was 3.446 MPa and the maximum tensile stress was 2.69 MPa. The neutral point of the side friction resistance appeared 8 m below the pile top, about 2/3 of the pile length. The maximum negative side friction resistance under the summer condition was 42.06 KPa, the maximum positive side friction resistance under the winter condition was 29.93 KPa, and the lateral resistance of the pile degraded in winter. Under the influence of thermal load, the final pile top displacements in the summer and winter were −0.7 mm (0.175%D) and 0.77 mm (0.193%D), respectively.

Application of Fiber Bragg Grating Sensing in Bidirectional Tests of Pile Foundations

Abstract The fiber Bragg grating (FBG) technique has emerged as a new strain monitoring technique in civil engineering applications. In this paper, based on the principle of the FBG technique and the bidirectional test method, FBG sensors and conventional vibrating wire strain gauges (VWSGs) were employed to monitor the strain variation law of a pile under different loads and analyze the relationship between the side resistance and pile-soil relative displacement of soil layers in bidirectional field testing of a large-diameter cast in situ pile. Four field bidirectional test cases of cast in situ piles, prestressed high-strength concrete (PHC) and steel pipe pile, and pile-base post-grouting pile are presented. The applicability and stability of the FBG sensing technique were verified by analyzing the side resistance distribution and bearing capacity. The results showed that FBG sensors can remove the influence because of residual force and the effect of temperature change and concrete expansion photosensitive material provides an improved sensitivity to the external environment compared with a VWSG. The strain and side resistance variation tendency of the cast in situ pile were consistent between the two kinds of sensors. The FBG sensing technique had high test accuracy in the four bidirectional O-cell field test cases of the cast in situ pile, PHC and steel pipe pile, and pile-base post-grouting pile. The pile-base post-grouting technique is an effective procedure that was implemented to increase the ultimate bearing capacity. The research results of this paper reveal a new strain monitoring technique for the bidirectional test method of pile foundations and represent a new test method for measuring the bearing capacity of PHC and steel pipe piles that has a remarkable application value in practical engineering.

Investigation on in-situ test of penetration characteristics of open and closed PHC pipe piles

To investigate the load transfer mechanism of open and closed Pre-stressed High-strength Concrete (PHC) piles jacked into layered soil, a full-scale in-situ test was conducted. In this test, the axial stress experienced by one open and two closed PHC pipe piles during jacking into layered soil was monitored using Fiber Bragg Grating (FBG) sensors mounted on pile shaft. The test results showed that (1) The jacking force on pile depended on the soil mechanical properties at the end of pile; (2) The variation of axial force on pile reflected that the compaction effect of soil for closed pile was larger than that for open pile; (3) At the beginning of penetration depth, the variation of shaft and end resistances on the open pile were different from those on the closed piles; (4) The variation of average shaft resistance relied on the mechanical properties of soil at the side of pile.

Shaft capacity of prestressed high strength concrete (PHC) pile-cemented soil column embedded in clayey soil

This paper presents a series of laboratory tests on cemented soil with high water content, and the water content of cemented soil samples were all larger than the liquid limit. A group of model PHC pile-cemented soil interface shear tests were then conducted to investigate the interface frictional capacity. Finally, a group of full-scale tests on PHC pile-cemented soil columns were conducted to study the frictional capacity of cemented soil-soil interface. The test results show that: the cohesion of cemented soil with high water content had a linear relationship with the unconfined compressive strength; the cemented soil strength had a large effect on the frictional capacity of PHC pile-cemented soil interface, and the peak skin friction of PHC pile-cemented soil interface increased linearly with the increasing cemented soil strength; the frictional capacity of cemented soil-soil interface was better than the frictional capacity of conventional pile-soil interface.

Field test of earth pressure at pile-soil interface by single pile penetration in silty soil and silty clay

Prestressed high-strength concrete (PHC) pipe pile with static press-in method is widely used in recent years. The earth pressure at the pile-soil interface caused by the process of pile jacking has an important influence on the jacked pile construction and even the bearing characteristics. Based on a construction project in Dongying, the change law of earth pressure at pile-soil interface caused by the penetration of PHC pipe pile is analyzed by monitoring the earth pressure at pile-soil interface during pile jacking, and the evolution characteristics of earth pressure at pile-soil interface in viscous soil foundation are revealed. The results show that the earth pressure at the pile-soil interface at different positions (h/B) caused by the penetration of PHC pipe pile gradually increases with time. In the process of pile jacking, the influence of pile jacking on the earth pressure at the pile-soil interface on the pile body and part that exceeds the relative distance of effective pile end (h/B) is weak. In the process of pile jacking, the earth pressure at the pile-soil interface is greatly affected by the relative height of the pile end (h/B), so it is necessary to consider the effect of the pile length to calculate the earth pressure at the pile-soil interface. The phenomenon of "lateral pressure degradation" exists with the increase of h/B in the earth pressure at the pile-soil interface. The research results have certain engineering guiding significance for construction and bearing capacity determination of jacked pile in viscous soil foundation.

Evaluation of Ultimate Bearing Capacity of Pre-Stressed High-Strength Concrete Pipe Pile Embedded in Saturated Sandy Soil Based on In-Situ Test

Estimation of ultimate bearing capacity (UBC) of pre-stressed high-strength concrete (PHC) pipe pile is critical for optimizing pile design and construction. In this study, a standard penetration test (SPT), static cone penetration test (CPT) and static load test (SLT) were carried out to assess, determine and compare the UBC of the PHC pipe pile embedded in saturated sandy layers at different depths. The UBC was calculated with three methods including the JGJ94-2008 method, Meyerhof method and Schmertmann method based on in-situ blow count (N) of SPT (SPT-N) which was higher than the values recommended in survey report regardless of pile length. The average UBC values calculated with cone-tip resistance and sleeve friction from CPTs was also higher than the value recommended in the survey report. Moreover, the actual UBC values directly obtained by load-displacement curves from SLTs were in line with the calculated values based on in-situ SPTs and CPTs, but approximately twice as high as the values recommended in the survey report regardless of pile length. For the SPT method, the application of bentonite mud in saturated sand layers is critical for the assessment of pile capacity in the survey phase, CPTs can provide reliable results regardless of soil characteristics and groundwater if the soil layer can be penetrated, and SLTs are necessary to accurately determine the UBC in complex stratum.

Study on the Influence of Polycarboxylate Superplasticizer Compound on the pump PHC

This aricle disclosed the effects of polycarboxylate superplasticizer (PCE), air entraining agent, defoamer, accelerator on the properties of prestressed high-strength concrete (PHC). The introduction of DAC and AMPS into the molecular structure of PCE is helpful to improve compressive strength of PHC. The air entraining agent can be used to improve the PHC workability. The Defoamers and accelerators are beneficial for increasing PHC compressive strength. The admixture compounding technology by using PCEs, air entraining agents, defoamers and accelerators can help to get excellent properties of PHC.

CHARACTERISTICS OF SLAG ACTIVATED PHC-PILE WITH REDUCED CEMENT CONTENT

ABSTRACT Ground Granulated Blast-furnace Slag (GGBS) powders, the massive by-products produced in the process of manufacturing cast iron are actively used as a substitute for cement, one of the major causes of greenhouse gas emissions. The purpose of this study was to investigate the proper mixing ratio of the activator, and to reduce the usage of cement in the production of high-strength steam cured concrete using GGBS. The cured products of various ratio were analyzed by XRD and TG/DTA, and the best ratio of the cured product were applied to the Pre-stressed high-strength concrete (PHC) Pile, and the performance was then evaluated. The results showed that the fluidity was improved by mixing with GGBS, and when an appropriate amount of activator was used, it was possible to produce high-strength cured products. In the PHC-Pile test, the replacement rate of 40% GGBS and combined 4% anhydrite gypsum and 2% Ca(OH)2 as an activator was excellent. The compressive strength (2MPa more), shear strength (1.3MPa more), and Axial force strength (2.7Mpa more) were more improved than for the normal Portland cement PHC-Pile, and this GGBS pile can be manufactured more economically (the cost can be saved in the amount of 14.5%). This is caused by the activating agent destroying the glassy film of the GGBS, and actively inducing the pozzolanic reaction, so that the hydrate materials are generated, such as C–S–H. In addition, by positively utilizing the inexpensive industrial by-product, GGBS, it is possible to manufacture environmentally friendly concrete products, while reducing the amount of cement used.

Experimental study on the seismic behavior of new PHC piles

In order to improve the bearing characteristics of prestressed high-strength concrete (PHC) piles, this paper proposes the use of steel strands and steel bars instead of steel reinforcement for PHC piles or PHC piles wrapped in BFRP. Eight PHC piles were tested under low-cycle loading to study their seismic performance. Through the analysis of the test phenomena, hysteresis curve, skeleton curve, strength and stiffness degradation, ductility and energy dissipation, it is found that the bearing capacity of the PHC pile coated with BFRP is much higher than that of the other types of piles. The PHC piles with steel bars and steel strands as the main reinforcement have the best seismic performance. ABAQUS was used to establish nonlinear finite element models. The results show that the experimental results are in good agreement with the finite element results. In addition, it is found that increasing the reinforcement ratio has no effect on improving the bending capacity of the PHC pile, while increasing the reinforcement ratio of the main reinforcement can improve the ultimate bending moment of the PHC pile.

Field Test of Excess Pore Water Pressure at Pile-Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor.

Prestressed high-strength concrete (PHC) pipe pile with the static press-in method has been widely used in recent years. The generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking have an important influence on the pile’s mechanical characteristics and bearing capacity. In addition, this can cause uncontrolled concrete damage. Monitoring the change in excess pore water pressure at the pile–soil interface during pile jacking is a plan that many researchers hope to implement. In this paper, field tests of two full-footjacked piles were carried out in a viscous soil foundation, the laws of generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking were monitored in real time, and the laws of variation in excess pore water pressure at the pile–soil interface with the burial depth and time were analyzed. As can be seen from the test results, the excess pore water pressure at the pile–soil interface increased to the peak and then began to decline, but the excess pore water pressure after the decline was still relatively large. Test pile S1 decreased from 201.4 to 86.3 kPa, while test pile S2 decreased from 374.1 to 114.3 kPa after pile jacking. The excess pore water pressure at the pile–soil interface rose first at the initial stage of consolidation and dissipated only after the hydraulic gradient between the pile–soil interface and the soil surrounding the pile disappeared. The dissipation degree of excess pore water pressure reached about 75–85%. The excess pore water pressure at the pile–soil interface increased with the increase in buried depth and finally tended to stabilize.

Service life of prestressed high-strength concrete pile in marine environment considering effects of concrete stratification and temperature

Prestressed high-strength concrete (PHC) piles have advantages of light weight and high load carrying capacity, which have been widely applied in coastal and offshore engineering. However, PHC piles serving in the coastal or offshore regions have already undergone severe durability problem. This paper presents a comprehensive method for predicting the service life of PHC piles in the marine environment, which reasonably takes the temperature effects and the concrete stratification caused by centrifugation of PHC piles into account. The pile service life is divided into the diffusion and corrosion periods. The service life of the diffusion period is predicted by solving the diffusion equation formulated for chloride ion diffusion in a two-layer ring medium, the results of which are compared with the data obtained from the elaborate experimental tests to examine the validity. The service life of the corrosion period is determined based on the ultimate corrosion expansion pressure produced by the accumulated corrosion products of steel bar. Various possible key factors, like protective cover thickness, initial chloride ion concentration, mortar layer thickness, etc., are considered to investigate how these parameters critically affect the pile service life. The results suggest that increasing mortar layer thickness can lead to a significant decrease of the service life in both diffusion and corrosion periods; and increasing protective cover thickness and sealing pile end are two effective ways to extend the service life of PHC piles.

Experimental Research Based on the Optical Fiber Sensing Technology for a Jacked PHC Pipe Pile Penetration Process

The aim of this work is to explore the influence of the end resistance and shaft resistance regarding the mechanism for jacked pile penetration and the load‐transfer rule during the penetration process. A full‐scale field test was conducted in an actual project located in Dongying, Shandong Province, China. In this test, the axial strain experienced by two closed Prestressed High‐strength Concrete (PHC) pipe piles during jacking into layered soil was monitored successfully using Fiber Bragg Grating (FBG) sensors mounted on the pile shaft. The experimental results show that FBG sensors have a good stability, strong antijamming performance, and can effectively monitor the pile stress. The variation law of the jacking force reflects the distribution of the soil layer, and the hardness of the soil layer at the pile end limits the pile force. When the pile end enters the silt layer from the clay layer, the jacking force and shaft resistance increase by 2.5 and 1.7, respectively. The shaft resistance accounted for 44.99% of the jacking force. The end resistance is affected by the mechanical properties of soil, and the end resistance of the silt layer is approximately twice that of the clay layer. The end resistance of the silt layer accounted for 59.84% of the jacking force. When the pile end enters the soft soil layer from the hard soil layer, the impact of the pile driving speed and the tangential force on the surface of the pile body must both be considered. During the pile penetration process, as the penetration depth increases, the radial stress on the pile side at a given depth is gradually released, while the shaft resistance at the pile side degrades significantly.

Impact of Marine Chloride Ion Erosion Environment on the Durability of Deep Sea Pile Foundation

Yan, Y.; Tian, T., and Wang, B., 2019. Impact of marine chloride ion erosion environment on the durability of deep sea pile foundation. In: Li, L.; Wan, X., and Huang, X. (eds.), Recent Developments in Practices and Research on Coastal Regions: Transportation, Environment and Economy. Journal of Coastal Research, Special Issue No. 98, pp. 6–9. Coconut Creek (Florida), ISSN 0749-0208.Subject to the chloride-induced corrosion, pipe pile foundations will suffer losses of the cross-sectional area of the steel bar, the bond strength between the steel bar and the concrete, as well as its bearing capacity. Even worse, it will malfunction. It is of great engineering significance to explore the corrosion initiation time and evaluate the service life of steel bars in pipe pile foundations under a marine environment. Based on Fick's law, this paper investigates the bilateral diffusion of chloride ions in pipe pile foundations in coastal areas and analyzes how the chloride ion diffusion effect is subjected to some key parameters such as bilateral erosion from chloride ions, mortar and concrete stratification, and temperature. In the end, the service life of prestressed high-strength concrete (PHC) pipe piles withstanding the bilateral diffusion of chloride ions is discussed. The findings show that when the temperature difference between the inner and outer walls of the pipe pile foundations is greater than 15°C, the wall temperature correction factor is quite different. It should be corrected according to the actual service conditions of the pipe pile foundations. Based on the hollow structure of PHC pipe piles, one should consider the simultaneous diffusion of chloride ions from the inner and outer walls and the centrifugal stratification of the pipe pile foundations when designing its service life. The mortar layer thickness and the ambient temperature are dominant factors that the service life of PHC pipe piles will be subject to. If the service year of the pipe pile foundation is 50 years, given the impact of the mortar layer, the thickness of the pipe pile foundation should be greater than 95 mm, and the mortar layer should be less than 10 mm; given the temperature effect, the pipe wall thickness should be greater than 100 mm. If the service year is 100 years, given the impact of the mortar layer, the wall thickness should be greater than 145 mm, and the mortar layer should be less than 25 mm; given the temperature effect, the wall thickness should be greater than 150 mm.

Influence of soil reinforcement on the uplift bearing capacity of a pre-stressed high-strength concrete pile embedded in clayey soil

This paper presents a field study on the uplift bearing capacity of a pre-stressed high-strength concrete (PHC) pile embedded in clayey soil, and on the soil around the PHC pile that was treated with cement paste. The PHC pile was inserted into a pile hole filled with cemented soil by its own weight (by gravity), and the soil compaction effect of a conventionally driven pile induced by the installation process was avoided. The test results showed that: the pile head displacement needed to fully mobilize the uplift bearing capacity of the test piles was about 0.1 D (pile diameter); the ultimate skin friction of the PHC pile–cemented soil interface was much larger than that of the cemented soil–soil interface; the PHC pile and the cemented soil around the pile behaved as an integral pile in the load transfer process; and the measured ultimate bearing capacity of the test piles was 0.91–0.94 times the American Petroleum Institute (API)’s proposed values for piles under compression and 0.79–0.80 times the values calculated with the effective stress method for piles under compression.

Flexural Performance Reinforcing Method of Damaged PHC Piles

Cracks may occur in a prestressed high-strength concrete (PHC) pile foundation because of factors, such as the aging of the domestic foundation structure, earthquake occurrence, and foundation ground expansion. The cracking of a pile foundation significantly red uces the bearing capacity of the pile such that its function as a structural foundation is lost. Recently, research on the development of reinforced methods for damaged piles has become essential in securing the stability of pile foundations. In this study, flexural strength tests were conducted on PHC piles with cracks, breakage, and cutting caused by various external forces to examine the reinforcing effects of various reinforcement conditions, such as mortar thickness, epoxy injection, and thickness of reinforcing steel, on the stability of a structure. Based on experimental results, a detailed reinforcing method for crack filling with epoxy, steel reinforcing, and mortar filling is suggested. Keywords: PHC Pile, Damaged Pile, Reinforcing Method, Flexural Strength Test

Flexural performances of prestressed high strength concrete piles reinforced with hybrid GFRP and steel bars

This study developed prestressed high-strength concrete (PHC) piles reinforced with high-strength materials (glass fiber-reinforced polymer (GFRP) bars) for flexural performance enhancement. Flexural strengths and behaviors of PHC piles reinforced with hybrid GFRP and steel bars were experimentally investigated, respectively. Large-scale specimens with total lengths of 12,000 mm and diameters of 600 mm were constructed and tested under bending, accompanied by evaluation of effects of non-prestressed reinforcement type and longitudinal reinforcement ratio. J-factors were calculated to evaluate deformability of all the specimens. PHC piles reinforced with GFRP bars were demonstrated to have much higher flexural capacity than those reinforced with steel bars. Moreover, strains at the midspans of cross sections of all the specimens basically conformed to the assumption of plane section. Failure of PHC piles reinforced with GFRP bars was attributable to gradual concrete crushing, while that of PHC piles reinforced with steel bars resulted from steel yielding. Results of this study were expected to provide theoretical basis for wide engineering applications of PHC piles reinforced with hybrid GFRP bars and steel bars in marine structures.