Suggested values of physical and mechanical parameters of bedrock soil layer.
The application of recycled compound concrete made of demolished concrete lumps (DCLs) and fresh normal concrete in pier foundation can effectively improve the utilization efficiency of construction waste resources. In this study, two prefabricated pier foundations based on recycled compound concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) and two cast-in-place pier foundations based on ordinary concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) were tested. Special attention was devoted to the load-settlement curve characteristics of the precast pier foundation of compound concrete, the load transfer law of the pier-soil system, the soil pressure distribution at the bottom of the pier, and the failure mode. The results showed that the Q-S curve of precast concrete pier foundation made of recycled compound concrete is slow deformation at loading, which is consistent with that of cast-in-place concrete pier foundation. The load transfer theory of pier-soil system is established, and its accuracy is verified by experimental analysis. The precast foundation of recycled compound concrete is the same as the cast-in-place foundation of ordinary concrete. The failure form of prefabricated pier foundation made of recycled compound concrete was a local shear failure, while the failure form of ordinary concrete cast-in-place pier foundation was piercing-type shear failure. The feasibility of relevant theoretical methods for calculating the vertical ultimate bearing capacity is examined.
- recycled compound concrete
- demolished concrete lump
- pier foundation
- vertical bearing capacity
With the growing interest in sustainable and environmentally friendly construction, recycled aggregate concrete (RAC) has been widely investigated and is gradually being incorporated into real engineering projects [1, 2, 3, 4]. A series of investigations has been conducted around the world on the mechanical properties, durability, and structural performance of RAC [2, 5, 6, 7]. Adopting demolished concrete blocks (DCBs) rather than recycled aggregates in structural members may reduce the cost of reuse of waste concrete. DCBs have distinctly larger size than the conventional recycled aggregates [8, 9, 10, 11]. Therefore, the utilization of construction and demolition waste has great significance in saving resources, protecting the environment, and realizing the sustainable development of the construction industry. Combined with the favorable geological conditions of Nanchang city, this paper made a preliminary exploration on casting large-size demolished concrete lumps (DCLs) and fresh concrete, e.g., recycled compound concrete, into pier foundation, which was used as a new type of building foundation form.
The research on pier foundation was first carried out in the United States, followed by the former Soviet Union, Japan, and other countries. In particular, a large number of experimental results were obtained in the research on pier foundations with enlarged bottom . The experimental research on pier foundation in China is relatively lagging behind. The theoretical research on pier foundation is more than the experimental research, mainly because it requires a large amount of financial and personnel resources to carry out the test. In terms of theoretical research, it mainly focuses on the soil failure theory , the design of pier foundation , the law of load transfer [15, 16], and the formula for calculating ultimate compressive bearing capacity [16, 17, 18, 19]. As for experimental study, it includes the research on end resistance reduction , settlement deformation [21, 22], working mechanism [23, 24, 25, 26], and bearing capacity formula [25, 27, 28]. Based on the experimental result and theoretical analysis, the paper provides a basis for the research on the vertical bearing capacity of the precast pier foundation with compound concrete.
In general, pier foundation is a kind of foundation form between rigid independent foundation and manual bored pile foundation [27, 29]. In multistory buildings, when the upper shallow foundation soil is relatively good and no weak layer beneath, when compared with shallow foundation, the artificial dig-hole pile foundation does not have the economic advantage, as it needs to use shallow foundation excavation of earthwork, which results in the high cost . However, the pier foundation has good seismic performance, large single pier bearing capacity, and less earthwork excavation ; it can meet the design requirements of the building, and the foundation cost is low . Hence, the research on the precast pier foundation made from recycled compound concrete has important engineering application value.
In this study, two prefabricated pier foundations based on recycled compound concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) and two cast-in-place pier foundations based on ordinary concrete (dimension of Ø800 × 2500 mm and Ø1000 × 2500 mm) were tested. Special attention was devoted to the load-settlement curve characteristics of the precast pier foundation of compound concrete, the load transfer law of the pier-soil system, the soil pressure distribution at the bottom of the pier, and the failure mode, and then the vertical bearing capacity of precast pier foundation filled with demolished concrete lumps is analyzed.
2. Geotechnical characteristics
The area is located in Jiangxi vocational technical college campus, Nanchang, Jiangxi province. The standard penetration test and the origin of a heavy dynamic penetration test were conducted to investigate the geotechnical characteristics. The soil samples collected by three apertures are shown in Figure 1.
According to the analysis of drilling soil samples in engineering geology, the soil stratigraphy consists of two main layers: Holocene filling soil (Q4ml) and Holocene alluvial layer (Q4al). According to its lithology and engineering characteristics, the distribution of each soil layer in the survey depth is described as follows:
Layer 1: silt (Q3al), brown and yellow, hard plastic, local hard, slightly dry, dry strength low, low toughness, shaking response medium, dull, components for clay, sand. The layer thickness is 0.40–1.80 m, and the depth of the top layer is 0.00 m, distributed throughout the field
Layer 2: middle sand (Q3al), brown and yellow, loose to slightly dense, slightly wet, particle composition is mainly quartz, the upper part contains more shale, exposed layer thickness 5.40–5.90 m, layer top buried depth 0.40–1.80 m, distributed throughout.
The main soil properties of each soil layer, derived from the geotechnical investigation, and the evaluation of in situ and laboratory tests are presented in Table 1.
|Soil layer name and number||Characteristic value of bearing capacity (kPa)||Soil heavy (KN/m3)||Compression modulus Es/deformation modulus E0 (MPa)||Bored pile (clean bottom, D = 800 mm)|
|Ultimate lateral friction||Extreme resistance|
|qsik (kPa)||qpk (kPa)|
3. Experimental procedures
3.1 Raw materials
Commercial concrete was used in this study as the fresh concrete, with the mix proportions shown in Table 2. The concrete used had a cement content of 220 kg/m3, a water-cement ratio of 0.46, and a maximum aggregate grain size of 31.5 mm. The slump was 180 mm, and the compressive strength was 35.2 MPa.
|Water-cement ratio||Water (kg/m3)||Cement (kg/m3)||Coarse aggregate (kg/m3)||Fine aggregate (kg/m3)||Fly ash (kg/m3)||Water reducer (kg/m3)|
3.1.2 Demolished concrete lumps
The DCLs used in the present study were obtained from the same batch of 150 mm cubic concrete samples reserved in the laboratory for more than 2 years. The cubic compressive strength of DCLs was 30.1 MPa. The demolished concrete cubes from an engineering quality inspection center were broken into lumps by excavator, as shown in Figure 2. Only concrete lumps with the sizes of 40–60 mm and 60–80 mm were selected for the inclusion into the recycled compound concrete, and these chosen lumps were used directly in the recycled compound concrete, i.e., without any further processing.
3.1.3 Reinforcement details
As shown in Figure 3, HRB 335 bar with a diameter of 12 mm was used for longitudinal bars of the pier foundation, while HRB 335 with a diameter of 8 mm was used for transverse reinforcement with a spacing of 250 mm. Four steel reinforcement cages were made, as shown in Figure 3, to serve as the structural reinforcement of the four pier foundations. Two of the piers are 800 mm in diameter and the other two are 1200 mm in diameter. All the four piers are 2400 mm in height. The longitudinal bar selected is HRB335 hot rolled thread steel bar with a diameter of 12 mm. The stirrup is the same type of steel reinforcement with a diameter of 8 mm, including 8 Φ 12 mm and longitudinal reinforcement stirrup Φ 8 @ 250 mm.
3.1.4 Concrete molds
Concrete molds were prepared from wood, as shown in Figure 4, one with an inner diameter of 800 mm and a height of 2500 mm and the other with an inner diameter of 1000 mm and a height of 2500 mm.
3.2 Pier foundation preparation
A total of four pier foundations were prepared, including two precast pier foundations filled with DCLs and fresh concrete (pier foundation dimension: ∅800 × 2500 mm, ∅1000 × 2500 mm) and two ordinary concrete cast-in-situ pier foundation (pier foundation dimension: ∅800 × 2500 mm, ∅1000 × 2500 mm. The schematic diagram of precast pier foundation design is shown in Figure 5. The details of pier foundations are presented in Table 3.
|Notation||Count||Dimension (mm × mm)||DCLs grading||η (%)|
|YD800||1||∅800 × 2500||40–60 mm:60–80 mm = 6:4||35.24||C25||30|
|YD1000||1||∅1000 × 2500|
|XD800||1||∅800 × 2500||—||35.24||—||—|
|XD1000||1||∅1000 × 2500||—|
For pier foundations made of recycled compound concrete (YD1000 and YD800), the replacement rate of DCLs was 30%, and the ratio of DCLs with diameters of 40–60 mm to 60–80 mm was 6:4. The precast pier foundation was cast near the test site. After 28 days of curing, the pier foundations were transported to the construction site. In order to ensure the lifting of the pier foundations during transportation, it is necessary to embed the lifting rings in the pier foundations before they are cast. As suggested in the literature , the bending center diameter of the lifting ring was set as 63 mm, the burial depth was set at 75 mm, and the bending hook was set as 25 mm. The casting process of the pier foundation is as follows:
Prepare reinforcement cages with diameters of 0.7 and 0.9 m for the pier foundations with diameters of 0.8 and 1.0 m, respectively. Strain gauges were pasted on the longitudinal reinforcement, and the layout of the strain gauges are presented in Figure 6. Moreover, the strain gauges were pasted to the outer sides of the longitudinal bars to prevent damage of the strain gauge during concrete casting and vibration.
Assemble and splice the wood mold with an inner diameter of 0.8, 1 m, and length of 2.5 m, and drill a hole with diameter 50 mm at the upper part of the mold as the outlet of strain gauge cables.
When the height of the mold was 2.5, scaffolding was set up for construction in the casting project. On the one hand, it could fix the mold; on the other hand, it could serve as an operating platform during the concrete casting process.
Put the steel reinforcement cage into the mold, and adjust the spacing between the steel reinforcement cage and the mold that there was a 50 mm gap between. The DCLs were wetted before casting to prevent the DCLs from absorbing too much water in the casting process. The fresh concrete is first casted to the bottom of the mold with a height of 250 mm as a cushion. The fresh concrete incorporation with the DCLs with the replacement rate of 30% was casted into the mold. During the casting process, the handheld vibrator is used for continuous vibration. When the casted layer reached 500 mm high, a steel wire mesh with a diameter slightly smaller than the diameter of the steel wire cage was laid on the fresh concrete. Repeat and alternate the above casting process until the mold is filled. The schematic diagram for the height setting of layered steel mesh is shown in Figure 7. The steps for the preparation of recycled compound concrete and ordinary concrete piers are shown in Figures 8 and 9, respectively. After 1 day of curing, the concrete was demolded. Carpet saturated with water was used to wrap the surface of the pier foundations. After 28 days of curing, the pier foundation was lowered into the hole, a little larger than the pile diameter.
The gap between the precast pier foundation and the hole shall be backfilled and compacted with undisturbed soil. At the same time, the hanging ring of the precast pier top was sawed off with the cutting machine to ensure the levelness of the pier top.
As the two cast-in-place piers of ordinary concrete are casted on the foundation, the pier foundation preparation is as follows. The test hole was dug firstly. Then, the steel cage was lowered into the hole, which was followed by fresh concrete casting. After casting, cover the top of the pier foundation with plastic film to prevent water evaporation.
3.3 Testing setup
The commonly used reaction methods for vertical compression and static load test of pier foundation include the platform reaction beam method, anchor pile reaction beam method, and anchor pile reaction beam loading method . Based on the actual situation of the site, the platform reaction beam method was adopted in this study, as shown in Figure 10. Load counterforce device consisted of the bearing plate, reaction beam system, 320 tons of double oil jack, and 63 MPa ultrahigh pressure pump station. Load counterforce device could provide the reaction force not less than 1.2 times of maximum load value. The load was applied at a rate of movement corresponding to a stress rate on the pier foundation as specified in the code . The compression force was measured by a static load tester (type: RS-JYB), along with the settlement measured by two displacement sensors. To understand the stress distribution along the pier foundation, six pairs of strain gauges were arranged along with the longitudinal reinforcement, as shown in Figure 6. To understand the soil pressure distribution of the foundation, the vibrating wire pressure sensors (JTM-v2000) were applied. The parameters of the sensor are shown in Table 4. The vibrating wire pressure sensors were arranged at 100 mm intervals at the bottom of the pier foundation, and the distribution of the earth pressure at the bottom of the pier foundation can be measured by considering half of the symmetrical arrangement. Two earth pressure boxes are required for the diameter of 0.8 m, and three earth pressure boxes are required for the diameter of 1 m. The detailed installation of vibrating wire pressure sensor is shown in Figure 11.
|Model||Dimension||Scale range||Resolution (% F.S.)||Temperature measurement range (°C)||Temperature measurement precision (°C)|
|JTMV2000||Ø117 × 28 mm||0–2.5 MPa||≤ 0.08||−25 to 60||0.5|
4. Results and analysis
4.1 Load-settlement relationship
The measured load-settlement relationships are plotted in Figure 12, and the characteristic values are listed in Table 5. All the pier foundations exhibit similar load-settlement curves, which belongs to slow variation. According to the geotechnical characteristic measured, the bearing layer of the pier foundation test was located in the middle sand layer, which is consistent with the Q-S curve characteristics of sandy soil . Regardless of the type of pier foundation (recycled compound concrete or ordinary concrete), the characteristic value of vertical bearing capacity of single pier increases with the increase of the diameter of pier foundations. For pier foundations with the same diameter, the vertical bearing capacity of the recycled concrete precast pier is lower than that of the ordinary concrete pier by 25–35%. This is mainly because ordinary concrete was cast-in-situ, and therefore there was relatively large friction between the pier foundation and the soil, while for precast pier foundation made of recycled compound concrete, the friction generated between the pier foundation surface and the soil can be ignored. Moreover, the rate of rebound deformation of the precast recycled compound concrete is higher than that of the cast-in-place ordinary concrete pier after unloading. It is mainly due to the precast pier, where the friction between the pier and the soil was insignificant, and consequently, the load transformed to the pier end was relatively larger. Upon the maximum loading, the precast recycled compound concrete pier had larger settlement than the cast-in-situ ordinary concrete pier. As shown in Table 5, the Δse/Δse-1 of YD1000 is 12.6, while that of XD1000 is 2.21, indicating that the foundation failure mode of the precast compound concrete pier is different from that of a cast-in-place ordinary concrete pier.
|Pier foundation||Qcr (kN)||Smax (mm)||Qu (kN)||Squ (mm)||Ra (kN)||Sra (mm)||Rebound deformation (%)||Δse/Δse-1|
4.2 Load transfer law of pier-soil system
4.2.1 Geological distribution of the test site
The geological distribution of the pier foundation test in the test site is silt in the range of 0.00–0.40 m and sandy soil in the range of 0.40–5.90, respectively.
4.2.2 Load transfer theory of pier-soil system
Based on the load transfer principle of large-diameter single pile without expanding bottom, and relevant literature [34, 35], the load transfer between the pier-soil system includes the following three stages:
At the initial stage, the load is mainly undertaken by the friction resistance between pier foundation and soil. The friction resistance of soil at the bottom of the pier does not occur, because of the relative displacement between the bottom of the pier and soil.
With the increase of the load, the compression deformation of the whole pier body and the relative displacement between the piers will increase, and thus the friction resistance at the bottom of the pier foundation will gradually increase, and the soil layer at the bottom of the pier will also experience compression settlement with the increase of pier end stress, and the pier end provides part of end resistance.
When the load is further increased, the settlement of the pier bottom increases, and correspondingly, the end resistance provided by the pier end increases significantly. On the other hand, with the compression of the soil layer at the pier end, the relative displacement of the piers also increases, and the friction resistance provided by the pier side will further increase.
4.2.3 Analysis of axial force and average friction resistance of pier body
Based on the mechanics of materials that the material satisfies Hooke’s law in the elastic deformation range, the relationship between stress and strain could be expressed as Eq. (1).
If the linear expansion coefficient of steel and concrete could be determined by the same deformation together, the cross-section of concrete and steel bar would have the same strain value . In this work, strain gauges placed on the longitudinal bars could serve the internal structure of the pier foundation. Along with the structural reinforcement, six sections are selected on both sides, and six strain gauges are placed on each side (Figure 6). By measuring the strain value, the axial force at the section is calculated, and then the distribution of friction resistance at the pier side can be obtained. Under the different levels of load, the axial force distribution diagram of pier body is presented in Figure 13.
As shown in Figure 13, it can be seen that with the increase of loading load, the axial force of the pier foundation increases. The axial force of the cast-in-place concrete pier foundation decreases with the depth of the pier foundation, indicating that the friction resistance at the pier side plays an important role. When the pier-soil system is damaged, the side resistance of the pier accounts for around 25% of the ultimate compressive bearing capacity. For the precast pier, the lateral resistance is almost zero, which is mainly caused by the gap between the precast pier foundation of compound concrete and the surrounding holes are not easy to be compacted when the undisturbed soil is backfilled.
The friction resistance of the pier body of the cast-in-situ pier foundation is analyzed in the following way: as the height of the test pier foundation is 2.5 m, and the soil layer distributed around the pier foundation has been presented that 0-0.4 m of the pier body is slit, and 0.4–2.5 m of the pier body is medium sand. Under the maximum load, the average friction resistance of the pier body in different soil layers is distributed along the pier body, as shown in Figure 14. In addition, with the increase of pier diameter, the greater is the friction resistance of the pier foundation at the same depth, the greater is the vertical compression bearing capacity of the pier foundation.
4.3 Analysis of soil pressure distribution of pier bottom
According to the measurement of the soil pressure distribution of pier bottom foundation, vibrating string earth pressure gauge. During the test, data were read for three to four times with vibrating string sensor readout instrument when each level of the load was reached, and its average value was taken. The recorded frequency was converted into pressure through Eq. (3):
where P is the earth pressure value (MPa), K is the calibration coefficient of the earth pressure gauge (MPa/HZ2),
Figure 15 presents the measured earth pressure distribution of the foundation at the bottom of the pier. The distribution of soil pressure of precast pier foundation of compound concrete and that of cast-in-place pier foundation of ordinary concrete has the same pattern, that is, the soil pressure of foundation of pier bottom is distributed in a parabolic shape. The main reason is that the bearing layer of the pier foundation was on the sand soil. In the process of loading, the sand grains at the bottom edge of the pier are easily extruded laterally, and the soil pressure at the edge of the foundation transfers to the middle part, leading to the stress redistribution. As a result, the soil pressure at the edges of the pier bottom was small, while the soil pressure at the middle of the pier bottom was large. At the initial stage of loading, the soil pressure value of the prefabricated pier foundation is much higher than that of the cast-in-place compound concrete pier foundation with the same diameter, which further indicates that the prefabricated pier foundation with recycled compound concrete is almost entirely provided by the resistance at the pier end. Also, with the increase of load, the failure of the bottom edge of the pier was earlier than that of the middle of the pier bottom.
4.4 Failure mode analysis of the pier foundation
The failure modes are presented in Figure 16, which the soil of both precast pier foundation and ordinary concrete cast-in-situ pier did not exhibit the bulging damage phenomenon. For precast concrete pier foundation, the settlement increased obviously in the former period, and Q-S curve has obvious turning point (Figure 12); therefore, along with the failure mode, the failure belongs to local shear failure damage. For ordinary concrete pier, as the load increases, the soil at the bottom of the pier foundation was compressed. When loaded into the pier-soil system damage, the rate of settlement remained unchanged, and Q-S curve did not show obvious turning point; therefore, along with the failure mode, the failure belongs to piercing-type shear failure forms.
4.5 Research on vertical bearing capacity formula
For the prediction of vertical bearing capacity of pier foundation, according to the analysis of load transfer law of pier-soil system in Section 3.2, it can be seen that the stress form of unexpanded bottom pier foundation tested in this paper is theoretically consistent with that of large-diameter pile foundation. Therefore, the calculation formula of vertical bearing capacity can refer to the code for building pile foundation technology (JGJ 94-2008) , specification for design of building foundation (GB 50007-2011) , and literature . In the following section, the feasibility of these calculation formulas is investigated.
4.5.1 Calculation formula of technical code for building pile foundation (JGJ 94-2008)
The formula for calculating the ultimate bearing capacity of a large-diameter single pile can be calculated according to Eq. (4):
|Soil type||Sand, gravel soil|
4.5.2 Calculation formula of code for design of building foundation (GB 50007-2011)
The preliminary design of the vertical bearing capacity characteristic value of a single pile is estimated according to Eq. (5):
However, the vertical ultimate bearing capacity of a single pile is divided by safety factor 2, that is, the characteristic value
4.5.3 Calculation formula in literature
|Sandy soil, gravel soil||1.6||1.7||1.8||1.9||2.1||2.3||2.5||2.7|
Additionally, the vertical ultimate bearing capacity of a single pile is divided by the safety factor 2, that is, the characteristic value Ra of the vertical ultimate bearing capacity of a single pile. Therefore, 2Ra is used to calculate the vertical ultimate bearing capacity of a single pile.
4.5.4 Pier foundation calculation formula selection
Based on Eqs. (4)–(6), Table 8 shows the comparison of the predicted values and tested results. For the value of Qus/Quj greater than 1, it indicates that the test value is greater than the calculated value, while for the value of Qus/Quj less than 1, it indicates that the vertical bearing capacity of pier foundation will be utilized fully in the design process. However, it should be noted that the minimum bearing capacity cannot be less than 0.5. Otherwise there will be safety risks and accidents might occur.
The statistical characteristic of the ratio between the test value and the calculated value by different methods are shown in Table 9. It can be seen from Table 9 that the variation coefficient of the formula by Liu  is the minimum, indicating that the calculated value of the formula has the minimum fluctuation compared with the experimental value. At the same time, its average value is also the smallest, which is greater than 1, indicating that it is appropriate to calculate the safety reserve.
|Formula||YD800||YD1000||XD800||XD1000||Average||Mean square error||Coefficient of variation|
Based on the investigations on the vertical bearing capacity of precast pier foundation filled with demolished concrete lumps, the following conclusions can be drawn:
The Q-S curve of prefabricated pier foundation made of recycled compound concrete is of slow deformation when loaded, which is consistent with that of cast-in-place concrete pier foundation. Through Q-S curve, the vertical bearing capacity of prefabricated pier foundation with recycled compound concrete is about 25–30% lower than normal concrete cast-in-situ pier foundation, and in the pier foundation of the same diameter, the rebound deformation of prefabricated pier foundation was higher than that of cast-in-situ pier.
With the increase in diameter, the characteristic value of the vertical bearing capacity of the prefabricated foundation made of recycled compound concrete increases. At the same time, when the vertical ultimate bearing capacity of the single pier is reached, the settlement value also increases, which is consistent with the performance law of the cast-in-situ ordinary concrete pier.
The load transfer theory of pier-soil system was established, and the accuracy of the theory was verified through experimental analysis. At the same time, the variation rules of axial force, average friction resistance, and the similarities and differences between cast-in-place concrete pier foundation and prefabricated pier foundation made of recycled compound concrete are analyzed by placing strain gauge on longitudinal bars of pier body structure.
By placing the earth pressure box at the bottom of the pier, it is analyzed that the prefabricated foundation of DCLs is the same as the cast-in-place foundation of the ordinary concrete. The earth pressure at the bottom of the pier is distributed in a parabola.
Based on the comprehensive analysis of the soil pressure distribution and Q-S curve change rule of the foundation at the bottom of the pier, the failure form of the soil mass of the prefabricated pier foundation of recycled compound concrete is local shear failure, while the failure form of the soil mass of the cast-in-situ pier foundation of ordinary concrete is piercing-type shear failure.
The method proposed in this study provides a reasonable prediction for the vertical compressive ultimate bearing capacity for both prefabricated pier foundation with recycled compound concrete and cast-in-situ pier foundation of ordinary concrete.
This work was supported by the National Natural Science Foundation of China (51668045 and 51808133) and Jiangxi Science and Technology Committee (20161BBG70056). The authors also acknowledge the research support from the Australian Research Council (DE150101751), Australia.