Behaviour of precast reinforced concrete pile caps

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Abstract

The objective of this investigation is to study the behaviour of precast reinforced concrete pile caps and the ultimate load-carrying capacity. Three pile cap units were cast and tested to failure. One unit was a control pile cap cast in situ and the other two were precast reinforced units with in situ concrete infill. The experimental results showed that the precast pile cap behaved in a similar manner as compared with the conventional cast in situ pile cap. Furthermore, all the three units failed at loads exceeding the failure loads predicted using conventional design methods and exhibited predicted failure modes. In addition, there was a substantial increase in productivity as the precast pile caps could be constructed quickly and thus reducing the risk of exposing the excavated pit to rain and possible failure of the unsupported sides.

Introduction

The current trend of increasing efficiency and productivity in the management of construction activities has placed considerable emphasis on the use of precast members where off-site manufacture, under controlled conditions, and uncoupled from site processes and delays, can provide a constant supply of precast elements. The use of precast elements is more crucial at locations where heavy rains can cause serious delays due to a difficult working environment. This is particularly evident for foundation works in soft or slimy soils where heavy rainfall can cause the sides of the excavation to fail and thus requires further time and effort to rectify the excavation.

The construction of conventional cast in situ pile caps (see Fig. 1) requires an excavation for the pile cap, base preparation with a layer of lean concrete, construction of forms, installation of a steel reinforcement cage and placing of fresh concrete. This sequence of work may easily take up to 2 days for a small pile cap of 1–2 m width. The steel cage may be pre-constructed and lifted into the pit to speed up this process. This current practice is vulnerable to heavy rains especially when the surrounding soil is weak. Flooding followed by failure of the sides of the pit is not uncommon.

An innovative system of precast pile caps is proposed where no extensive ground preparation or external forms are required. The steel cage can be constructed separately and cast with a thin layer of concrete on the sides to form a precast reinforced concrete shell as illustrated in Fig. 2, Fig. 3. This shell serves as a permanent form for the pile cap and rests directly on the cut-off piles. The precast shell is then infilled with in situ concrete to complete the construction of the pile cap. A lean concrete layer, which is normally required to provide a firm base, may not be necessary with this system.

The objectives of this project are to compare the ultimate load-carrying capacity of precast reinforced concrete pile caps with conventional cast in situ pile caps and to study the behaviour of these precast units. No previous experimental work on precast pile caps was reported in the literature.

Section snippets

Design concept

The concept of this precast pile cap is to cast a thin concrete shell together with the steel reinforcement cage to provide a permanent form to hold the fresh concrete. The sides of the steel cage are cast with a thin layer of concrete of approximately 70 mm to provide an outer cover of at least 50 mm to the steel bars. Inner cover to the steel is not required, as the in situ concrete will protect the bars. The bottom of the steel cage is left open to enable it to rest on the top of the piles,

Methods

Three pile cap units for a four-pile group were fabricated. The first unit is a conventional cast in situ pile cap of 1000×1000×400 mm designed in accordance with BS8110 and to fail in flexure. Four 150-mm concrete cubes were utilised to represent the piles. The second unit was of similar dimensions and steel reinforcements, but precast with a shell thickness of 70 mm. The third unit of 1000×1000×300 mm was precast and was constructed with a larger amount of reinforcement to investigate failure

Results

The observed load-deflection relationships at the pile cap centre, for the three units, are shown in Fig. 5. The observed crack widths and ultimate loads of the three pile cap units are tabulated in Table 2, Table 3, respectively.

Pile cap A was predicted to fail at a total load of 890 kN by the design equations of BS8110, with flexure being critical. However, the unit failed at 38% higher load of 1230 kN. The load-deflection curve was linear up to a point of more than 800 kN, where a definite

Crack behaviour

The pile caps typically had very few cracks prior to failure. Units A and B failed in flexure with flexural cracks extending diagonally between the piles. Failure of unit C was with a square crack pattern within the four piles indicative of punching shear failure.

Fig. 6 shows the deformation pattern at the soffit of the pile caps at failure and Fig. 7 shows the crack patterns at the sides of the pile caps. The flexural cracks originated from the centre of the soffit of pile cap A, extending

Discussion

The comparison of crack widths indicates that the precast unit B has slightly larger crack widths compared to the conventional cast in situ unit A. However, the first crack occurred at a marginally higher load in the precast unit. The loads at which these cracks occurred in units A and B were higher than the estimated working loads that the pile caps were designed for. It is evident from the crack widths, loads at which these cracks occurred and the crack patterns that the precast pile cap

Summary and conclusions

A comparison of the observed failure loads of the two precast test units with predictions from the British code indicates that the failure load of precast pile caps was approximately 40% and 7% higher when the units failed in flexure and shear, respectively. The behaviour of the precast unit is similar to the corresponding cast in situ unit with only a slight increase in crack widths. It is therefore expected that current design equations for conventional cast in situ construction can be used

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