30-01-2013, 04:35 PM
Underwater Concrete in Drilled Shafts: the Key Issues and Case Histories
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ABSTRACT:
In construction of drilled shafts under water, placing concrete in the shafts
is technically demanding and involves complex construction logistics. Past construction
experience has demonstrated that high quality concrete can be placed in drilled shafts
under water with a proper concrete mix and proper placement techniques. However, a
significant number of failures have occurred which have resulted in excessive cost
overruns and delays. These problems may have occurred because proper underwater
concrete construction techniques have not been widely disseminated within the industry.
This is a technical area where competent design and sound construction planning can
achieve a significant reduction in both risk and cost. This paper will discuss some key
technical issues in the concrete mix design, concrete production and placement for the
drilled shaft construction. The paper also describes two lesson-learned case histories from
drilled shaft construction projects.
INTRODUCTION
Placing concrete in the shafts is one of the most critical and complex operations that
often determine success or failure of many drilled shaft construction projects. If the
concrete is placed under water, the construction is even more technically demanding and
involves complex construction logistics. A number of failures have occurred due to
improper concrete mix or improper construction procedures. The following sections
present some important technical issues that are frequently encountered in underwater
construction of drilled shafts.
CONCRETE MIX DESIGN
Because concrete placed underwater is inherently susceptible to cement washout,
laitance, segregation, cold joints, and water entrapment, it must possess some unique
properties that are not otherwise required. The following list outlines the essential and
unique requirements for concrete placed underwater:
1. Flowability: the concrete must be able to flow around and fully encase reinforcing
steel bars. In practice, the interpretation of flowability is specific to the
application for a given project. Therefore, the level of concrete flowability must be specifically defined and fully understood by all the parties involved in the
design and construction for a given project.
2. Self-compaction: the concrete must solidate itself under its own weight without
entrapping any water and laitance. It should be pointed out that the self-weight of
concrete is substantially reduced by its buoyancy in water. This differs from the
so-called self-comsolidating concrete normally placed in the dry.
3. Adequate cohesion: Cohesive concrete prevents segregation, excessive bleeding
or cement wash-out. A high degree of cohesiveness in concrete improves
homogeneity and strength of the underwater concrete. The required degree of
concrete cohesion, however, depends on many variables for a specific project. For
a given mixture, an increase in flowability tends to reduce cohesion and vice
versa. Nevertheless, modern engineering practice has proven that proper mixture
proportions can provide for highly flowable concrete with adequate cohesion
characteristics.
4. Retention of workability: underwater concrete often entails transportation of a
large quantity of materials over distance, which may create logistic problems with
regard to the time lapse between concrete mixing and concrete placement. In
such cases, the concrete mixtures must be able to maintain all the desirable
properties (such as flowability, cohesiveness, and self-compacting characteristics)
over a reasonable work window.
Concrete Production, Delivery, and Placement
Proper concrete production, supply, and placement are critical in achieving high quality
concrete and cost-effective construction. The choice of a proper underwater concreting plan for a
project has to be ultimately determined by site conditions, engineering requirements, availability
of equipment and contractor's preference. Only general comments are given in this paper.
Good quality underwater concrete is obtained through a continuous placement at a
constant placement rate. Any prolonged interruption in concrete placement imposes high
risks for defective concrete. Efforts should be made to ensure adequate and continuous
concrete supply to the placement. The location of a concrete batch plant is an important
consideration in logistics planning and has a significant impact on construction cost, risk,
and quality control. An on-site batch plant has the main advantage of providing more
reliable control of the concrete workability at the point of placement, because the time
between concrete batching and placing is relatively short. This option, however, requires
a significant investment in equipment.