BrownSchindler GeoDenver 2007
BrownSchindler GeoDenver 2007
BrownSchindler GeoDenver 2007
CONSTRUCTION
Dan Brown1, Anton Schindler1
ABSTRACT: Drilled shaft foundations on major bridge projects often require the use of
underwater placement of large volumes of concrete through densely placed rebar, with
the cage often made even more congested with the placement of additional tubes for postconstruction integrity testing. Such conditions represent the most difficult circumstances
for constructors in terms of concrete placement to achieve a shaft which is free of defects
or anomalies in the integrity testing. The concept of high performance for drilled shaft
concrete is that the mixture should address the most critical performance requirements
related to workability. This paper describes the important considerations in concrete
mixture design for drilled shaft construction including the use of admixtures and mixture
components associated with self-consolidating concrete (SCC). Some case histories of
difficulties associated with concrete are also described.
INTRODUCTION
Large diameter drilled shafts are becoming increasingly popular on major bridge
projects due to increased availability of drilling equipment and skilled contractors and
inherent advantages of high capacity shafts in supporting axial and lateral loads. Shaft
diameters of up to 4 m (13 ft) and lengths of up to 80 m (260 ft) are no longer unusual.
These shafts often require the use of underwater placement of large volumes of concrete
through densely placed rebar, with the cage usually made even more congested with the
placement of additional tubes for post-construction integrity testing. Such conditions
represent the most difficult circumstances for constructors in terms of concrete placement
to achieve a shaft which is free of defects or anomalies in the integrity testing.
This paper provides an overview of the most important characteristics of concrete
mixture design and construction practices for drilled shaft construction. In drilled shaft
applications, the most critical performance requirements for concrete are related to
workability and thus the concept of high performance material emphasizes the
construction aspects of the mixture in addition to hardened properties required to meet
structural design requirements. Important components of a good mixture design and
installation plan include requirements for workability for the duration of the concrete
placement operations, passing ability, resistance to bleeding, and low heat of hydration.
Strength requirements and aggregate properties that are consistent with these
considerations are important, as are concrete placement techniques used by the
contractor. If any single important issue is not adequately addressed in the mixture
design and placement procedures, placement difficulties and/or anomalies in integrity test
measurements can result.
concrete
C oncrete w ith
inadequate w orkability
Workable concrete for tremie placement in drilled shafts must be a flowable, cohesive,
self consolidating mixture that is easily placed without external vibration. Although the
use of the term self-consolidating concrete or SCC has been used in recent years with
reference to mixtures with ultra workability in conventional concrete applications, drilled
shaft concrete has always been intended as a self consolidating mixture. Traditionally,
drilled shafts have been constructed using slump as the sole indication of workability.
Alternative methods to describe workability may have application in large diameter
drilled shafts.
Concrete slump ranging from 175 to 225 mm (7 to 9 inches) has been found to provide
adequate workability for drilled shafts up to 2.5 m (8 ft) in diameter if the reinforcing
cage has openings not less than 150 mm (6 inches). For mixtures requiring greater
workability, the use of slump flow and/or the L-box (or J-Ring) tests may be more
suitable for assessing the properties of the fresh concrete. Figure 4 provides an
illustration of these control tests, now in routine use for SCC mixtures. The slump flow
is a simple test performed with a conventional slump cone, but measurements are
performed on the diameter of the resulting fluid concrete mixture rather than the height of
the cone. Based on some initial field trials of drilled shaft construction using SCC-type
mixtures (Brown et al. 2005), slump flow requirements in the range of 450 to 600 mm
(18 to 24 inches) appear suitable for drilled shaft construction.
Figure 4 Slump Flow (left) and L-Box (right) Measurements of Workability and
Passing Ability
Concrete mixtures can be designed with high workability by using suitable aggregates
and gradation and the proper dosage of water reducing admixtures. Some of the key
components for high workability drilled shaft mixtures are as follows:
Rounded gravel aggregate sources are much preferred over crushed stone in these
mixtures. Coarse aggregates with a No. 67 or No. 78 gradation have performed
better than a No. 57 in terms of workability.
In general, an increase in the sand content in proportion to coarse aggregate will
provide increased workability and passing ability with less tendency for
segregation; a sand to total aggregate ratio (by volume) from 0.44 to 0.50 has been
found to work well in drilled shaft mixtures.
WORKABILITY RETENTION
For large diameter shafts which can often require 300 to 500 m3 (400 to 650 yd3) of
tremie-placed underwater concrete, retention of workability is critical. The dosage of
retarding or hydration control admixtures must be selected to ensure that the concrete
retains adequate workability to allow the tremie placement to be completed. Loss of
workability will lead to difficulties in maintaining flow through the tremie, with attendant
flaws in the shaft as described above and illustrated in Figure 5.
Trem ie
Slurry
Fresh,fluid concrete
Trapped Laittance
O ld,stiffconcrete
600
Tremie Placement
Loading Traveling Hopper # 1
500
400
300
Waiting Period
12:05 AM: tremie stuck, rigging failure resulted
12:30 AM: tremie re-rigged, pulled free.
12:35 AM: tremie over-flowed , pour stopped,
cable up and down.
12:40 AM: flow resumed, but last 10CY in
hopper would not come out,
sprayed and wasted.
1:00 AM: new hopper, flow resumed.
5 hrs 34 min.
200
100
7:00 AM
6:00 AM
5:00 AM
4:00 AM
3:00 AM
2:00 AM
1:00 AM
12:00
AM
11:00
PM
10:00
PM
9:00 PM
8:00 PM
7:00 PM
6:00 PM
Crack
Crack
accelerate the rate of hydration significantly and reduce the concretes workability. This
effect is nonlinear and rate of hydration increases dramatically with temperature in excess
of 70F. The measurements presented on Figure 10 demonstrate the effect of initial
temperature on the heat generated within the concrete as a function of time. This
generated heat produces more rapid setting in the mixture and a significantly higher heat
of hydration in mass concrete.
14
T0 = Initial Temperature
T0 = 90 F
= 28.0 hrs
= 1.50
u = 0.850
12
10
T0 = 80 F
8
6
T0 = 70 F
T0 = 50 F
2
0
0
10
20
30
40
50
60
70
C oncrete A ge (hours)
Figure 10: The effect of different initial mixture temperatures on the temperature
development during adiabatic conditions (Schindler 2002)
Besides the concern about setting time, high heat of hydration is a potential concern
for drilled shaft concrete. Shafts larger than about 1.2 m (4 ft) diameter have
characteristics of mass concrete in which the heat of hydration can feed on itself and
generate large temperatures within the shaft. Recent measurements in Florida (Mullins,
2006) have shown temperatures within the interior of 3 m (10 ft) diameter shafts as high
as 180F. Concrete members made with plain portland cement that reach temperatures
above 158F may exhibit delayed ettringite formation (DEF). DEF can significantly
reduce long term durability of the hardened concrete. The temperature development of
an in-place mixture within an actual shaft can be evaluated on test specimens by using
adiabatic or semi-adiabatic calortimery (Schindler and Folliard 2005).
In-place temperatures can be controlled by: 1) limiting the total cementitious materials
content , 2) controlling the fresh concrete placement temperature, and 3) proper selection
of the cementitious material types.
The amount of total cementitious materials has implications relative to design
compressive strength. However, concrete design stresses are often quite low in drilled
shafts and so it is prudent that the mixture design requirements not exceed the actual
performance requirements for design. Because of concerns for setting time and heat of
hydration, the use additional portland cement to accommodate an unnecessarily high
project should have a specific mixture developed to meet the requirements for that
project. Fresh concrete performance grades, as proposed for hardened properties
(Goodspeed, Vanikar, and Cook 1996) should be established for drilled shaft concretes.
By preselecting the required performance grade for each fresh property, this will enable
designers, contactors, and concrete suppliers to communicate the specific unique
requirements of the concrete placement before bidding of the project costs. A single
mixture design cannot be simply transferred from one locale to another without
consideration of the specific source materials and project requirements.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the contributions of Joe Bailey of Applied
Foundation Testing, Prof. Gray Mullins of the Univ. of S. Florida, Billy Camp and Aaron
Goldberg of S&ME, Charleston, Jeff Sizemore of SCDOT, Mark McClelland of
TXDOT, and Steve Dapp of Dan Brown & Associates.
REFERENCES
Brown, D., (2004). Zen and the Art of Drilled Shaft Construction, GeoSupport 2004,
ASCE, GSP 124, pp. 19-33.
Brown, D., Bailey, J., and Schindler, A. (2005). The Use of Self-Consolidating
Concrete for Drilled Shaft Construction: Preliminary Observations from the Lumber
River Bridge Field Trials, Proc, Geo Construction QA/ QC Conf, ADSC: The Intl
Assoc. of Foundation Drilling, Dallas/Ft. Worth, TX, pp. 437-448.
Camp, W., Brown, D., and Mayne, P. (2002). Construction Method Effects on Axial
Drilled Shaft Performance, Deep Foundations 2002, ASCE, GSP 116, pp 193-208.
Deese, G., and Mullins, G. (2005). Factors Affecting Concrete Flow in Drilled Shaft
Construction, Proceedings, Geo Construction Quality Assurance / Quality Control
Conference, ADSC: The International Association of Foundation Drilling, Dallas/Ft.
Worth, TX, pp. 144-155.
F&ME Consultants (2004). Report of Drilled Shaft Coring Unpubl Report to SCDOT
Goodspeed, C.H., Vanikar, S., and Cook, R.A. (1996). High-Performance Concrete
Defined for Highway Structures, Concrete International, Vol. 18, No. 2, pp. 62-67.
Holland, T. and Gerwick, B. (1983). Cracking of Mass Concrete Placed Under Water,
Concrete International, April, pp. 29-36.
Mullins, G. (2006). Personal communication.
Schindler, A.K. (2002). Concrete Hydration, Temperature Development, and Setting at
Early-Ages, Ph.D. Dissertation, The University of Texas at Austin, Texas.
Schindler A.K., and Folliard K.J. (2005). Heat of hydration models for cementitious
materials, ACI Materials Journal, Vol. 102, No. 1, 2005, pp. 24-33.
Yao, S. and Gerwick, B. (2004). Underwater Concrete, Concrete Intl, Feb., pp. 77-82.