Piles and Foundation
1. It is not necessary to design nominal reinforcement to piles. Is it true?
In BS8110 and BS5400 Pt.4, they require the provision of nominal reinforcement for
columns. However, for pile design the requirement of nominal reinforcement may not be
necessary. Firstly, as piles are located underground, the occurrence of unexpected loads to
piles is seldom. Secondly, shear failure of piles is considered not critical to the structure
due to severe collision. Moreover, the failure of piles by buckling due to fire is unlikely
because fire is rarely ignited underground.
However, the suggestion of provision of nominal reinforcement to cater for seismic effect
may be justified. Reference is made to J P Tyson (1995).
2. How do rock sockets take up loads?
The load transfer mechanism is summarized as follows:
When a socketed foundation is loaded, the resistance is provided by both rock socket wall
and the socket base and the load distribution is a function of relative stiffness of foundation
concrete and rock mass, socket geometry, socket roughness and strength. At small
displacements the rock-socket system behaves in an elastic manner and the load
distribution between socket wall and socket end can be obtained from elastic analysis. At
displacements beyond 10-15mm, relative displacement occurs between rock and
foundation and the socket bond begins to fail. This results in reduction of loads in
rock-socket interface and more loads are transferred to the socket end. At further
displacements, the interface strength drops to a residual value with total rupture of bond
and more loads are then distributed to the socket end.
3. In designing mini-piles, should the strength of grout be neglected during assessment
of loading carrying capacity?
In designing min-piles, there are two approaches available:
(i) In the first approach, the axial resistance provided by the grout is neglected and steel
bars take up the design loads only. This approach is a conservative one which leads to
the use of high strength bars e.g. Dywidag bar. One should note that bending moment
is not designed to be taken up by min-piles because of its slender geometry.
(ii) In the second approach, it involves loads to be taken up by both grout and steel bars
together. In this way, strain compatibility requirement of grout and steel has to be
satisfied.
4. What are the considerations in determining whether casings should be left in for
mini-piles?
Contrary to most of pile design, the design of min-piles are controlled by internal capacity
instead of external carrying capacity due to their small cross-sectional area.
There are mainly two reasons to account for designing mini-piles as friction piles:
200 Questions and Answers on Practical Civil Engineering Works Vincent T. H. CHU
67
(i) Due to its high slenderness ratio, a pile of 200mm diameter with 5m long has a shaft
area of 100 times greater than cross-sectional area. Therefore, the shaft friction
mobilized should be greater than end resistance.
(ii) Settlements of 10%-20% of pile diameter are necessary to mobilize full end bearing
capacity, compared with 0.5%-1% of pile diameter to develop maximum shaft
resistance.
Left-in casings for mini-piles have the following advantages:
(i) Improve resistance to corrosion of main bars;
(ii) Provide additional restraint against lateral buckling;
(iii) Improve the grout quality by preventing intrusion of groundwater during concreting;
(iv) Prevent occurrence of necking during lifting up of casings during concreting.
5. What is the purpose of post-grouting for mini-piles?
Post-grouting is normally carried out some time when grout of the initial grouting work has
set (e.g. within 24 hours of initial grouting). It helps to increase the bearing capacity of
mini-piles by enhancing larger effective pile diameter. Moreover, it improves the behaviour
of soils adjacent to grouted piles and minimizes the effect of disturbance caused during
construction. In essence, post-grouting helps to improve the bond between soils and grout,
thereby enhancing better skin friction between them.
During the process of post-grouting, a tube with a hole at its bottom is lowered into the pile
and grout is injected. The mechanism of post-grouting is as follows: the pressurized grout
is initially confined by the hardened grout and can hardly get away. Then, it ruptures the
grout cover and makes its way to the surrounding soils and into soft regions to develop an
interlock with harder soil zones. In order to enhance the pressurized grout to rupture the
initial grout depth, a maximum time limit is normally imposed between the time of initial
grouting and time of post-grouting to avoid the development of high strength of initial
grout. Consequently, the effect of soil disturbance by installation of casings and subsequent
lifting up of casings would be lessened significantly.
6. In designing the lateral resistance of piles, should engineers only use the earth
pressure against pile caps only?
In some design lateral loads are assumed to be resisted by earth pressure exerted against the
side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do
contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance
of piles, earth pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs are adopted with
modulus of reaction kept constant or varying with depth. The normal practice of using a
constant modulus of reaction for soils is incorrect because it overestimates the maximum
reaction force and underestimates the maximum bending moment. To obtain the profile of
modulus of subgrade reaction, pressuremeter tests shall be conducted in boreholes in site
investigation. Reference is made to Bryan Leach (1980).
Saturday, 10 January 2009
Accelerating admixtures for concrete
Accelerating
Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. Chemical compositions of accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine.
Among all these accelerating materials, calcium chloride is the most common accelerator used in concrete. Most of the available literature treats calcium chloride as the main accelerator and briefly discusses the other types of accelerators. However, growing interest in using "chloride-free" accelerators as replacement for calcium chloride has been observed. This is because calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement, especially in moist environments. However, the use of good practices, i.e. proper proportioning, proper consolidation, and adequate cover thickness can significantly reduce or eliminate problems related to corrosion.
Calcium Chloride. Calcium chloride (CaCl2) is a byproduct of the Solvay process for sodium carbonate manufacture.
CaCO3 + 2NaCI Na2CO3 + CaCI2limestone
brine solution
Calcium chloride is available in two forms. Regular flake calcium chloride (ASTM D 98 Type 1) contains a minimum of 77% CaCl2; concentrated flake, pellet, or granular calcium chloride (ASTM D 98 Type 2) contains a minimum of 94% CaCl2 (ACI Comm. 212 1963). A 29% solution of CaCl2 is the most frequent form of liquid product commercially available. In solid or liquid form, the product should meet the requirement for ASTM C 494, Type C and ASTM D 98 (Admixtures and ground slag 1990).
Calcium chloride has been used in concrete since 1885 (Rixom and Mailvaganam 1986) and finds application mainly in cold weather, when it allows the strength gain to approach that of concrete cured under normal curing temperatures (Rixom and Mailvaganam 1986). In normal conditions, calcium chloride is used to speed up the setting and hardening process for earlier finishing or mold turnaround.
Effects of calcium chloride on concrete properties are also widely studied and quantified. Aside from affecting setting time, calcium chloride has a minor effect on fresh concrete properties. It has been observed that addition of CaCl2 slightly increases the workability and reduces the water required to produce a given slump (Ramachandran 1984) and reduces bleeding. Initial and final setting times of concrete are significantly reduced by using calcium chloride. Effects of calcium chloride on initial and final setting of cement paste are shown in Figure 2.4 (Ramachandran 1984). The total effect of adding calcium chloride depends on dosage, type of cement used, and temperature of the mix.
Compressive and flexural strengths of concrete are substantially improved at early ages by using calcium chloride. Laboratory tests have indicated that most increases in compressive strength of concrete resulting from the use of 2% of calcium chloride by weight of cement range from 400 to 1,000 psi (2.8 to 6.9 MPa) at 1 through 7 days, for 70° F (21° C) curing (ACI Comm. 212 1963). Long-term strength is usually unaffected and is sometimes reduced, especially at high temperatures (Admixtures and ground slag 1990).
There is evidence that drying shrinkage of mortar or concrete is increased by using calcium chloride, especially at early ages. The large shrinkage at earlier periods may be attributed mainly to more hydration. Some work has shown that it is possible to reduce drying shrinkage by the addition of sodium sulfate (Ramachandran 1984). At early ages concrete with 2% CaCl2 shows a higher resistance to freezing and thawing than that without the accelerator, but this resistance is decreased with time. It has been found, however, that addition of CaCl2 up to 2% does not decrease the effectiveness of air entrainment (Ramachandran 1984).
Because of its corrosion potential, calcium chloride—especially in prestressed concrete—has been strictly limited in use. ACI Committee 222 (1988) has determined that total chloride ions should not exceed 0.08% by mass of cement in prestressed concrete. British Standard CP.110 strongly recommends that calcium chloride should never be added to concrete containing embedded metals.
Nonchloride Accelerators Although calcium chloride is an effective and economical accelerator, its corrosion-related problem limited its use and forced engineers to look for other options, mainly nonchloride accelerating admixtures. A number of compounds—including sulfates, formates, nitrates, and triethanolamine—have been investigated. These materials have been researched and successfully used in concrete. Triethanolamine (N(C2H4OH)3) is an oily, water-soluble liquid with a fishy odor and is produced by the reaction between ammonia and ethylene oxide. It is normally used as a component in other admixture formulations and rarely, if ever, as a sole ingredient (Rixom and Ramachandran 1986).
Calcium formate is another type of nonchloride accelerator used to accelerate the setting time of concrete. At equal concentration, calcium formate (Ca[OOOCH] 2) is less effective in accelerating the hydration of C3S than calcium chloride and a higher dosage is required to impart the same level of acceleration as that imparted by CaCl2 (Ramachandran 1984). An evaluation study of calcium formate as an accelerating admixture conducted by Gebler (1983) indicated that the composition of cement, in particular gypsum (SO3) content, had a major influence on the compressive strength development of concretes containing calcium formate. Results showed that the ratio of C3A to SO3 should be greater than 4 for calcium formate to be an effective accelerating admixture; and that the optimum amount of calcium formate to accelerate the concrete compressive strength appeared to be 2-3% by weight of cement (Gebler 1983). Calcium nitrate and calcium thiosulfate are also considered accelerators.
Calcium nitrite accelerates the hydration of cement, as shown by the larger amounts of heat developed in its presence. Calcium nitrite and calcium thiosulfate usually increase the strength development of concrete at early ages (Ramachandran 1984).
Recommendations
Verification tests should be performed on liquid admixtures to confirm that the material is the same as that which was approved. The identifying tests include chloride and solids content, ph and infrared spectrometry.
Calcium chloride should not be used where reinforcing steel is present.
Calcium chloride should not be used in hot weather conditions, prestressed concrete or steam cured concrete.
In applications using calcium chloride, the dosage rate should be limited to 2 percent by weight of cement.
Care must be taken in selecting non-calcium chloride accelerators since some may be soluble salts which can also aggravate corrision.
References
Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.
ACI Committee 212. 1963. Admixtures for concrete. ACI Journal Proceedings 60 (11):1481-524.
ACI Committee 222. 1988. Corrosion of metal in concrete. ACI manual of concrete practice. Part 1. ACI 222R-85. Detroit: American Concrete Institute.
Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council
Gebler, S. 1983. Evaluation of calcium formate and sodium formate as accelerating admixtures for portland cement concrete. ACI Journal 80 (5):439-44.
Ramachandran, V. S. 1976. Calcium chloride in concrete. Science and technology. Essex, England: Applied Science Publishers.
Ramachandran, V. S. 1984. Accelerators. In Concrete admixtures handbook: Properties, science, and technology, ed. V. S. Ramachandran. Park Ridge, N.J.: Noyes Publications.
Rixom, M. R., and N. P. Mailvaganam. 1986. Chemical admixtures for concrete. Cambridge, England: The University Press.
This page last modified on June 14, 1999
Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. Chemical compositions of accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine.
Among all these accelerating materials, calcium chloride is the most common accelerator used in concrete. Most of the available literature treats calcium chloride as the main accelerator and briefly discusses the other types of accelerators. However, growing interest in using "chloride-free" accelerators as replacement for calcium chloride has been observed. This is because calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement, especially in moist environments. However, the use of good practices, i.e. proper proportioning, proper consolidation, and adequate cover thickness can significantly reduce or eliminate problems related to corrosion.
Calcium Chloride. Calcium chloride (CaCl2) is a byproduct of the Solvay process for sodium carbonate manufacture.
CaCO3 + 2NaCI Na2CO3 + CaCI2limestone
brine solution
Calcium chloride is available in two forms. Regular flake calcium chloride (ASTM D 98 Type 1) contains a minimum of 77% CaCl2; concentrated flake, pellet, or granular calcium chloride (ASTM D 98 Type 2) contains a minimum of 94% CaCl2 (ACI Comm. 212 1963). A 29% solution of CaCl2 is the most frequent form of liquid product commercially available. In solid or liquid form, the product should meet the requirement for ASTM C 494, Type C and ASTM D 98 (Admixtures and ground slag 1990).
Calcium chloride has been used in concrete since 1885 (Rixom and Mailvaganam 1986) and finds application mainly in cold weather, when it allows the strength gain to approach that of concrete cured under normal curing temperatures (Rixom and Mailvaganam 1986). In normal conditions, calcium chloride is used to speed up the setting and hardening process for earlier finishing or mold turnaround.
Effects of calcium chloride on concrete properties are also widely studied and quantified. Aside from affecting setting time, calcium chloride has a minor effect on fresh concrete properties. It has been observed that addition of CaCl2 slightly increases the workability and reduces the water required to produce a given slump (Ramachandran 1984) and reduces bleeding. Initial and final setting times of concrete are significantly reduced by using calcium chloride. Effects of calcium chloride on initial and final setting of cement paste are shown in Figure 2.4 (Ramachandran 1984). The total effect of adding calcium chloride depends on dosage, type of cement used, and temperature of the mix.
Compressive and flexural strengths of concrete are substantially improved at early ages by using calcium chloride. Laboratory tests have indicated that most increases in compressive strength of concrete resulting from the use of 2% of calcium chloride by weight of cement range from 400 to 1,000 psi (2.8 to 6.9 MPa) at 1 through 7 days, for 70° F (21° C) curing (ACI Comm. 212 1963). Long-term strength is usually unaffected and is sometimes reduced, especially at high temperatures (Admixtures and ground slag 1990).
There is evidence that drying shrinkage of mortar or concrete is increased by using calcium chloride, especially at early ages. The large shrinkage at earlier periods may be attributed mainly to more hydration. Some work has shown that it is possible to reduce drying shrinkage by the addition of sodium sulfate (Ramachandran 1984). At early ages concrete with 2% CaCl2 shows a higher resistance to freezing and thawing than that without the accelerator, but this resistance is decreased with time. It has been found, however, that addition of CaCl2 up to 2% does not decrease the effectiveness of air entrainment (Ramachandran 1984).
Because of its corrosion potential, calcium chloride—especially in prestressed concrete—has been strictly limited in use. ACI Committee 222 (1988) has determined that total chloride ions should not exceed 0.08% by mass of cement in prestressed concrete. British Standard CP.110 strongly recommends that calcium chloride should never be added to concrete containing embedded metals.
Nonchloride Accelerators Although calcium chloride is an effective and economical accelerator, its corrosion-related problem limited its use and forced engineers to look for other options, mainly nonchloride accelerating admixtures. A number of compounds—including sulfates, formates, nitrates, and triethanolamine—have been investigated. These materials have been researched and successfully used in concrete. Triethanolamine (N(C2H4OH)3) is an oily, water-soluble liquid with a fishy odor and is produced by the reaction between ammonia and ethylene oxide. It is normally used as a component in other admixture formulations and rarely, if ever, as a sole ingredient (Rixom and Ramachandran 1986).
Calcium formate is another type of nonchloride accelerator used to accelerate the setting time of concrete. At equal concentration, calcium formate (Ca[OOOCH] 2) is less effective in accelerating the hydration of C3S than calcium chloride and a higher dosage is required to impart the same level of acceleration as that imparted by CaCl2 (Ramachandran 1984). An evaluation study of calcium formate as an accelerating admixture conducted by Gebler (1983) indicated that the composition of cement, in particular gypsum (SO3) content, had a major influence on the compressive strength development of concretes containing calcium formate. Results showed that the ratio of C3A to SO3 should be greater than 4 for calcium formate to be an effective accelerating admixture; and that the optimum amount of calcium formate to accelerate the concrete compressive strength appeared to be 2-3% by weight of cement (Gebler 1983). Calcium nitrate and calcium thiosulfate are also considered accelerators.
Calcium nitrite accelerates the hydration of cement, as shown by the larger amounts of heat developed in its presence. Calcium nitrite and calcium thiosulfate usually increase the strength development of concrete at early ages (Ramachandran 1984).
Recommendations
Verification tests should be performed on liquid admixtures to confirm that the material is the same as that which was approved. The identifying tests include chloride and solids content, ph and infrared spectrometry.
Calcium chloride should not be used where reinforcing steel is present.
Calcium chloride should not be used in hot weather conditions, prestressed concrete or steam cured concrete.
In applications using calcium chloride, the dosage rate should be limited to 2 percent by weight of cement.
Care must be taken in selecting non-calcium chloride accelerators since some may be soluble salts which can also aggravate corrision.
References
Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.
ACI Committee 212. 1963. Admixtures for concrete. ACI Journal Proceedings 60 (11):1481-524.
ACI Committee 222. 1988. Corrosion of metal in concrete. ACI manual of concrete practice. Part 1. ACI 222R-85. Detroit: American Concrete Institute.
Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council
Gebler, S. 1983. Evaluation of calcium formate and sodium formate as accelerating admixtures for portland cement concrete. ACI Journal 80 (5):439-44.
Ramachandran, V. S. 1976. Calcium chloride in concrete. Science and technology. Essex, England: Applied Science Publishers.
Ramachandran, V. S. 1984. Accelerators. In Concrete admixtures handbook: Properties, science, and technology, ed. V. S. Ramachandran. Park Ridge, N.J.: Noyes Publications.
Rixom, M. R., and N. P. Mailvaganam. 1986. Chemical admixtures for concrete. Cambridge, England: The University Press.
This page last modified on June 14, 1999
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