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International Concrete Abstracts Portal

Showing 1-5 of 16 Abstracts search results

Document: 

SP167-09

Date: 

March 1, 1997

Author(s):

Buquan Miao and Pierre-Claude Aitcin

Publication:

Symposium Papers

Volume:

167

Abstract:

High performance concretes (HPC) are increasingly used in high-rise building columns. Some creep and shrinkage data of HPC has been published based on laboratory tests. Very few long term field results are available. During the construction of the new library of Concordia University (Montreal, Canada) in 1990, two pairs of circular 850 mm diameter reinforced, concrete columns made of lOO-MPa silica fume concrete and of 80.MPa non silica fume concrete were instrumented with vibrating wire extensometers. One column (called active column) of each pair was part of the structure; the other (called mockup column) was never loaded and used for shrinkage and thermal strain measurements. Strains and temperatures at different locations in concrete and in reinforcing bars as well as load in concrete were recorded for about 5 years. Data obtained on these HPC columns is presented in this paper. Creep, shrinkage, thermal expansion coefficient of the two field concretes are discussed. Stresses in concrete and in reinforcing bars are analyzed. Axial loads in active columns are calculated from experimental data and compared with the specified loads used in the structural design.

DOI:

10.14359/6287


Document: 

SP167-15

Date: 

March 1, 1997

Author(s):

Y. L. MO and W. L. Hwang

Publication:

Symposium Papers

Volume:

167

Abstract:

Prestressed concrete frames are commonly used in bridge design. However, very little is known about their behavior under reversed cyclic loads, particularly when the frame is made of high strength prestressed concrete and is subjected to severe earthquakes. Most bridge codes do not provide the required design guidelines. Results from small scale models of eight prestressed concrete frames (divided into two groups), tested under various displacement histories simulating earthquake forces are presented. The primary curves (horizontal force-displacement relationships) and the hysteretic loops are determined experimentally. Concrete strength are 35 MPa and 52 MPa, for the two groups, respectively, and the effective prestress is 51 percent of the ultimate strength of prestressing steel. It is found that prestressed frames with high strength concrete provide greater ductility and dissipated energy than those with normal strength concrete. The effect of displacement history on the mechanical behavior is significant.

DOI:

10.14359/6293


Document: 

SP167-07

Date: 

March 1, 1997

Author(s):

Mohamed Lachemi, Michel Lessard, and Pierre-Claude Aitcin

Publication:

Symposium Papers

Volume:

167

Abstract:

The amount of heat developed in any concrete structure due to cement hydration is of concern for two reasons. First, concrete sets at a much higher temperature than ambient so its strength can be quite different than that of standard specimens and it shrinks as it cools. Second, cooling is not done at the same rate in all parts of the structure resulting sometimes in thermal gradients large enough to cause cracking. This paper describes briefly some features of an ongoing study of the thermal behavior of high performance concrete and presents some results. It reports measurements of temperature induced by the heat of hydration in a high performance concrete viaduct built near Montreal, Quebec, Canada. The experimental results are compared with the results of a finite element analysis with regard to early-age temperature developments in a concrete structure. The results obtained numerically are in good agreement with the experimental results. Once the validity of the numerical model is established, it becomes a powerful research tool which can be used to study different aspects related to the thermal behavior of concrete structures.

DOI:

10.14359/6285


Document: 

SP167-14

Date: 

March 1, 1997

Author(s):

Bernard Espion and Pierre Halleux

Publication:

Symposium Papers

Volume:

167

Abstract:

This paper reviews the ACI-318 Building Code requirements concerning the design of slabs post-tensioned with unbonded tendons. The design of a simply supported one-way slab is considered in detail. By taking into account all requirements (in service and at ultimate), it is shown that use of high strength concrete results in savings in the number of tendons or in slab depth when compared to a design in normal strength concrete. Tests up to failure of two similar two-span slabs, one in normal strength concrete (f'c = 40 MPa), the other in high strength concrete (f'c = 75 MPa I reveal a better ultimate load behavior for the high strength slab which exhibited more ductility than the normal strength slab. ACI requirements proved to be adequate for estimating the service load and conservative for predicting the actual carrying capacity.

DOI:

10.14359/6292


Document: 

SP167-13

Date: 

March 1, 1997

Author(s):

Naysan Khoylou and George England

Publication:

Symposium Papers

Volume:

167

Abstract:

Moisture migration in non-uniformly heated concrete is a complex phenomenon. It depends upon many factors, both intrinsic to the concrete mix and its local environment. At temperatures above 100°C pore vapour pressures dominate the mass transfer behaviour and lead to creation of dry zones containing superheated steam and zones of excessive wetness and physical saturation where condensation has occurred. Spalling of concrete, in fire, is strongly related to the water content of concrete at the time of heating and its moisture flow properties. During heating, as the temperature rises, the free water, contained in the porous structure of concrete, will expand whilst sustaining an increasing saturated vapour pressure. The continuous expansion of water together with the moisture flow frequently leads to physical saturation of the pores. Further heating will then generate additional strains in the solid envelope surrounding the pores and can lead to cracking and hydraulic fracture of the solid skeleton. High strength concrete is particularly vulnerable to this behaviour because of its inherent, low porosity, low permeability to water flow and high percentage of initial pore saturation. This paper describes numerical/theoretical modelling procedures, for the prediction of temperature-dependent moisture flow in non-uniformly heated concrete. The flow is considered to be governed dominantly by the pore pressures. A mathematical description is also provided to help understand the spalling process caused by the hydraulic fracture of the solid skeleton during heating of the water in saturated pores.

DOI:

10.14359/6291


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