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International Concrete Abstracts Portal

Showing 1-10 of 16 Abstracts search results

Document: 

SP167-15

Date: 

March 1, 1997

Author(s):

Y. L. MO and W. L. Hwang

Publication:

Special Publication

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.

10.14359/6293


Document: 

SP167-14

Date: 

March 1, 1997

Author(s):

Bernard Espion and Pierre Halleux

Publication:

Special Publication

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.

10.14359/6292


Document: 

SP167-13

Date: 

March 1, 1997

Author(s):

Naysan Khoylou and George England

Publication:

Special Publication

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.

10.14359/6291


Document: 

SP167-12

Date: 

March 1, 1997

Author(s):

Edward Francis O’Neil, Christophe Evian Dauriac, and Scott Keith Gilliland

Publication:

Special Publication

Volume:

167

Abstract:

A public-private partnership has been chosen to ignite the introduction of RPC into the United States construction market. This research and development project is being conducted under the Construction Productivity Advancement Research (CPAR) program of the US Army Corps of Engineers. The project was initiated in the fall of 1994 and it will run for three years. The program goal is to verify product integrity and gain industry acceptance and commercialization by developing and demonstrating the technical and economic viability of RPC for producing culvert/sewer pipes, pressure pipes and piles. T h e primary technology transfer has been completed, US component material source identification has been brought into action and material property verification has been initiated. Other US products development efforts have been initiated. These applications include : 0 0 spun cast concrete poles, impact resistant railroad ties and grade crossing planks.

10.14359/6290


Document: 

SP167-11

Date: 

March 1, 1997

Author(s):

Roberto C. A. Pinto and Ken Hover

Publication:

Special Publication

Volume:

167

Abstract:

This investigation was performed to access the applicability of conventional maturity functions to high strength concretes incorporating silica fume and superplasticizer. Concrete specimens were allowed to cure under three temperatures simulating hot weather, laboratory, and cold weather conditions. Both linear and exponential strength-maturity functions predicted different values of strength for different concrete curing temperatures for the same value of maturity, as has been observed by other researchers. Of these two unctions, the exponential performed somewhat better in this regard for elatively low values of maturity. Concrete strength-gain behavior was influenced by the presence of silica ume and the high amount of super-plasticizer in the mixture. Strength-maturity equations already developed for normal strength concrete underestimated strength at low maturity ages and overestimated strength at high maturity. It is suggested that further studies should be done to evaluate such effects in those relationships.

10.14359/6289


Document: 

SP167-10

Date: 

March 1, 1997

Author(s):

F. Michael Bartlett and James G. MacGregor

Publication:

Special Publication

Volume:

167

Abstract:

The ratio between the in-place compressive strength of high performance concretes and the strength of standard 28-day cylinders is investigated. Strength data for 771 cores from 3 1 large elements cast using 22 concrete mixes reported in five investigations by others are analysed. It is observed that the ratio of in-place strength to standard cylinder strength decreases as the maximum temperature sustained during hydration increases. If the concrete mix contains silica fume, Class C fly ash, or slag, the ratio of the in-place strength at 28 days to the standard 28-day cylinder strength of the same concrete is markedly less than that observed for concretes which do not contain supplementary cementitious materials. In all elements investigated, the average in-place strength continued to increase after 28 days. The relative strength gain of silica fume concretes after 28 days was significantly less than that of conventional concretes.

10.14359/6288


Document: 

SP167-09

Date: 

March 1, 1997

Author(s):

Buquan Miao and Pierre-Claude Aitcin

Publication:

Special Publication

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.

10.14359/6287


Document: 

SP167-08

Date: 

March 1, 1997

Author(s):

Anik Delagrave, Jacques Marchand, Eric Samson, Michel Pigeon, and Jean-Pierre Ollivier

Publication:

Special Publication

Volume:

167

Abstract:

The diffusion mechanisms of chloride ions into ordinary and high performance mortars were studied. Four different mortar mixtures were tested. Test parameters included the water/binder ratio (0.25 and 0.45) and the use of silica fume. An ASTM type III cement was used in the preparation of the 0.25 water/binder ratio mortars while the 0.45 water/binder ratio mixtures were prepared with an ASTM type I. For all mixtures, the sand volume fraction was maintained constant at 50%. The diffusion properties of the mortars were studied according to two different experimental procedures. In a first series of tests, apparent diffusion coefficients were calculated from chloride ion profiles measured after a 12-month immersion period. In a second series, a migration test (where the chloride ion penetration is accelerated by the application of an electrical potential of 10 volts) was used to investigate the transport properties of the four mortars. All test results clearly show that the reduction of the water/binder ratio and the use of silica fume contribute significantly to the reduction of the chloride ion penetration. The consequences of these results on the long-term durability of high-performance concrete structures and, more specifically, on their ability to resist to reinforcing steel corrosion are discussed. The ability of the accelerated migration test to reliably predict the penetration of chlorides in cement-based materials after only a 14-day test period is also discussed.

10.14359/6286


Document: 

SP167-07

Date: 

March 1, 1997

Author(s):

Mohamed Lachemi, Michel Lessard, and Pierre-Claude Aitcin

Publication:

Special Publication

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.

10.14359/6285


Document: 

SP167-06

Date: 

March 1, 1997

Author(s):

C. E. Ospina, S. D. B. Alexander, and James G. MacGregor

Publication:

Special Publication

Volume:

167

Abstract:

Reinforced concrete columns are typically made with higher strength concrete than are the floor slabs that they support. In construction, the slab is usually cast continuous through the region of the slab-column joint. As a result, load in the column above the slab must pass through a layer of weaker slab concrete before reaching the column below the slab. The column-slab joint may be viewed as a “sandwich” column, with high strength concrete above and below a layer of lower strength concrete. In design, the effective column concrete strength is based on a special weighted average of the column and slab concrete strengths. Because of confinement, the slab concrete in the joint region is assumed to be capable of carrying stresses well in excess of its specified strength. This confinement is, in turn, affected by gravity loading of the slab. Existing design procedures are based on tests of slab-column joints in which no load was applied to the slabs. This paper presents the results of a series of tests on interior column-slab joints in which service level loads were applied to the slabs prior to loading the columns. The major conclusions of this study are: (1) tests of sandwich slab-column joints with unloaded slabs consistently overestimate the strength of the connection and (2) the AC1 3 18-89 provisions for interior column-slab joints are unconservative for high ratios of column to slab strength and/or high ratios of slab thickness to column size.

10.14359/6284


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