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International Concrete Abstracts Portal

Showing 1-5 of 11 Abstracts search results

Document: 

SP95-10

Date: 

October 1, 1986

Author(s):

H.C. Fu and N. J. Gardner

Publication:

Symposium Papers

Volume:

95

Abstract:

When the shore/reshore method of construction is used high early age short duration loads are imposed upon the supporting slabs. These loads can be calculated to be of comparable magnitude to the design service loads and are applied to concrete slabs which have not achieved their specified concrete strength. Due to the slab concrete being immature with a reduced modulus of elasticity, the immediate deflections due to the construction loads are relatively large. Creep effects are dependent upon the magnitude of the applied stress relative to the concrete developed strength. Hence creep deflections due to construction loads should be large. Deflection due to concrete shrinkage also must be considered. Five, nominally identical, one-way slabs were fabricated and subjected to different load histories to compare immediate and time dependent deformations. The load histories were modelled to represent different construction methods. All slabs were designed for a live load/dead load ratio of 0.5. Taking account of the actual moduli of elasticity, the immediate deflections were consistent. The time dependent deflections were significant and of the order of 5 to 7 times the immediate deflections. Shrinkage deflections were also greater than the immediate deflections. A method is given to determine the total long term deflection of one-way slabs in terms of the peak construction load relative to the slab strength.

DOI:

10.14359/6278


Document: 

SP95-07

Date: 

October 1, 1986

Author(s):

W Hansen

Publication:

Symposium Papers

Volume:

95

Abstract:

The effects of the following variables on the elastic modulus versus compressive strength relationship were investigated: (w/c) ratio, duration of curing, water-to-cement paste content, silica fume content, and crushed limestone versus river gravel. From a set of empirical equations the effect of paste and silica fume content may be used to predict this relationship. The results are compared to the current ACI equation ACI 318-83). The proposed equations are applicable over a strength range from approximately 500 psi to 11,000 psi. Further it was found that for the concretes investigated the static modulus can be predicted from the dynamic modulus using the resonant frequency technique. In addition the entire static modulus Venus strength relationship for each concrete can be predicted from early age (up to 3 days) measurements of dynamic modulus and compressive strength.

DOI:

10.14359/6275


Document: 

SP95-05

Date: 

October 1, 1986

Author(s):

S.P. Shah, R.A. Miller, and T.E. Virding

Publication:

Symposium Papers

Volume:

95

Abstract:

A study was conducted to determine if the current design formulas can adequately predict the capacity of reinforced concrete beams tested at early ages and to determine the effectiveness of stirrups in young beams. Three sets of reinforced concrete beams were tested-with the main variables being beam age and stirrup spacing. It was found that the ACI-318 ultimate strength design method provides a lower bound to flexural strength of young reinforced concrete beams. For beams failing in shear, however. the current code provisions appear unsatisfactory. For example,the contribution of shear reinforcement was observed to be less than that predicted by the code when beams failed in shear at early ages. A change in mode of failure was often observed at early ages and the ductility of young reinforced concrete beams , was less than that for the 28 day old beams.

DOI:

10.14359/6273


Document: 

SP95-09

Date: 

October 1, 1986

Author(s):

K. Mather

Publication:

Symposium Papers

Volume:

95

Abstract:

Bars (25 mm square) of normal-consistency paste made using Type I and Type II portland cements and pastes consisting of 70 percent cement and 30 percent of a Class F fly ash, by solid volume, at the same water-to-solids ratio, were stored under four temperature regimes: the three accelerated curing regimes given in ASTM: C 684 (warm-water, boiling-water, and autogenous) and at 23 + 1.70~. Modified-cube compression tests were made and samples of the paste examined by X-ray diffraction and scanning electron microscopy at the end of accelerated curing and at 3, 7, 28, 91, 180, and 365 days. Coefficients of determination for the regression equations average approximately 0.9 for the warm-water, 0.8 for the boiling-water regimes, and were about 0.6 for the autogenous regime. The boiling-water method affects the nature of the hydration products that are present especially by degrading the crystallinity of the ettringite. The autogenous method does not provide uniform acceleration. It was concluded that the warm-water method was to be preferred. It is now used by the Corps of Engineers. The degradation of the ettringite in the boiling water regime was accompanied by the production of hydrogarnet. The micrographs of one-day old pastes cured by all relevant regimes show very large amounts of empty space in spite of the pastes having water-to-solids ratios of 0.23 and 0.25 by mass.

DOI:

10.14359/6277


Document: 

SP95-03

Date: 

October 1, 1986

Author(s):

E Acker; C. Foucrier; and Y. Malier

Publication:

Symposium Papers

Volume:

95

Abstract:

The DITEX computer program was developed to predict the temperature distribution in a concrete structural element during manufacture and to evaluate the resulting stresses. This program takes account of several types of boundary conditions : variable ambient temperatures, and various kinds of formwork and heat treatment. A simple and inexpensive specific test has been developed to determine the heat of hydration of the cement, the kinetics of which are a function of position. The test consists of recording the temperature during setting of a concrete sample subjected to adiabatic conditions. These tools have recently been applied to actual cases (bridges, tunnels, pressure vessels, etc.) and have demonstrated the variety of their applications: - optimization of the temperature cycle, making possible a significant reduction in fuel consumption; - better distribution of heating resistors to avoid cracks completely; - selection of the type of cement based on quantitative analysis - a judicious staging of the casting of large structures.

DOI:

10.14359/6271


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