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Presion del hormigon sobre encofrado

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pepenachog
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#1

Mar Ene 13, 2009 7:45 pm

Buenas colegas. Una consluta, estoy haciendo el diseño de un moldaje de madera. Para diseñarlo aun mejor necesito la amabilidad de alguno de Uds para calcualra la presion que efectua el hormigon sobre las paredes de un encofrado. Puedo usar la formula de Pascal y tratar al hormigon como fluido, pero ademas se que hay directa dependencia con la temperatura y velocidad de colocacion, pero no he encontrado alguna formula o tabla que me relacione esas variables. Gracias


mgf2004
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México

#2

Mar Ene 20, 2009 9:57 pm

R--------
Buenas noches:
El diseño de cimbras, moldes, etc. Es muy complejo
Por todos factores que intervienen: Propiedades del
Concreto durante el vaciado, durante el fraguado,
etc., etc. Y esto da lugar a que El diseño se base
en criterios personales (experiencia en diseño
De cimbras y moldes).
Hay una gran diversidad de teorías-formulas, pero no hay
Una que garantice que el diseño nos dará el molde
Mas ligero-resistente-y con poca deformación.

Si optas por considerar el concreto como un líquido
Tendrás un molde con demasiado peso y esto
No es práctico para su manejo en la obra.
Además de considerar que su costo se eleva
si requieres de x moldes o x ml
de cimbra para muro.

Por lo cual no es aceptable o/ razonable considerar el concreto
Como un líquido.

Un saludo.

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ilustrador2003
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#3

Mié Feb 04, 2009 7:15 pm

pepenachog.
Te recomiendo.
1. Formwork construction and practice
Autor: Richardson, J. G. ; Instituto mexicano del cemento y del concreto, A.C.
Autor: Risoul R, Juan J (Revisor); Fiscal, María (Traductor); Fernandez, Javier (Revisor)
Editorial: LIMUSA , MEXICO, MEXICO
2. Lateral Pressures for Formwork Design
Concrete International , Jun 2007 by Hurd, M K
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A review of the formulas to determine the pressure of fresh concrete

(ProQuest-CSA LLC: ... denotes formulae omitted.)

Fresh concrete exerts pressure on vertical form surfaces, and an assessment of that pressure is needed for designing forms. In the simplest theory, fresh concrete acts as a fluid exerting pressure equally in all directions at whatever point the measurement is made-essentially assuming a hydrostatic pressure effect. This is reasonable because the fresh concrete behaves much like a fluid at least briefly during vibration, or for a longer time if flowability of the mixture has been enhanced through use of admixtures or special proportioning and materials selection.

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But concrete is not a true fluid, and some method of evaluating the concrete's actual pressure is needed. Evaluating pressure has been a significant part of the work of ACI Committee 347, Formwork for Concrete. As early as 1958, Committee 347 (then Committee 622) studied available field measurements of lateral pressure on formwork and used the data to develop pressure formulas that could be safely used for form design. A report was published in 1958,1 and the formulas, with some modifications, were included in ACI's first formwork standard, ACI 347-63.2 In the days before the advent of the personal computer, the committee considered it important to keep the equations simple, reasoning that this would encourage their use and minimize mathematical errors.

These formulas were carried forward through successive ACI standards until 2001, when accumulating data3 on lateral pressures enabled the committee to introduce new coefficients for unit weight and chemistry of the mixture, expanding coverage of the formulas to mixtures with cement replacements, admixtures, or both. I wrote about those changes for CI readers in October 2002.4 Further modifications were issued in ACI 347-04,5 however, and clarification of the present status may be needed. To avoid possible confusion, the formulas presented here are given the same identifying numbers as in ACI 347-04.

BASIC FORMULA

Although the pressure at any given point within the form varies over time, the designer usually doesn't need to know the variation in detail. Hence, the equations for lateral formwork pressure provide only the maximum to be used for design.

The basic formula is:

p = wh (2.1a), in.-lb units

p = ρgh (2.1b), SI units

For the equation in in.-lb units, p is the lateral pressure, in lb/ft^sup 2^; w is unit weight of the fresh concrete, in lb/ft^sup 3^; and h is the depth in feet of fluid or plastic concrete from the top of a placement to the point under consideration in the form. For the equation in SI units, p is the lateral pressure, in kPa; ρ is the concrete density, in kg/m^sup 3^; g is the gravitational constant of 9.81 N/kg; and h is the depth of fluid or plastic concrete in meters from the top of a placement to the point under consideration in the form. If a form is filled rapidly before any stiffening of the concrete takes place, h should be taken as the full height of the freshly placed concrete. If multiple placements are to be made, h should be taken as the distance between construction joints. This formula is applicable for all conditions other than those specifically defined for the use of Eq. (2.2), (2.3), and (2.4).

COLUMN FORM PRESSURE

For the purpose of pressure determination, Committee 347 defines columns as elements with no plan dimension exceeding 6.5 ft (2 m).

For concrete with a slump of 7 in. (175 mm) or less and placed in column forms with normal internal vibration to a depth of 4 ft (1.2 m) or less, ACI 347-04 recommends the following equation for calculating the maximum pressure p^sub max^ in lb/ft^sup 2^ (kPa) to be used for column form design:

p^sub max^= C^sub w^C^sub c^ [150 + 9000R/T] (2.2), in.-lb units

... (2.2), SI units

with a minimum of 600Cw lb/ft^sup 2^ (30C^sub w^ kPa), but in no case greater than Eq. (2.1). C^sub w^ and C^sub c^ are the unit weight and chemistry coefficients shown in Tables 1 and 2, respectively; R is the rate of placement, in ft/h (m/h); and T is the temperature of the concrete during placement, in °F (°C).

With rapid placement and intensive vibration or with self-consolidating concrete, it is possible to have concrete remaining in a fluid condition for the full duration of the placement, in which case the only theoretical pressure limit will be as in Eq. (2.1). Committee 347 didn't have sufficient test data to develop separate provisions for self-consolidating concrete. A number of studies have been or are being conducted -for example, Reference 6- but definitive results have yet to be found.

WALL FORM PRESSURE

For purposes of pressure determination, ACI 347-04 defines a wall as a vertical structural element with at least one plan dimension greater than 6.5 ft (2 m).

ACI 347-04 gives two equations for wall form pressure. As in the case of the column formula, both are applicable to concrete with a slump of 7 in. (175 mm) or less and vibration to a depth of 4 ft (1.2 m) or less. The first, Eq. (2.3), applies to walls with a rate of placement less than 7 ft/h (2.1 m/h) and a placement height of 14 ft (4.2 m) or less:
Checalos, y comentalos
saludos


ivan1970
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#4

Jue Sep 18, 2014 10:49 pm

Hola buenas noches, se que este tema un muy interesante y complejo. Quisiera hacer una pregunta, cuando se desea encofrar una columna aislada de seccion 60x60 cm es necesario anclar la formaleta al piso para evitar que el concreto eleve la formaleta y arruinar el trabajo?

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