Newsgroup: it.discussioni.ingegneria.civile
Titolo: resistenza calcestruzzo armato a xx giorni

Ciao a tutti, esiste una tabella indicativa della resistenza, in percentuale 
rispetto a quanto e' stata progettata una struttura, che dica dopo tot 
giorni che carico sopportera'?
mi basta ogni 5 giorni .
grazie 


vado a memoria: sul Calzona - Cestelli Guidi "il calcolo del cemento armato"
nelle prime pagine c'è qualcosa in merito sotto forma di diagramma.
Penso che però qualsiasi altro testo decente di tecnica delle costruzioni
affronti l'argomento.

il riferimento bibliografico è: CEN, prEN 1992-1-1:2003 E, final draft,
december 2003
(formule in formato excel ;-).
3.1.2   Strength
(1)P  The compressive strength of concrete is denoted by concrete
strength classes which relate to the characteristic (5%) cylinder
strength fck, or the cube strength fck,cube, in accordance with EN
206-1.
(2)P  The strength classes in this code are based on the characteristic
cylinder strength fck determined at 28 days with a maximum value of
Cmax.
Note: The value of Cmax for use in a Country may be found in its
National Annex.  The recommended value is C90/105.
(3)  The characteristic strengths for fck and the corresponding
mechanical characteristics necessary for design, are given in Table 3.1.
(4)  In certain situations (e.g. prestressing) it may be appropriate to
assess the compressive strength for concrete before or after 28 days, on
the basis of test specimens stored under other conditions than
prescribed in EN 12390.
If the concrete strength is determined at an age t > 28 days the values
Alpha_cc and  Alpha_ct defined in 3.1.6 (1)P and
3.1.6 (2)P should be reduced by a factor kt.
Note: The value of kt for use in a Country may be found in its National
Annex. The recommended value is 0,85.
(5)  It may be required to specify the concrete compressive strength,
fck(t), at time t for a number of stages (e.g. demoulding, transfer of
prestress), where
fck(t) = fcm(t) - 8 (MPa) for       3 < t < 28 days.
fck(t) = fck                     for     t >= 28 days
More precise values should be based on tests especially for t <= 3 days
(6)  The compressive strength of concrete at an age t depends on the
type of cement, temperature and curing conditions. For a mean
temperature of 20 °C and curing in accordance with EN 12390 the
compressive strength of concrete at various ages fcm(t) may be estimated
from Expressions (3.1) and (3.2).
fcm(t) = Beta_cc(t)*fcm              (3.1)
with
Beta_cc(t)=exp[s*(1-radq(28/t)]       (3.2)
where:
fcm(t)   is the mean concrete compressive strength at an age of t days
fcm is the mean compressive strength at 28 days according to Table 3.1
Beta_cc(t)   is a coefficient which depends on the age of the concrete t
t is the age of the concrete in days
s is a coefficient which depends on the type of cement:
   = 0,20 for cement of strength Classes CEM 42,5 R, CEM 53,5 N and CEM
53,5 R (Class R)
   = 0,35 for cement of strength Classes CEM 32,5 R, CEM 42,5 N (Class
N)
   = 0,38 for cement of strength Classes CEM 32,5 N (Class S)
Where the concrete does not conform with the specification for
compressive strength at 28 days the use of Expressions (3.1) and (3.2)
is not appropriate.
This clause should not be used retrospectively to justify a non
conforming reference strength by a later increase of the strength.
For situations where heat curing is applied to the member see 10.3.1.1
(3).
(7)P  The tensile strength refers to the highest stress reached under
concentric tensile loading. For the flexural tensile strength reference
should be made to 3.1.8 (1).
nota: non potendo riportare la tabella 3.1 per intero, dovrebbe essere
sufficiente la seguente formula:
fcm = fck + 8 (MPa)
saluti,
v.