This paper presents an experimental study of early-age tensile strengt
h (i.e., modulus of rupture) of low, medium, and high-strength concret
es. In this study, low strength indicates a 28-day concrete compressiv
e strength of 30 MPa (4300 psi), medium strength indicates a 28-day co
ncrete compressive strength of 70 MPa (10,150), and high strength indi
cates a 28-day concrete compressive strength of 100 MPa (14,500 psi).
Tests on modulus of rupture specimens were carried out at frequent int
ervals during the first 3 days after casting to observe the influence
of concrete strength. The influence of three different curing conditio
ns--temperature-matched curing, sealed curing, and air-dried curing--w
ere investigated. It was found that the gain in modulus of rupture of
temperature-matched cured concrete beams is higher than that of sealed
and air-dried beams. After an initial retardation period, the 70 and
100 MPa (10,150 and 14,500 psi) concretes showed a higher rate of flex
ural strength gain than the 30 MPa (4300 psi) concrete. It was observe
d that the current ACI Code expression for modulus of rupture overesti
mates the modulus of rupture for very early-age concrete and underesti
mates the modulus of rupture for concrete strengths above 15 MPa (2175
psi). It was also found that the equation proposed by the ACI 363 Com
mittee overestimates the modulus of rupture for all concrete strengths
and curing conditions investigated. Relationships are proposed to pre
dict the modulus of rupture as a function of the concrete compressive
strength. These relationships are applicable at very early ages up to
an age of 91 days and account for a wide range of concrete compressive
strengths and curing conditions.