Civil Engineering Reference
In-Depth Information
erties of concrete under given job conditions are governed
by the quantity of mixing water used per unit of cement or
cementing materials (
Abrams 1918
).
The strength of the cementitious paste binder in
concrete depends on the quality and quantity of the
reacting paste components and on the degree to which the
hydration reaction has progressed. Concrete becomes
stronger with time as long as there is moisture and a favor-
able temperature available. Therefore, the strength at any
particular age is both a function of the original water-
cementitious material ratio and the degree to which the
cementitious materials have hydrated. The importance of
prompt and thorough curing is easily recognized.
Differences in concrete strength for a given water-
cementing materials ratio may result from: (1) changes in
the aggregate size, grading, surface texture, shape,
strength, and stiffness; (2) differences in types and sources
of cementing materials; (3) entrained-air content; (4) the
presence of admixtures; and (5) the length of curing time.
Strength
The specified compressive strength,
˘
, at 28 days is the
strength that is expected to be equal to or exceeded by the
average of any set of three consecutive strength tests.
ACI
318
requires for ù to be at least 17.5 MPa (2500 psi). No
individual test (average of two cylinders) can be more
than 3.5 MPa (500 psi) below the specified strength. Spe-
cimens must be cured under laboratory conditions for an
individual class of concrete (
ACI 318
). Some specifications
allow alternative ranges.
The average strength should equal the specified
strength plus an allowance to account for variations in
materials; variations in methods of mixing, transporting,
and placing the concrete; and variations in making,
curing, and testing concrete cylinder specimens. The
average strength, which is greater than ˘, is called Â; it
is the strength required in the mix design. Requirements
for  are discussed in detail under “Proportioning” later
in this chapter. Tables 9-1 and 9-2 show strength require-
ments for various exposure conditions.
Table 9-1. Maximum Water-Cementitious Material Ratios and Minimum Design Strengths for Various Exposure
Conditions
Maximum water-cementitious material
Minimum design compressive strength,
f
c
, MPa (psi)
Exposure condition
ratio by mass for concrete
Concrete protected from exposure to
Select water-cementitious material ratio
Select strength based on structural
freezing and thawing, application of
on basis of strength, workability,
requirements
deicing chemicals, or aggressive
and finishing needs
substances
Concrete intended to have low
permeability when exposed to water
0.50
28 (4000)
Concrete exposed to freezing and
thawing in a moist condition or deicers
0.45
31 (4500)
For corrosion protection for reinforced
concrete exposed to chlorides from
0.40
35 (5000)
deicing salts, salt water, brackish water,
seawater, or spray from these sources
Adapted from
ACI 318
(2002).
Table 9-2. Requirements for Concrete Exposed to Sulfates in Soil or Water
Minimum design
Water-soluble
Maximum water-
compressive
Sulfate
sulfate (SO
4
) in soil,
Sulfate (SO
4
)
cementitious material
strength,
f
c
, MPa (psi)
exposure
percent by mass*
in water, ppm*
Cement type**
ratio, by mass
Negligible
Less than 0.10
Less than 150
No special type required
—
—
II, MS, IP(MS), IS(MS), P(MS),
Moderate
†
0.10 to 0.20
150 to 1500
0.50
28 (4000)
I(PM)(MS), I(SM)(MS)
Severe 0.20 to 2.00 1500 to10,000 V, HS 0.45 31 (4500)
Very severe Over 2.00 Over 10,000 V, HS 0.40 35 (5000)
* Tested in accordance with the Method for Determining the Quantity of Soluble Sulfate in Solid (Soil and Rock) and Water Samples, Bureau
of Reclamation, Denver, 1977.
** Cement Types II and V are in ASTM C 150 (AASHTO M 85), Types MS and HS in ASTM C 1157, and the remaining types are in ASTM C 595
(AASHTO M 240). Pozzolans or slags that have been determined by test or service record to improve sulfate resistance may also be used.
† Seawater.
