1. By: Edgar B. Manubag, CE, PhD

2. CONCRETE
Concrete is a mixture of water, cement,
sand, gravel, crushed rock, or other
aggregates. The aggregates (sand, gravel,
crushed rock) are held together in a rocklike
mass with a paste of cement and water.

3. As with most rocklike mass concrete has a
very high compressive strength but have a.
very low tensile strength. As a structural
member, concrete can lie made to carry
tensile stresses (as in beam in flexure). In
this regard, it is necessary to provide steel
bars to provide the tensile strength lacking
in concrete. The composite member is called
reinforced concrete.

4. Aggregates used in concrete may be fine
aggregates (usually sand) and coarse
aggregates (usually gravel or crushed stone).
Fine aggregates are those that passes
through a No. 4 sieve (about 6 mm in size).
Materials retained are coarse aggregates.

5. The nominal maximum sizes of coarse
aggregate are specified in Section 5.3.3 of
NSCP. These are as follows: 1/5 the
narrowest dimension between sides of
forms, 1/3 the depth of slabs, or 3/4 the
minimum clear spacing between individual
reinforcing bars or wires, bundles of bars, or
prestressing tendons or ducts. These
limitations may not be applied if, in the
judgment of the Engineer, workability and
methods of consolidation are stich that
concrete can be placed without honeycomb
or voids

6. According to Section 5.3.4, water used in
mixing concrete shall be clean and free from
injurious amounts of oils, acids, alkalis, salts,
organic materials, or. Other substances that
may be deleterious to concrete or
reinforcement. Mixing water for prestressed
concrete or for concrete that will contain
aluminum embedments, including that
portion of mixing water contributed in the
form of

7. free moisture on aggregates; shall not
contain deleterious amounts of chloride ion.:
Non-potable (non-drinkable) water shall not
be used in concrete unless the following are
satisfied: (a) Selection of concrete
proportions shall be based on concrete mixes
using water from the same source and (b)
mortar test cubes made with non-potable
mixing water shall have 7-day and 28-day
strengths equal to at least 90 percent of
strengths of similar specimens made with
potable water.

8. Proportions of
materials for concrete shall be established to provide: (a) workability and consistency
to permit concrete to be worked readily into forms and around reinforcement under
conditions of placement to be employed, without segregation or excessive bleeding,
(b) Resistance to special exposures; and (c) conformance with strength test
requirements.

9. Concrete lighter in weight than ordinary
sand-and-gravel concrete is used principally
to reduce dead load, or for thermal
insulation, nailability, or fill. Disadvantages
of lightweight structural concretes include
higher cost, need for more care in placing,
greater porosity, and mote drying shrinkage,
For a given percentage of cement, usually
the lighter the concrete, the lower the
strength.

11. Concrete weighing up to about 60.5 kN/m3
can be produced by using heavier than-
ordinary aggregate. Theoretically, the upper
limit can be achieved with steel shot as fine
aggregate and steel punching as coarse
aggregate. The heavy concrete is used
principally in radiations shields and
counterweights.

12. Unlike steel and other materials, concrete
has no definite modulus of elasticity. Its
value is dependent on the characteristics of
cement and aggregates used, age of concrete
and strengths.

13. Ec= wc 1.5 0.043 √f’c (in MPa) Eq 1-1
Where:
f’c = the 28-day compressive strength of concrete
in MPa,
wc = the unit weight on concrete. in kg/m3.
For normal weight concrete:
Ec = 4700√f’c
Modulus of elasticity Es for nonprestressed
reinforcement:.
Es=200,000 MPa

15. Depending on the mix (specially the water-
cement ratio) and the time and quality of
curing, compressive strengths of concrete
can be obtained up to 97 MPa or more.
Commercial production of concrete with
ordinary aggregates is usually in the 21 to 83
MPa range with the most common ranges for
cast-in-place buildings from 21 to 41 MPa. On
the other hand, precast and prestressed
applications often expect strengths of 27.6
to 55.1 Mpa.

16. The 28-day compressive strength of concrete
can be estimated from the 7-day strength by
a formula proposed by W.A Sater:
S28 = S7 + 2.5√S7 Eq. 1.2
Where:
S28 = 28-day compressive strength in MPa
S7 = 78-day compressive strength in MPa
The stress-strain ·diagram for concrete of a
specified compressive strength is a curved
line as shown in Figure 1.1. Maximum stress
is reached at a strain of 0.002 mm/ mm,
after which the curve descends.

18. Concrete strength is influenced chiefly by
the water-cement ratio; the higher this
ratio, the lower the strength. In fact, the
relationship is approximately linear when
expressed in terms of C/W, the ratio of
cement to water by weight. For a workable
mix, without the use of water reducing
admixtures:
S28 = 18.61 C – 5.24 Eq. 1-3
W

19. With the absence of any required data,
concrete proportions shall be based on
water-cement ratio, limits in Table 1.3, if
approved by the engineer. ·

22. Required average compressive strength f’cr
used as the basis for selection of concrete
proportion shall be the larger of Eq. 1-4 or
Eq. 1-5 using a standard deviation calculated
in accordance with Sec. 5.5.3.1.1 or Sec.
5.5.3.1.2 of the Code.
f’cr = f’c + 1.34 s Eq. 1-4
or
f’cr = f’c + 2.33s – 3.5 Eq. 1-4
Where s = standard deviation, MPa

24. Metal reinforcement in concrete shall be
deformed, except that plain reinforcement be
permitted for spirals or tendons; and
reinforcement consisting of structural steel,
steel pipe, or steel tubing. Reinforcing bars to
be welded _shall be indicated on the drawings
and welding procedure to be used shall be
specified. PNS reinforcing bar specifications shall
be supplemented to require a report of material
properties necessary to conform to welding
procedures specified in "Structural Welding Code
- Reinforcing Steel" · (PNS/AWS D 1.4) of the
American Welding society and/ or "Welding of
Reinforcing Bars (PNS/ A5- 1554)' of the
Philippines National Standard.

25. Deformed, reinforcing bars shall conform to
the standards specified in Section 5.3.5.3 of
N$CP. Deformed reinforcing bars with a
specified 'yield strength fy exceeding 415
MPa shall be permitted, provided fy shall be
the stress corresponding to a strain of 0.35
percent and the bars otherwise conform.t6
one of the ASTM and PNS specifications listed
in Sec. 5.3.5.3.1.

26. Plain bars for spiral reinforcement shall
conform to the specification listed in Section
5.3.5.3.1 of NSCP. For wire with specified
yield strength fy exceeding, 415 Mpa, fy
shall be the stress corresponding to a strain
of 0.35 percent if the yield strength
specified in the design exceeds 415 MPa.'

29. According to Section 5.7~6 of NSCP, the
minimum clear spacing between parallel bars
in a layer should be db but not less than 25
mm. Where parallel reinforcement is placed
in two or more layers, bars in the upper
layers should be placed directly above bars
in the bottom layer with clear distance
between layers not less than 25 mm. In
spirally reinforced or tied reinforced
compression members, clear distance
between longitudinal bars shall be not less
than 1.5db nor 40mm.

30. Groups of parallel reinforcing bars bundled
in contact to act as a unit shall be limited to
four in any .one bundle. Bundled bars shall.
be enclosed within stirrups or ties and bars
larger than 32 mm shall not be bundled in
beams. The individual bars within a bundle
terminated within the span of flexural
members should terminate at different
points with at least 40db stagger. Since
spacing limitations and minimum concrete
cover of most members are based on a single
bar diameter db, bundled bars shall be
treated as a single bar of a diameter derived
from the equivalent total area.

31. Diameter of single bar equivalent to bundled
bars according to NSCP to be used for
spacing limitation and concrete cover.

32. Cast-in-place Concrete (nonprestressed). The
following minimum concrete cover shall ~
provided for reinforcement:

33. Precast concrete (Manufactured Under Plant
Conditions). The following minimum
concrete shall be provided for
reinforcement:

34. The following minimum concrete cover shall·
he provided for prestressed. And
nonprestressed reinforcement, ducts and
end fittings.

35. For bundled bars, the minimum concrete
cover shall be equal to the equivalent
diameter of the bundle, but need not be
greater than 50 mm, except for concrete
cast against and permanently exposed to
earth, the minimum cover shall be 75 mm.

36. The term standard hook refers to one. of the
following:
(a) 180°bend plus 4db extension but not less than
65 mm at free end,
(b) 90° bend plus 12db extension, at free end of
bar,
(c) For stirrups and tie hooks:
(1)16 mm bar and smaller, 90°· bend plus
6db extension at free end of bar,
or
(2) 20 mm and25 mm bar, 90° bend plus
6db extension at free end of bar, o
(3)25 mm bar and smaller, 135° bend plus
6db , extension at free end of bar.

37. The diameter of bend measured on the
inside of the bar, other than for stirrup and
ties in sizes 1O mm through 15 mm shall not
be less than the following:
(a) 6db for 10 mm to 25 mm bar,
(b) 8db for 28 mm to 32 mm bar, and
(c) 10 db for 36 mm bar.

38. The inside diameter of bend of stirrups and
ties shall not be less than 4db' for 16mm bar
and mm bar and smaller. For bars larger than
16 mm, the diameter of bend shall be in
accordance with the previous paragraph.

39. The most important and most critical task of
an engineer is the determination of the loads
that can be applied to a structure during its
life, and the worst possible combination of
these loads that might occur simultaneously.
Loads on a structure may be classified as
dead loads or live loads.

40. Dead loads are loads of constant magnitude
that remain in one position. This consists
mainly of the weight of the structure and
other permanent attachments.

41. Live loads are loads that may change in
magnitude and position. Live loads that move
under their own power are called moving
loads. Other live loads are those caused by
wind, rain, earthquakes, soils, and
temperature changes. Wind and earthquake
loads are called lateral loads.

42. Live loads may be applied only to the floor or
roof under consideration, and the far ends of
columns built integrally with the structure
may be considered fixed. It is permitted by
the code to assume the following
arrangement of live loads:
a) Factored dead load on all spans with
full factored live load on two adjacent spans,
and
(b) Factored dead load on all spans
with full factored live load on alternate
spans.

48. Notes:
1. In all figures shown, the wind comes from
the left.
2. In the formula for pressure coefficient on
the windward slope:
(a) e is the angle of slope with the horizontal
in degrees;
(b) The wind force is a pressure if coefficient
is positive;
(c) The wind force is suction if coefficient is
negative.

53. Structure and structural members should be
designed to have design strengths at all
sections at least equal to required strengths
calculated for the factored loads and forces
in any combination of loads.

55. And
but not be less than 1.4DL,+ 1.7LL

56. And
but not be less than 1.4DL,+ 1.7LL

57. Where DL or LL reduce the effect of H
but not be less than 1.4DL,+ 1.7LL

58. but required strength U shall not be less
than U=1.4(DL + T)

59. The design strength provided by a concrete
member shall be taken as the nominal strength
multiplied by a strength reduction factor having
the following values:

60. U = 1.4DL + 1.7LL Eq, 1 -14
U = 0.75(1.4DL + 1.7LL + 1.7W) Eq. 1-15
U = 0.9DL + 1.3W Eq 1-16
U = 1.1DL + 1.3LL + 1.1E Eq. l-17
U.= 0.9DE + 1.1E Eq. 1 -18
U = l.4DL + 1.7LL + 1.7H Eq.1-19
U = 1.75(l.4DL + 1.4T + 1.7LL) Eq.1-20
but U shall not be less than
U=1.4(DL + T) Eq. 1-21
Where:
W= wind load
E= earthquake load
H= soil pressure
T= settlement, creep, shrinkage,
expansion of shrinkage-compensating
concrete or temperature change