By: Edgar B. Manubag, CE, PhD

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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

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.

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. 1.2Where:

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.

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

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. ·

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

orf’cr = f’c + 2.33s – 3.5 Eq. 1-4

Where s = standard deviation, MPa

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.

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.

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.'

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.

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.

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

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

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

And

but not be less than 1.4DL,+ 1.7LL

And

but not be less than 1.4DL,+ 1.7LL

Where DL or LL reduce the effect of H

but not be less than 1.4DL,+ 1.7LL

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

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:

U = 1.4DL + 1.7LL Eq, 1 -14U = 0.75(1.4DL + 1.7LL + 1.7W) Eq. 1-15 U = 0.9DL + 1.3W Eq 1-16U = 1.1DL + 1.3LL + 1.1E Eq. l-17U.= 0.9DE + 1.1E Eq. 1 -18U = l.4DL + 1.7LL + 1.7H Eq.1-19U = 1.75(l.4DL + 1.4T + 1.7LL) Eq.1-20 but U shall not be less than

U=1.4(DL + T) Eq. 1-21Where:

W= wind loadE= earthquake loadH= soil pressureT= settlement, creep, shrinkage,

expansion of shrinkage-compensating concrete or temperature change