CE 373– Concrete ILecture 2: 2.1 – 2.162.1-2.3) Introduction, Cement, and Aggregates2.4) Proportioning and Mixing Concrete2.5) Conveying, Placing, Compacting, and Curing2.6) Quality Control2.7) Admixtures2.8-2.9) Properties in Compression and Tension2.10) Strength under Combined Stress2.11) Shrinkage and Temperature Effects2.12) High-Strength Concrete2.13-2.16) Reinforcing Steel, Welded Wire, Prestressing SteelCE373 – Concrete I Dr. Ammar T. Al-Sayegh

2.1) Introduction RC members are composed of concrete reinforced with steel bars or prestressed

with steel wires. Thus, understanding of the material characteristics of both concrete and steel behavior under loading is fundamental for understanding the performance of RC structures.

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2.2) Cement Cement has adhesive and cohesive properties necessary to bond inert aggregates

to form strong and durable mass.

RC used hydraulic cement, which needs water for the chemical process (hydration) to turn cement powder into one solid mass.

Most common type of hydraulic cement is portland cement first presented in England in 1824, which is a finely powdered material consisting mainly of calcium and aluminum silicates.

Five standard types of portland cement have been developed. Common are:

1. Type I: Most common type (90% of construction) – require 28 days for full design strength, can hold reasonable loads. 0.40-0.60 water-cement ratio.

2. Type III: High early strength – require 14 days for full design strength, good for speeding up construction, differ from Type I with proportion and material fineness. 0.20-0.40 water-cement ratio.

Excessive amount of water causes concrete pores (weakness).

Hydration of concrete produces heat.

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2.3) Aggregates Aggregates occupy around 65%-75% of the volume of the hardened mass of the

concrete. Remainder occupied by cement, water not used by hydration, and air voids. The latter two do not contribute to the strength of the concrete.

Natural aggregates are classified as:

Fine aggregates: typically sand.

Coarse aggregates: material coarser than sand.

Maximum aggregates size is limited by the requirement that it should easily fit into forms and between reinforcement bars.

Requirements for satisfactory aggregates are stated in ASTM C33.

The unit weight for of concrete with natural stone aggregates can generally be assumed to be 2320 kg/m3.

Lightweight and Heavyweight concrete is also available.

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2.4) Proportioning and Mixing Concrete The components of concrete mix are

proportioned so that the resulting concrete has:

Adequate Strength.

Workability for placing.

Cost effectiveness (minimum cement).

The better gradation of aggregates the less cement is needed to fill the voids.

More water increases workability, but decreases strength.

Water-Cement ratio is chief factor controlling the strength of the concrete.

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2.5) Conveying, Placing, Compacting, and Curing Conveying is usually done by pumping concrete from the concrete mix truck to

the forms using steel pipelines. The prime danger in conveying is segregation. In overly liquid concrete, coarse aggregates tend to settle while water and finer aggregates tend to rise.

Placing is the process of transferring the concrete from the conveying device to the final place in the form. Proper placing must avoid segregation, displacement of the forms, or displacement of reinforcement in the forms.

Compacting is achieved through consolidating the concrete components by high-frequency vibrators.

Curing is the maintenance of proper conditions for concrete to gain its strength rapidly in the first few days after placing. This is very critical because concrete gains 70% of its strength in the first 28 days after placing.

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2.6) Quality Control The quality of steel must be ensured by its manufacturer.

Concrete is produced at or close to the building site, and its final qualities are effected by a number of factors. Thus, systematic quality control must be instituted at the construction site.

The main measure of the concrete quality is its compressive strength.

Compressive strength of concrete is tested using cylindrical samples. Standard samples about 150 mm in diameter and 300 mm in height.

The form is not perfectly150 mm in diameter, so before testing, we measure the actual diameter.

During test, measure the applied load P and the deformation (shortening) δalong a certain height h (usually 200 mm).

Applied stress and corresponding strain can be calculated as:

→ 𝜎𝜎 = 𝑃𝑃𝐴𝐴

; 𝜀𝜀 = 𝛿𝛿ℎ

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2.7) Admixtures Admixtures are additives that improve concrete performance by:

Accelerating or retarding setting and hardening.

Increase strength.

Improve workability.

Improve durability.

Decrease permeability.

Air-entraining agents are most commonly used admixtures. Note that this type of agent increases workability and durability, and reduce segregation, but it reduces concrete strength as well.

Accelerating admixtures are used to reduce setting and strength development time.

Set-retarding admixtures are used to keep concrete workable for a long time and during high ambient temperatures.

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Plasticizers and Superplasticizers are used to reduce the water requirement of a concrete mix. This can help in producing:

High-Strength Concrete.

Flowable concrete.

Improve workability.

Improve durability.

Decrease permeability.

Viscosity-modifying admixtures can be combined with superplasticizers to produce self-consolidating concrete (SCC) which does not require vibration to remove entrapped air. This type of concrete is widely used in:

Members with congested reinforcement.

Prestressed concrete (note issues with steel in upper portion of members).

Fly ash and silica fumes are pozzolans or mineral admixtures, which qualify as supplementary cementitious material and can partially replace portland cement. Ground granulated blast-furnace slag (GGBFS) is another supplementary cementitious material.

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2.8) Properties in Compression

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Concrete is primarily used for compression, so its stress-strain diagram is of a primary interest to us.

Concrete stress-strain curve starts initially with a straight elastic portion.

Concrete reaches its maximum stress (compressive strength) in the horizontal portion at strain ranges between 0.002 to 0.003.

𝑓𝑓𝑐𝑐′ ranges between 21 to 35 MPa for normal-density concrete and up to 55 MPa for prestressed concrete. High strength concrete can reach 100 MPa or more.

The modulus of elasticity, 𝐸𝐸𝑐𝑐 (in MPa), is the slope of the initial straight portion. It becomes larger as concreate strength increases and can be computed for normal-densityas:

→ 𝐸𝐸𝑐𝑐 = 4700 𝑓𝑓𝑐𝑐′ (2.4) <- Fair Accuracy

→ 𝐸𝐸𝑐𝑐 = (3320 𝑓𝑓𝑐𝑐′ + 6900) 𝑤𝑤𝑐𝑐2300

1.5(2.5) <- Higher accuracy

2.9) Properties in Tension

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Tensile strength is important in predicting crack propagation in RC members.

Tension failure in concrete involved both formation and propagation of cracks.

Modulus of rupture, 𝑓𝑓𝑟𝑟, measures the tensile strength of concrete, and can be calculated for normal-weight concrete as:

→ 𝑓𝑓𝑟𝑟 = 0.53 𝑓𝑓𝑐𝑐′

Range of tensile strength for concrete can be estimated as:

2.10) Strength under Combined Loading

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In most cases, RC members are subjected simultaneously to various stresses in multiple directions.

Combined stresses can be reduced to 3 principle stresses acting at right angles to one another on a properly oriented cube. Several states can rise from this:

Uniaxial stress state.

Biaxial stress state.

Triaxial stress state.

In most cases, the uniaxial properties such as 𝑓𝑓𝑐𝑐′ and 𝑓𝑓𝑡𝑡 are given from strength tests.Thus, it is desirable to calculate all stress states from these properties.

Interaction diagrams are used shows the strength in concrete in direction 1 as a function of the stress applied in direction 2 where all the stresses are normalized in terms of uniaxial 𝑓𝑓𝑐𝑐′.

2.11) Shrinkage & Temperature Effects As concrete dries, it shrinks in volume. Shrinkage continues at a slowing rate

with time. If not adequately controlled, it will cause cracks.

The key factor in concrete shrinkage is the unit water content in the fresh concrete.

Since aggregates do not contribute to shrinkage, increase in aggregate content can significantly decrease shrinkage.

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2.12) High Strength Concrete High-Strength Concrete refers to concrete with strength that varies in the range

of 56 to 138 MPa or higher.

Most common application for high-strength concrete is the columns for tall buildings to avoid large columns and loss of floor space and to save in the amount of reinforcement.

High-strength concrete is also useful for concrete bridges to minimize dead load and therefore long of spans.

The essential requirement for high-strength concrete is low water-cement ratio around 0.25 or lower.

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2.13) Reinforcing Steels for Concrete Tension and compression yield strength of steel, 𝑓𝑓𝑦𝑦, is around 15 times the

compressive strength of concrete, 𝑓𝑓𝑐𝑐′. However, steel is much more expensive than concrete. Thus, in RC members, steel rests in tensile stresses regions to resist tension while concrete is left to resist compression.

For effective action, it is essential that concrete and steel bond well together. This strong bonding can be achieved by:

Chemical adhesion that develops at the steel-concrete interface.

Natural roughness of hot-rolled reinforcing steel bars.

Surface deformation created by manufacturers on steel bars a provide a high degree of interlocking.

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2.14) Reinforcing Bars The most common type of reinforcing steel is in the form of rounded bars

(rebars).

In Kuwait, bars are referred to by their nominal diameter, and are available is the following sizes: 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, ..

6 mm bars are not for structural use.

Steel is locally produced, but does not cover need. Additional amounts are imported

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2.15) Welded Wire Reinforcement Welded wire reinforcement is used for reinforcing slabs.

Welded wire reinforcement consists of sets of longitudinal and transverse cold-down steel wires and right angles to each other and welded together at all points of intersection.

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2.16) Prestressing Steel Prestressing steel is used in 3 forms:

Round wires.

Stranded cables.

Alloy steel bars.

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Example L2-1The specified concrete strength for a new building is 𝑓𝑓𝑐𝑐′ = 42 MPa. Calculate the required average strength 𝑓𝑓𝑐𝑐′ for the concrete if :

a) There are no prior test results for concrete with a compressive strength within 7 MPa of 𝑓𝑓𝑐𝑐′ made with similar materials.

b) 20 test results for concrete with 𝑓𝑓𝑐𝑐′ = 35 MPa made with similar materials produce a sample standard deviation 𝑠𝑠𝑠𝑠 = 4 MPa.

c) 30 tests with 𝑓𝑓𝑐𝑐′ = 38 MPa made with similar materials produce a sample standard deviation 𝑠𝑠𝑠𝑠 = 4.1 MPa.

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