Prestessed Concrete

Materials for Prestressed Concrete

1. Concrete, Strength Requirement
 In practice, 28-day cylinder strength of 28 to 55 MPa are required for PC.
 Higher strength is necessary for PC for several reasons.
 First: Commercial anchorages for prestressing steel always designed on the basis of high strength concrete. Weaker concrete either will require special anchorages or may fail under the application of pre-stress.

Second: High strength concrete offers high resistance in tension and shear, as well as bond and bearing.
 Third: High strength concrete is less liable to the shrinkage cracks.・・・?If very good curing in a factory
 Fourth: It also has a higher modulus of elasticity and smaller creep strain, resulting in smaller loss of prestress.

 Concrete strength of 28 to 41 MPa can be obtained without excessive labor or cement.
 It is a general practice to specify a lower strength of concrete at transfer than its 28 day strength. This is desirable in order to permit early transfer of pre-stress to the concrete.

2. Concrete, Strain characteristics
 In PC, the strains are produced as well as stresses. This is necessary to estimate the loss of prestress in steel.
 Such strains can be classified into 4 types: elastic strains, lateral strains, creep strains, and shrinkage strains.

Elastic strains – just, take a look
 Review
 The stress-strain curve for concrete is seldom a straight line even at normal levels of stresses . The lower portion of the instantaneous s-s curve, being relatively straight may be called elastic.
 It is then possible to obtain the values for the modulus of elasticity.
 The modulus varies with several factors: the strength, the age, the properties of aggregate and cement and the definition of modulus.

 Tangent, initial, or secant modulus.
 The modulus may vary with the speed of load application and type of specimen (a cylinder or a beam).
 Hence it is almost impossible to predict it with accuracy.

 As an average value for concrete at 28 days old, and compressive stress up to 40% strength, the secant modulus has been approximated by the following formula.
 A. ACI code (2-1). Ec=w1.5×0.043√f
 B. By Jansen
 C. By Hognestad
 D. JSCE. Given by a Table based on the strength
 The modulus in tension is same as in compression before cracking.

Lateral strains
 Lateral strains are computed by Poisson’s ratio. The loss of prestress is slightly decreased in biaxial prestressing.
 Poisson’s ratio varies from 0.15 to 0.22, averaging about 0.17

Creep strains- just take a look
 Defined as its time-dependent deformation resulting from the presence of stress.
 A brief summary of an investigation carried out at the UC extending over 30 years.
 1.Creep continued over the entire period. Of the total creep in 20 years,
 18-35%(ave: 25) occurred in the first 2 weeks of loading,
 40-70%(ave. 55), within 3 months
 60-83%ave 76), within 1 year.

 2. Creep increased with a higher W/C ratio and with a lower aggregate cement ratio, but was not directly proportional to the total water content.
 3. Creep of concrete with type Ⅳ (low heat) shows greater.
 4. Creep of concrete was greater for crushed sandstone.

Creep strains Esp. from 28 to 90 days at time of loading, from 2-8 MPa, 50%RH
 1. Those loaded at 90 days had less creep than those at 28 days, by roughly 10%.
 3. The total amount of creep strain at the end of 20 years ranged from 1 to 5 (averaging. 3 in Japanese definition 2).
 4. The creep at 50% RH was about 1.4 times that in air at 70% RH and about 3 times that for storage in water.
 5. Creep decreased as the size of specimen increased.

Shrinkage strain
 As distinguished from creep, shrinkage in concrete is its contraction due to drying and chemical changes dependent on time and moisture conditions, but not on stresses.
 It may ranges from 0.0000 to 0.0010 and beyond. Stored under very dry condition, 0.0010 can be expected.

 Shrinkage of concrete is somewhat proportional to the amount of water.
 Hence, the water cement ratio and the cement paste should be kept to minimum.
 Thus aggregate of larger size, well graded for minimum void, will need a smaller amount of cement paste, and shrinkage will be smaller.
 Cement: shrinkage is small for cements high in C3S and low in the alkalis and the oxides of sodium and potassium.

 The amount of shrinkage varies , depending on the individual conditions.
 For the purpose of PC design, shrinkage strain would be 0.0002 to 0.0006.
 The rate of shrinkage depends chiefly on the weather conditions- swelling during rainy seasons and shrinking during dry ones.

3. Concrete, special manufacturing techniques
 Most of the techniques for good concrete can be applied to PC.
 There is a few factors peculiar to PC.
 1. They must not decrease the high strength required.
 2. They must not appreciably increase the shrinkage and creep.
 3. They must not produce adverse effects, such as inducing corrosion in the wires.

Compacting
 Compacting the concrete by vibration is usually desirable and necessary.
 Usually, without using an excessive amount of mortar, a low water cement ratio and a low slump concrete must be chosen.
 There are only a few isolated applications in which concrete of high slump is employed.

Curing
 Too early drying of concrete may result in shrinkage cracks before applying prestress.
 Only by the careful curing can the specified high strength can be attained.
 (As I explained, high strength concrete is easier to be cracked.)
 Steam curing and also auto-clave curing is often resorted to in the pre-casting factory.

Early hardening
 To speed plant production or to hasten field construction.
 High-early strength cement or steam curing is commonly employed.
 Accelerators should be employed with caution. For example, calcium chloride will cause corrosion.

Pre-cast segmental construction for prestressed bridges (cantilever)
 Breaking up a bridge superstructures into segments reduces the individual weight and facilitates casting and handling.
 They are used for longer spans , thus enabling them compete with structural steel on these larger spans.
 The joints are very thin epoxy-filled space with the surfaces being match cast.
 Prestressing tendons are threaded through.

4. Self-stressing cement
 Types of cements that expand chemically after setting and during hardening are known as expansive or self-stressing cement.
 If used, the steel is prestressed in tension, concrete is in compression, known as chemical or self-stressed concrete.
 When concrete made with expanding cement is unrestrained, the amount will be 3-5%, and the concrete will disintegrate by itself.

When restrained, the amount of expansion can be controlled but not so much.
 By applying restraint in one direction, the growth in the other two directions can be limited because of the crystalline nature of hardened paste. (maybe, not well understood)
 When high-strength steel is used to produce the prestress, say 1035 MPa and an Es of 186×103 MPa, an expansion of 1035/186×103 =0.55% (5500μ) will be required (very difficult to achieve).

Because of the expansion in all three directions,
 It seems difficult to use the cement for complicated structures.
 Expanding cement has been successfully for many interesting projects. In Japan, sewage structures, crack control or even destroying concrete.
 While many problems are remained, esp. about long term stability.

Steels for prestressing
High strength steel.
The production of high-tensile steel is by alloying. Carbon is an economical element for alloying.
Beneficial results have been obtained by quenching from the rolling heat.
The most common method is by cold drawing.
The process of cold drawing tends to realign the crystals.

 High strength steel for PC takes one of five forms: wires(Prestressing wire is a single unit made of stee), strands(Two, three or seven wires are wound to form a prestressing strand), tendon ( A group of strands or wires are wound to form a prestressing tendon), cable (A group of tendons form a prestressing cable) or bars (A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire)

 

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