Constancia en el expediente académico

C4.20 Curing by Moisture Retention Without Supplemental Heat

4.20.1 General C4.20.1 General

For curing of the concrete without supplemental heat, the surface of the concrete shall be kept covered or moist until such time as the compressive strength of the concrete reaches the strength specified for transfer of prestress or stripping.

Acceptable methods of curing are:

• Leave the unit in the forms and keep the top surfaces continuously moist by fogging, spraying, or covering with wet mats, or by covering the top surface with an impermeable cover or membrane curing compound.

• Remove the side forms and cure all exposed surfaces by the applicable methods described above.

4.20.2 Moisture Retention Enclosures C4.20.2 Moisture Retention Enclosures Enclosures used for the purpose of retaining moisture

during the curing period shall ensure that free water is present on all concrete surfaces at all times. Moisture retention enclosures shall be resistant to tearing and shall be positively fastened in place to avoid displacement by wind or other means during the curing cycle. Moisture retention enclosures shall remain in place until the completion of the curing cycle as described above.

4.20.3 Curing with Membrane Curing Compound The use of membrane curing compound to retain moisture within the concrete during curing shall be as follows:

1. The coating of membrane curing compound shall cover the entire exposed surface with a uniform film. The coating shall remain in place without gaps or omissions until the full curing cycle is complete.

Positive means shall be taken to detect and re-coat areas of incomplete coating.

C4.20.3 Curing with Membrane Curing Compound Curing compounds should only be used on surfaces where discoloration or staining will not result in an unsatisfactory product. Some curing compounds function as bond breakers and may interfere with adhesion of repairs or surface coverings such as paint, fabric, insulatio n materials, or other types of protective coatings. Curing compounds with bond-breaking characteristics should only be used if a definite bond break is required.

2. The membrane curing compound shall be applied to the exposed concrete surface at the minimum coverage rate recommended by the manufacturer.

3. The membrane curing compound shall be applied as soon as the surface bleed water sheen disappears.

4. The membrane curing compound shall be compatible with coatings or other materials to be applied to the product in later construction stages.

Membrane curing compounds should not be used in areas where joint sealants or other required adhesive materials are to be applied, unless entirely removed at the end of the curing period. Removal should be performed by sandblasting or with an approved solvent, unless conclusive test results demonstrate that the residue of the membrane does not reduce bond.

4.21 Post-Tensioning Tendon Grout C4.21 Post-Tensioning Tendon Grout

4.21.1 Scope and Purpose C4.21.1 Scope and Purpose

Grout for post-tensioning tendons used in prestressed concrete members shall provide protection of post-tensioning steel and develop bond between the prestressing steel and the surrounding concrete.

Post-tensioning is sometimes used to supplement pretensioning in precast concrete units. It may be used to provide required prestress force in excess of stressing bed capacity, in construction of segmental elements, and in situations that require close control of camber.

4.21.2 Materials for Post-Tensioning Tendon Grout

C4.21.2 Materials for Post-Tensioning Tendon Grout

The materials used for making grout to be used in post-tensioning systems shall meet the following minimum requirements:

1. Portland cement shall be in accordance with ASTM C150. The cement shall be Types I, II or III.

Type III cement is typically only used for cold weather grouting. When Type III cement is used, trial mixes should be performed to determine an appropriate mix design.

2. Water used in the grout shall be potable, clean, and free of materials known to be deleterious to cement or cause corrosion of the prestressing steel.

Admixtures may be used to reduce the water requirement and improve flowability at a given water content. While such admixtures are often used, research of horizontal tendons in semirigid ducts indicates that satisfactory grout quality may be achieved without admixtures.

DIVISION 4 CONCRETE

Standard Commentary

Page 4.46 MNL-116 4th Edition

3. Admixtures, if used, shall impart the properties of low water content, good flowability, minimum bleeding, and expansion, when required.

Admixtures shall not contain chemicals in quantities that may be deleterious to the cement or cause corrosion of prestressing steel.

Admixtures containing chloride ions (as Cl^-) in excess of 0.5% by weight of the admixture, based on 1 lb (0.45 Kg) of admixture per 94 lbs (42.6 Kg) sack of cement, shall not be used.

Admixtures that contain fluorides, sulfites, and nitrates shall not be used.

A common international standard of practice for measuring the bleedwater of grouts is to use a metal or glass cylinder with an internal diameter of approximately 4 in. (100 mm) filled to a height of approximately 4 in. (100 mm) with grout. Recent research in the United States indicates that more representative test results may be achieved by using a grout specimen container with an internal diameter of approximately 1.25 in. (32 mm) filled to a height of approximately 20 in. (508 mm). The container should be covered during the test to prevent evaporation. The following are suggested limits for determining the acceptability of the grout:

1. The total bleedwater shall not exceed 2% by volume of the grout, measured 3 hr. after mixing.

2. Maximum total bleedwater shall not exceed 4% by volume of the grout.

All admixtures shall be used in accordance with the manufacturer’s recommendations.

An approved gas -evolving material that is well dispersed throughout the admixture may be used for obtaining 5%

to 10% unrestrained expansion of the grout.

4.21.3 Proportioning of Grout C4.21.3 Proportioning of Grout Grout proportioning shall be based on tests made on

trial mixtures, or may be selected on the basis of prior documented experience with similar materials, equipment, and under comparable field conditions (weather, temperature, tendon configuration, length, etc.).

Grout that is cured at a temperature of 65°F (18°C) and at a relative humidity of approximately 70% should produce a minimum compressive strength at 28 days of 4,000 psi (27.6 MPa), unless otherwise specified.

The water content shall be the minimum necessary for proper placement. When Type I or Type II cement is used, the w/cm shall not exceed 0.45 by weight of materials.

The water content required for Type III cement shall be established on the basis of trial mixtures.

The water demand for Type III cement grout mixtures is generally more variable than for Type I or II cement mixtures. This is due to the increased variability in fineness of Type III cement.

4.21.4 Grout Mixing and Pumping Equipment 1. The grouting equipment shall be capable of

producing uniformly mixed grout that is free of lumps and undispersed cement. The equipment shall be able to continuously mix and pump the grout in a manner that will reliably and completely fill the tendon ducts.

C4.21.4 Grout Mixing and Pumping Equipment A thixotropic grout undergoes significant changes in fluidity depending on whether the grout is in motion or quiescent. This property is caused by the use of additives.

Grouts developed with admixtures and other additives often develop thixotropic properties. Thixotropic grouts can undergo a significant loss of fluidity when mechanical mixing stops.

2. Means for accurately measuring the mix constituents (batch quantities) shall be provided.

3. The grouting shall be performed with a positive displacement type pump that is capable of producing a minimum outlet pressure of 150 psi (1.05 MPa). The pump shall be properly sealed to prevent introduction of oil, air, or other foreign substance into the grout and to prevent loss of grout or water.

The grouting equipment should utilize gravity feed to the pump inlet from a hopper attached directly over top. This is a good way to ensure a continuous supply of grout to the pump.

4. A pressure gauge having a full scale reading of no greater than 300 psi (2.10 MPa) shall be in the grout line between the pump outlet and the tendon duct inlet.

Care should be taken to not overpressurize post-tensioning ducts, to ensure against damaging the surrounding concrete.

5. Grouting equipment shall contain a screen, with a maximum clear opening of 0.125 in. (3 mm) to screen the grout prior to pumping. This screen shall be easily accessible for inspection and cleaning.

If a thixotropic grout is to be used, the screen opening size can be increased to 0.188 in. (5 mm).

6. The hopper of the grout pump shall be kept at least partially full of grout at all times during the pumping operation to prevent air from being drawn into the post tension duct.

Grout bleed tubes are generally used at tendon profile high points, to ensure that air is not trapped in these locations.

7. Unless special conditions require special measures to be developed, the grouting equipment shall be capable of continuously grouting the largest tendon on the project within a maximum time of 20 minutes.

4.21.5 Mixing of the Grout C4.21.5 Mixing of the Grout 1. Water should be added to the mixer first,

followed by the cement and admixture, unless otherwise required by the admixture manufacturer.

2. Mixing duration shall ensure a uniform, thoroughly blended grout, without excessive temperature increase or loss of expansive properties. The grout shall be continuously agitated until it is pumped.

3. Water shall not be added to grout that has lost fluidity due to a delay.

Typical grout mixers for this type of application require 1.5 to 3 minutes to satisfactorily mix the grout.

DIVISION 4 CONCRETE

Standard Commentary

Page 4.48 MNL-116 4th Edition

4. The pumpability of the grout shall be established by an approved method or on the basis of previous experience with the grout mix prior to grouting.

The pumpability of the grout may be determined in accordance with the U.S. Corps of Engineers Methods CRD-C79. With this method, the efflux time after mixing should not be less than 11 seconds. The flow cone test does not apply to thixotropic grout.

4.21.6 Grout Temperature

The temperature of grout should not be above 95°F (35°C) during mixing or pumping. If necessary to comply with the temperature limit, the mixing water shall be cooled.

C4.21.6 Grout Temperature

Difficulties in pumping grout may occur when the grout temperature in the mixer exceeds 90°F (32°C).

Standard Commentary