Lateral Pressure of Concrete Forms for Wall

Подпись: Table 7.1 Pressure Values for Concrete Walls: Relation among the Rate of Filling Wall Forms, Maximum Pressure, and Temperature (ACI) Rate of filling forms, ft/h Maximum concrete pressure, lb/ft2 Temperature, °F 40 50 60 70 80 90 100 1 375 330 300 279 262 250 240 2 600 510 450 409 375 350 330 3 825 690 600 536 487 450 420 4 1050 870 750 664 600 550 510 5 1275 1050 900 793 712 650 600 6 1500 1230 1050 921 825 750 690 7 1725 1410 1200 1050 933 850 780 8 1793 1466 1246 1090 972 877 808 9 1865 1522 1293 1130 1007 912 836 10 1935 1578 1340 1170 1042 943 864 15 2185t 1858 1573 1370 1217 1099 1004 20 2635t 2138t 1806 1570 1392 1254 1144 t These values are limited to 2000 lb /ft2. Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.

Formwork should be designed to resist the lateral pressure loads exerted by the newly placed concrete in the forms. If concrete is placed rapidly in wall or column forms, the pressure can be equiva­lent to the full liquid head pressure. This requires that rate of place­ment exceed the initial set time of the concrete mix. Excessive deep vibration can liquefy the initial set of concrete within the ef­fective coverage of the vibrations. Retarder additives or cool weather can also delay the initial set and result in higher than antic­ipated lateral pressure. The formula for wall pressure established by the American Concrete Institute (ACI-347) considers the mix temperature and the rate of placement of concrete. The rate of placement is expressed in terms of feet per hour of concrete rise in the forms. Table 7.1 shows pressure values for concrete walls of different temperature and rate of filling.

Подпись: P Lateral Pressure of Concrete Forms for Wall

1. For columns and walls with rate of placement less than 7 ft/h (2.1 m/h);

with a maximum of 3000 psf (1.47 kgf/cm2) for columns, 2000 psf (0.98 kgf/cm2) for walls, a minimum of 600 psf (0.29 kgf/cm2), but no greater than 150h (0.24hst).

where p = lateral pressure (lb/ft2)

R = rate of placement, ft/h T = temperature of concrete in the form °F h = height of the form, or the distance between construction joints, ft

or

8 0R

pM = 0.073 + – r (metric equivalents)

Lateral Pressure of Concrete Forms for Wall Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.

Tq + 17.8

Подпись: (metric equivalents)+ 2.49^s, Tc + 17.8

with a maximum of 2000 psf [0.98 kgf/cm2], a minimum of 600 psf [0.29 kgf/cm2], but no greater than 150h (0.24h*).

3. For rate of placement > 10 ft/h: p = 150h or

pM = 0.24hst (metric equivalents)

The above three formulas can only be applied if concrete sat­isfies the following conditions:

• Weighs 150 pcf (2403 kg/m3)

• Contains no admixtures

• Has a slump of 4 in. (100 mm) or less.

• Uses normal internal vibrator to a depth of 4 ft (1.22 m) or less

Подпись: Copyright © Marcel Dekker, Inc. All rights reserved.

Подпись: or
Lateral Pressure of Concrete Forms for Wall
Подпись: PM

If concrete is pumped from the base of the form, the form should be designed to resist the lateral hydrostatic pressure of fresh concrete plus minimum allowance of 25 percent to account for pump surge. Caution must be taken when using external vibra­tion or concrete made with shrinkage-compensating or expansive cements as pressure higher than the hydrostatic pressure is ex­pected to occur. It is a good practice to reduce the allowable stresses to half its original value when using external vibrators.