ACI 224.3R Joints in Concrete

Construction (Non-Contractual)

Chapter 3 - Buildings

� For monolithic concrete, a good

construction joint might be a bonded interface that provides a

watertight surface, and allows for flexural and shear continuity

through the interface.

� Without this continuity, a weakened region results that may

serve as a contraction or expansion joint.

ACI 224.3R

3.2.1 Joint construction

To achieve a well-bonded watertight interface, a fewconditions should be met before the placement of freshconcrete.

The hardened concrete is usually specified to be cleanand free of laitance (ACI 311.1R). Older joints needadditional surface preparation. Concrete that has setshould be prepared using a wet sand blast or ultra-highpressure water jet (ACI 311.1R).

ACI 318 states that existing concrete should be moistenedthoroughly before placement of fresh concrete. Concretethat has dried out may require saturation for a day ormore, but concrete that has been placed recently willnot require additional water.

Only if a few hours elapse between successiveplacements, a visual check is needed to be sure thatloose particles, dirt, and laitance are removed. The newconcrete will be adequately bonded to the hardenedgreen concrete, provided that the new concrete isvibrated thoroughly. Cleaning by an air-water jet or wirebrooming can be done when the concrete is still softenough that laitance can be removed, but hard enoughto prevent aggregate from loosening.

Other methods may also be useful for preparing aconstruction joint for new concrete.

ACI 224.3R

3.2.1 Joint construction

3.2.2.2 Vertical Elements —Although placements with a depth of30 ft (10 m) have been made with conventional formwork, it isgeneral practice to limit concrete placements to a height of onestory. Horizontal Construction joint should be located at theundersides of footings. Construction joints are provided at the topof floor slabs for capitals should be placed monolithically with thePile Cap.

The placement of fresh concrete on a horizontal surface canaffect structural integrity of the joint.

Although it is not always necessary, common practice has beento provide a bedding layer of mortar, of the same proportions asthat in the concrete, before placement of new concrete abovethe joint. ACI 311.1R recommends using a bedding layer ofconcrete with somewhat more cement, sand, and water than thedesign mix for the structure. Aggregate less than 3/4 in. (20 mm)can be left in the bedding layer, but larger aggregate should beremoved. This mixture should be placed 4 to 6 in. (100 to 150 mm)deep and vibrated thoroughly with the regular mixture placedabove.

Shear transfer and bending at Horizontal joints should beaddressed in much the same way it is for beams and slabs. Thereinforcement should continue through the joint, with adequatelength to ensure a complete splice. If the joint is subject to lateralshear, load transfer by shear friction or dowel action is added.

ACI 224.3R

CHAPTER 10—MASS

CONCRETE

10.3—Construction joints

Proper preparation of construction joints before

placing fresh concrete upon the construction joint

surfaces is important in assuring the integrity of a

concrete structure. There are several methods for

preparing construction joints, including green

cutting, sandblasting, and high-pressure water jet.

Detailed methods of preparation of horizontal

construction joints in massive concrete structures

are given in ACI 207.1R.

ACI 207.1R-05 Guide to Mass

Concrete (Non-Contractual)

Mass Concrete:

� ACI 116R defines mass concrete as

“any volume of concrete with dimensions large enough to

require that measures be taken to cope with generation of heat from

hydration of the cement and attendant volume change to

minimize cracking.”

ACI 207.1R-054.3 Placing

The surface should be left free from protruding rock,

deep footprints, vibrator holes, and other surface

irregularities

In general, the surface should be relatively even, with

gentle slope for drainage.

Before of the placement of the next lift, the surface

film and contamination should be removed to expose

a fresh clean mortar and aggregate surface. Over

cutting to deeply expose aggregate is unnecessary

and wasteful of good material.

ACI 207.1R-054.3 Placing

Strength of bond is accomplished by cement grains, not by

protruding coarse aggregate. Usually removal of a few

millimeters of inferior material will reveal a satisfactory surface.

The best methods of obtaining such clean surface are by means

of wet sandblasting or high-pressure water jet of at least 6000 psi.

This has the advantage of being able to clean concrete of any

age. The water-jet method may not work as efficiently after the

concrete is more than a week old.

The initially acceptable surface may become dull with lime

coatings or can become contaminated to such an extent that it

may be necessary to use sandblasting or high-pressure water jets

to re-clean it.

Recommendations upon

placement

ACI 224.3R: 3.2.1 Joint construction

� Pools of water should not be left standing onthe wetted surface at the time ofplacement; the surface should just bedamp. Free surface water will increase thewater-cement ratio of new concrete at theinterface and weaken the bond strength.

� Form construction plays an important role inthe quality of a joint. It is essential to minimizethe leakage of grout from under bulkheads(Hunter, 1953). If the placement is deeperthan 6 in. (150 mm), the possibility of leakageincreases due to the greater pressure headof the wet concrete. Grout that escapesunder a bulkhead will form a thin wedge ofmaterial, which must be cut away beforethe next placement. If not removed, thiswedge will not adhere to the fresh concrete,and, under load, deflection in the elementwill cause this joint to open.

ACI 207.1R: 4.3 Placing

� The concrete surface should be

approaching dryness and be free from surface moisture at the time

new concrete is placed on it.