1、1 The principles governing proper placement of concrete are:2 Segregation must be avoided during all operations between the mixer and the point of placement, including final consolidation and finishing. 3 The concrete must be thoroughly consolidated, and should fill all angles and corners of the for
2、ms. 4 Where fresh concrete is placed on hardened concrete, a good bond must be developed.5 The temperature of fresh concrete must be controlled from the time of mixing through final placement, and protected after placement. 。6 Concrete may be conveyed from a mixer to point of placement by any of a v
3、ariety of methods and equipment, if properly transported to avoid segregation. Selection of the most appropriate technique for economy depends on jobsite conditions, especially project size, equipment, and the contractors experience.In building construction， concrete usually is placed with hand-or p
4、ower-operated buggies; drop bottom buckets with a crane; inclined chutes; flexible and rigid pipe by pumping; shot-crete, in which either dry materials and water are sprayed separately or mixed concrete is shot against the forms; and for underwater placing, tremie chutes (closed flexible tubes).For
5、mass concrete construction, side-dump cars on narrow-gage track or belt conveyers may be used For pavement, concrete may be placed by bucket from the swinging boom of a paving mixer, directly by dump truck or mixer truck, or indirectly by trucks into a spreader. 7 Even within the specified limits on
6、 slump and water-cementitious materials ratio, excess water must be avoided. In this context, excess water is presented for the conditions of placing if evidence of water rise (vertical segregation) or water flow (horizontal segregation) occurs.Excess water also tends to aggravate surface defects by
7、 increased leakage through form openings. The result may be honeycomb, variations in color, or soft spots at the surface.8 In vertical formwork, water rise causes weak planes between each layer deposited. In addition to the deleterious structural effect, such planes, when hardened, contain voids whi
8、ch water may pass through.9 In horizontal elements, such as floor slabs, excess water rises and causes a weak laitance layer at the top. This layer suffers from low strength, low abrasion resistance, high shrinkage, and generally poor quality. 10 The purpose of consolidation is to eliminate voids of
9、 air and to ensure intimate complete contact of the concrete with the surfaces of the forms and the reinforcement.Intense vibration, however, may also reduce the volume of desirable entrained air; but this reduction can be compensated by adjustment of the mix proportions 11 Powered internal vibrator
10、s are usually used to achieve consolidation. For thin slabs, however, high-quality, low-slump concrete can be effectively consolidated, without excess water, by mechanical surface vibrators. For precast elements in rigid external vibration is highly effective. External vibration is also effective wi
11、th in-place forms, but should not be used unless the formwork is specially designed for the temporary increase in internal pressures to full fluid head plus the impact of the vibrator. 12 Except in certain paving operations, vibration of the reinforcement should be avoided. Although it is effective,
12、 the necessary control to prevent over-vibration is difficult. Also, when concrete is placed in several lifts of layers, vibration of vertical rebars passing into partly set concrete below may be harmful.Note, however, that re-vibration of concrete before the final set, under controlled conditions,
13、can improve concrete strength markedly and reduce surface voids.This technique is too difficult to control for general use on field-cast vertical elements, but it is very effective in finishing slabs with powered vibrating equipment. 13 The interior of columns is usually congested; it contains a lar
14、ge volume of reinforcing steel compared with the volume of concrete, and has a large height compared with its cross-sectional dimensions.Therefore, though columns should be continuously cast, the concrete should be placed in 2-to 4-ft-deep increments and consolidated with internal vibrators. These s
15、hould be lifted after each increment has been vibrated.If delay occurs in concrete supply before a column has been completed, every effort should be made to avoid a cold joint. When the remainder of the column is cast, the first increment should be small, and should be vibrated to penetrate the prev
16、ious portion slightly. 14 In all columns and reinforced narrow walls, concrete placing should begin with 2 to 4 inches of grout. Otherwise, loose stone will collect at the bottom, resulting in the formation of honeycomb.This grout should be proportioned for about the same slump as the concrete or sl
17、ightly more, but at the same or lower water-cementitious material ratio.Some engineers prefer to start vertical placement with a mix having the same proportions of water, cement, and fine aggregate, but with one-half the quantity of coarse aggregate, as in the design mix, and to place a starting lay
18、er 6 to 12 inches deep.When concrete is placed for walls, the only practicable means to avoid segregation is to place no more than a 24-in layer in one pass. Each layer should be vibrated separately and kept nearly level. 15 For walls deeper than 4 ft, concrete should be placed through vertical. The
19、 concrete should not fall free more than 4 ft or segregation will occur, with the coarse aggregate ricocheting off the forms to lodge on one side. Successive layers after the initial layer should be penetrated by internal vibrators for a depth of about 4 to 6 in to ensure complete integration at the
20、 surface of each layer. Deeper penetration can be beneficial (re-vibration), but control under variable jobsite conditions is too uncertain for recommendation of this practice for general use. 16 The results of poor placement in walls are frequently observed: slope layer lines; honeycombs, leaking,
21、if water is present; and, if cores are taken at successive heights, up to a 50% reduction in strength from bottom to top. Some precautions necessary to avoid these ill effects are: 17 Do not move concrete laterally with vibrators18 For deep, long walls, reduce the slump for upper layers 2 to 3 in be
22、low the slump for the starting layer. 19 On any delay between placing of layers, vibrate the concrete thoroughly at the interface.20 Concrete should be inspected for the owner before, during, and after casting. Before concrete is placed, the formwork must be free of ice and debris and properly coate
23、d with bond-breaker oil. The rebars must be in place, properly supported to bear any traffic they will receive during concrete placing. Conduit, inserts, and other items to be embedded must be in position, fixed against displacement.Construction personnel should be available, usually carpenters, bar
24、 placers and other trades, if piping or electrical conduit is to be embedded, to act as form watchers and to reset any rebars, conduit, or piping displaced. 21 As concrete is cast, the slump of the concrete must be observed and regulated within prescribed limits, or the specified strengths based on
25、the expected slump may be reduced. An inspector of placing who is also responsible for sampling and making cylinders, should test slump, temperatures, and unit weights, during concreting and should control any field adjustment of slump and added water and cement. The inspector should also ascertain
26、that handling, placing, and finishing procedures that have been agreed on in advance are properly followed, to avoid segregated concrete.In addition, the inspector should ensure that any emergency construction joints made necessary by stoppage of concrete supply, rain, or other delays are properly located and made in accordance with procedures specified or approved by the engineer. 22 Inspection is complete only when concrete is cast, finished, protected for curing, and attains full strength.
