Apparatus for pre-stressing concrete structural member

The disclosure is of apparatus, and the positioning and adjustment thereof within a form in which a pre-tensioned pre-stressed concrete structural member is to be cast, for holding a tensioned strand extending through the concrete against movement after release of the tensioning force, thereby to prevent the formation in the cast body of the weakened zone known as the "development length."

BACKGROUND OF THE INVENTION 
In the manufacture of pre-tensioned, pre-stressed concrete structural 
members it is conventional to provide wedge means external to the piece to 
be cast for holding the tensioned steel from movement after the tensioning 
apparatus is removed from the strand, which may be a wire, cable, rod or 
the like and which is referred to hereinafter, as in the industry, by the 
term "the steel." 
The release of tension on the steel causes two phenomena to occur, the 
first being that the embedded steel tends to expand in diameter, and the 
other being that the steel tends to move into the cast concrete in a 
direction longitudinal to the steel. These effects result in weakening of 
the concrete in a zone surrounding the strand and extending inwardly from 
the external surface of the cast member for a distance which may be a 
large multiple of the diameter of the steel. This weakened zone is known 
in the industry as the "development length." 
In FIG. 1 of the drawings there is shown, for purpose of illustration of 
the prior art, the conventional assembly of parts used in the manufacture 
of pre-tensioned, pre-stressed pre-cast concrete structural members. Such 
an assembly includes a form 2 within which the concrete member is cast, 
and which is closed by end plates 4, each of which is provided with one or 
more openings 6 leading to the interior of the form. A tensioned steel 
strand 8 is passed through the openings 6 and through the form and 
protrudes from the plate at each end of the form. Outside each end of the 
form there is provided means for holding the tensioned steel, and each 
such means comprises a wedge assembly including barrel 10 which rests on a 
fixed buttress 11 and has an internal frusto-conical surface 12 on which a 
plurality of wedges 14 are positioned in surrounding relation to the steel 
8. A tensioning force in the direction of the arrow of FIG. 1 is applied 
to the steel, the concrete is cast within the form, and after the concrete 
is fully set the tensioning force is released and the wedge assemblies are 
removed. The purpose of the wedge assemblies to prevent retraction of the 
steel into the cast body of concrete, but it has been found that when the 
tensioning force is released the steel tends to enlarge in diameter and to 
retract into the cast body, and these effects cause a weakening of the 
cast body in a zone extending from the inner surface of each end plate 
inwardly of the cast body for a distance that may be as much as 100 times 
the diameter of the steel. This zone is known in the industry as the 
"development length" and is so designated in FIG. 1. 
It is usually desirable or necessary to attach an end plate to the cast 
structural member, and because of the existence of the development length 
zone additional means must be provided for securing such an end plate to 
the structural member. This is conventionally done by welding one or more 
elongated rods 20 to the inner surface of the end plate, each rod having a 
length greater than the depth of the development length zone, as shown in 
FIG. 1. The attachement of this reinforcing and connecting rod may be 
effected by welding or other suitable means before the form is closed and 
the casting begins. 
It has been the object of this invention to provide apparatus by which the 
development length zone is prevented in the manufacture of concrete 
structural members such as beams, columns, piles and the like. The 
elimination of the development length zone has important advantages which 
may not be immediately apparent. For example, it is often desirable and 
necessary to connect two such structural members in end-to-end relation, 
and in order to do this it is necessary to secure end plates to the 
structural members. In following known procedures in the manufacture of 
such pre-stressed members the formation of the development length zone at 
the end of the member prevents secure attachment of an end plate to the 
member without the use of additional attachment means, while utilization 
of the present invention results in attachment of each end plate to the 
cast body by the tensioned steel itself without the necessity of any 
additional connecting means. Further, the formation of the development 
length zone following conventional procedures and using known apparatus 
results in the weakening of the end parts of the structural member, 
thereby requiring special techniques in building and other structures, 
such as the use of special reinforcing rods within the development length 
zone. This is entirely obviated by the use of the present invention, as 
with such use the zone is eliminated. 
SUMMARY OF THE INVENTION 
The invention provides, in combination with a pre-cast, pre-tensioned, 
pre-stressed concrete structural member, an end plate in 
surface-to-surface engagement with the concrete member through which the 
tensioning steel passes into and through the cast member, and wedge means 
internally of the plate and engaging it and surrounding the tensioning 
steel and positioned to resist expansion of the steel and retraction of 
the steel into the cast body when the tensioning force on the steel is 
released.

DESCRIPTION OF THE INVENTION 
FIG. 2 of the drawings discloses the casting and pre-stressing assembly of 
parts shown in FIG. 1 together with the means added by the present 
invention and omitting, of course, the reinforcing rods 20 and the 
indication of the development length zones, both of which are obviated by 
the present invention. As shown in this figure the means provided by the 
invention are located internally of the end plate 4 and in surrounding 
relation to the steel 8, and one of such means is illustrated in enlarged 
detail in FIG. 3 and is described in detail as follows. 
In accordance with the invention the inner surface of the end plate 4 has 
integrally formed thereon a cylindrical flange 30 which extends inwardly 
of the form 2 from the inner surface of the plate and which is positioned 
concentrically with an opening 6 in the end plate through which the steel 
8 extends, it being understood that any number of such openings and 
surrounding flanges may be formed in the end plate at each end of the 
form. The flange 30 is threaded, as shown, for threaded connection to a 
cylindrical barrel 34 which is provided with a central opening 
therethrough which adjacent the end plate 4 has a frusto-conical surface 
36 with its larger end adjacent the end plate. Within the central opening 
in the barrel there are provided a plurality of wedge members 40 which are 
positioned in circular arrangement surrounding and in close engagement 
with the steel 8 and with their outer surfaces in engagement with the 
frusto-conical surface 36 of the barrel, the larger ends of the wedges 
being adjacent the plate 4. 
In the use and operation of the described apparatus in the formation of a 
pre-tensioned, pre-stressed, pre-cast concrete structural member all of 
the parts illustrated in FIGS. 2 and 3 are assembled as shown in the two 
figures, the barrel 34 loosely surrounding the steel 8 within the form. 
Tension is now applied to the steel and the wedge and other steel holding 
means 10, 11, 12, 14 external to the form provide their usual function. At 
this point the wedge means 40 are assembled within the barrel 34 as shown 
in FIGS. 2 and 3 and the barrel is threaded onto the flange 30 causing the 
wedges 40 to move toward the plate 4, and this operation will continue 
until the outer, larger ends of the wedges are in engagement with the 
inner surface of the plate. Concrete is now poured within the form 2 and 
allow to set. 
The tension on the steel 8 is now released, for example by cutting the 
steel external to the form. The effect of this on the steel is to cause 
the steel to tend to expand transversely of its length and to tend to move 
into the cast body with the adverse results described above, including the 
formation of the development length zone, but the apparatus provided by 
the invention prevents these adverse results. The enlargement of the steel 
cannot take place because of the interposition of the fixed, rigid 
assembly of wedges and barrel between the steel and the plate, and 
movement of the steel into the cast body is prevented by the gripping 
pressure of the wedges on the steel. As there is no movement of the steel 
into the cast body there is no formation of a development length zone. 
Furthermore, the end plate is firmly attached to the cast body through the 
flange, barrel, wedges and strand, thus requiring no additional connecting 
means between the plate and the cast body such as the rods of FIG. 1. 
In the embodiment of the invention disclosed in FIGS. 2 and 3 the assembly 
of barrel and wedges is connected to the end plate by a threaded 
connection between the barrel and a flange formed integrally with the end 
plate extending into the form, and the wedges are moved into operative 
position against the inner surface of the end plate by moving the barrel 
with respect to the flange. Other forms of operative relation of the 
wedges to the end plate are within the scope of the invention and examples 
of these are disclosed in FIGS. 4 and 5. 
In the embodiment disclosed in FIG. 4 an internally threaded opening 50 is 
provided in end plate 4, and within this opening and in the general plane 
of the plate there is threaded a cylindrical plug 52 which has a central 
un-threaded opening through which the steel 8 passes into the cast body 
and which extends within the body below the inner surface of the end 
plate, thus forming an interior abutment surrounding the steel 8. A barrel 
54 is internally threaded at its upper end for adjustable attachment to 
the external threads of the plug 52, and below these threads, or 
internally of the form, the barrel is provided with an internal 
frusto-conical surface 56 which is concentric with the steel 8 and plug 52 
and has its larger end adjacent the inner end of the plug. A plurality of 
wedge segments 58 are provided within the barrel and have external 
surfaces corresponding to the internal frusto-conical surface of the 
barrel. 
In the embodiment of the invention disclosed in FIG. 5 the upper end of the 
barrel 60 is externally threaded for adjustable connection within an 
internally threaded opening 62 in plate 4, and is also internally threaded 
to receive an externally threaded plug 62 which has a central opening 
through which the steel 8 passes. The assembly of wedges 64 surrounds the 
steel 8 within the barrel and co-operates with the frusto-conical surface 
66 of the barrel in the same manner as in other forms of the invention. 
It is important to note that in all forms of the invention, the wedges 
cannot become loose after the parts are adjusted prior to pouring 
concrete, because the barrel and wedges are moved toward the end plate 
until the large ends of the wedges abut a fixed part, which is the end 
plate in the embodiment of FIGS. 2 and 3, the plug 52 of FIG. 4, or the 
plug 62 of FIG. 5. Thus, after such adjustment the end plate, barrel and 
wedges form a fixed, rigid assembly which prevents expansion of the steel 
and retraction of the steel into the cast concrete after release of the 
external means which applies tension to the steel.