Slippage controlled threaded rebar joint in reinforced concrete

A rebar splice joint embedded in a reinforced concrete structure and having improved seismic resistance is made between a first rebar with an internal thread and a second rebar element having a male thread between a stop shoulder and the end of the rebar. The bar length between the stop shoulder and the male threaded bar end is shorter than the length of the internal thread. The male thread is turned into the internal thread and the first rebar is torqued against the stop shoulder to near the elastic limit of the bars to elastically deform the male and female threads into more uniform contact along their opposing thread surfaces thereby increasing contact area between the threads to reduce or eliminate relative axial movement between the rebars under cyclic axial loading, such as seismic loading.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to the field of reinforced concrete construction 
and more specifically concerns improved splice joints between male and 
female threaded ends of steel reinforcement bars used in such 
construction. 
2. Background of the Invention 
In reinforced concrete construction it is commonly necessary to splice 
together steel reinforcement bars to meet the need for bar lengths longer 
than available stock, or to tie together adjacent sections of poured 
concrete. Such rebar splices are frequently made by providing mating 
threads at the rebar ends to be joined. One rebar end has a male screw 
threading, and a second rebar end is enlarged to form a socket with a 
female thread. A threaded splice can also be made by joining two male 
threaded rebar ends by means of an internally threaded sleeve. Threaded 
joints of this type include those with parallel threads, i.e. where the 
thread diameter is constant along its length, and tapered threads which 
diminish in diameter towards the end of the rebar in the case of male 
threading and where the female thread has a maximum diameter at the 
opening of the socket and diminishes in diameter towards its interior. 
Tapered threads quickly separate as soon as they are loosened and the joint 
depends on tight frictional engagement between the male and female thread 
surfaces to preserve integrity of the rebar splice. Consequently, tapered 
threads require that the splice joint be torqued together in order to 
maintain the joint. 
Parallel threads are not subject to this limitation, however, and the 
male/female threads remain in mutual engagement without being torqued 
together. Separation of the male/female threads requires that the male 
thread be actually fully unscrewed from the female thread, which does not 
occur simply as a result of loose engagement between the thread surfaces. 
This is reflected in the building codes presently in effect which do not 
require torquing of parallel thread rebar joints, and present industry 
practice in fact does not call for such torquing. Typically, the male 
thread is simply turned until the end of the rebar reaches the bottom of 
the female bore, a condition which under present practice is deemed to 
constitute a sufficient and adequate splice joint. 
Rebar threads are cut or rolled into the steel bars to relatively low 
tolerances, due to the nature of the steel alloys used for manufacture of 
concrete reinforcement bars and the limitations of efficient high volume 
production of rebars at competitive cost. The result is that typical rebar 
thread surfaces have a significant degree of small-scale irregularity 
which at a micro level prevents full surface-to-surface contact between 
opposing male/female thread surfaces. These small scale irregularities do 
not normally reduce the tensile strength of the rebar splice joint, and 
current practice produces threaded joints which readily meet, for 
instance, building code requirements of a 60,000 psi yield strength and 
90,000 psi ultimate tensile strength. 
An aspect of threaded splice joints which has been largely overlooked until 
recently is the behavior of the splice joint under seismic or fatigue 
conditions where the joint is subjected to rapidly alternating tension and 
compression force cycles, i.e. where the load on the joint is rapidly and 
repeatedly reversed. Under such conditions, the spliced rebars do not 
perform as a single unbroken bar. For example, irregular contact between 
opposing thread surfaces in a parallel thread splice joint may be 
evidenced by clicking sounds as the joint is alternately subjected to 
stress and strain. If such a splice joint is embedded in a concrete 
structure, one or both of the rebars may slip axially relative to the 
surrounding mass of concrete when subjected to cyclic loading along the 
splice axis. Any movement of a rebar relative to the concrete it is 
intended to reinforce is undesirable and potentially weakens the 
structure. 
Current building codes only specify that a rebar splice must exceed 125% of 
its yield strength under continuous load conditions without breaking. 
Rebar splices are not tested for slippage of the rebars relative to the 
concrete nor for splice joint performance under peak loads typical of 
seismic conditions. 
What is needed is a threaded rebar splice joint which is more resistant to 
cyclic loading conditions, with performance more closely approximating 
that of an unbroken, continuous steel bar. 
SUMMARY OF THE INVENTION 
This invention addresses the aforementioned need by providing a method for 
making a seismic resistant splice between steel reinforcing bars in a 
concrete structure. A first rebar has an internal or female thread at a 
first rebar end. A second rebar has an exterior or male thread extending 
between a stop element and a second rebar end. The length of the male 
thread between the stop element and the second rebar end is shorter than 
the length of the internal thread on the first rebar end. The splice is 
made by threading the male thread into the internal thread, and then 
torquing the first rebar end against the stop element on the second rebar 
sufficiently to deform the male and female threads into substantially more 
uniform contact along opposing thread surfaces of the two rebar ends, 
thereby to increase the area of contact between the threads and thus to 
substantially reduce or eliminate movement of the rebars relative to each 
other under cyclic axial loading. At least portions of the first and 
second rebars including the spliced together first and second rebar ends 
are embedded in a concrete structure for reinforcing the concrete 
structure. 
The stop element may be annular about the second rebar, such as a 
circumferential shoulder which preferably is integrally formed with the 
rebar and defines a stop surface facing the threaded end of the bar. 
The torquing of the first rebar end against the stop preferably includes 
application of sufficient torque to achieve an axial loading of the rebar 
ends approximating but lesser than the characteristic yield strength of 
the rebars so that the deformation of the male and female threads remains 
elastic. 
The resulting splice joint is characterized by a substantially increased 
area of contact between the male and female threads over the undeformed 
threads of the splice joint without torquing. As a result, axial loads on 
the splice joint are distributed more uniformly over the helical length of 
both male and female threads as opposed to a more irregular and spotty 
contact between the originally undeformed threads. The deformation of the 
mated threads in effect reduces or removes the original manufacturing 
tolerances of the rebar threads in the torqued splice and thus minimizes 
the freedom of movement between the joined rebar ends relative to each 
other and to the surrounding concrete under cyclic loading conditions. 
The optimum torque to the splice joint is such as to axially preload the 
joined rebar ends to a load approximating but lesser than the 
characteristic yield strength of the rebar material so as to retain 
elasticity of the deformed male and female threads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a splice joint J made according to current practice 
between a conventional dowel bar 10 and a conventional dowel-in 12. The 
dowel bar has an enlarged head 14 terminating in a flange 16. A bore 18 
extends axially into the head 14 from the center of the flange 16 and 
terminates in a blind inner end 20, which in this example is concavely 
conical in shape. An internal or female thread 22 is cut in the bore 18 
from the open end 24 of the bore to a thread end 26 which is spaced from 
the conical end surface 20 of the bore 18. The dowel-in 12 has an external 
or male thread 28 which extends from the end 30 of the dowel-in to a male 
thread end 32. 
According to conventional practice, the dowel-in 12 and dowel bar 10 of 
FIG. 1 are spliced together by turning the male thread 28 into the bore 18 
until the end 30 of the dowel-in reaches the thread end 26 of the interior 
thread 22. The splice joint is typically made by simply hand turning the 
dowel-in, although in construction projects where large numbers of splices 
need to be made at a given time, power tools such as a centrifugal chuck 
on an electric or air powered drill motor may be used to speed 
installation. In either case, no particular attention is given to torquing 
of the splice joint J other than to turn the dowel-in 12 until it is fully 
threaded into the dowel bar end 14. 
Normal thread tolerances for steel reinforcing bar is class 2, specifically 
class 2A for the female thread, and class 2B for the male thread, which 
are the standard tolerances for nut and bolt combinations in the fastener 
industry. Class 2 tolerances permit a variation or delta of 0.001" in the 
mean thread diameter, measured halfway between the crest and valley of the 
thread. 
This tolerance level coupled with relatively rough surface finish of the 
typical reinforcing bar stock threading results in uneven and 
discontinuous contact between the opposing male and female thread surfaces 
along the helical threads. FIG. 2 illustrates in magnified detail the 
cross section of the mated male/female threads of the prior art splice of 
FIG. 1. Contact between the opposing thread surfaces is imperfect due in 
part to the deviation of the threads from a true helical shape, and in 
part due to the small scale irregularities in the thread surfaces. The 
result is that a substantial portion of the male and female thread 
surfaces are in mutually facing relationship but spaced from each other to 
varying degrees without making contact with each other. The combined 
effect of these imperfections in the splice joint threading often permits 
a small degree of relative axial displacement between the two rebar 
elements when a sufficient axial load is applied to the spliced bars. This 
phenomenon is aggravated under conditions of cyclic loading where the 
splice joint is repeatedly and rapidly subjected to high peak axial loads. 
Under such circumstances, an audible clicking sound may be heard as the 
male thread alternately strikes against one side of the female thread 
groove and then against the other side due to the imperfect fit within the 
thread groove. When such a prior art rebar splice is embedded in a 
concrete structure and therein subjected to cyclic loading, the slight 
degree of axial freedom between the splice ends may lead to loss of 
cohesion between one or both of the rebar elements 10, 12 and the 
surrounding concrete mass C. Separation of the reinforcing bar from the 
concrete weakens the structure and if the condition becomes widespread 
among the reinforcing bars of a particular structure, may lead to 
catastrophic failure of the same. 
FIG. 3 illustrates a dowel-in 12' improved according to this invention and 
a dowel bar 10 which is similar to the dowel bar 10 of FIG. 1. The 
improved dowel-in 12' has an external thread 28 between a bar end 30 and 
an annular flange 34 which is formed integrally with the bar 12' and which 
provides an annular stop surface 36 facing the end 30 of the bar. 
FIG. 4 shows an improved rebar splice joint S formed by threading the end 
of the improved dowel-in 12' into the conventional dowel bar 10. The male 
thread 28' of the dowel-in is mated to the female thread 22 of the dowel 
bar and advanced into the bore 18 until the stop surface 36 of the flange 
34 comes against the face 38 of the flange 16, as better seen in FIG. 5. 
Contact between the stop face 36 and flange face 38 is annular over the 
entire stop surface 36, and is radially symmetrical with respect to the 
longitudinal axis of the splice joint S. 
An important feature of the improved dowel-in 12' is that the axial length 
of the male thread 28', measured between the end 30 and the stop surface 
36, is shorter than the axial length of the female thread 22 measured 
between the flange face 38 and the inner thread end 26. This feature 
prevents the end of the male thread 28' from jamming into the end 26 of 
the female thread 22, as typically occurred in prior art splices such as 
illustrated in FIG. 1. Once contact is made by the stop surface 36 against 
the flange surface 38, the splice joint S of FIGS. 4 and 5 is completed by 
torquing the dowel-in 12' in relation to the dowel bar 10 with a force 
sufficient to deform the male and female threads, 28', 22 respectively, 
into substantially more uniform contact along the mated thread surfaces. 
The annular flange 34 and stop surface 36 serve an important role in 
achieving uniform distribution of the torque force along the entire 
helical length of the male and female threads 28', 22 respectively. The 
symmetrical annular contact of the stop surface 36 against the face 38 of 
the flange 16 allows the tensile forces created by torquing of the two bar 
elements relative to each other to distribute themselves in a rather 
uniform manner along the threads. 
Torquing of the splice joint strains the mated threads which are drawn 
tightly against each other and mutually correct towards a more perfectly 
helical shape. Also, the opposing thread surfaces may shift slightly or 
locally deform at a micro scale against each other to achieve a better 
meshing together of the small scale surface irregularities, further 
enhancing the extent of surface contact between the mated threads. The 
extent of deformation of the mutually engaged threads in the splice of 
FIG. 1 is directly related to the torque applied to the joint. The greater 
the torque, the greater the deformation of the threads into increased 
mutual contact along the thread surfaces. The condition of the engaged 
threads shifts from the imperfect, irregular mutual contact of FIG. 2, to 
a more uniform contact of the opposing thread surfaces illustrated in FIG. 
5. A practical limit, however, is imposed by the characteristics of the 
rebar material, specifically the characteristic yield strength of the 
rebar elements 10, 12'. If the torque applied results in an axial loading 
of the engaged rebar ends beyond the characteristic yield strength of the 
material involved, the deformation of the threads becomes inelastic, at 
which point the threads deform permanently and may be damaged in a manner 
detrimental to the integrity of the splice joint. For a given pair of 
male/female threads, optimum deformation of the threads for maximizing the 
area of contact between the opposing thread surfaces is achieved by 
applying sufficient torque so as to reach an axial loading of the spliced 
rebar ends which approximates but is lesser than the characteristic yield 
strength of the rebar material. Mathematical expressions applicable to 
tightening of threaded fasteners, such as bolt and nut joints, are also 
applicable to the rebar splice joint of FIG. 4. In particular, the 
equation commonly referenced for relating assembly torque to the axial or 
clamp load of threaded fasteners is: 
EQU T=KDW 
where 
T is assembly torque measured in inch-pounds; 
K is torque coefficient; 
D is the nominal bolt diameter in inches; and 
W is the target axial tension or clamp load. 
For optimum performance of the splice joint S of FIGS. 4 and 5 under cyclic 
loading, the value of W should normally be set at slightly less than the 
characteristic yield strength of the particular rebars 12' and 10 being 
spliced. 
K has been experimentally derived, and for mild steel of the type commonly 
used for concrete reinforcing bars is usually assumed to have a value of 
0.20. 
Based on this equation, approximate values of the torque necessary to 
achieve optimum splice joint performance under cyclic loading can be 
computed, and the splice joint S can be torqued to the computed value with 
the aid of suitable tools, such as calibrated torque wrenches, for 
example. 
It should be appreciated that this result is quite different from that 
obtained by torquing a prior art splice such as shown in FIG. 1. In the 
prior art splice J, the end of the male thread 28 reaches the end 26 of 
the female thread 22 and is prevented from advancing beyond that point 
into the bore 18. As torque is applied to the dowel-in 12 relative to the 
dowel bar 10, the end of the male thread is jammed against the end of the 
female thread. The torque force is concentrated at this point, which is 
radially offset from the longitudinal axis of the splice, and sets-up a 
reactive force urging the dowel-in 12 diametrically away from the thread 
end 26 within the bore 18. The result is an asymmetrical distribution of 
the torque forces which tend to become localized near the end 30 of the 
dowel-in 12, and do not spread uniformly along the length of the mated 
male and female threads 28, 22. Deformation of the threads is generally 
limited to the vicinity of the end 30 and does not produce the superior 
results of the torqued splice S of FIGS. 4 and 5. 
FIG. 6 shows an alternate dowel-in 12" where the annular stop surface 36' 
is provided by a flaring transition 40, with the bar 12" gradually 
increasing in diameter to form a shoulder 42 terminating in the stop 
surface 34, which functionally performs in the manner of the stop surface 
34 described in connection with the dowel-in 12' in FIG. 4. 
While particular embodiments of the invention have been described and 
illustrated for purposes of clarity and example, it must be understood 
that many changes, modifications and substitutions to the described 
embodiments will become apparent to persons having ordinary skill in the 
art without thereby departing from the scope and spirit of the present 
invention which is defined only by the following claims.