Method of forming concrete floors and product of the method

A method for forming a concrete floor as a monolithic unit is disclosed wherein wet concrete mix is deposited and screeded to achieve a generally uniform thickness of concrete with the aggregate being densified or settled to produce a thin upper layer formed substantially from sand and cement, thereafter floating the concrete to substantially remove surface irregularities, and then grinding the upper surface of the concrete floor after it is hardened to produce a flat surface having a "sanded" finish.

The present invention relates to a method for forming concrete floors and 
more particularly to such a method employed for forming concrete floors 
upon a suspended substrate in high-rise buildings. 
In the prior art, a wide variety of techniques, methods and apparatus have 
been employed for forming concrete structures and particularly for forming 
concrete floors. Of these many techniques, two are described below in 
detail in order to contrast and demonstrate the novelty and unexpected 
utility of the present invention. These two techniques include a method of 
forming floors as occasionally practiced in Europe with a final step of 
grinding the surface of the floor to produce a flat surface and a 
technique commonly practiced for example in the United States in forming 
concrete floors for high-rise buildings having structural steel skeletons 
and reinforced substrates for supporting the concrete floor. 
In the European technique first mentioned above, the concrete is deposited 
upon a supporting structure of removable forms with reinforcing steel or 
the like being placed directly in the concrete medium. According to this 
technique, the concrete is generally deposited in substantial thicknesses 
of six to twelve inches, for example, and contains reinforcing steel or 
the like for forming a structural element after removal of the forms. 
According to this technique, wet concrete mix is deposited upon the forms 
and a rough leveling or screeding operation is accomplished by means of a 
heavy vibrating screed which is generally necessary in order to assure 
densification or compaction of the concrete throughout its substantial 
depth and to assure integral contact between the concrete and the 
reinforcing steel. Because of the need for a relatively heavy vibrating 
screed, this technique requires supporting forms which establish the upper 
surface of the floor and support the ends of the vibrating screed. The 
forms are normally placed up to about five meters apart with the concrete 
being deposited therebetween and leveled by means of the vibrating screed. 
The concrete may be further smoothed by means of conventional float 
equipment immediately after the vibrating screed and then allowed to stand 
until "bleed water" has risen from the concrete to form shallow pools upon 
its upper surface. The concrete is allowed to stand until the bleed water 
has substantially risen to its surface. As the bleed water rises through 
the concrete, it tends to produce some disruption in the surface 
continuity of the concrete. Accordingly, a second floating operation is 
carried out after the bleed water has risen in order to form a smooth 
"creamy" surface layer upon the concrete including primarily cement and 
sand. 
After this second floating operation, the concrete is then allowed to stand 
until it is hardened but not finally cured. The upper surface of the 
concrete is then particularly susceptible to being finished by means of a 
power tool such as a rotary grinder. Such a rotary grinder may for example 
include a rotating platform including means for replaceably mounting 
grinding stones upon its lower surface to contact or abrade the concrete 
and produce a flat "sanded" surface. 
In practicing this conventional European technique, it will be apparent 
that a substantial amount of time elapses between the initial deposition 
of the concrete upon the floor and the final or second floating operation 
after which the concrete may be allowed to stand prior to its being 
finished in a grinding operation. More importantly, it is necessary to 
assure access of an operator to the surface of the concrete when the 
second floating operation is to be carried out. For these reasons, it has 
been common practice to form large floors in alternating strips, those 
strips being finished and allowed to harden so that the concrete surfaces 
are not susceptible to damage. On a subsequent day, the intermediate 
strips may then be formed in the same manner so that the final floor is 
formed with alternating strips having parallel joints extending throughout 
the floor. These joints are usually uneven and entail extensive finishing 
time, in addition to the time required for multiple forms. The finished 
floor may then be allowed to stand for a suitable period of time, for 
example, an additional 24 hours up to one week or even longer before the 
final grinding operation is accomplished. Within this time period, the 
concrete floor is sufficiently hardened to support the weight of the 
grinder without rupturing or tearing but not finally cured so that the 
grinding operation may be accomplished in a relatively efficient manner. 
The "European" technique described in some detail above has not been 
employed in the construction of high-rise buildings for example in the 
United States for a number of reasons. In construction of high-rise 
buildings, the use of vibrating screeds is generally not feasible or 
economical. A suspended substrate normally provides a base upon which the 
concrete floor is formed. The substrate may become an integral portion of 
the floor along with a relatively thin layer of concrete, for example, 
approximately three to six inches. In addition, within such high-rise 
buildings, it has not been found satisfactory to provide forms of the type 
employed with the heavy vibrating screeds commonly used in the European 
technique. Accordingly, a substantially different technique is commonly 
employed for forming concrete floors in such high-rise buildings. That 
technique is described below and referred to generally as the "trowel" 
technique. 
In conventional trowel technique described below, an entire floor or 
integral portion of a floor is commonly formed as a unit, without the use 
of intermediate forms or the like for controlling the height or thickness 
of the floor. According to this technique, the wet concrete mix is 
deposited upon the substrate which may be either a corrugated steel sheet 
or reinforced concrete for example. Thereafter, the concrete is first 
leveled in a rough operation commonly employing a manual screed. The 
thickness of the concrete is controlled by the operator who may employ 
occasional reference points with which he visually compares the height of 
the concrete being screeded. 
The concrete floor is then floated shortly after it is poured and screeded. 
The floating operation may optionally include a first densifying step 
using a "jitterbug" or the like to prevent the occurrence of voids. 
Whether or not a densifying step is performed, the concrete is commonly 
smoothed by means of a flat floating tool such as a "bull float." 
According to this technique, the concrete is then allowed to stand a 
suitable period of time, for example, approximately one-half to two hours, 
(four to six hours in cold, damp weather), during which time bleed water 
rises from the concrete and the concrete sets up sufficiently to support 
the weight of an operator. At that time, initial trowelling is performed 
either manually or with a power machine while the surface of the concrete 
may still be worked. The concrete is then trowelled a number of times as 
it dries to form a continuous, glazed surface which is substantially 
different from the "sanded" surface effect produced by a grinding 
operation such as that described above. 
There have been found to be a number of disadvantages within the trowel 
technique described immediately above. Initially, the production of 
concrete floors is obviously labor intensive. Referring to the technique 
described above, it will be apparent that in order to allow the concrete 
to stand for a sufficient period of time prior to the final trowelling 
operation, it is necessary either to discontinue the pouring of concrete 
early during a work shift or to continue the final trowelling operation 
into an extended or second work shift. In either event, substantial 
additional costs are incurred which add significantly to the final cost of 
the floors and the overall cost of the building. Even further, because of 
the interruption of the pouring operation, substantial additional time of 
many days may be required for finishing the large number of floors in such 
a building. This time delay by itself can be a significant factor in the 
overall construction cost of the building because of the increased time 
before the building is ready for occupancy. 
The trowel technique described above has been commonly employed at 
cnstruction sites in the United States. Power grinding has consistently 
been avoided in these operations because of the substantial cost for this 
operation alone. However, the present invention, as described in greater 
detail below, has produced the unexpected result of actually reducing 
overall costs for forming concrete floors through use of a method 
including power grinding as an essential step while eliminating all 
trowelling of the floor. 
In any event, there was found to remain a need for a relatively efficient 
method for forming concrete floors in the construction of high-rise 
buildings and elsewhere while substantially decreasing the overall time 
for construction. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
forming concrete floors wherein an entire floor or substantial floor 
portion is formed as a monolithic unit, the method including the steps of 
depositing wet concrete mix and roughly leveling the concrete by means of 
a screed and then floating the surface of the concrete to produce a 
generally flat surface, these combined steps being performed in a manner 
to effectively densify the concrete or to settle the aggregate in order to 
produce a thin upper layer which is formed substantially from sand and 
cement, the concrete then being allowed to stand for a period of at least 
approximately 24 hours or more after which the surface of the concrete is 
treated by means of a power grinder in order to remove a thin surface 
portion from the concrete and produce a flat "sanded" surface thereupon. 
Within the context of the present invention, the term "monolithic" is 
employed to mean that the entire expanse of a floor, or a substantial 
portion of the floor which may be formed within a single working shift, is 
produced without joints of the type caused by the pouring or depositing of 
wet concrete adjacent concrete which has already set up. This condition 
tends to prevent formation of a completely integral bond with the freshly 
deposited concrete. As will be apparent from the following description, a 
joint of this type would be produced by the present invention for example 
if an entire floor is poured in a circular pattern having a common 
beginning and ending point. However, even within such a floor, its entire 
expanse between the beginning and ending points would be formed as a 
monolithic structure. 
In forming such a floor according to the present invention, initial 
densification of settling of the aggregate may be accomplished in a number 
of ways. For example, the first floating operation may be accomplished 
with a corrugated or ribbed float such as a bull float, a vibrating bull 
float or other smooth float apparatus employed with a tamping action. All 
of these different techniques or tools accomplish the same effect of 
settling or densifying the aggregate within the concrete in order to 
produce a thin upper layer formed substantially from cement and sand which 
facilitates the final grinding step as discussed in greater detail below. 
Densification or settling of the aggregate may also be accomplished by 
employing concrete having a relatively high water content. Although such a 
high water content may reduce final strength characteristics of the 
concrete, the additional water tends to cause the aggregate to settle 
within the concrete medium in order to accomplish densification as desired 
by the present invention. 
It is also a further object of the invention to preferably employ a final 
floating operation in order to produce a generally flat surface upon the 
concrete before it is allowed to set up sufficiently for the final 
grinding operation and thereby minimize the amount of surface material 
removed from the concrete. In connection with this floating operation, it 
is important to note that the floor is continuously poured as a monolithic 
unit and portions of the floor rapidly become inaccessible to an operator 
unless the operator is upon the surface of the concrete itself. In the 
"European" technique described above, this presents a substantial problem 
solved by the formation of small areas or alternate strips as described 
above so that the operator could have access to all portions of the 
concrete in order to perform the floating operations when desired. 
According to the present invention, the final floating operation may be 
accomplished immediately after the concrete is first deposited, screeded 
and initially floated. In following such a technique, bleed water would 
subsequently rise from the concrete and be allowed to merely evaporate 
from the surface of the concrete as it sets up prior to the final grinding 
operation. 
On the other hand, it has also been found that final floating may be 
carried out by means of a power tool such as a rotary float or the like 
after the bleed water has risen from the concrete provided that additional 
time elapses so that the concrete sets up sufficiently to support the 
weight of an operator. With the concrete being in this condition, the use 
of a heavy power float is necessary in order to adequately work the 
surface of the concrete in this condition. 
It is also an object of the present invention to provide a variation of the 
method for forming concrete floors in applications permitting an entire 
dimension of the concrete floor to be spanned by a vibrating screed. 
Vibrating screeds of this type are commercially available with spans of 
over one hundred feet. Such a vibrating screed may be employed with the 
present invention when there are substantially no projections or 
obstructions throughout the expanse of the floor. Such floors may commonly 
be encountered in large shopping malls and other on-grade sites or even in 
the floors of multi-level buildings where elevator shafts, other service 
areas and the like are not in a central portion of the floor. Within such 
a combination, it is a relatively simple matter to provide forms along 
opposite sides of the floor so that the entire width of the floor may be 
poured and screeded simultaneously. 
The vibrating screed accomplishes the basic function of densifying or 
settling aggregate in accordance with the present invention. Because of 
the substantial span for the vibrating screed, access to the entire wet 
concrete surface from an external point is not possible after the concrete 
is poured and screeded. Accordingly, the concrete is allowed to stand 
until bleed water has risen to the surface and the concrete is set up 
sufficiently to support the weight of an operator. At that time, the 
surface of the concrete is smoothed with a power float which is 
sufficiently heavy to permit working of the concrete surface. The floor is 
then allowed to stand for approximately 24 hours or more after which its 
surface is treated with a power grinder as described above in order to 
form a flat surface having a porous, sanded quality. 
Many large concrete floors can be formed in a single pass through the use 
of such a vibrating screed. In some applications, however, the dimensions 
of the floor may be so great that a vibrating screed of the type described 
above will be incapable of spanning the entire floor. Accordingly, in 
these applications, the floor may be formed in multiple segments, each 
segment being produced in the same manner as described above to produce a 
monolithic unit. 
The methods described above thus contemplate formation of concrete floors 
in multi-level buildings and even in on-grade applications such as large 
floors for shopping centers. The present invention may also be employed 
where the concrete floor is formed upon a reinforcing substrate of metal 
or preformed concrete which becomes an integral portion of the floor. Even 
in these applications, some reinforcing metal is often disposed within the 
poured slab of concrete. In addition, the method or methods of the present 
invention may also be used where the concrete is poured upon removable 
forms and reinforcing metal or material is placed within the concrete. In 
these applications, the removable forms are of course removed after the 
concrete and self-contained reinforcing material provide sufficient 
strength for supporting the weight of the floor. 
It is also an object of the present invention to provide a concrete floor 
as a product of the method or methods described above, the floor being 
further characterized as a monolithic structure and having a surface which 
is flat, porous and of a "sanded" character. Such surface characteristics 
are particularly desirable for the application of tile, carpeting or the 
like upon the finished floor. 
Additional objects and advantages of the invention are made apparent in the 
following description having reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention contemplates an efficient and cost saving method for 
forming concrete floors wherein the floor is initially poured and rough 
leveled or screeded in a manner selected to densify or settle aggregate in 
the concrete to produce a thin upper layer formed substantially from fluid 
components of sand, cement and water. The concrete is then smoothed by a 
bull float or other similar smooth-surfaced float and allowed to stand at 
least aproximately 24 hours or more after which the surface of the 
concrete floor is treated with a power grinder in order to remove a 
portion of the concrete surface and produce a finished surface which is 
flat, porous and has a sanded characteristic. 
In the following description, reference is first made to FIG. 1 in order to 
better define the "European" system discussed at some length above. One 
variation of the present invention is then described with reference to 
FIG. 2, the floor of FIG. 2 preferably being one level of a multi-level 
building wherein the floors are interrupted by service areas such as 
elevators and the like. For such an application, the present invention 
contemplates a method wherein a concrete slab is formed upon the floor as 
a monolithic unit, the method including a first step of depositing the 
concrete upon the floor and initially leveling the concrete by means of a 
manual screed, usually where the operator visually establishes the rough 
level of the floor. 
Another variation of the invention is then described having reference to 
FIG. 3 showing a generally unobstructed floor and permitting the use of a 
vibrating screed extending entirely across one dimension of the floor. As 
will be described in greater detail below, this method involves initial 
rough leveling of the concrete with the vibrating screed. After the 
concrete sets up sufficiently to support the weight of an operator, the 
floor is then smoothed by means of a power float device and then allowed 
to stand 24 hours or more before being treated with a power grinder to 
produce a flat, porous surface having a sanded characteristic. 
A floor of the type produced according to the present invention is 
illustrated in FIG. 6. FIGS. 4 and 5 are referred to below within the 
descriptions of the preferred methods as examples of apparatus which may 
be employed within the method of the present invention. 
Referring now to the drawings, FIG. 1 is included in the drawings to assure 
a proper understanding of the European method referred to above. Assuming 
that the floor to be formed is about 30 meters square, for example, forms 
12 may be arranged at 5-meter intervals across one dimension of the floor 
and extend the full length of the floor in the other direction. 
Spaced-apart strips 14, 16 and 18 may be formed during a first work shift 
or day in accordance with the preceding description. It is to be 
particularly noted that operator access to the strips 14, 16 and 18 is 
provided throughout the entire operation by means of the alternate strips 
20, 22 and 24. Such access is generally essential in the European 
technique. After the concrete in the spaced-apart strips 14, 16 and 18 is 
sufficiently hardened, the workers may return on another day to similarly 
pour the concrete floor in the alternate strips 20, 22 and 24. Since the 
concrete in the first strips 14, 16 and 18 is substantially set up before 
concrete is poured in the alternate spaces 20, 22 and 24, joints tend to 
remain as indicated at 26 between each adjacent pair of strips. Within 
this prior art technique, it will be obvious that the individual strips 
may be interrupted to allow for obstructions such as service areas in the 
floor 10. 
Referring now to FIG. 2, one variation of a method for forming concrete 
floors according to the present invention is described immediately below 
having reference to FIG. 2 preferably representing one floor 110 of a 
multi-level building (not otherwise shown) wherein the floors are 
interrupted by a service area generally indicated at 112. Commonly, the 
floor 110 is formed by reinforcing material such as corrugated sheet metal 
or preformed concrete which becomes an integral portion of the floor 
including the concrete slab formed according to the present invention. 
However, as was noted above, the present invention may also be employed in 
applications where the floor is to be poured on fabricated forms which are 
removed after the floor, including self-contained reinforcement, attains 
sufficient strength to support its own weight. 
According to the method of the present invention, concrete is poured upon 
the floor 110 and initially rough leveled or struck off by means of a 
conventional manual screed (not shown). With such a screed, the operator 
visually established the level of the floor by comparison with one or more 
reference points (also not shown). 
Immediately after the concrete is screeded, it is initially treated in an 
additional step to densify or settle aggregate in order to produce a thin 
upper layer formed substantially from sand, cement and water. Preferably, 
this step involves the use of a corrugated float of the type illustrated 
in FIG. 4. Referring momentarily to FIG. 4, it may be seen that the float 
device 30 has a smooth surface 32 for engaging the upper surface of the 
concrete and a plurality of spaced apart corrugations or ribs 34 which 
project through the surface of the concrete to urge the aggregate 
downwardly into a densified or settled condition as described above. The 
corrugated float 30 of FIG. 4 is a preferred device for accomplishing this 
step. However, it will be apparent that the concrete may be similarly 
densified with other concrete tools such as a vibrating bull float or any 
of a number of smooth floats employed with a tamping action. After the 
concrete is deposited, screeded and densified as described above, the 
surface of the concrete is then smoothed by a conventional float 
operation. Preferably, this operation may be accomplished by means of a 
smooth float as illustrated at 40 in FIG. 5. The float 40 has a smooth 
uninterrupted surface 42 for engaging the surface of the concrete and 
producing a smooth surface. This step is desirable in order to minimize 
the amount of concrete to be removed during the subsequent grinding 
operation. 
After the concrete is smoothed in accordance with the preceding step, it is 
then allowed to stand for a period of at least approximately 24 hours and 
up to one week or longer. Preferably, the concrete is allowed to stand for 
a period of approximately two to five days after which it is treated in a 
grinding operation as described immediately below. A conventional power 
grinder of the type contemplated by the present invention is illustrated 
in U.S. Pat. No. 3,098,329 issued July 23, 1963 and that reference is 
incorporated herein for the purpose of disclosing such a floor finishing 
machine. In any event, the power grinder is employed to remove a thin 
surface layer from the concrete floor and produce a flat, porous surface 
having a sanded quality as illustrated in FIG. 6. 
The delay period prior to the grinding operation is selected to permit the 
concrete to set up so that it is not ruptured or "torn" by the grinder. At 
the same time, it is also desirable to accomplish the grinding operation 
before the concrete has completely cured since it then becomes difficult 
to remove a surface layer from the concrete. Accordingly, the grinding 
operation would be more inefficient if delayed until after complete curing 
of the concrete. 
Referring again particularly to FIG. 2, the pattern in which the concrete 
is poured and treated may be selected according to the configuration of 
the floor. In FIG. 2, it may be seen that the floor is to be formed 
entirely around a central service area 112. With a single crew, it would 
of course be possible to begin pouring at a starting line 114 and continue 
around the entire periphery of the floor ending again at the line 114. 
Alternatively, the present invention also contemplates the use of multiple 
crews. For example, two finishing crews could begin in opposite directions 
as indicated by the solid arrow 116 and the broken line arrow 118. Both 
crews would then meet at an intermediate line 120. With the floor being 
formed according to this variation, it is immediately apparent that both 
crews would be simultaneously pouring wet concrete mix as they approach 
line 120 so that there would not be a joint formed at that point. 
Accordingly, with two such crews working, the entire slab for the floor 
110 would be formed as a monolithic unit. 
A number of variations are of course possible within the method described 
above having reference to FIG. 2. As noted above, the first step of the 
invention contemplates the depositions, screeding and densifying of the 
concrete, preferably by mechanical densification means. However, it is 
also possible to achieve densification through the use of a concrete mix 
provided with excess water tending to settle aggregate within the mix. 
Such a mix could result in relatively low strength characteristics for the 
finished concrete. However, if sufficient strength is provided for example 
by means of the reinforcing substrate, such a technique could also be 
employed for achieving densification. 
As another variation of the technique described above in connection with 
FIG. 2, it is to be noted that the smooth manual float of FIG. 5 is 
preferably employed immediately after densification with the corrugated 
float 30 of FIG. 4 in order to produce a smooth surface upon the concrete. 
Within the scope of the present invention, it is also possible to use a 
power float of generally similar construction as the power grinder 
referred to above. Both of the devices are provided with a rotating 
platform adapted to mount grinding stones in the case of the power grinder 
or smooth float members in the case of a power float. With the final float 
operation being delayed until after the concrete develops sufficient 
strength to support the weight of an operator, the surface of the concrete 
is substantially more difficult to work because of its reduced plasticity. 
Accordingly, a relatively heavy power float of the type described 
immediately above is employed to smooth the surface of the concrete. 
Thereafter, the concrete could again be allowed to stand in accordance 
with the preceding description before carrying out the final grinding 
operation. 
Another variation of the present invention is described immediately below 
having reference to FIG. 3. FIG. 3 contemplates a floor 210 of large 
dimensions which is generally free from obstructions such as the service 
area 112 in the floor 110 of FIG. 2. In such an application, the present 
invention contemplates the use of a conventional vibrating screed 212 as 
is schematically illustrated at 212 in FIG. 3. The vibrating screed 212 
has a strike-off bar 214 of sufficient length to span one dimension of the 
floor 210 so that it may be supported at its opposite ends by means of 
forms 216 and 218 extending the entire length of the floor 210. Vibrating 
means 220 are mounted on the strike-off bar. In this variation of the 
method according to the present invention, both rough leveling and 
densification are accomplished by means of the vibrating screed 212. 
After the concrete sets up sufficiently to support the weight of an 
operator, a power float of the type described above is then employed to 
produce a flat or level surface after which the concrete is allowed to 
stand in accordance with the preceding description. Thereafter, the floor 
is treated with a power grinder also as described above in order to 
produce a flat, porous surface having a sanded characteristic as 
illustrated in FIG. 6. 
Numerous modifications and variations in addition to those described above 
are of course obvious within the scope of the present invention which is 
accordingly defined only by the following appended claims.