Rapid protection system for freshly-placed concrete and a method of practicing the system

Abstract

A system and method for the rapid protection of freshly placed concrete. The system includes a mat which extends the full width of the freshly placed concrete and is mounted to be displaced and positioned so that it is located at approximately 4 inches above the top surface of the freshly placed concrete for a set period of time and then placed on the top surface of the freshly placed concrete for a further set period of time.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application relies on priority from provisional application, No. 60/323,094, which was filed on Sep. 19, 2001.

GOVERNMENT SPONSORSHIP

[0002] The present invention was developed in conjunction with the Texas Department of Transportation Research Project 0-2116 “Effects of Wet-Mat Curing Time and Earlier Loading on Long-Term Durability of Bridge Decks.” As such, the Texas Department of Transportation may have rights in the present invention.

TECHNICAL FIELD

[0003] The present invention relates, in general, to an apparatus and mechanism allowing near-immediate application of a protector mat for wet and/or elevated heat curing concrete surfaces. More specifically, the apparatus permits the automated addition of fluid, energy, and the like throughout the entire curing process. The specific apparatus intends to improve the curing process by creating ideal curing conditions that result in a more durable slab of concrete in a potentially shorter amount of time. “Rapid protection” equates to a reduced interval between placement of fresh concrete to the time a protection system is securely put in place. The present invention also relates to an improved method for affecting the curing of freshly-placed concrete.

BACKGROUND OF THE INVENTION

[0004] A concrete slurry is created by mixing water and concrete ingredients, including cement and aggregates. The slurry begins to harden by a chemical reaction called hydration. While hydration occurs, the concrete is said to be “curing.” The curing conditions directly facilitate hydration and are essential to the realization of high quality concrete. For example, the curing conditions can impact the concrete's durability, water tightness, strength, and resistance to wear. In fact, proper curing techniques will improve nearly all the desirable properties of concrete. Though concrete can cure for extended periods of time—perhaps even years—the most significant curing for long-term durability occurs during the first few weeks after initial placement. To ensure the hydration process extends until the concrete becomes as durable as practically and/or inherently possible, certain curing protection measures are employed.

[0005] Maintaining the proper conditions for concrete curing requires the ability to satisfactorily control moisture content and temperature levels. Under proper conditions, the concrete will retain moisture as it continues to strengthen and harden during the curing process. In addition, improperly cured concrete will shrink to such an extent that cracks can occur, damaging the strength and aesthetics of the finished concrete. Therefore, evaporation of mix-water from the concrete slurry must be decreased or eliminated, as the retained moisture will also prevent the concrete from shrinking during drying. Unfortunately, a by-product of traditional curing construction methods has been long construction delays as the concrete slowly cures. Therefore, there is a demand and need for methods that can quickly cure concrete while still remaining strong and durable.

[0006] Numerous techniques have been employed in order to accelerate the curing process. For example, according to one technique, an electric current has been conducted through the mix itself. More typical methods include:

[0007] Water Cure—a concrete slab is flooded or mist sprayed in order to prevent mix-water evaporation. However, the process is time consuming and does not address potential problems with ambient conditions such as freezing temperatures.

[0008] Water Retaining Methods—a covering (such as sand, straw, etc.) is placed on the concrete and kept continuously wet. This method is ineffective because the material can mar or become imbedded in the still soft concrete. Moreover, surface evaporation, poor temperature conditions, and the inconsistent application of the material to the concrete remain as problems.

[0009] Waterproof Paper or Plastic Film Seals—continuous layers or sheets are applied to the wet concrete surface to retain moisture. Again, the concrete has to be hard enough to resist surface damage. The plastic films may also cause discoloration and can be problematic if heat or energy needs to be applied to the concrete. A system does not currently exist that can quickly apply the layers to prevent surface evaporation.

[0010] Chemical Membranes—a chemical mixture, or ‘curing compound’, is added to the curing concrete. The curing compound must be developed and used for specific concrete applications. This curing technique alone does not account for the need to control the temperature and moisture content of the curing concrete.

[0011] In addition to the various problems noted above, these traditional approaches have been deficient in producing an adequate final compressive strength. Also, the necessary apparatus can be too costly or ineffective. An improved mechanism for curing concrete that can combine these approaches in an economical and timely manner is needed.

[0012] The economics of curing are particularly important in regards to the building of larger structures, such as pre-stressed beams and bridge decks. Larger structures are built by casting the concrete in pre-made forms or pouring the concrete on-site. In either case, the investment in money and man-hours is significant and reducing the time for curing increases efficiency. Yet, the strength of the structure cannot be compromised for the sake of reducing production time.

[0013] While curing ensures a stronger concrete product, a smooth, hard surface must also be achieved by subjecting the concrete to screeding. Historically, screeding was the process whereby a rigid piece of metal or wood was used to level the concrete and remove excess material. A typical modem “screed machine,” as illustrated in FIG. 1, moves along the face of an uncured concrete structure on a set of rails. Fresh concrete is placed in front of the screed machine. The surface of the fresh concrete is unlevel and must be vibrated, smoothed, and finished (i.e., textured or grooved). For roadway and bridge deck applications, finishing is required for a smooth riding surface while texturing/grooving of the pavement is performed for vehicle traction control. The screed machine performs all of these functions at a rate of approximately twenty longitudinal feet per hour, depending on slab thickness, deck width, etc. A manned platform often follows the screed machine. The platform operator adds a liquid curing compound and/or sprays water on the concrete. As mentioned, premature “drying” of the concrete before the curing process is substantially complete can lead to excessive shrinkage, cracking, and undesirable micro-structural features in the concrete. However, curing compounds are not very effective at sealing the concrete and the water is prone to surface evaporation.

[0014] In some instances, heavy wet mats or blankets are applied to the concrete to retain moisture and facilitate curing. The wet mats are applied as soon as possible but, in order to protect the finished surface of the concrete pour, these wet mats generally cannot be applied until three to four hours after the concrete is poured. Applying the mat before the concrete had set would mar the concrete.

[0015] The problems with wet-mats are compounded by a current industry trend toward more “high-performance” concretes, which typically require a much shorter interval (e.g., less than fifteen minutes) between the time of the concrete placement and the application of the wet mats. The result is that a more immediate and higher level of protection is required for the freshly placed concrete. However, the extremely short interval of time required for the placement of curing mats or the like currently makes it impossible to do without marring the surface of the concrete. It is also possible that the immediate application of mats directly on top of the surface of the concrete could adversely influence the long-term durability of the cured concrete.

[0016] Finally, heat, though desirable in cold weather concreting, is rarely applied to the top surface of curing concrete due to practical and logistical limitations with the current techniques for curing. However, with the use of more complex “high performance” concretes, there is a need to substantially shorten the interval between the time the concrete is placed and the time heat is applied. The typical interval is currently around three to four hours. A process or mechanism that shortened that interval would be desirable in many environments.

[0017] There currently exists a need for preventing moisture loss and surface evaporation from finished concrete. In addition, in certain applications, heat, chemicals, or water may also need to be added after a protective mat is deployed. Present day seals or blankets cannot be applied to concrete in a timely manner without marring the surface of the concrete and potentially weakening the finished product. Also, the technology does not exist to apply heat in a timely manner. Therefore, a mechanism that facilitates the placement of wet mats and allows for the addition of heat, water, chemicals, and the like is needed

SUMMARY OF THE INVENTION

[0018] For the first time, a curing protection system for concrete is accomplished at a fraction of the time that it presently takes and in an efficient and relatively inexpensive manner.

[0019] Existing methods and structural arrangements are utilized, and modifications introduced to effect the objective of shorter time spans for the installation of the curing protection system over the concrete while maintaining the quality of the concrete. Both a unique structure and method of proceeding are introduced by the present invention.

[0020] Curing mats, which are presently employed for curing concrete, are used according to the present invention, but in a modified structural manner, resulting in the unique structure. The procedure according to which the curing mats are used is also modified, resulting in the unique method.

[0021] In essence, the unique structure allows the curing mats to be used in conjunction with the laying of the concrete but with a short time delay. The curing mats are positioned relative to the freshly laid concrete but not in initial contact so that the surface quality of the concrete is not affected, but curing is. Subsequent to a short delay during which the application of moisture and/or a curing compound can proceed, contact of the curing mats with the concrete is made and a potentially more rapid curing takes place.

[0022] The present invention then, relates to a rapid protection system for freshly placed concrete to improve durability. In greater detail, and in a preferred embodiment of the invention, the present invention employs a screed machine where the screed machine and a curing compound platform currently used in the industry remain essentially the same. The present invention follows the application of moisture or a curing compound with the immediate placement of a mat to protect the curing cement. The invention rolls sections of curing mats along screed rails that are already in place and leveled for the concrete placement screed machine. The mats are suspended through the use of either 1) a collapsible, embedded structural system or 2) an attachable structural system. Either system can be designed in a number of ways to allow the mats to be rolled up rather compactly and also to span the deck surface of the concrete pad when rolled out. Once the mat is suspended across the deck, water vapor, heat, chemicals, and the like may be applied.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The following eight figures are chosen as representative of the various embodiments. The foregoing, and additional objects, features, and advantages of the present invention will become apparent to those of skill in the art from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which:

[0024] FIG. 1 is a perspective view exemplary of a screed machine in-use on a concrete bridge deck;

[0025] FIG. 2 is a perspective view of a screed machine combined with the rapid protection system in accordance with one embodiment of the present invention;

[0026] FIG. 3 is a cross section of a screed machine combined with the rapid protection system in accordance with one embodiment of the present invention;

[0027] FIG. 3A is an exploded cross section of the rapid protection system for freshly placed concrete in accordance with one embodiment of the present invention;

[0028] FIG. 4 is a perspective view of a screed machine with a deployed rapid protection system in accordance with one embodiment of the present invention wherein the typical curing compound platform is removed and a protective mat is immediately deployed after a screed machine passes over freshly placed concrete;

[0029] FIG. 5 is a cross section of a screed machine with a deployed rapid protection system in accordance with one embodiment of the present invention wherein the typical curing compound platform is removed and a protective mat is immediately deployed after the screed machine passes over freshly placed concrete;

[0030] FIG. 5A is an exploded cross section of the rapid protection system for freshly placed concrete in accordance with one embodiment of the present invention;

[0031] FIG. 6 is a cross section of a collapsible truss in accordance with one embodiment of the present invention;

[0032] FIG. 7 is a perspective view of a screed machine combined with the rapid protection system using tensioned support wires to support a protective mat in accordance with one embodiment of the present invention;

[0033] FIG. 8 is a cross section of a rapid protection system mat suspended over curing concrete in accordance with one embodiment of the present invention; and

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0034] While the invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.

[0035] Generally a curing compound is applied immediately after a screed machine smooths the poured or placed concrete, followed much later by the application of wet mats or some similar protector when the concrete has achieved sufficient strength to support that operation. The mats operate to retain moisture in the concrete necessary for a number of desirable characteristics, including durability and the prevention of cracks. However, this curing method has the significant disadvantage of not being able to provide moist curing when it is most needed immediately after a curing compound is added. The result is significant moisture loss for freshly placed concrete, especially under adverse conditions. In the case of high performance concretes with low water-cement ratios, the curing method may deny much needed moisture to the concrete resulting in autogenous shrinkage and cracks.

[0036] The present invention is developed to retain and provide moisture for curing much earlier than it is done under the conventional methods. It is a two-stage method where, in stage 1, a curing mat is suspended on a collapsible or attachable truss that will prevent it from damaging the still “green” concrete. In this case, the mat is able to provide more effective sealing of the concrete than a curing compound would. Thus, nearly one-hundred percent humidity over the paved concrete can be maintained. Further, the rapid protection system of the present invention can be placed on existing rails used by a screed machine, and it can be placed immediately after the paver has moved away from the paved area.

[0037] In stage 2, the rapid protection system curing mat is lowered to the slab when the slab is able to support the weight of the mat. This could be earlier than the final setting time because construction personnel do not have to get on the slab to place the mat. The quick placement will enable the rapid protection system to provide additional moisture to prevent autogenous shrinkage of high performance concrete. The short interval of time between the paving of the concrete and placement of the mat could double or triple the lifespan of the concrete slab or deck.

[0038] There are illustrated drawings wherein the numerals have been used throughout the several views to designate the same or similar parts. FIG. 1 depicts a slab of curing concrete 10 with an affiliated screed machine 12. The dimensions of slab 10 and screed machine 12 are, of course, dependent upon the particular dimensions of the roadway, bridge deck, or other item being constructed. In any event, screed machine 12 is disposed in operative position over concrete slab 10. For the purposes of illustrating the present invention, the item to be constructed is a bridge deck consisting of an abutment 4 supporting steel beams 6.

[0039] Screed machine 12 includes a main trusswork or frame 16 having a first end 16a and a second end 16b. Frame 16 extends across and over the surface of concrete slab 10. While not depicted here, screed machine 12 can incorporate a texturing and/or grooving unit mounted on frame 16. In addition, means to vibrate and smooth slab 10 would be included and such means are well known in the art.

[0040] As commonly practiced, rough concrete 20 is placed upon the unfinished bridge deck. The slurry or rough concrete 20 is held in place by forms 22. Screed machine 12 consists of screed frame 16, screed rails 18 and a curing compound platform 24. Frame 16 rests on screed rails 18 at a first end 16a and second end 16b. Rails 18 are preferably formed at the top of forms 22 or as a separate component along with forms 22. Frame 16 then progresses from a first end of slab 10 to a second end by means known in the art. Grooving, texturing and/or vibrating means convert rough concrete 20 into flat slab 10. Next, curing compound platform 24 passes along screed rails 18 behind screed frame 16. An operator stands on platform 24 in order to spray water and/or curing compounds onto slab 10 to facilitate curing. Once the surface of slab 10 has hardened for several hours, a wet mat 8 can be applied to the surface of concrete slab 10 to prevent the evaporation of any moisture or curing compounds located in the slab 10 or on the surface thereof.

[0041] Turning now to a detailed description of the present invention, as illustrated in FIGS. 2-9, a rapid protection system for freshly placed concrete allows for a substantial decrease in the time interval between concrete placement and mat and heat application. First, FIGS. 2 and 3 show a screed machine 12 employing the principals of a rapid protection system (‘RPS’) 14 of the present invention. RPS 14 includes a roller 30, a beam 32 holding a RPS mat 34, and a support structure (i.e. a truss 44). Mat 34, sized to cover slab 10, is pre-wound onto beam 32, which is supported by frame members 36 above slab 10. Frame members 36 and a lead bar 38 are also parts of the RPS 14 support structure. The illustrated frame members 36 are ‘A’ shaped and are supported by wheeled bases 52 that are adapted to move along screed rails 18. However, frame members 36 supporting beam 32 could be shaped in a number of other ways. More important is the fact that beam 32 can move along slab 10 using the pre-existing screed rails 18. Therefore, mat 34 can be installed in a timely and cost effective manner.

[0042] In usage, mat 34 is unrolled from beam 32 and lead by lead bar 38 toward slab 10. Ultimately, the unrolled mat 34 is secured to slab 10 at a first end and along the edges of slab 10. Specifically, hangers 26 are positioned over screed rails 18. Support beam 32 is then rolled along screed rails 18 from a first end of slab 10 to a second end, i.e. along and toward the direction of travel of screed machine 12. Mat 34 unrolls from roller 30. A series of collapsible hangers 26 in the form of a truss 44 (FIG. 6) support mat 34 above slab 10. Collapsible truss 44 could either be attached to mat 34 and, therefore, truss 44 could “fold-down” as mat 34 is being rolled up. Then, as mat 34 is unrolled, each truss 44 would “fold-out.” A small amount of cross bracing could be included between adjacent truss hangers 26 once truss 44 is in an expanded position. Another embodiment, referring to FIG. 3, would be to have a second worker to the left of RPS mat roller 30. As mat 34 is being unrolled (in this case without hangers 26 embedded into the big roll), a person attaches truss 44 hangers to mat 34.

[0043] FIG. 3 represents a cross view of one embodiment of the present invention and depicts a bridge deck represented by abutment 4 and supporting steel beams 6. The concept of a bridge deck could be replaced by a roadway or any mostly horizontal item that requires a slab of concrete. Forms 22 retain the concrete poured onto the bridge deck while screed rails 18 rest on top of the form 22 as noted. Screed machine 12 moves along screed rails 18. Curing compound platform 24 follows the same direction of travel as screed machine 12. Next, roller 30 also travels along screed rails 18 and mat 34 is unwound from beam 32.

[0044] FIGS. 3a and 3b illustrate heater and water outlet mechanisms integrated into mat 34. The distance of mat 34 over slab 10 can vary with each application with a distance of two to four inches above the deck for most applications. As mentioned above, additional water may be necessary to maintain the proper moisture content for the curing concrete. Water outlet 42 allows for the continuous application of water along the entire length of concrete slab 10. In addition, a heater 40 can be used in appropriate environments to either speed the curing process or prevent mix water from freezing during hydration. Of course, additional water from water outlet 42 could also freeze in some environments without heater 40.

[0045] FIG. 3a also shows in greater detail mat 34 suspended above a concrete slab by means of hangers 26. Screed rails 18 are formed much like a steel rail for a train, so they are somewhat rounded at the top. Thus, hangers 26 for truss 44 simply “hang” over rail 18 at both edges. Protective mats cannot be directly applied to concrete for three to four hours following screeding because the contact can potentially mar and weaken the final concrete product. Each hanger 26 has a mat lowering mechanism 38 (FIGS. 3a and 5a). Though any number of methods could be employed to lower mat 34 to the slab 10, including electric motors, a simple latch is illustrated in the figure. Once the concrete has hardened to a sufficient degree, mat 34 is lowered into direct contact with slab 10. Unless grooving operations will be performed under mat 34 later, truss 44 supports can be removed.

[0046] For extremely wide pours, the truss/hanger system may need to “spoke” into the concrete at certain locations. FIG. 6 demonstrates means whereby mat 34 is suspended by lightweight, collapsible truss 44. Truss 44 spans the width of slab 10 from a first form 22 to a second form 22. A spoke 46 extends vertically into the concrete to support truss 44. Small diameter holes caused by spokes 46 would be sealed with grout, epoxy or the like after the curing operation is complete and the lifespan of slab 10 would not be compromised. Another possible method to suspend RPS mat 34, especially for extremely wide placements, would be the use of tension support wires 48 as shown in FIG. 7. Tensioned wires 48 assist in elevating mat 34 above slab 10. In FIG. 7, wires 48 are attached to the curing compound platform 24. However, they could be attached to other locations that are capable of providing the necessary support. Mat 34 lies on the wires after roller 30.

[0047] FIGS. 4 and 5 show a second embodiment of the present rapid protection system for freshly placed concrete where the curing compound platform 24 has been eliminated from screed machine 12. High performance concretes are intended to cure and set quicker than typical concretes. A protective covering must be placed over the concrete almost immediately following screeding. Mat 34 unwinds from beam 32 by passing over helper roll 50. Helper roll 50 is designed to layout mat 34 in front of the direction of travel the roller 30. Mat 34 suspension would still be affected by use of truss 44 (with or without spokes 46 depending on width of the placement) or wires 48.

[0048] Although the present invention has been described in terms of a preferred embodiment, it will be understood that numerous variations and modifications may be made without departing from the invention. Thus, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.

Claims

1. A rapid protection system for freshly placed concrete, comprising:

a pair of elongated parallel extending support tracks extending along the elongated, parallel edges of the freshly placed concrete;

a mat extending the full width of the freshly placed concrete and between the elongated parallel edges thereof; and

means for displacing and positioning said mat along said elongated parallel extending tracks such that said mat is positioned above the top surface of the freshly placed concrete at a distance approximately 4 inches above the top surface of the freshly placed concrete.

2. The rapid protection system for freshly placed concrete as defined in claim 1, further comprising:

a platform which extends between the elongated, parallel edges of the freshly placed concrete and is displaced along said elongated parallel extending tracks in conjunction with the displacement and positioning of said mat such that the freshly placed concrete can be treated in advance of the positioned mat above the top surface of the freshly placed concrete.

3. The rapid protection system for freshly placed concrete as defined in claim 1, further comprising:

a pair of elongated parallel extending forms adjacent to said support tracks for defining the outer width limit of the freshly placed concrete.

4. The rapid protection system for freshly placed concrete as defined in claim 3, wherein said support tracks are formed as part of said forms, and are located as the top edge of said forms respectively.

5. The rapid protection system for freshly placed concrete as defined in claim 1, wherein said mat forms a roller supported on said support tracks.

6. The rapid protection system for freshly placed concrete as defined in claim 5, wherein said means for displacing and positioning said mat comprises a pair of frame members situated on opposed ones of said support tracks with said roller extending between and supported by said frame members.

7. The rapid protection system for freshly placed concrete as defined in claim 6, wherein said means for displacing and positioning said mat comprises a feed bar which extends from said frame members outwardly from said roller, the engagement of said mat with said feed bar effecting the positioning of said mat relative to the top surface of the freshly placed concrete.

8. A rapid protection system for freshly placed concrete in conjunction with a screed machine, the screed machine producing the freshly placed concrete, comprising:

a pair of elongated parallel extending support tracks extending along the elongated, parallel edges of the freshly placed concrete for supporting the screed machine for displacement along said support tracks;

a mat extending the full width of the freshly placed concrete and between the elongated parallel edges thereof; and

means for displacing and positioning said mat along said elongated parallel extending tracks such that said mat is positioned upstream of the screed machine and above the top surface of the freshly placed concrete at a distance approximately 4 inches above the top surface of the freshly placed concrete.

9. The rapid protection system for freshly placed concrete as defined in claim 8, further comprising:

a platform which extends between the elongated, parallel edges of the freshly placed concrete and is displaced along said elongated parallel extending tracks in conjunction with the displacement of the screed machine and the displacement and positioning of said mat such that the freshly placed concrete can be treated in advance of the positioned mat above the top surface pf the freshly placed concrete.

10. The rapid protection system for freshly placed concrete as defined in claim 8, further comprising:

a pair of elongated parallel extending forms adjacent to said support tracks for defining the outer width limit of the freshly placed concrete.

11. The rapid protection system for freshly placed concrete as defined in claim 10, wherein said support tracks are formed as part of said forms, and are located as the top edge of said forms, respectively.

12. The rapid protection system for freshly placed concrete as defined in claim 8, wherein said mat forms a roller supported on said support tracks.

13. The rapid protection system for freshly placed concrete as defined in claim 12, wherein said means for displacing and positioning said mat comprises a pair of frame members situated on opposed ones of said support tracks with said roller extending between and supported by said frame members.

14. The rapid protection system for freshly placed concrete as defined in claim 13, wherein said means for displacing and positioning said mat comprises a feed bar which extends from said frame members outwardly from said roller, the engagement of said mat with said feed bar effecting the positioning of said mat relative to the top surface of the freshly placed concrete.

15. A method for protecting freshly placed concrete using a mat, comprising the steps of placing the mat above the top surface of the freshly placed concrete such that said mat is approximately 4 inches above the top surface;

maintaining the mat suspended above the top surface of the freshly placed concrete for a set period of time; and

placing the mat on the top surface of the freshly placed concrete after sid set period of time for a further set period of time.

16. The method for protecting freshly placed concrete as defined in claim 15, further comprising the steps of:

forming the mat into a roller;

supporting the roller above the top surface of the freshly placed concrete such that it extends along the fill width of the freshly placed concrete; and

dispensing the mat to achieve said suspension.

17. The method for protecting freshly placed concrete as defined in claim 16, further comprising the step of:

releasing the mat after said suspension to effect said placement.