NO348116B1 - Semi-precast elevated concrete element system - Google Patents

Description

Technical field

The present invention relates to a semi-precast elevated concrete element system.

Background art

Typical semi-precast elevated concrete elements consist of prefabricated reinforced concrete elements with an upper layer of reinforced concrete, which is either bonded or unbonded to the prefabricated reinforced concrete elements.

Fig. 1A illustrates a bonded version of a prior art elevated concrete element - a slab. This known type of semi-precast elevated concrete element is implemented as follows:

the prefabricated reinforced concrete elements are produced in a factory and transported to a building site;

the prefabricated reinforced concrete elements are erected on temporary supports, typically aided by a crane;

additional reinforcement (e.g., negative moment reinforcement, two-way reinforcement, punching shear reinforcement, etc.) is then installed over the erected prefabricated reinforced concrete elements by workers, and the upper layer of concrete is placed, typically by means of a pump or skip, levelled and finished.

There are numerous drawbacks in the installation and performance of this type of semi-precast elevated concrete elements. During installation significant time, material, and space on the construction site, and resources (i.e., workers, cranes, etc.) are used for the placement of reinforcement. The required placement of reinforcement is a possible source of construction errors as it must be appropriately placed, within specified tolerances. Further, congestion of reinforcement is a common problem which leads to complications in placing and adequately compacting the upper layer of concrete. Structural performance of this type of semi-precast elevated concrete element is also affected by the interaction between the two layers resulting from a commonly known behavior of concrete, which is shrinkage. Under normal service conditions concrete is exposed to air, resulting in drying. As a result of the concrete drying, shrinkage strains occur due to the development of capillary stresses in the concrete pores. This behavior of concrete is time dependent with the majority of shrinkage taking place rapidly after exposure and subsequently at a reduced rate versus time. As the in-situ cast upper layer of concrete is placed on prefabricated reinforced concrete elements that are typically cast months prior, the prefabricated elements restrain the shrinkage deformations of the in-situ cast concrete, leading to a differential shrinkage. As illustrated in Fig. 1A, this differential shrinkage results in a negative pre-camber (i.e., a downward deflection) of the element.

The shrinkage of the in-situ cast upper layer of concrete is additionally restrained internally by the reinforcement. The restraint of the shrinkage of the upper concrete layer results in the development of tensile stresses and cracking in said upper layer, as illustrated in Fig. 1A. Cracking of the upper layer of concrete may have durability and serviceability implications. When the element is exposed to loads, cracks in the compression zone of the element first need to close before compressive stresses are transferred. Some amount of element deflection is required to close these cracks which leads to an overall increased deflection for this type of semi-precast elevated concrete element.

More recently, the above described semi-precast elevated concrete element has been improved in Volkel et al. 1998 and Riese and Droese 1996. The reinforcement bars in the in-situ cast upper layer of concrete were replaced by steel fiber reinforcement. This resulted in savings in time and resources during installation as the reinforcement placement was substituted by intermixing reinforcement (i.e., fibers) into the concrete. Further, the internal restraint from the continuous reinforcement bars in the upper layer was eliminated; however, shrinkage of the upper concrete layer still occurs and cracking of the upper layer can still be induced via external restraint from the precast concrete layer. Drawbacks in the installation process are addressed by these improvements. However, load-induced deflections of the semi-precast elevated concrete elements described above are negatively impacted by the in-situ cast upper layer of concrete. As a result, currently there is a practical limit in the possible span length of these elements due to serviceability limit state considerations (i.e., deflection limits).

There is also known a precast concrete slab system disclosed in US5448866. The main drawbacks of the known system are that placement of reinforcement requires significant time, material, space on the construction site, and resources (i.e., workers, cranes, etc.); also the required placement within specified tolerances of reinforcement is a possible source of construction errors; further, congestion of reinforcement is a common problem which leads to complications in placing and adequately compacting the upper layer of concrete. Moreover as the in-situ cast upper layer of concrete is placed on prefabricated reinforced concrete elements that are typically cast months prior, the prefabricated elements restrain the shrinkage deformations of the in-situ cast concrete, leading to a differential shrinkage, which results in a negative precamber (i.e., a downward deflection) of the slab. The restraint of the shrinkage of the upper concrete layer results in the development of tensile stresses and cracking in said upper layer, as illustrated in Figure 1(a). Furthermore there is a practical limit in the possible span length of these slabs due to serviceability limit state considerations (i.e., deflection limits).

Disclosure of the invention

It is an object of this invention to provide integrated semi-precast elevated concrete composite elements with the improved structural performance of reduced deflections under equivalent loads.

Other objects and advantages of this invention are to provide integrated semiprecast elevated concrete composite elements with:

improved deflection control, allowing increases in allowable span lengths and corresponding reduction in the number of columns needed,

an increased load capacity while maintaining the same geometry (i.e., span length and total thickness), and

enhanced space efficiency due to reduced number of columns which are cost effective, commercially viable, and attractive for implementation by architects and engineers.

In accordance with the above listed objects and advantages, the invention provides an integrated semi-precast elevated concrete element system comprising a precast concrete panel with partially embedded reinforcing elements for shear transfer and reinforcement for additional load capacity, and an in-situ cast upper layer of a modified concrete having positive length change, i.e. net dilation of at least 75 με at an infinite time; wherein the in-situ cast upper layer of concrete comprises: hydraulic cement being a Portland cement or blended cement: 200 to 700 kg/m<3 >of concrete, more preferred 225 to 600 kg/m<3 >of concrete and even more preferred 250 to 500 kg/m<3 >of concrete; water, at a ratio of watenhydraulic cement of 0.30 to 0.75 by weight, preferably from 0.30 to 0.65 by weight; aggregate, with a maximum size of 40 mm, preferably with maximum size of 30 mm and aggregate:concrete ratio from 0.45 to 0.85 by volume; fiber reinforcement, 10 to 200 kilograms of steel fiber per cubic meter of concrete, more preferred 15 to 150 kilograms of steel fiber (e.g. 25-90 mm in length and 0.5-1. 5 mm in diameter) per cubic meter of concrete; and/or 0.2 to 30 kilograms of synthetic polymer fiber (5-30 mm in length and 0.01-1.0 mm in diameter) per cubic meter of concrete; and/or 1 to 100 kilogram of synthetic mineral fiber (3-90 mm in length and 0.01-1.5 mm in diameter) per cubic meter of concrete; chemical prestressing additive: 1 to 30% by mass of hydraulic cement and sufficient to achieve the positive net length change target above. The chemical prestressing additive is a mineral admixture adapted to create hydration products, which are volumetrically larger than the volume of the reactants, where the length change at an infinite time and drying conditions of 50% RH and 20°C is estimated by plotting the length change data collected in accordance with ASTM C 157 (Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete; Book of Standards Volume: 04.02, ASTM International) as a function of the inverse square root of time and extrapolating the infinite shrinkage after 28-days of drying.

Some known chemical prestressing additive include magnesium oxide, calcium aluminate, tricalcium aluminate or combinations of these or other substances causing chemical prestressing of restrained concrete elements.

The precast concrete panel preferably comprises Portland cement concrete optionally with pozzolan(s) and/or chemical admixture(s) mixed therein. The precast concrete panel is optionally prestressed. The precast concrete panel may comprise a hollow core, or flat panel, or double-T, or U-shaped, or 1-shaped precast concrete profile.

The partially embedded reinforcing elements of the precast concrete panel are preferably geometrically asymmetric with an increased area of reinforcement in the part of the reinforcing element embedded in the precast concrete panel. Said reinforcing elements may be lattice girders or reinforcing profiles made of metal or fibre-reinforced plastic.

The in-situ cast upper layer may further contain metallic or non-metallic bars or other reinforcing elements.

Brief description of drawings

Fig. 1 is a schematic view of the impact of varying length change behaviors of the upper in-situ concrete layer on the induced deformations of a bonded semi-precast elevated concrete element (impact of self-weight on deflection is not shown), where Fig. 1A illustrates shrinkage of the prior art in-situ concrete layer (deflection); Fig. IB shows no shrinkage of in-situ concrete layer; and Fig. 1C illustrates overcompensation of shrinkage of in-situ concrete layer (negative deflection) according to the present invention;

Fig. 2A shows a frontal and Fig, 2B - side views of an example of a steel lattice girder, which can be used in the precast element;

Fig. 3 - pictures showing load-induced flexural deflection testing of semi-precast elevated concrete element system panels; where application of uniform load was realized using water-filled palette tanks and deflection instrumentation was implemented by dial gauges;

Fig. 4 - chart showing reduction in flexural load-induced deflections for 5 m span of the invented semi-precast elevated concrete element system (P3 line) compared to traditional reinforced concrete systems of the same thickness with (N2 line) and without (N1 line) negative moment reinforcement.

Fig. 5 - chart showing reduction in flexural load-induced deflections for 7.5 m span of the invented semi-precast elevated concrete element system (P8 line) compared to traditional reinforced concrete systems of the same thickness with (N7 line) and without (N6 line) negative moment reinforcement.

Fig. 6 - chart showing experimental length change results of the concrete for the in-situ cast upper concrete layer; the figure presents the length change of the concrete versus inverse square root of the specimen age, allowing for estimation of the length change at an infinite time by means of linear extrapolation.

The scale, apparent dimensions (both individually and relatively between the two layers) and shapes of the precast element and the in-situ concrete layer are solely for demonstration purpose and do not represent a limitation of the presented invention.

Figure 1 shows a schematic view of the semi-precast elevated concrete composite elements, consisting of a precast concrete panel and an in-situ cast upper layer of a concrete. The precast concrete panel has reinforcing elements, like that shown in Figure 2, partially embedded in its top surface for the purpose of transfer of shear between the precast concrete panel and the in-situ cast upper layer of concrete, and longitudinal reinforcement for improved load capacity. A steel lattice girder (Fig. 2) is partially embedded into the precast concrete with the lower chord contained in the precast element and the upper chord and diagonals protruding from the precast element.

The first aspect of the invention relates to the combination of these components (i.e., precast concrete panel and an in-situ cast upper layer of a concrete) wherein the in-situ cast upper layer of concrete includes fiber reinforcement and chemical prestressing additive. Experimental results demonstrate the invention provides surprising and unexpected improvements in the flexural rigidity compared to previously known semi-precast elevated concrete composite elements.

Figures IB and 1C illustrate the theoretical deflected shape induced by the length change of the in-situ cast upper layer of concrete in an integrated (i.e., bonded) semi-precast elevated concrete composite element wherein the upper layer of concrete has no shrinkage and an overcompensated shrinkage (i.e., net expansion), respectively.

As illustrated in Figure IB, it is believed that the elimination of shrinkage in the in-situ cast upper layer of concrete results in the mitigation of shrinkage induced cracking and differential shrinkage-induced deflection of the element As a result of the mitigation of cracking in the in-situ cast upper layer of concrete, it is believed that compressive stresses are immediately transferred through the compression zone of the element without the need for first closing cracks. Thereby, it is believed that eliminating shrinkage in the in-situ cast upper layer of concrete, two components of the total element deflection are removed, being the element deflection necessary to close cracks in the compression zone and the detrimental element deflection induced by differential shrinkage.

Further, by overcompensating shrinkage in the in-situ cast upper layer of concrete (Figure 1C) a beneficial negative pre-camber can be realized. In this case, a special type of concrete with a permanent net expansion is used for the in-situ cast upper layer of concrete. Therefore, a beneficial differential length change is realized wherein the precast concrete panels that are typically cast months prior are combined with an in-situ cast upper layer of concrete with net expansion, yielding a negative precamber.

It has been experimentally determined that an in-situ cast upper layer of concrete comprising hydraulic cement, water, aggregate, mineral admixtures (optional), and chemical admixtures (optional), fiber reinforcement and chemical prestressing additive;

combined with a precast concrete element comprising:

- a concrete mix consisting of components including hydraulic cement, water, aggregate, mineral admixtures (optional), chemical admixtures (optional), and fiber reinforcement (optional) and

- a partially embedded steel element that is geometrically asymmetric with an increased area of steel in the embedded part of the steel element (see Fig. 2 for an example of an asymmetric steel element);

yields unexpected levels of beneficial negative deflection (i.e., positive pre-camber), significantly increases the effective bending stiffness of the integrated semi-precast elevated concrete composite element and ultimately allows longer span lengths than previously feasible.

Table 1 - Composition of concretes presented in Figure 6.

Table 2 - Test parameters for comparison of the invented thin semi-precast elevated concre reinforcement bar reinforced concrete system of the same thickness with and without negative mome

1) Including lower chords of lattice girders

2) Including upper chord of lattice girders

3) The lattice girder designation E 16-06 6 10 means (See Figure 2 for girder dimensions): Type E, gir diameter = 06 mm, diagonal diameter = 6 mm, upper chord diameter = 10 mm.

4) C25/30 concrete with 40 kilograms of HE 75/50 steel fiber per cubic meter of concrete and 37.5 ki additive (MgO) per cubic meter of concrete.

Table 2 provides parameters of comparison tests of the invented thin semiprecast elevated concrete element system and prior art reinforcement bar reinforced concrete system of the same thickness with and without negative moment (i.e., upper) reinforcement. Two span lengths, 5 m and 7.5 m, were investigated at total element thicknesses of 22 and 32 cm, respectively. The prefabricated reinforced concrete elements consisted of a 5 cm thick concrete thickness with partially embedded lattice girders as described in Table 2. The prefabricated reinforced concrete elements were supported on the ends, to the described span lengths, and with additional evenly spaced temporary supports to allow placement of the upper layer of concrete. Two different materials were used for the upper layer concrete, a typical C25/30 (i.e., 25 MPa cylinder strength) concrete and C25/30 concrete including 40 kilograms of HE 75/50 steel fiber per cubic meter of concrete and 37.5 kilograms of chemical prestressing additive per cubic meter of concrete. In particular examples magnesium oxide was used as chemical prestressing additive, which is preferable, however other chemical prestressing additives may be used instead (e.g. calcium aluminate, tricalcium aluminate).

The upper layer of concrete was placed over the prefabricated reinforced concrete elements and cured in accordance with common concreting practice. After curing, the temporary supports were removed and a uniform loading was simulated as shown in Figure 3 by means of water-filled palette tanks to a level of 5.9 kN/m (600 kg/m). Table 2 introduces sample identifications for the resulting 6 systems tested.

Figures 4 and 5 present the experimental evidence of this significant increase in the effective bending stiffness of the integrated semi-precast elevated concrete composite element across two span lengths, 5 and 7.5 m, respectively (cf. also Table 2).

Figure 4 presents results from the 5 m span samples Nl, N2 and P3. Comparing the load-deflection response for samples Nl and N2 illustrates the impact cracking, introduced by the internal restraint from the upper reinforcement detail in the N2 sample, has on flexural rigidity. The instantaneous deflection is increased by approximately 3.5 mm from Nl to N2. While the deflection is better controlled by Nl, it is noted that there is a lack of negative moment reinforcement in Nl, resulting in a system that is less robust overall. The P3 sample realizes a still further increased flexural ridigity than the Nl sample, with 2.1 mm less instantaneous load-induced deflection, while still providing significant reinforcement through the incorporation of steel fiber into the mix. The increased level of resistance to load-induced deflections for the P3 sample was not expected as structural analysis of sample Nl and P3 (wherein the in-situ cast upper layer is treated as unreinforced concrete) would estimate an identical immediate deflection. However as shown, P3 was unexpectedly found to have a significantly reduced deflection compared to N 1.

Figure 5 presents results for 7.5 m span samples N6, N7 and P8. Similar as for the 5 m span samples, the P8 sample offers the highest level of flexural rigidity with an instantaneous deflection of 7.7 mm, followed by the samples without (N6, 17.9 mm of instantaneous deflection) and with (N7, 20.5 mm of instantaneous deflection) negative moment reinforcement. Again, the increased level of resistance to load-induced deflections for the P8 sample was not expected as structural analysis of sample N6 and P8 (wherein the in-situ cast upper layer is treated as unreinforced concrete) would estimate an identical immediate deflection.

The amounts of the individual components in the concrete for the in-situ cast upper layer and the precast concrete element may vary widely.

Ranges of the content of hydraulic cement, a product complying with standard specifications including e.g., EN 197, ASTM C 150, ASTM C 595, etc., to yield the embodiments of the presented invention include 200 to 700 kg/m<3>, more preferred 225 to 600 kg/m<3 >and even more preferred 250 to 500 kg/m<3>. Hydraulic cement may consist of Portland cement or blended cements consisting of Portland cement interground or mixed with mineral admixtures. Mineral admixtures, which are known to one skilled in the art, may include groundgranulated blast furnace slag, fly ash, silica fume, limestone powder, burnt shale and may comprise between 5-95% by mass of cement in blended cement.

The ratio of water:hydraulic cement is from 0.30 to 0.75 by weight, preferably from 0.30 to 0.65 by weight.

The ratio of aggregate:concrete is preferably from 0.45 to 0.85 by volume. Aggregate, which is a term familiar to one skilled in the art, comprise fine and coarse aggregates up to a maximum aggregate size of 40 mm, preferably up to 35 mm and more preferably maximum aggregate size of 30 mm.

Chemical admixtures are a series of products known by one skilled in the art complying to standard specifications including e.g., EN 934 series, ASTM C 260, ASTM C 494, ASTM C 1017, ASTM C 1582. Chemical admixtures may be included, as necessary, to achieve other desired performances in fresh concrete, e.g., enhanced workability, accelerated or retarded setting, and hardened concrete, e.g., resistance to damage from freeze-thaw cycling, enhanced strength properties.

As known by one skilled in the art, fibers for fiber reinforced concrete are created from numerous materials by various methods and to various shape, lengths and aspect ratios (i.e., ratio of length: diameter or length: equivalent diameter). Appropriate materials for fibers to realize the beneficial embodiments of the presented invention including various steel types, including stainless, carbon steel and galvanized; mineral and polymer materials, including acrylic, aramid, basalt, carbon, nylon, polyester, polyethylene, and polypropylene; processed and unprocessed natural fibers, including coconut, bamboo, jute, flax and wood fiber. Preferred materials for fibers are steel, mineral fibers, polymer fibers, or combinations thereof. Particularly preferred are cold-drawn carbon steel fiber with hooked-ends or undulations, monofilament synthetic mineral or synthetic polymer fibers, or combinations thereof.

Ranges of the content of fibers to yield the beneficial embodiments of the presented invention vary widely and depend on the material type. Preferred contents of steel fiber reinforcement are 10-200 kilograms of steel fiber per cubic meter of concrete, more preferred is 15-150 kilograms of steel fiber per cubic meter of concrete. The contents of synthetic polymer fiber are 0.2-30 kilogram of synthetic polymer fiber per cubic meter of concrete. Preferred contents of synthetic mineral fiber are 1-100 kilogram of synthetic mineral fiber per cubic meter of concrete.

Additives for chemical prestressing, which is a concept known to one skilled in the art, are added to the concrete mix for the in-situ cast upper layer.

Figure 6 shows experimental length change results from various concretes with different contents of chemical prestressing additives and fibers. Table 1 provides the concrete compositions used for results presented in Figure 6. The results are plotted versus the inverse square root of the specimen age which allows for the estimation of the net length change at an infinite time by means of linear extrapolation as shown by the broken lines in the figure. A beneficial embodiment of the presented invention, based on flexural load-induced deflection experiments described below, is an unexpected level of beneficial negative deflection of the integrated semi-precast elevated concrete composite element prevails when the concrete for the upper layer has an estimated net expansion of at least 75 με at an infinite time. Ranges of chemical prestressing additives will vary widely to achieve the performance described in Figure 6, with preferred contents between 0.5-50% by mass of hydraulic cement and more preferred 1-30% by mass of hydraulic cement.

The observed improved flexural deflection resistance offered by the said integrated semi-precast elevated concrete composite element provides a beneficial structural performance and larger span length than previously feasible can be achieved.

A second aspect of the invention therefore relates to the design process for the integrated semi-precast elevated concrete composite element, wherein the improved flexural resistance is considered. Limits on the flexural deformation of concrete elements is known in the design process for structural reinforced concrete. In certain cases and for particular elements, flexural deformation limits may control geometric parameters; mainly span length and also element thickness, or both; of the element.

In one embodiment of the invention the structural design and architectural considerations of a structure are improved as the allowable span length of the said integrated semi-precast elevated concrete element can be increased while maintaining the deflection control, load capacity, and total thickness compared to use of ordinary concrete for the in-situ cast upper layer.

In an alternative embodiment, the allowable load level of the said integrated semi-precast elevated concrete element is increased while maintaining the deflection control, span length, and total thickness compared to use of ordinary concrete for the in-situ cast upper layer.

In a further embodiment, design options including combinations of the above embodiments; being increased span length and increased capacity are also possible. For instance, a design option of an increased load capacity and increase allowable span length, while maintaining sufficient deflection control, is possible.

The following examples of the implementation of the invention further elaborate on these embodiments. Some of the benefits of the invention are highlighted by the provided examples by means of comparisons to outputs from design approaches in current building design codes using ordinary concrete for the in-situ cast upper layer. Other implementations of the invention are also possible.

Examples of implementation of the invention

Example 1

The precast concrete element comprises a 50 mm thick flat plate of a C25/30 concrete with a minimum age of 28 maturity days and two individual evenly spaced partially embedded steel element in the concrete. The steel element consists of the triangular shaped girder, as in Figure 2, with a upper chord diameter of 10 mm, a diagonal diameter of 6 mm, a lower chord diameter of 6 mm, a girder width of 80 mm and a girder height of 160 mm. Said in-situ cast upper layer of concrete comprises a 220 mm thickness of a steel fiber reinforced concrete with 45 kilograms of steel fibers per cubic meter of concrete, 20 kilograms of a chemical prestressing additive (MgO) per cubic meter of concrete and 300 kilograms of CEM I 42.5N cement per cubic meter of concrete. In this example, the final integrated semi-precast elevated concrete composite elements are used as a multiple continuous span elevated element with a span length of 7.5 meters subject to 5 kilonewtons per square meter uniform loading. The increased effective bending stiffness provided by the invention yields a span-to-deflection ratio under the said load condition of approximately 940.

For the same concrete compressive strength properties for both the precast and in-situ cast concrete, a similar total thickness of 250 mm (compared to 270 mm), and using a concrete mixture without chemical prestressing additive for the in-situ cast upper layer of concrete, the allowable span length is 6 meters. In order to permit a 7.5 meter span with a concrete mixture without chemical prestressing additive for the in-situ cast upper layer of concrete, the in-situ cast concrete shall have a thickness of 270 mm and consequently the total thickness shall be 320 mm.

Therefore, in this embodiment the increased rigidity of the invented system means that a 20 mm increase in the total thickness is sufficient to accommodate a 1.5 meter increase in the allowable span length compared to the design based on current building design codes. The same 1.5 meter increase in the allowable span length using the design approach in current building design codes requires a 70 mm increase in the total thickness.

This example speaks to the attractiveness of the invention for implementation by architects and engineers as the spacing between columns can be significantly increased through the use of the invention, reducing the total number of columns needed. Through the implementation of the invention, engineers and architects will realize reductions in limitations on column-free open spaces and overall construction time and cost.

Example 2

The precast concrete element comprises a 50 mm thick flat plate of a C25/30 concrete with a minimum age of 28 maturity days and two individual evenly spaced partially embedded steel elements in the concrete. The steel elements consist of the triangular shaped girders, as in Figure 2, with a upper chord diameter of 10 mm, a diagonal diameter of 6 mm, a lower chord diameter of 6 mm, a girder width of 80 mm and a girder height of 130 mm. The said in-situ cast upper layer of concrete comprises a 160 mm thickness of a steel fiber reinforced concrete with 10 kilograms of synthetic fibers per cubic meter of concrete, 35 kilograms of a chemical prestressing additive (tricalcium aluminate) per cubic meter of concrete and 325 kilograms of CEM II/A-V 52.5R cement per cubic meter of concrete. In this example, the final integrated semi-precast elevated concrete composite elements have a total thickness of 210 mm and used as a multiple continuous span elevated element with a span length of 6 meters. The increased effective bending stiffness provided by the invention allows a uniform loading of 5 kilonewtons per square meter.

In comparison, design approaches using ordinary concrete for the in-situ cast upper layer allows a uniform loading of 3 kilonewtons per square meter for the same total thickness of 210 mm and span length of 6 meters.

Therefore, in this example the invention allows for an increase of 2 kilonewtons per square meter in the allowable load level while maintaining the span length, deflection control, and total element thickness compared to the design approach using ordinary concrete for the in-situ cast upper layer.

A third aspect of the invention is the method to produce the concrete for the in-situ cast upper layer with fiber reinforcement and chemical prestressing additives.

In a preferred embodiment, said method comprises the steps of: (i) providing an initial concrete mixture including hydraulic cement, aggregate, water, and optional chemical admixtures by a ready-mix concrete facility and transporting said mixture to the jobsite (e.g. in a rotating drum ready-mix truck); (ii) incorporating fiber reinforcement and chemical prestressing additives into the initial concrete mixture on site using purpose-built equipment and (iii) the concrete placing, consolidating (compacting), leveling, finishing and curing.

In an alternative embodiment, said method comprises the steps of: (i) providing a concrete mixture including hydraulic cement, aggregate, water, optional chemical admixtures, and fiber reinforcement by a ready-mix concrete facility and transporting said mixture to the jobsite, (ii) incorporating chemical prestressing additives into said concrete mixture on site, and (iii) the concrete placing, consolidating (compacting), leveling, finishing and curing.

In a further embodiment, said method comprises the steps of: (i) providing a concrete mixture including hydraulic cement, aggregate, water, optional chemical admixtures, fiber reinforcement, and chemical prestressing additive by a ready-mix concrete facility and transporting said concrete mixture to the jobsite, and (ii) the concrete placing, consolidating (compacting), leveling, finishing and curing.

References Cited

1. Wolfgang Volkel, Anja Riese, and Siegfried Droese, "Neuartige Wohnhausdecken aus Stahlfaserbeton ohne obere Bewehrung," Betonund Stahlbetonbau 93 (1998), Heft 1.

2. Anja Riese, and Siegfried Droese, "Wohnhausdecken ohne obere Bewehrung - Belastungsversuche an neuartigen Deckenplatten," Betonund Stahlbetonbau 91 (1996), Heft 12.

3. US 5,448,866. Trusses And Precast Concrete Slabs Reinforced Thereby.

Claims (10)

Claims

1. An integrated semi-precast elevated concrete element system with enhanced flexural rigidity, comprising the following components:

(i) a precast concrete panel with partially embedded reinforcing elements for shear transfer and reinforcement for additional load capacity, and

(ii) an in-situ cast upper layer of a concrete having an estimated net dilation of at least 75 με at an infinite time;

wherein the in-situ cast upper layer of concrete comprises:

- hydraulic cement being a Portland cement or blended cement: 200 to 700 kg/m<3 >of concrete, more preferred 225 to 600 kg/m<3 >of concrete and even more preferred 250 to 500 kg/m<3 >of concrete;

- water, at a ratio of water:hydraulic cement of 0.30 to 0.75 by weight, preferably from 0.30 to 0.65 by weight;

- aggregate, with a maximum size of 40 mm, preferably with maximum size of 30 mm and aggregate:concrete ratio from 0.45 to 0.85 by volume;

- fiber reinforcement, 10 to 200 kilograms of steel fiber per cubic meter of concrete, more preferred 15 to 150 kilograms of steel fiber per cubic meter of concrete; and/or 0.2 to 30 kilograms of synthetic polymer fiber per cubic meter of concrete; and/or 1 to 100 kilogram of synthetic mineral fiber per cubic meter of concrete;

- chemical prestressing additive: 1 to 30% by mass of hydraulic cement and sufficient to achieve the positive net length change target above;

wherein the chemical prestressing additive is a mineral admixture adapted to create hydration products, which are volumetrically larger than the volume of the reactants when added to plastic concrete; where the chemical prestressing additive is selected from the group consisting of magnesium oxide, calcium aluminate and tricalcium aluminate; where the length change at an infinite time and drying conditions of 50% relative humidity and 20° C is estimated by plotting the length change data collected in accordance with ASTM C 157 as a function of the inverse square root of time and extrapolating the infinite shrinkage after 28-days of drying.

2. The element as claimed in claim 1, wherein the concrete in the precast concrete panel comprises blended cement with at least one mineral admixture selected from the group consisting of ground-granulated blast furnace slag, fly ash, silica fume, limestone powder, burnt shale, calcined clay or rice husk ash.

3. The element as claimed in claim 1, wherein the precast concrete panel is prestressed.

4. The element as claimed in claim 1, wherein the precast concrete panel comprises hollow core, or flat panel, or double-T, or U-shaped, or 1-shaped precast concrete profile.

5. The element as claimed in claim 1, wherein the partially embedded reinforcing elements of the precast concrete panel are geometrically asymmetric with an increased area of reinforcement in the embedded part of the reinforcing element.

6. The element as claimed in claim 5, wherein the partially embedded reinforcing elements of the precast concrete panel are geometrically asymmetric lattice girders or reinforcing profiles made of metal or fibre-reinforced plastic.

7. The element as claimed in claim 1, wherein the steel fiber is 25-90 mm in length and 0.5-1.5 mm in diameter.

8. The element as claimed in claim 1, wherein the synthetic polymer fiber for the in-situ cast layer is 5-30 mm in length and 0.01-1.0 mm in diameter.

9. The element as claimed in claim 1, wherein the synthetic mineral fibers for the in-situ cast upper layer are 3-90 mm in length and 0.01-1.5 mm in diameter.

10. The element as claimed in claim 1, wherein the in-situ cast upper layer further contains metallic or non-metallic bars or other reinforcing elements.