Nowadays, high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) are ranked among advanced concrete technologies. The application of the mentioned advanced technologies may have potential to improve the construction efficiency from several points of view. For instance, reducing of construction time and construction material, construction quality improving, environmental impact minimizing, and increasing of both durability and lifetime of structures as well as reducing of total construction costs may be obtained. Particular advanced concrete technologies are described and the possibilities of their utilization in both monolithic structures and precast units are presented in the article. The main benefits of modern methods of construction (MMC) based on advanced concrete technologies application in precast elements production are presented. Regarding the selected aspects of construction efficiency assessment, a comparison of conventional and advanced concrete technologies that are applied in monolithic structures and precast units is made. The results of this comparison, estimated in semantic differential scale, are presented in the article. By the results of the comparison, the significance of applying the advanced concrete technologies in modern methods of concrete structures production is demonstrated in order to improve construction efficiency.
Concrete structures as well as concrete structure units have been already made by precast method for several decades. Utilization of precast concrete structures has enabled meeting a demand of social requirement for high intensity of building-up. Construction based on concrete precast (PC) unit presents one of forms of modern methods of construction (MMC) [
MMC has been described as including all “new products and technologies” that result in “delivery methods which set out to improve product and process” [
The aim of development of new concrete technologies was the requirement for high durability of concrete structures. One of main ways to meet that is to achieve sufficiently compacted concrete by qualified workers. A lot of failures and faulty work occurred because of bad availability of qualified workers as well as under the pressure of construction acceleration. Therefore, it was necessary to reduce the human factor by fresh concrete processing, significantly. Technology of Self-Compacting Concrete (SCC) is known since eighties of last century from Japan [
Last advances in material engineering permit producing the concrete whose performance is higher than of the HPC. Those are so-called ultra-high-performance concrete (UHPC). Higher strength, durability, and esthetic value of monolithic and/or precast concrete structures can be achieved by using materials based on advanced concrete technologies (HPC and UHPC). Specific properties of concrete based on mentioned technologies enable designing lighter and thinner elements utilizing material flexibility [
High added value of HPC lies in its technological and technical parameters. The technical parameters are typical for a finished structure and are in this case represented by mechanical properties, for example, compressive strength (strength class C 55/67 up to C 100/115 according to EN 206), flexure strength, modulus of elasticity, resistance to environment influences (almost zero permeability of the composite), and high durability [
HPC characterizing parameters may be achieved only by suitable composition of the concrete. The main principle consists in performance of low water/binder ratio (up to 0.4) together with the lower viscosity of fresh concrete. High fluidity may be achieved by only right ratio of gravel aggregate fraction (grain-size curve), usage of fine-grained additions, and especially usage of effective plasticizers and water-reducing admixtures. Moreover, it is necessary to use a suitable reinforcement to ensure the static function of a structure. Within the HPC technology it is actual to use a reinforcement based on high-strength artificial fibers (known as dispersed reinforcement) in addition to traditional reinforcement bars (steel bars, modern synthetic bars). Nowadays, steel, glass, and synthetic fibers are mostly applied as dispersed reinforcement. Exclusive application of it increases efficiency of concrete structures production even more and thus improves construction process as a whole.
The UHPC technology is characterized by enormous strengths of hardened concrete and zero permeability. A typical UHPC material has a design compressive strength 200 MPa and a flexural strength 10–15 MPa, which is achieved by optimizing both the bulk density of the matrix and the length-diameter spectrum of the fiber reinforcement [
Despite bearing function, concrete structures participate in architectural aspect of a building sometimes. In case of visible surfaces of a monolithic concrete structure or precast concrete element, there can be required specific, predefined feature of the surface [
The progressive concrete technologies present a great interest for precast concrete industry as a substantial material base of modern methods of construction. They allow new designs for the precast products which cannot be done with common concrete. By using these new materials, it is possible to produce lighter products with thinner section. The advantages of modern method of construction from precast concrete elements can be emphasized by application of advanced concrete technologies in production of precast concrete elements. The technical parameters of concrete based on HPC and UHPC are significantly better than parameters of common concrete.
By thoroughly proposed composition and application of special admixtures and additions, the modern concrete usually achieves excellent mechanical characteristics. The initial compression strength can reach the value of 20 MPa after several hours of setting. The formwork of such precast concrete elements may be soon dismantled. Thus, the cycle time of such element production can be significantly reduced. Moreover, early age compression strength of concrete can reach 50 MPa after two or three days. This offers possibility to transport and assemble the element really soon after production. It is profitable mainly when the precast elements are produced continuously during assembly of the building, or in case of some special element need during construction. High final compressive strength, often more than 200 MPa, provides possibility to build modern structures from more subtle and light precast elements and so not only minimize the base material consumption but also modify the technological characteristics of the precast elements (as lower weight of bigger measured element, etc.). These are the reasons why it is possible to make construction process faster, to reduce laboriousness, and to reduce demands on machines and equipment (e.g., crane with less capacity or higher radius).
Within the environment of precast elements production, it is possible to apply not only special methods of fresh concrete production, transport, placing, compacting, and surfaces finishing, but also special methods of concrete curing (assurance of optimal conditions in time of concrete feeding and hardening, as temperature, humidity, cleanness of production environment, etc.). This improves markedly the properties of completed products, especially mechanical parameters as well as durability. Required method of curing is usually impossible in building site conditions, or it is demanding in terms of technical equipment, time, and cost. However, the precast elements involve some specifics, too. The specifics imply possible problems in assembly. The assembly method, or the method of connecting the elements into existing structures as well as connecting with each other, is the most distinguished factor influencing the final quality of a building [
The durability of a commercially available UHPC was independently evaluated in [
The UHPC is characterized by high compressive strength and excellent durability properties resulting in lighter structures and longer life. Different approaches have been adopted by researchers in achieving ultra-high strength and associated other improved performances. As a result, several types of UHPC are available today [
A suitable comparative base is an unavoidable assumption for assessment of efficiency of the advanced concrete technologies application in construction. In this case, the comparative base is represented by conventional concrete technology of monolithic concrete structures execution.
Excellent mechanical properties of advanced concrete technologies contributing to utility characteristics of building may increase the construction process efficiency. On the other hand, higher cost connected with new technologies in construction reduces the value of construction efficiency. Estimation of expected cost of construction can be based on economic data of completed construction projects or on economic indicators of a production unit. When estimating the construction efficiency, a problem may arise in connection with quantification of utility technical, technological, economic, and environmental characteristics of final building structures in relation to applied technology.
The aim of the study is to find out if the professionals in Slovakia apperceive the widely declared strengths of advanced concrete technologies in comparison with conventional technology of monolithic concrete structures. Professional’s perception of advanced concrete technologies as well as their utilization potential can form the basis of strategy to achieve wider application of mentioned types of concrete. In order to unveil the perception of professionals, a questionnaire survey was conducted. Rating group consist of professionals from different fields of building industry, including architects (designers), ready-mix concrete producers, precast concrete units producers, contractors, and technologists. Overall, 113 questionnaires were sent and 50 of them returned. Thus, returned/sent ratio is 0.44. Widely presented benefits of modern methods of construction (MMC) are construction time reducing, workforce reducing, on-site laboriousness reducing, favorable ratio of cost and returns, and smaller environmental impact as well as better quality control and higher production quality [
For individual aspects of construction efficiency assessment, the comparison of selected technical and technological characteristics of the advanced concrete technologies (HPC and UHPC) with conventional concrete technology (common concrete (CC)) and mutual comparison of two alternatives of concrete structures execution (monolithic structures from ready-mix concrete and assembled precast units) are performed. The results of the expert group assessment are expressed by numbers on the basis of semantic differential. It is the method to measure the intensity of psychological and sociological attitudes of a person to some existing situation. The method is based on specific defined point scale which presents the intensity of a person attitude into existing situation. The terminals of the scale represent the opposing terms (e.g., short and long). In Table
View of valuation subject on characteristic of HPC and UHPC in comparison with CC.
Characteristics of HPC and UHPC in comparison with CC | Extremely reduced | Significantly reduced | Moderately reduced | Same as CC | Moderately improved | Significantly improved | Extremely improved |
---|---|---|---|---|---|---|---|
Basic range of score assessment | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
The more comprehensive assessment of the advanced concrete technologies influence on construction efficiency in real construction conditions requires not only detailed analysis of data from already constructed buildings but also the selection of relevant utility and efficiency indicators.
Characteristics of concrete technologies are evaluated within traditional method of construction (RMX (ready-mix concrete)) as well as modern method of construction (PC (precast) concrete elements) in the paper. Assessment is presented by view of professional community over the declared benefits of advanced concrete technologies which have positive influence on construction efficiency, incorporating the following: construction time reduction, reduction of number of workers as well as on-site equipment, cutting down the labor content of on-site processes, better efficiency of control during the process of precast elements production, increasing of quality, durability, and endurance of structures.
Within the evaluation of characteristics, view of professional community on environmental impact of concrete production and processing and also selected economic indicator of efficiency (costs to returns ratio) were determined, in respect to advanced concrete technologies as well as modern methods of construction. Average score evaluation of common concrete, high-performance concrete, and ultra-high-performance concrete technologies, respectively, are listed in Table
Average score evaluation of characteristics of CC, HPC, and UHPC, respectively, compared with reference basis.
Evaluated characteristics of concrete technologies (CC, HPC, and UHPC) within particular methods of construction | Traditional methods of construction (using ready-mix concrete) | Modern methods of construction (using precast concrete elements) | ||||
---|---|---|---|---|---|---|
RMX- | RMX-HPC | RMX-UHPC | PC-CC | PC-HPC | PC-UHPC | |
Construction time | | 4.86 | 4.92 | 5.52 | 5.56 | 6.24 |
Number of workers at process | | 5.10 | 5.38 | 6.28 | 6.30 | 6.34 |
Labor content of site processes | | 4.86 | 4.98 | 5.34 | 5.86 | 6.02 |
Environmental impact | | 4.78 | 5.00 | 4.88 | 5.10 | 5.36 |
Control efficiency within the process of building structure production | | 4.80 | 5.96 | 5.34 | 6.18 | 6.44 |
Quality, durability, and endurance of structure | | 5.36 | 6.22 | 4.78 | 5.90 | 6.22 |
Costs/returns ratio | | 3.06 | 1.70 | 5.02 | 4.18 | 1.54 |
In Figure
Minimum and maximum values of score evaluation as well as its standard deviation.
The results of assessment show that the construction of concrete structure using the precast concrete elements are more profitable as traditional method of construction using ready-mix concrete, regardless of concrete technology. Precast method of construction brings benefits within construction time, both work power and equipment on-site requirement, total labor content of processes, environmental impact, and efficiency of control of structures, respectively. Similarly, evaluated characteristics of ready-mix concrete method using HPC and UHPC are better than reference RMX-CC, excepting cost/return ratio. These results correspond with declared benefits of MMC and advanced concrete technologies very well.
Assessment of benefits of ready-mix concrete based on both HPC and UHPC processed on building site is not clear. Excellent workability of fresh concrete enables decreasing the number of workers needed for concrete processing. On the other hand, worker’s qualification has to be much higher, and frequency and range of testing on building site increase. Thus, total labor content of process raises. Applying of MMC in conjunction with HPC and UHPC decreases the environmental impact, too.
Difference of view on indicator of economic efficiency of advanced concrete technologies (e.g., costs/returns ratio) results from contrary inputs and information. For example, material costs of UHPC can be several times higher than costs of common concrete. Hereby, volume need for material for more subtle elements made from UHPC is much lower. Moreover, quality and durability of UHPC based structures are much higher. It is necessary to compare cost to particular technological processes and primarily determine real relation between costs and utility value of structure, because higher economic evaluation of enhanced utility value of structure affects value of returns, directly. Total costs consisting of material cost, labor cost, equipment cost, maintenance facility cost, and other costs (faster construction, bigger useful area of building) should be considered.
Improvement of efficiency and assurance of sustainable development in construction depends on wider application of progressive technologies, materials, and modern methods of construction in construction practice.
The common concrete has a long history in precast elements production. The advantages of precast construction are multiplied by combination of modern methods of concrete structures construction with advanced concrete technologies (HPC, UHPC). Therefore, the use of advanced concrete technologies in precast construction has the significant potential to improve construction efficiency. On the other hand, HPC and UHPC technologies are utilizable also in monolithic concrete construction thanks to their technical and technological properties. Meaningful reduction in weight and volume of the structures leads to elimination of environmental impact resulting from lower material consumption.
The result of the study based on professionals’ opinions exposes that the most significant contribution of modern concrete technologies based on HPC and UHPC consists in improvement of final products quality (mechanical properties, durability). Reduction of construction time, labor content, and number of both on-site workers and equipment as well as reduction of environmental impact and improvement of quality present some of benefits which result from synergistic effect of utilization of both precast technology and high sophisticated materials, as high-performance concrete and ultra-high-performance concrete are, even though much higher costs of structures based on advanced concrete technologies related with need for more specific materials cause these technologies to be utilized for only special architectonic buildings with specific shape and/or mechanical requirements.
For a more comprehensive evaluation of impact of the advanced concrete technologies based application on the construction process efficiency, it is necessary to prepare and analyze a wider set of entry data from different construction projects, realized within various areas also beside Slovakia.
The authors declare that they have no competing interests.
The article presents a partial research result of project VEGA 1/0677/14 “Research of Construction Efficiency Improvement through MMC Technologies.”