THE PROPERTIES OF THE CEMENT SCREEDS WITH THE ADDITION OF POLYPROPYLENE FIBRES AND THE SHRINKAGE-REDUCING ADMIXTURE

Iscrpno i realno definisanje zahteva koji se postavljaju za građevinske materijale, u zavisnosti od funkcije koju u složenom sistemu objekta konkretan materijal ima, kao i tehnički podaci o svim relevantnim karakteristikama, osnov su za pravilan izbor, primenu i ugradnju pogodnih građevinskih materijala. Danas se savremene konstrukcije grade u kombinacijama raznovrsnih, fizički posebnih ali funkcionalno međusobno povezanih materijala, što nameće potrebu da se materijali posmatraju u sprezi s konstrukcijom. Paralelno s tzv. tradicionalnim materijalima, u primeni je čitav niz novih, industrijski proizvedenih materijala, prilagođenih zahtevima i funkcijama savremenog graditeljstva. Iako se danas cementna košuljica-estrih u velikom obimu izvodi primenom „gotovih”, fabrički proizvedenih materijala, što predstavlja veliki korak napred, uz koje se najčešće ne dobijaju tehnički listovi s podacima dovoljnim za poznavanje, pravilan izbor i primenu, u okviru klasičnog postupka njihovog izvođenja najčešće se ne propisuju neophodne fizičko-mehaničke, reološke i tehnološke karakteristike datih malterskih kompozicija. Brojna ispitivanja pokazuju da se svojstva cementnih košuljica, koja po definiciji predstavljaju sloj dobro zbijene


SVOJSTVA CEMENTNIH KOŠULJICA S DODATKOM POLIPROPILENSKIH VLAKANA I KOMPENZATORA SKUPLJANJA THE PROPERTIES OF THE CEMENT SCREEDS WITH THE ADDITION OF POLYPROPYLENE FIBRES AND THE SHRINKAGE-REDUCING ADMIXTURE
Rada M. RADULOVIĆ Dragica Lj.JEVTIĆ Vlastimir RADONJANIN

INTRODUCTION
Exhaustive and realistic definition of the demands that are placed in front of building materials, depending on the function that in the complex system of the facility of specific material, as well as technical data on all relevant characteristics, are the basis for proper selection, implementation and building of suitable building materials.Today, modern structures are built in combinations of various, physically special but functionally interrelated materials, which impose the needs that materials should be considered in conjunction with the construction.In parallel, with the so-called "traditional" materials, a series of new, industrially produced materials are implemented and tailored according to the requirements and functions of modern architecture.
Today, although cement screed to a large extent is carried out using a "finished", factory produced materials, which represents a major step forward, which usually lack technical sheets with data sufficient for knowledge, proper selection and application; the framework of classical procedure of their execution is not normally provided by the necessary physical and mechanical, rheological and technological characteristics of the given mortar composition.mešavine, najčešće cementa i sitnog agregata, mogu poboljšati korišćenjem novih materijala boljih svojstava u poređenju sa tzv.tradicionalnim komponentama ove malterske kompozicije.
Since cement screed by definition represent a wellcompacted layer of the mixture, numerous tests have shown that its properties can be improved by using new materials with improved performance compared to the so-called "traditional" components of this mortar composition.
Thus, for example, the thermal properties of the screed of recycled glass and its importance for the energy efficiency of the building have been studied by A. Alan and others [1]. A. Morier and others investigated the enhanced thermal and acoustic performance of cement screed, as well as the possibility of reducing their weight by adding cork granules obtained as waste during production [12].Using waste materials as constituents of the screed have been studied both by Boehme and others who were dealt in impact assessment of recycled concrete as aggregate, made with fine fraction in cement screeds [6].
Since industrial and commercial facilities, high-bay warehouse, station and hospital lobbies and other similar facilities are exposed to extremely heavy traffic, cement screed is very often and very successfully embedded in these floor structures which is often exposed to significant loads, including concentrated loads, which acting through a relatively small area produce high local stresses of pressure responsible for damage and defects in the screed [20].
In construction practice, there is often a need for rehabilitation and repair of floor structures, which over the time, are subjected to various loads and other influences that result in various forms of deformation.The damage is usually manifested during the exploitation, in some cases immediately after manufacture, in which most often occur on the final floor covering made through the screed.Poorly designed cement screed has resulted in huge costs of reconstruction of this floor, as well as the substantial losses caused by the complete cessation of activity in the house which is the subject of renovation.
Despite modern devices for floor coverings manufacture, we are faced with numerous shortcomings regarding manufacturing quality of cement screed.Many errors are reflected already in the choice of materials, production mixture, technical installations, and their dimensioning and embedding preparations.
Since the screed repairs must be carried out in the shortest possible period of time, it is necessary to determine the true reason why there has been a defect and propose an adequate solution for the repair, in order to avoid the same problem.Damage to buildings jeopardizing its functionality, which manifest themselves during the exploitation, led to a change in the way of designing and maintaining a relationship with the building [18].
Throughout experimental research and theoretical analysis this paper has an objective to compare the behaviour of cement screeds made in traditional way without additives, and cement screeds made with microfibre reinforcement and additives to reduce shrinkage, embedded in different ways, in order to define optimal combination for the formation of high-quality cement screed.

TYPES OF CEMENT SCREED
Cement screed is one of the most frequently used type of floor surface due to its good properties such as, it evenly rest against the construction, there is no extreme deformation, it is flat enough and it can carry a large number of different types of floor coverings.Depending on the mode of performance, it has a supporting function, and can also represent an element for protection from moisture, heat and sound insulation.
Cement screed in the standard SRPS EN 13318: 2011 [26] is defined as a horizontally constructed layer of cement mortar, which is placed on a support, usually concrete surface or by partial or insulating layers, whose purpose is to fulfil one or more of the following functions: • to achieve a certain defined level, • to be used as the basis for floor coverings, • to be used as a bearing surface.Depending on the way the cement screeds mounted on the carrier substrate can be divided into three basic groups: • monolithic, • bounded, • unbounded.Monolithic screeds are made while the concrete base is still in a plastic state, thus at the same time when performing and concrete basis.In monolithic screed thickness greater than 40mm there is a risk of loss of adhesion between the covering and the base, due to differential shrinkage.In order to obtain good adhesion between the covering and the concrete base structure, it is important to know the presence of the phenomenon of appearance of water on the upper surface of the fresh concrete and to solve this problem by one of the following way: • immediately after compacting concrete, prior the water appears on the surface, • after solving the problem with surface water, when it is removed or evaporated.
Immediate setting is suitable because there is no need for preparing base construction, as opposed to the other way when you need extensive cleaning, or adequate surface preparation.
Bounded (coupled) screeds are applied to the layer of hardened concrete or on a concrete foundation.The required thickness of this kind of screed depends on the structural requirements, but should not be less than 25mm or more than 50mm.In cases where the thickness of the bounded screed, at the request of the designer should be larger than 60mm, the use of reinforcement that should prevent cracking is justified.Production of the bounded liner requires perfect preparation of the substrate, which must be firm, free of grease and dust and coated with appropriate contact coating.This screed is used in cases where there is no requirement for sound or thermal insulation; it is necessary to provide a specific height datum or make a flat surface.
Unbounded (floating) cement screeds are placed on the top of insulating layer that separates screed from concrete base and should have a minimum thickness of 50 mm.In the case when it is placed on the compressible layer, such as, for example, an insulating plate, the thickness of screed should be at least 70 mm.
Dozvoljena odstupanja vezana za pravilnost površine estriha, vodeći računa o vrsti poda koja će se primeniti, zavise od mnogih faktora.Za veće površine podova, koji se koriste u uobičajene svrhe, dozvoljeno odstupanje country due to high prices in comparison with the developed countries of Europe.The liquid screed can be as fully finished product purchased in concrete plants and transported to the construction site with mixers or packed in sacks and mixed with water on the site and installed.
Levelling masses are used when the surface should be levelled only, when the layer should be less thick and cannot be performed screed or when the screed surface is badly treated.

THE PROPERTIES OF CEMENT SCREED
The basic properties of the cement screed vary depending on the type of used aggregate, the extent of mixing of the components, the amount of used water, degree of porosity, amount and type of additives but primarily on realized compactness of hardened material [19].
The screed density ranges from 1850 kg/m 3 to 2100 kg/m 3 in a common configuration when the grain size distribution curve of aggregates is in the area recommended by Regulation for concrete and reinforced concrete [17].For screeds that have larger class strength, the density is higher at about 2300 kg/m 3 .Porosity of conventionally rendered cement screed in most cases changes by their thickness, the lowest being on the top surface, and the largest in lower areas.This is due to the screed technology construction, when applying the procedure of smoothing the upper surface by engine smoother (choppers) or manually.In the (earth-damp) consistency mixture, removal of residual air present in fresh mortar composition is very difficult, which results in a porosity of more than 3%.
The coefficient of specific heat c, which actually means the speed of heating or cooling of materials, for cement screed amounts to c=1050 J/kgK o .
The thermal conductivity coefficient λ, which represents the ability of materials to transfer heat through their mass, as a result of the temperature difference between two surfaces for cement screed is about λ=1,4W/mK o [10].The thermal coefficiet of linear expansion α t for cement screed is α t = 10-12x10 -6 (1/ o C) and represents a dilatation in temperature change about 1 o C. In the case of cement screed thermal stability is higher when the screed is more hemogenous and more compact, respectively when the thermal coefficient α t is lower.
Diffusion resistance of water vapour µ is the resistance that construction element provides to water vapour and whose value is the density of the material in comparison with the air for which the coefficient is µ = 1st Resistance factor to diffusion of water vapour in almost all cement composition should be µ =30 [14].
In the case of thin concrete elements, in which a large surface is exposed to evaporation, as it is the case with cement screed, it is very difficult to avoid the appearance of cracking.The sizes of shrinkage, expressed in the terms of changes in unit length of the observed materials for cement screed amounts to 0.5-1.0mm/m 1 .The permitted tolerances related to regularity in the surface of the screed depend on many factors taking into account the type of floor to be applied.For larger floor surfaces that are used in common purpose, the je 15mm.Veća tačnost može biti zahtevana u manjim prostorijama po linijama pregradnih zidova, u blizini otvora za vrata i gde treba da se instalira specijalizovana oprema direktno na podu, kao i u slučajevima visokih zahteva kod industrijskih podova.
permitted tolerance is ± 15mm.Greater accuracy may be required in smaller rooms along the lines of partition walls, near the door opening and where the specialized equipment is installed directly on the floor, as well as in the cases of high demand for industrial floors.

THE EXPERIMENTAL RESEARCH
In order to determine the influence of microfibre reinforcement and shrinkage-reducing admixture to the physical-mechanical and deformation characteristics of cement screed, Laboratory of Materials of Civil Engineering, University of Belgrade, has realized a program of experimental research of mortar compositions based on cement as a binder, river aggregate (0/4mm), water, polypropylene fibres and additives against shrinkage.
When planning the experimental part of the research we started with the assumption that the optimum composition of cement screed can be defined based on the analysis of component materials, selecting the proper embedding procedure, as well as the fact that optimal amount of microfibre reinforcement and shrinkage-reducing admixture have a favourable influence on the properties of cement screed.

The composition of the tested mortar mixtures
For the purposes of the experiment, and according to the research program, the preparation and testing of the properties of three different types of mortar mixture for cement screed installed in two ways, manually and by vibrations, has been envisaged.The series are identified as follows: • Series I -reference mixture, common cement mixture for cement screed (cement+aggregate+water), • Series II -cement mixture with the addition of microfibre reinforcement (cement+aggregate+water+ microfibre reinforcement), • Series III -cement mixture with the addition of microfibre reinforcement and shrinkage-reducing admixture (cement+aggregate+water+microfibre reinfor-cement+shrinkage-reducing admixture).
In the design phase of ingredients of mortar composition, water-cement ratio 0.38 has been taken as the starting point which is proved by experience, because of the conditions that the consistency of cement screed should fit the description "wet as a ground."The quantities of component materials are calculated approximately based on an assumed density and watercement ratio, as well as the adopted constant proportions of mixing cement and aggregate 1: 3 (Table 1) using the so-called mass equation [14].

Composite materials
As the primary objective of the research was to determine the effect of additives on the properties of cement screed, all series of mortar mixtures are made with the same type of cement, type and granulometric composition of aggregate and drinking water.
is a natural sand "Moravac" of fraction 0/4mm, which was previously tested in the laboratory of the specific mass γ sa =2600kg / m 3 .Clean water should be used exclusively while preparing mortar composites for testing cement screed.Thus, water from the municipal water supply was used for the preparation of the samples; therefore, there was no need to conduct a test of its physical and chemical composition.

Fibres
It is known that mortars, as composite materials, very poorly accept and bear the tensile stress, as opposed to compressive stress.Therefore, the use of fibre has become an indispensable element in the preparation of the screed.Its usage largely enables removal of common disadvantages of conventional plaster and concrete -low tensile strength and low toughness.Furthermore, its usage results in some additional effects such as: reducing the risk of shrinkage cracks of cement, providing thixotropic mixture of fresh state [25], and may also contribute to increase of abrasion resistance of composites, better adhesion to the substrate to contact the "old" and "new "concrete, increased resistance to dynamic impacts, increasing resistance to fire [8].
The results of numerous tests carried out on plasters and concrete have demonstrated efficacy of polypropylene fibres in controlling plastic shrinkage of concrete [2] and that their presence is necessary, especially in thin concrete elements [24] such as cement screeds.
The common phenomena of separation of excess water on the surface after the preparation and implementation of plaster ("bleeding"), using polypropylene fibres can be greatly alleviated, because fibres with their exceptional number and homogeneous pattern intersect the system of capillary pores and thus slow down the diffusion of water [15].
The main parameters that influence the quality of the mortar, related to fibres are the quantities and the socalled aspect ratio (l/d), which provides the ratio between the fibre length (l) and their diameter (d).What polypropylene fibres specially recommended as fibre reinforcement for the mortar mixture for cement screeds is very convenient aspect ratio l/d, because the effect of reducing the total shrinkage deformation is higher with fibres of higher length, i.e. higher values of the aspect ratio (l/d) [8].
Polypropylene is a good thermal insulation and electrical insulation material; it has a high chemical resistance, it is non-porous, and in principle, has a hydrophobic surface.It is chemically inert, has a high resistance to the conditions of aggressive action of acid, and the salts as well.Moreover, alkaline environment, which is characteristic of mortars, has insignificant effect on the change of quality or durability of polypropylene fibres.The drawback to these fibres is volatility at elevated temperatures, low modulus of elasticity and creep deformation.
When using the polypropylene fibres, their length should be 3-3.5 times higher than the coarsest aggregate in the mix (4mm), due to which were used fibres including the length of 12 mm.Therefore, the monofilament, wavy polypropylene SIKA FIBER fibres, of the length of 12mm, of the usual dosage of 900 to 1000 g/m 3 mortar were used (Table 2).

Shrinkage-reducing admixture
Given that the shrinkage is an inevitable feature of the mortar hardening, shrinkage-reducing admixture (shrinkage compensators) started its application in the eighties of the last century in order to avoid cracking and deflection due to drying shrinkage.
In the case of thin concrete elements, in which a large area is exposed to evaporation, as is the case with cement screeds, it is very difficult to avoid the occurrence of cracks in the fresh mortar.If the loss of surface moisture exceeds 0.5 kg/m 2 /h negative capillary pressures are developed within the concrete, causing internal pressures [11].The shrinkage-reducing admixture of the new generation do not act in swelling concrete in the early stages of hardening, but in preventing the evaporation of water that causes it, furthermore, in preventing shrinkage of wetted surface capillary in water evaporation.They are particularly effective in combination with superplasticizers of new generation, usually when the shrinkage of concrete can Napravljeni su na bazi propilen-glikola i deluju tako da kad vodom napunjene pore, veličine od 2,5 do 50 nm, počnu da gube vlagu, oblikuju zakrivljene meniskuse, a površinski napon vode povlači zidove pora ka unutra (slika 3).Prema [22] sa smanjenjem površinskog napona vode smanjuje se i sila koja deluje tako što istiskuje vodu iz kapilarnih pora, što rezultira smanjenjem skupljanja.Dodatak aditiva protiv skupljanja, osim što utiče na skupljanje od sušenja, povećava obradljivost betona [16]; kompatibilan je s drugim dodacima i poboljšava vodonepropusnost betona.
Kako se ne treba oslanjati samo na pozitivna dejstva be reduced by 30% to 70% [3].They are designed on the basis of the propylene glycol and operate so that when the water-filled pores, a size of 2.5 to 50 nm start losing moisture and form a curved meniscus, the surface tension of walls pulls the walls of the pore towards inside (Figure 3).According to [22], with a reduction in the surface tension of water, the force acting in the way that displaces the water out of the capillary pores is reduced as well, which results in reduced shrinkage.The addition of additive against shrinkage affects of drying shrinkage, and enhances the workability of concrete [16] aditiva protiv skupljanja, već treba sagledati uticaj i na ostala svojstva [4] pokazano je u istraživanjima drugih autora [21] iz kojih se vidi da ova vrsta dodataka može delovati na produžavanje vremena vezivanja i smanjenje čvrstoće pri pritisku.Preporučena potrošnja od 2% na masu cementa (6-10 kg/m³ betona) relativno je velika doza, zbog čega je uzeta u obzir prilikom pripreme recepture.Dodavanje aditiva doprinosi povećanju poroznosti, pa je količina vode smanjena za količinu upotrebljenog dodatka.
additives and improves waterproof quality of concrete.
One should not rely only on the positive effects of additives against shrinkage, but should consider the impact of other properties [4]; it has been shown in the studies by other authors [21] who show that this type of additives can act on extending the setting time and compressive strength reduction.
The recommended consumption of 2% of cement mass (6-10 kg / m³ of concrete) is relatively a large dose, which is taken into account during the preparation of the recipe.Adding additives contributes to an increase of porosity, and the amount of water is reduced for the amount of used additive.

TEST RESULTS
Testing the physical-mechanical and deformation characteristics of the samples of mortar mixtures for cement screed was conducted over the extracted cylinders-cores diameter of Ø50mm, which in the days of tests were 2, 7 and 28 days old and over prismatic samples of dimensions 4x4x16cm of different age.From each of the boxes-molds, in which the samples of cement screeds are made, were pulled out (Figure 4), in accordance with the standard of SRPS U.M1.049: 2000 per 9 cylinders (Figure 5), which were then tested.The samples made in this way have enabled us to notice the importance of the method of compacting when comparing the framework of the same species.U laboratorijskim uslovima, zapreminska masa cementnog maltera u očvrslom stanju određuje se, saglasno proceduri opisanoj u standardu SRPS EN 12390-7:2010, merenjem masa osušenih uzoraka koji u konretnom slučaju imaju ili oblik cilindra Ø50 mm ili prizme 4x4x16 cm, uz pomoć poznatog obrasca:

Density
In laboratory conditions, the density of cement mortar in the hardened state is determined in accordance with the procedure described in the standard SRPS EN 12390-7: 2010, by measuring the mass of dried samples, which in the case of concrete, have either form of a cylinder Ø50mm or prism 4x4x16 cm, with the help of the form: Na osnovu izmerenih čvrstoća pri pritisku i poznatih zapreminskih masa uzoraka, računskim putem dobijene su vrednosti koeficijenata konstrukcijske povoljnosti K kp koji predstavlja odnos čvrstoće pri pritisku materijala (u MPa) prema njegovoj zapreminskoj masi γ (u t/m 3 ) (tabela 3) On the basis of the measured compressive strength and well-known density of samples, the values of coefficients of structural favorability K kp were obtained by computation, which represents the ratio of material compressive strength (in MPa) according to its density γ (t/m 3 ) (Table 3).
From the attached test results it can be seen that the density of the placed-hand samples (1), regardless of their composition, are significantly lower, ranging from about 1400 kg/m 3 to 1700kg/m 3 , from the density placed by vibrating the samples (2), which is moving about 2000 kg/m 3 .Since the mechanical characteristics, as well as the other properties of concrete (mortar) strongly depend on the realized structures [13], on the basis of its density it can be concluded that in the cases of mortar mixture for cement screeds incorporated by hand and unsatisfactory achieved compactness, which directly affects the porosity of the composite, it can be reasonably expected a lower nominal quality.Adding microfibre reinforcement and shrinkage-reducing admixture fails to influence the change of its density (up 3.6%).
The coefficient of structural favorability K kp is significant because it directly affects the weight of the structure, and it is more suitable in manually embedded composites compared to mixtures embedded by vibrating and changes insignificantly by adding fibres and additives.
Based on the results shown in the Table 4 it can be concluded that: • regardless of their composition, the samples of all the series embedded manually (1), have far less compressive strength compared to the samples which were subjected to the process of compaction (2), which is a logical consequence of lower compactness; • mortar mixtures containing the fibres (II) have a higher strength by about 10% compared to an etalon (I), provided that the gain of strength is more pronounced among "young" composite and is about 30%; • addition of additives against shrinkage has considerably influenced the decrease of compressive strength, the series III have lower strength of 1.5% compared to the Series II, provided that the compressive strength of series III are greater of the reference mixture (I) for about 8.5%.

Uzorak
Compressive strength is in functional correlation with density (Figure 7) and they are higher in samples of larger density (compacted samples) which is proved by testing of other authors [14].Slika 7. Funkcionalna zavisnost između čvrstoće pri pritisku i zapreminske mase, pri starosti od 28 dana Figure 7. Functional dependence between compressive strength and density, at the age of 28 days

Ultrasonic pulse velocity test
Non-destructive testing method of ultrasound has a very wide application in construction and other fields in technique.In practice, ultrasound is used as a method of determining a range of material properties, typical for cement screeds, such as: homogeneity, detection of voids and "nest" beneath the surface, determining the depth of the cracks, as well as indirect determination of density γ, compressive strength f p and dynamic modulus of elasticity E D of concrete.
Due to the value of γ, f p and E D stand to a certain interdependence, which is mostly pronounced between the relation of density -the physical and mechanical properties, the method of ultrasound in principle can be applied to determine all of the properties that are in functional relation to the density [14].The test results were obtained in accordance with the procedure described in the standard SRPS.U.M1.042 using the device "Pundit" (Portable Ultrasonic Non-destructive Digital Indicating Tester), on the samples of prismshaped dimensions 4x4x16cm.
The results of testing the speed of propagation of ultrasound waves through series of mortar composites at different ages are presented in the form of a diagram (Figure 8).Accordingly, it can be noticed that: • the speed of passage of the ultrasound increases in time, which is a logical consequence of composite hardening, • the values of ultrasonic pulse velocity are in the range of 4300 m/s to 4500 km/s, which is characterized by the quality of concrete (mortar), which is referred to as good [5], • the additives in the form of microfibre reinforcement and shrinkage-reducing admixture lead to a slight decrease (less than 5%) of the ultrasonic pulse velocity.Slika 8. Rezultati ispitivanja brzine prostiranja ultrazvuka Figure 8.The results of testing the ultrasonic pulse velocity

Shrinkage
Numerous factors, such as: temperature and humidity of the environment, dimensions of the elements, type and quantity of cement, water-cement ratio, size distribution and mineral composition of aggregates, strength of mortar, method of compacting and curing conditions and the like influence the final shrinkage values of cement screed, which represents a rheological deformation, reduction in size of plaster over the time without the action of external forces.This property, which is primarily associated with the state of humidity of cement stone, consists of three components: plastic, hydration and hydraulic shrinkage [23].
Hydraulic shrinkage occurs after the end of the cement bonding as a result of evaporation of water from the capillary pores, which causes contraction of cement stone.Hydration shrinkage is caused by the occurrence of physical and chemical processes of hydration of cement.Since polypropylene fibres, which are composed of two of the three study mixtures, as inert material have no effect on the cement hydration processes; this supplement can contribute to the possible reduction of purely hydraulic components shrinkage of tested mortar.
Testing of timely deformation of shrinkage was done according to the standard of SRPS B.C8.029, on the samples made of cement mortar of dimension 4x4x16cm, with special, stainless benchmarks, with which the preparation of the samples were carried out in a room where the temperature was 20±3C0, and a relative humidity 55±5%.
• test results of series II, which contains the microreinforcement, indicate a decrease in shrinkage of only 1.5% in relation to the etalon (Series I), the 126-th day of testing, while in earlier stages (4-th day) the dilatation shrinkage is 26.2% less in comparison with etalon.It can be explained by the fact that the synthetic fibre is very effective in the earliest stage, while their elastic modulus is lower than the modulus of concrete elasticity, so cracking and shrinkage are prolonged for the subsequent period when the concrete strength is greater.Larger hydraulic shrinkage could be explained exactly by backward, prolonged shrinkage of the early stages of hardening [7].
The functional dependence of deformation shrinkage of the time tested series show a strong correlation (R 2 >0.9) for the displayed logarithmic functions (Figure 10).

CONCLUSION
The main objective of this study was to determine the impact of the application of microfibre reinforcement and shrinkage-reducing admixture, as well as a way of its placement in the formation of high-quality cement screed.The samples of mortar compositions made in the traditional way without additives were compared with the samples made with additions of microfibre reinforcement and shrinkage-reducing admixture placed by hand and by vibration due to define the improved, optimal properties of the screed.
Based on aforementioned, it can be generally concluded that the impact of incorporation procedure is crucial to the achieved structure of cement stone.Given that all the respective mixtures have been made with low water cement ratio, solid and slightly plastic consistency, test results of density and compressive strength clearly show that only incorporation of vibration procedures result in obtaining the value of the properties that guarantee high quality of hardened cement composites, provided that the coefficients of structural favorability are disadvantageous.
Density of the samples placed by hand are much lower (1400 kg/m 3 to 1700kg/m 3 ) than the density of the samples placed by vibration, which are about 2000 kg/m 3 .Consequently, it can be concluded that in the cement screed which was placed manually and where there is insufficient compactness lower nominal quality can be expected.Addition of microfibre reinforcement and shrinkage-reducing admixture has insignificant influence on the change of density (uppermost 3.6%).
The coefficient of structural favorability K kp is significant because it directly affects the weight of the structure; it is more suitable in manually embedded composites compared to the mixtures embedded by vibration and changes insignificantly by adding fibres and additives.
Compressive strength is in functional correlation with the density and it is higher in the samples of larger density.However, the density should be viewed as a function of the degree of compactness of the mixture during the application because certain mixtures achieve better compactness than the others at the same intensity and duration of vibrations during its application.
The influence of polypropylene fibres in mortar mixtures at the compressive strength is expected.Cement composites containing microfibre reinforcement show an increase in strength by about 10% compared to reference one, at the age of 28 days, and in earlier stages this increase amounts up to 30%.
The addition of shrinkage-reducing admixture show a slight impact on the decrease of compressive strength (only 1.5%) compared to the samples made with fibre.
Test results show that the effect of shrinkagereducing admixture is more important than the impact of the addition of microfibre reinforcement on the shrinkage of cement composites.Namely mixtures containing compensators of shrinkage indicate decrease in shrinkage dilatation of 42.3% compared to the reference, while the series containing fibre reinforcement have less shrinkage in only 1.5%, as compared to the reference on the 126-th day of the test.In the earlier stages of testing, dilatation of shrinkages are 26.2%lower than the reference in series with fibre, which is a contribution of fibre in reducing the appearance of cracks in the "young" composites.
Although shrinkage-reducing admixture affects the decrease of compressive strength, its impact on reducing the dilation of shrinkage is large, which justifies the use of the additive when preparing mortar and cement screed, as well as the use of polypropylene fibres.At the same time, the basic problem of damage and deformation occurrence of the screed can be successfully solved only by means of incorporating some of the methods of vibration.

Slika 10 .
Funkcionalna zavisnost skupljanja Ɛsk od vremena Figure10.Functional dependence of shrinkage Ɛsk of time ; it is compatible with other . Density of samples in the form of cylinder Ø50mm and structural advantages coefficients K kp , at the age of 2, 7 and 28 days Slika 6. Funkcionalna zavisnost čvrstoće pri pritisku od vremena Figure6.Functional dependence of compressive strength of time