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Properties Of Concrete

This study presents the effect of incorporating metakaolin (MK) on the mechanical and durability properties of high strength concrete for a constant water/binder ratio of 0.3. MK mixtures with cement replacement of 5, 10 and 15 % were designed for target strength and slump of 90 MPa and 100 ± 25 mm. From the results, it was observed that 10 % replacement level was the optimum level in terms of compressive strength. Beyond 10 % replacement levels, the strength was decreased but remained higher than the control mixture. Compressive strength of 106 MPa was achieved at 10 % replacement. Splitting tensile strength and elastic modulus values have also followed the same trend. In durability tests MK concretes have exhibited high resistance compared to control and the resistance increases as the MK percentage increases. This investigation has shown that the local MK has the potential to produce high strength and high performance concretes.

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Effect of Metakaolin Content on the Properties of High Strength Concrete

THE STRENGTH OF CONCRETE MADE FROM SOUND, DURABLE MATERIALS DEPENDS UPON THE EXTENT TO WHICH THE SOLID PARTICLES ARE BONDED TOGETHER AND THE AMOUNT OF SOLID MATERIALS IN THE CONCRETE. THE BOND DEPENDS UPON THE PROPORTION OF CEMENT IN A GIVEN QUANTITY OF CONCRETE AND THE AMOUNT OF MIXING WATER. USING THESE BASIC PRINCIPLES SEVERAL METHODS FOR DESIGN ARE AVAILABLE. IN ANY CASE, THE MAINTENANCE OF A CONSTANT RATIO OF CEMENT TO MIXING WATER IS ESSENTIAL TO UNIFORMITY OF RESULTS. THE NEED FOR ACCURATE MEASUREMENT OF ALL MATERIALS IS EMPHASIZED AND ADEQUATE METHODS ARE DESCRIBED. AN EXAMPLE OF THE USE OF THE MORTAR-VOID METHOD OF DESIGNING TO MEET AN UNUSUAL CONDITION IS INCLUDED.

Design of concrete mixtures

The primary goal of concrete mixture design is to strike a balance among workability, compressive strength, durability, economic efficiency and sustainability. In this paper, for a given strength grade, the optimum paste consisting of cement, fly ash and slag, and the optimum ratio among paste, fine aggregate and coarse aggregate were optimized using the simplex centroid design method based on rheological properties. Results showed that the optimum content of total cementitious materials in concrete could be obtained according to the relationships between the workability, yield stress, plastic viscosity and the paste volume fraction. The optimum replacement of supplementary cementitious materials could be determined according to the rheological properties and compressive strength of concrete with ternary cementitious components. It is an effective way to optimize the mixture design of concrete based on the rheological properties using the simplex centroid design method.

Mixture design of concrete using simplex centroid design method

Determination of steel fibres distribution in self-compacting concrete beams using X-ray computed tomography

The article shows the effectiveness of the use of polymer additives for the production of fine-grained concrete mixtures and concretes based on using coal fly ash, which can be used as working mixtures for a 3D printer. Using mathematical planning of experiments, a set of experimental–statistical models was obtained that describes the influence of mixture composition factors including copolymer additive on the most important properties of ash-containing concrete mixtures and concretes for 3D concrete printing in the presence of a hardening accelerator additive. It is shown that when the dry mixture is mixed in water, the redispersed polymer powders are converted into an adhesive polymer dispersion, which, when the solution cures, creates “rubber bridges” in its pores and at the border with the base. They have high tensile strength and elastically reinforce the cement stone; in addition, they are also capable of not only significantly increasing the adhesion between the layers of the extruded mixture, but also significantly smoothing out such shortcomings of the cement stone as increased brittleness, low ultimate elongation, and a tendency to cracking.

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Effectiveness of Polymer Additives in Concrete for 3D Concrete Printing Using Fly Ash

Today, numerous design solutions require joining thin-walled sheets or profiles as the traditional methods of welding with a consumable electrode in gas shielding, most often used in production processes, do not work well. The reason for this is that a large amount of heat is supplied to the joint, causing numerous welding deformations, defects, and incompatibilities. Moreover, the visual aspect of the connections made more and more often plays an equally crucial role. Therefore, it is important to look for solutions and compare different joining processes in order to achieve production criteria. The paper compares the properties of a 1.5 mm thick steel sheet joined by the manual and robotic MAG 135 and 138 welding process, manual and robotic laser welding, CMT welding with the use of solid or flux-cored wire, and butt welding. The macro- and microstructure, as well as the microhardness distribution of individual regions of the joints, were analyzed depending on the type of joining technology used. Furthermore, the mechanical properties of individual zones of joints were investigated with the use of a digital image correlation system. On the basis of the obtained test results, it was found that the joints made by the processes of manual laser welding and butt welding were characterized by a very regular weld shape, the smallest joint width, and greater grain refinement compared to other analyzed processes. Moreover, this method was characterized by the narrowest zone of hardness increase, only 3 mm, compared to, e.g., a joint made in the process of robotic welding CMT, for which this zone was more than twice as wide. Furthermore, the heat-affected zone for the joints made in this way, in relation to the welds produced by the MAG 135/138 method, was, respectively, 2 and 2.7 times smaller.

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Microstructure and Mechanical Properties of Joints Depending on the Process Used

The paper considers the influence of β-hemihydrate and phosphogypsum binder on the properties of dry pack mortars for self-levelling floors. As preliminary research demonstrated that adding to phosphogypsum (PG) admixtures of lime and PA or PC superplasticizer permits to obtain binder with high compressive (up to 24 MPa) and bending strength (up to 12 MPa). The manufacturing of dry pack mortars for self-levelling floor underlayment was determined as one of the areas of PG application. There has been investigated composite binder consisted of PG, Portland cement and metakaolin.
There have been determined linear deformations of PG/ based binder comparing to β-hemihydrate calcium sulphate binder and bending to compressive strength ratio.
The method of planned experiment was applied for determination the optimal proportion of the components of the mortar. There were considered the sand / binder ratio, cement and metakaolin portions in binder, water-binder ratio and content of Tylose (water-retaining admixture). There was tested composite binder consisted of PG (or β-hemihydrate), PC and metakaolin. There were tested flow spread on Suttard's viscometer, time of application, and compressive strength at the age of 1, 3 and 28 days. The flow spread diameter of the mixtures was within the range of 180-200 mm, and time of application was between 30 and 45 min.
The compressive strength of mortar specimens is up to 4 MPa, at the age of 3 days is 10 MPa and at the age of 28 days reaches 17 MPa. The increasing of Portland cement dosage and metakaolin makes the most positive effect on strength parameters.
The mortar based on PG is applicable for underlayment of different flooring compounds types.

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Dry pack mortars for self-levelling floor compounds based on β-hemihydrate and modified phosphogypsum binder

Using stone crushing waste products allows more rational and complex use of mineral resources and increases the ecological safety level of quarry works and production of stone materials. This paper focuses on an experimental investigation of the composition effect of vibro-pressed fine-grained concrete incorporating granite siftings on compression and flexural strength. Analysis of regression equations, obtained using mathematical experiments planning, shows that dust particles included in the siftings can behave as micro-filler. This is especially the case at low cement-water ratios therefore yielding an increase in concrete strength. The positive effect of the dust fraction in siftings decreases as the cement-water ratio and the fineness modulus of sand that is a part of the aggregate become higher. The hardening kinetics of vibro-pressed concrete with granite siftings is analyzed. The results of the present study may be used for preliminary assessment of granite sifting efficiency as aggregates in vibro-pressed concrete and design of their optimal compositions.

Using granite siftings for producing vibro-pressed fine-grained concrete

Adding different fiber types may yield improvement of steel fiber reinforced concrete (SFRC) features. Therefore, the investigation of hybrid fiber reinforced concrete (HFRC) mechanical properties is relevant. The effect of adding hybrid steel and basalt fiber on the mechanical properties of fine-grained concrete is studied. It is shown that hybrid fiber reinforcement using optimal steel and basalt fiber ratio allows preventing concrete mixtures' segregation and improving their structure homogeneity. This, in turn, allows achieving higher concrete strength values. In most cases, the design of such concrete compositions is based on engineering experience that limits the designers' capabilities. Therefore, an effective methodology for proper HFRC composition design should be developed. The present study is focused on developing such a methodology. The developed methodology includes using the mathematical experiments planning method to design optimal composition of high-strength fine-grained fiber reinforced concrete with hybrid steel and basalt fiber reinforcement. It is demonstrated that the proposed method can be effectively used for the design of optimal compositions of HFRC.

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Leonid Dvorkin

Papers

Effectiveness of Polymer Additives in Concrete for 3D Concrete Printing Using Fly Ash

Microstructure and Mechanical Properties of Joints Depending on the Process Used

Dry pack mortars for self-levelling floor compounds based on β-hemihydrate and modified phosphogypsum binder

Using granite siftings for producing vibro-pressed fine-grained concrete

Mechanical Properties and Design of Concrete with Hybrid Steel and Basalt Fiber