Concrete. Chapter 10 Properties of Fresh Concrete. Materials of Construction-Concrete 1. Wikipedia.org

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1 Wikipedia.org Concrete Chapter 10 Properties of Fresh Concrete Materials of Construction-Concrete 1

2 What is Concrete? Concrete is a composite material that consists of a cement paste within which various sizes of fine and coarse aggregates are embedded. Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to desired shape. Concrete contains entrapped air and may contain purposely-entrained air by the use of air-entraining admixtures. Materials of Construction-Concrete 2

3 Concrete Constituents Cement Water Fine Agg. Coarse Agg. Admixtures Materials of Construction-Concrete 3

4 Advantages Ability to be cast Economical Durable Fire resistant Energy efficient On-site fabrication Materials of Construction-Concrete 4

5 Disadvantages Low tensile strength Low ductility Volume instability Low strength to weight ratio Materials of Construction-Concrete 5

6 Order of Operations for Concrete Specific operations must be performed in a certain order final quality is influenced by every step I. mix design (proportioning) II. trial mixes & testing In lab. III. batching start the clock IV.mixing In plant V. transporting VI. pouring (placing) VII. vibrating (consolidating) initial set here VIII. finishing In site final set here IX. curing X. maintenance Sampling is here Materials of Construction-Concrete 6

7 Fresh? Hardened? The plastic state of the concrete starting from the time that the concrete making materials are mixed until the concrete gains rigidity is called fresh concrete The state of the concrete starting from the time that it gains rigidity is called hardened concrete Materials of Construction-Concrete 7

8 Basic Properties of Fresh Concrete Workability Consistency Segregation Bleeding Setting time Unit weight Uniformity Materials of Construction-Concrete 8

9 FRESH CONCRETE Fresh concrete must have the desired degree of workability for a given job. It must; be easily mixable and transportable, be sufficiently flowable so that it can fill the forms completely for which it was designed, have the ability to be compacted fully without an excess amount of energy being applied, be uniform, and the materials in it must not show segregation during mixing, placing and consolidation and, its surface must be easily finishable to the required amount of smoothness. Materials of Construction-Concrete 9

10 FRESH CONCRETE Other fresh concrete properties; It must show very little or no bleeding. It must have a proper setting time. It must have the required amount of unit weight. Materials of Construction-Concrete 10

11 WORKABILITY Workability is the ease with which fresh concrete can be mixed, placed, compacted and finished without harmful segregation. There is no single test method that can simultaneously measure all the properties involved in workability. It is determined to a large extent by measuring the consistency of the mix. Materials of Construction-Concrete 11

12 Factors Affecting the Workability - Quantity of the mixing water, - Quantity and properties of the cement, - Gradation and D max of the aggregate, - Shape and surface texture of aggregate particles, - Quantity of mineral admixtures, - Quantity and characteristics of chemical admixtures, - Quantity of the entrained air, - Temperature of concrete. Materials of Construction-Concrete 12

13 Consistency Consistency is the fluidity or degree of wetness of concrete. It is generally dependent on the shear resistance of the mass. It is a major factor in indicating the workability of freshly mixed concrete. Materials of Construction-Concrete 13

14 Test Methods for Measuring Consistency Flow test measures the amount of flow Kelly-Ball test measures the amount of penetration Slump test (Most widely used test!) Materials of Construction-Concrete 14

15 10 cm 30 cm Concrete 20 cm Slump cone (Abram s cone) The slump cone is filled in 3 layers. Every layer is evenly rodded 25 times. Concrete Concrete Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen. Materials of Construction-Concrete 15

16 Slump Test is related with the ease with which concrete flows during placement (TS EN , ASTM C 143) Materials of Construction-Concrete 16

17 Slump Classes acc. to TS EN Class Slump, mm S S S S S 5 > 220 Materials of Construction-Concrete 17

18 SLUMP TEST VIDEO RECORD Materials of Construction-Concrete 18

19 In a properly proportioned concrete, the water content required to produce a given slump depends on several factors: Shape and surface texture of aggregate, D max of well graded aggregate, Entrained air. Materials of Construction-Concrete 19

20 Segregation Seperation of the coarse aggregates from the mortar in the fresh concrete mixture. The segregation may take place either by the settling of heavy particles to the bottom of the fresh concrete or by the seperation of the coarse aggregates from the body of the concrete due to improper placing or vibration of the concrete. Materials of Construction-Concrete 20

21 Factors leading to segregation Large maximum particle size (over 25 mm) and proportions of the large particles. Decreased amount of sand and cement (decreased amount of paste volume in concrete mix). Too wet and too dry mixtures. Improper mixing, improper placing, and improper consolidation (prolonged vibration) of the concrete. Materials of Construction-Concrete 21

22 Bleeding Bleeding is a form of segregation in which some of the water in the mix tends to rise to the surface of the freshly placed concrete. It is caused by the inability of the solid particles in the fresh concrete mix to hold all of the mixing water as the solid particles settle downward with the effect of gravitional forces. Materials of Construction-Concrete 22

23 Harmful effects of bleeding With the movement of water towards the top, the top portion of the concrete becomes weak and porous (relative w/c ratio increment in bleeded region!) Since the top part of the concrete section is porous (and permeable), the resistance of the concrete to freezing and thawing or to sulfates is decreased. Relatively weak and non durable surface, which is not resistant to abrasion, may be obtained. Materials of Construction-Concrete 23

24 Bleeding can be reduced! By increasing the cement fineness and/or by using finely divided mineral admixtures. By entraining ait into the concrete mix (use of air entraining chemicals). By reducing the water content of the concrete mixture, if this can be done while maintaining an acceptable workability. By placing the concrete in relatively shallow layers. Materials of Construction-Concrete 24

25 Setting Time Setting time is the length of time from the moment of mixing concrete making material until the moment that concrete gains rigidity. During this time concrete is plastic and shapeable. The setting time for ordinary concrete is usually 3-5 hours. The setting time of concrete is affected by: - mix proportions, - type and amount of cement, - type and amount of admixtures, - the ambient temperature. Materials of Construction-Concrete 25

26 Unit Weight Unit weight of fresh concrete is the weight of it per unit volume (including the volume of the solid particles and the spaces between them). It is expressed in kg/m 3 or ton/m 3. Unit weight of concrete is affected by: - the gradation of the aggregates, - the specific gravity of the aggregates, - the degree of compaction applied to the concrete - the air content of concrete and, - obviously mix proportions of concrete. Materials of Construction-Concrete 26

27 Unit Weight The unit weight of concrete made with normal weight aggregates is around 2400 kg/m 3. A concrete that has a relatively higher unit weight is a more compact concrete, which leads to higher strength and durability. The unit weight of concrete is generally used to check the uniformity of concrete batches (produced using same types and proportions of materials) and in mix design calculations. Materials of Construction-Concrete 27

28 Uniformity Uniformity: Concrete should be uniform and similar in appearance and quality within-batch and batch-to-batch. Within-batch variations: Differences in the quality of concretes obtained from one section of a concrete batch to another section of the same batch. Materials of Construction-Concrete 28

29 Determination of uniformity Concrete uniformity is checked by conducting tests on fresh and hardened concretes. Slump, unit weight, air content tests Strength tests Materials of Construction-Concrete 29

30 Order of Operations for Concrete Specific operations must be performed in a certain order final quality is influenced by every step I. mix design (proportioning) II. trial mixes & testing In lab. III. batching start the clock IV.mixing In plant V. transporting VI. pouring (placing) VII. vibrating (consolidating) initial set here VIII. finishing In site final set here IX. curing X. maintenance Sampling is here Materials of Construction-Concrete 30

31 Proportioning of Concrete Mixes (Mix Design) Determine proportions of mix ingredients be economical be practical use available materials satisfy requirements & specs acceptable workability of fresh mix quality (durability, strength, appearance) of hardened concrete economy 31

32 Mix Design: Volumetric Method 1. Strength requirements 2. Determine W/C 3. Estimate coarse aggregate mass 4. Air entrainment requirements 5. Workability needs 6. Estimate water content 7. Determine cement content requirements 8. Evaluate admixture needs 9. Estimate fine aggregate mass 10. Determine moisture corrections 11. Trial Mix

33 Trial Mixes check proportions with trial batches air content slump 28 day compressive strength: 3 cylinders 15cm in dia x 30 cm adjust for optimum workability & economy Materials of Construction-Concrete 33

34 Batching Concrete Batching is the process of measuring concrete mix ingredients by either mass or volume and introducing them into the mixer. To produce concrete of uniform quality, the ingredients must be measured accurately for each batch. Most specifications require that batching be done by mass rather than by volume Materials of Construction-Concrete 34

35 Mixing Concrete All concrete should be mixed thoroughly until it is uniform in appearance, with all ingredients evenly distributed. Mixers should not be loaded above their rated capacities and should be operated at the mixing speed recommended by the manufacturer. If the blades of a mixer become worn or coated with hardened concrete, mixing action will be less efficient. These conditions should be corrected. Materials of Construction-Concrete 35

36 Stationary Mixing Concrete is sometimes mixed at the jobsite in a stationary mixer or a paving mixer. Stationary mixers include both onsite mixers and central mixers in ready mix plants. They are available in sizes up to 9.0 m 3 and can be of the tilting or nontilting type or the open-top revolving blade or paddle type. All types may be equipped with loading containers and some are equipped with a swinging discharge chute. Materials of Construction-Concrete 36

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39 Stationary Mixing Careful attention should be paid to the required mixing time. Many specifications require a minimum mixing time of one minute plus 15 seconds for every cubic meter. Short mixing times can result in nonhomogenous mixtures, poor distribution of air voids (resulting in poor frost resistance), poor strength gain, and early stiffening problems. Materials of Construction-Concrete 39

40 Stationary Mixing Under usual conditions, up to about 10% of the mixing water should be placed in the drum before the solid materials are added. Water then should be added uniformly with the solid materials, leaving about 10% to be added after all other materials are in the drum. Materials of Construction-Concrete 40

41 Ready-Mixed Concrete Ready mixed concrete is proportioned and mixed off the project site and is delivered to the construction area in a freshly mixed and unhardened state. It can be manufactured by any of the following methods: 1. Central-mixed concrete is mixed completely in a stationary mixer and is delivered either in a truck agitator, a truck mixer operating at agitating speed or a truck 2. Shrink-mixed concrete is mixed partially in a stationary mixer and completed in a truck mixer. 3. Truck-mixed concrete is mixed completely in a truck mixer Materials of Construction-Concrete 41

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43 Pouring (Placing) Chute Conveyor Materials of Construction-Concrete 43

44 Pouring (Placing) During (or just before) the placing, slump and air content of the fresh concrete should be determined and samples for further strength tests should be prepared. Test on site Slump Air content Prepare samples for determining concrete strength class Standard cylinders (or cubes) Beams (if required) Materials of Construction-Concrete 44

45 Pouring (Placing) Materials of Construction-Concrete 45

46 Directly into form Pump Wheel barrow/ buggy Bucket Materials of Construction-Concrete 46

47 Placing Issues Drop height < 90 cm Horizontal movement Limit to prevent segregation Pumping Adjust mix design tremie chute to limit drop height Materials of Construction-Concrete 47

48 Vibration of Concrete Consolidation (compaction) should be completed before initial set Manually by ramming tamping Mechanically using vibrators Internal poker 5 sec to 2 min in one spot <10 sec. typical avoid segregation through entire depth penetrate layer below if still plastic External by vibrating the mould itself Materials of Construction-Concrete 48

49 Finishing Concrete smoothing and imprinting the surface of the concrete with the desired texture. must be completed before final set Materials of Construction-Concrete 49

50 Screeding strike concrete off to desired level Power float Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. Materials of Construction-Concrete 50

51 Curing Concrete Maintain moisture and temperature in the concrete to promote continued hydration and strength gain Hydration will resume if curing is stopped and resumed Curing affects: durability strength water-tightness abrasion resistance volumetric stability resistance to freezing and thawing resistance to de-icing chemicals Materials of Construction-Concrete 51

52 Curing Approaches 1. Maintaining presence of water in the concrete 2. Seal the surface so mix water can t escape. 3. Providing heat & additional moisture to accelerate the strength development Materials of Construction-Concrete 52

53 Approach 1. Maintaining Presence of Water Must water periodically Also provides cooling Methods ponding: smaller jobs flat-work (floors and pavement) and laboratory spraying or fogging: expensive and a lot of water wet coverings: burlap, cotton, rugs, etc. Materials of Construction-Concrete 53

54 Spraying Fogging Wet Covering Materials of Construction-Concrete 54

55 Approach 2. Seal the Surface impervious paper or plastic sheets membrane forming compounds leave forms in place Plastic Sheets Materials of Construction-Concrete 55

56 Approach 3. Heat Insulate Hot steam application (around 60C 0 ) good for early strength gain and in freezing weather Electrically heated forms Usually in precast plants only Can be applied in very cold weather conditions Materials of Construction-Concrete 56

57 Compressive strength of concrete at different ages & curing levels Materials of Construction-Concrete 57