Report Student Work Lightweight concrete Table of Contents PDF

Preview

Summary

Arab International University Faculty of civil engineering Student Work Report Lightweight concrete Supervisors: Dr. Basem Ali Student :Jaber Hasan Al-Sodi 1 Table of Contents List of tables…………………………………………………………………………………………………………………….3 List of figures …………………………………………….…………............................

Description

Arab International University Faculty of civil engineering

Student Work Report Lightweight concrete

Supervisors: Dr. Basem Ali

Student :Jaber Hasan Al-Sodi

1

Table of Contents List of tables…………………………………………………………………………………………………………………….3 List of figures …………………………………………….…………............................................................4 1.Introduc on........................................................................................................................... 5 2.TYPES OF LIGHTWEIGHT CONCRETE ..........................................................................................6 . 2.1. NO-FINES CONCRETE .......................................................................................................6 2.2. LIGHTWEIGHT AGGREGATE CONCRETE……………………………….…………………………………………….….6 2.3. AERATED CONCRETE …………………….………………………………………………………………………………….….7 3.ADVANTAGES AND DISADVANTAGES OF LIGHTWEIGHT CONCRETE …………………………………….……….8 4.Mechanical Properties of Structural Lightweight Concrete ……………………………….…………………………..9 4.1. Compressive strength (unheated specimens) ……………………………………………………………..9 4.2. Compressive strengths (heated specimens) ……………………………………………………………………..10 5.Effect of reinforcement on behavior of lightweight concrete ………………………………………………………13 5.1.Compressive Strength…………………………………………………………………………………………………...…. 13 5.2.Flexural Strength…………………………………………………………………………………………………………………14 5.3. Splitting Tensile Strength………………………………………………………………………………………….………..16 6.Effect of mineral admixture on properties of lightweight concrete ………………………………………..……17

References…………………………………………………………………………………………………………………….……18

2

List of Tables: Table 3.1: Advantages and Disadvantages of Lightweight Concrete ……………………………….8 Table 4.1 Average Compressive Strength of Unheated Test Specimens ……………………..…...9 Table 5.1. Compressive Strength at 7 and 28 days ………………………………………………………..……13 Table 5.2. Flexural Strength at 28 days ………………………………………………………………………..……...15 Table 5.3. Spli ng tensile at 28 days………………………………………………………………………………………………16

3

List of Figures:

Figure 2.1 No-fines concrete……………………………………………….……………….………….…….6 Figure 2.2 Lightweight Aggregate Concrete………………………………………………….………..7 Figure 2.3. Aerated Concrete………………………………………………………………………………...8 Figure 4.1. Varia on of Strength with Age at Ambient Temperature……………………………....10

Fig. 4.2.Varia on of Strength with Temperature for Different Mix Ra os. …………..11 ,12 Figure 5.1 Compressive Strength at 7 and 28 days………………………………………………….13 Figure 5.1.2 Show the shape of concrete crush with Fibers and without Fibers……..13 Figure 5.2.1 Flexural Strength at 28 days……………………………………………………………………………14

Figure 5.2.2 Show the shape of concrete failure with Fibers and without Fibers…...14 Figure 5.3.1 Spli ng tensile strength at 28 days………………………………………………………………….16 Figure 5.3.2 Rela onship between the steel fibers content and increasing in tensile splitting strength………………………………………………………………………………………………………………………………..16

4

1.Introduction In concrete construction, the concrete represents a very large proportion of the total load on the structure, and there are clearly considerable advantages in reducing its density. One of the ways to reduce the weight of a structure is the use of lightweight aggregate concrete (LWAC)(Mouli and Khelafi, 2008) Lightweight concrete (LWC) has been used for more than 2,000 years (ACI 213R) (American Concrete Ins tute [ACI], 2003). Early notable LWC structures include the Port of Cosa, the Pantheon Dome, and the Coliseum. Lightweight concrete can be defined as a type of concrete which includes an expanding agent in that it increases the volume of the mixture while giving additional qualities such as nailibility and lessened the dead weight [1]. .

It is lighter than the conven onal concrete with a dry density of 300kg/m3up to 1840 kg/m3; 87

to 23% lighter. It was first introduced by the Romans in the second century where ‘The Pantheon’ has been constructed using pumice ,the most common type of aggregate used in that particular year [2]. From there on, the use of lightweight concrete has been widely spread across other countries such as USA, United Kingdom and Sweden. The lower density and higher insulating capacity are the most obvious characteristics of Lightweight Aggregate Concrete (LWAC) by which it distinguishes itself from ‘ordinary’ Normal Weight Concrete (NWC). However, these are by no means the only characteristics, which justify the increasing attention for this (construction) material. If that were the case most of the design, production and execution rules would apply for LWAC as for normal weight concrete, without any amendments. Lightweight Aggregate (LWA) and Lightweight Aggregate Concrete are not new materials. In recent years, more attention has been paid to the development of lightweight aggregate concrete (Lo et al.,2007). The specific gravity of concrete can be lowered either by using porous, therefore lightweight aggregates instead of ordinary ones, or introducing air into the mortar, or removing the fine fractions of aggregate and compacting concrete only partially. In all cases, the main goal is to introduce voids into the aggregate and the mortar or between mortar and aggregate. A combination of these methods can also be made in order to reduce further the weight of concrete. The use of lightweight aggregates is by far the simplest and most commonly used method of making a lightweight concrete (Gündüz and Ugur, 2005).

5

2. TYPES OF LIGHTWEIGHT CONCRETE Lightweight concrete can be prepared either by injecting air in its composition or it can be achieved by omitting the finer sizes of the aggregate or even replacing them by a hollow, cellular or porous aggregate. Particularly, lightweight concrete can be categorized into three groups: i) No-fines concrete ii) Lightweight aggregate concrete iii) Aerated/Foamed concrete 2.1. NO-FINES CONCRETE No-fines concrete can be defined as a lightweight concrete composed of cement and fine aggregate. Uniformly distributed voids are formed throughout its mass. The main characteristics of this type of lightweight concrete is it maintains its large voids and not forming laitance layers or cement film when placed on the wall. Figure 2.1 shows one example of No-fines concrete.

Figure 2.1 No-fines concrete No-fines concrete usually used for both load bearing and non-load bearing for external walls and partitions. The strength of no-fines concrete increases as the cement content is increased. However, it is sensitive to the water composition. Insufficient water can cause lack of cohesion between the particles and therefore, subsequent loss in strength of the concrete. Likewise too much water can cause cement film to run off the aggregate to form laitance layers, leaving the bulk of the concrete deficient in cement and thus weakens the strength. 2.2. LIGHTWEIGHT AGGREGATE CONCRETE Porous lightweight aggregate of low specific gravity is used in this lightweight concrete instead of ordinary concrete. The lightweight aggregate can be natural aggregate such as pumice, scoria and all of those of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, 6

vermiculite and clinker aggregate. The main characteristic of this lightweight aggregate is its high porosity which results in a low specific gravity [4]. The lightweight aggregate concrete can be divided into two types according to its application. One is partially compacted lightweight aggregate concrete and the other is the structural lightweight aggregate concrete. The partially compacted lightweight aggregate concrete is mainly used for two purposes that is for precast concrete blocks or panels and cast in-situ roofs and walls. The main requirement for this type of concrete is that it should have adequate strength and a low density to obtain the best thermal insulation and a low drying shrinkage to avoid cracking [2]. Structurally lightweight aggregate concrete is fully compacted similar to that of the normal reinforced concrete of dense aggregate. It can be used with steel reinforcement as to have a good bond between the steel and the concrete. The concrete should provide adequate protection against the corrosion of the steel. The shape and the texture of the aggregate particles and the coarse nature of the fine aggregate tend to produce harsh concrete mixes. Only the denser varieties of lightweight aggregate are suitable for use in structural concrete [2]. .

Figure 2.2 shows the feature of lightweight aggregate concrete.

Figure 2.2 Lightweight Aggregate Concrete.

2.3. AERATED CONCRETE Aerated concrete does not contain coarse aggregate, and can be regarded as an aerated mortar. Typically, aerated concrete is made by introducing air or other gas into a cement slurry and fine sand. IN commercial practice, the sand is replaced by pulverized fuel ash or other siliceous material, and lime maybe used instead of cement [2].

7

There are two methods to prepare the aerated concrete. The first method is to inject the gas into the mixing during its plastic condition by means of a chemical reaction. The second method, air is introduced either by mixing-in stable foam or by whipping-in air, using an air-entraining agent. The first method is usually used in precast concrete factories where the precast units are subsequently autoclaved in order to produce concrete with a reasonable high strength and low drying shrinkage. The second method is mainly used for in-situ concrete, suitable for insulation roof screeds or pipe lagging.

Figure 2.3 shows the aerated concrete.

Figure 2.3. Aerated Concrete 3. ADVANTAGES AND DISADVANTAGES OF LIGHTWEIGHT CONCRETE Table 2 shows the advantages and disadvantages of using lightweight concrete as structure [2]. Table 3.1: Advantages and Disadvantages of Lightweight Concrete Advantages i) rapid and relatively simple construction.

Disadvantages i) Very sensitive with water content in the mixtures.

ii) Economical in terms of transportation as well as reduction in manpower. iii) Significant reduction of overall weight results in saving structural frames, footing or piles.

ii) Difficult to place and finish because of the porosity and angularity of the aggregate. In some mixes the cement mortar may separate the aggregate and float towards the surface.

iv) Most of lightweight concrete have better nailing and sawing properties than heavier and stronger conventional concrete.

iii) Mixing time is longer than conventional concrete to assure proper mixing.

8

The use of lightweight aggregate in concrete has many advantages. These include: (a) Reduction of dead load that may result in reduced footings sizes and lighter and smaller upper structure. This may result in reduction in cement quantity and possible reduction in reinforcement. (b) Lighter and smaller pre-cast elements needing smaller and less expensive handling and transporting equipment. (c) Reductions in the sizes of columns and slab and beam dimensions that result in larger space availability. (d) High thermal insulation. (e) Enhanced fire resistance (Kayali, 2007; ACI 213,2003). 4. Mechanical Properties of Structural Lightweight Concrete

4.1. Compressive strength (unheated specimens) Table 4.1 shows summary of average compressive strength of unheated test specimens. It is observed that at 7-day curing age, the compressive strength values of the unheated concrete specimens

with

1:2:2

mix

and

w/c ra os

of 0.6

and

0.8

were

2.85

and

2.60

N/mm2respectively. At 21-day curing age, average compressive strength of specimens with w/c ra o of 0.6 and 0.8 were 4.46 and 3.65 N/mm2 respec vely. At 90-day curing age, concrete with 1:2:2 mix and water/cement ra o of 0.6 showed an average compressive strength value of 4.69 N/mm2 while for 1:2:2 mix and at 0.8 water/cement ra o, the average strength was 4.56 N/mm2. ( 3) Table 4.1 Average Compressive Strength of Unheated Test Specimens ( N/mm2) w/c Ratio Curing Age (days)

Mix Ra o 1:2:2

Mix Ra o 1:2.5:2

7

21

90

7

21

90

Compressive Strength

0.6

2.85

4.46

4.69

5.34

6.00

7.34

(N/mm2)

0.8

2.6

3.95

4.56

4.88

5.62

6.52

In all test cases, the average compressive strengths of test specimens with w/c of 0.6 were higher than the corresponding values for test specimens with 0.8 w/c ra o. The decrease in strength of test specimens with w/c = 0.8 rela ve to test specimens prepared with w/c = 0.6 could be attributed to presence of excess moisture for hydration process in the specimens prepared with 0.8 w/c ra o. ( 3) The results of strength variation with curing age for different mixes at 21oC laboratory temperature (unheated specimens) are presented in Fig. 4.1. The figure indicates that the test 9

specimens for 1:2 ½:2 mix at w/c ra o of 0.6 have the highest compressive strength values. At 7-day curing age, the average values for compressive strength are 5.34N/mm2and 4.88 N/mm2for 0.6 and 0.8 w/c ra os respec vely. This indicates a 9.20% more than the strength of the specimens with 0.8 w/c ra o. At 90 day curing age, the strength values are7.34 N/mm2 and 6.52 N/mm2 at w/c ra o of 0.6 and 0.8. This indicates a difference of 12.42% in strength values an indica on that the smaller the w/c ratio value, the higher the strength of the mixes provided the mix were prepared under the same condition. ( 3) Also, for test specimens prepared from 1:2:2 mix with w/c ra o of 0.6, the average compressive strength at 7-day curing age was 2.85 N/mm2as against 2.60 N/mm2 for specimens with 0.8 w/c ra o. This indicates a reduc on of 8.77% of compressive strength of test specimens with 0.6 w/c ratio. This trend of decrease in strength values for mix with 0.6 w/c ratio when compared with the mix with 0.8 w/c ratio was also observed at 21- and 90-day curing ages. ( 3)

Figure 4.1. Varia on of Strength with Age at Ambient Temperature.

4.2. Compressive strengths (heated specimens) Figures 4.2 present results of compressive strengths with increase in temperature. It is observed that the compressive strengths of test specimens reduced with increase in temperature. At 7-day curing age, the 1:2½:2 mix test specimens cast with 0.6 w/c ra o have average compressive strength of 5.34 N/mm2 at ambient (21oC) temperature while at 800oC temperature, the average compressive strength of test specimens reduced to 3.67N/mm2 at the same age. This shows 31.27% reduc on in strength. An average of 3.48% reduc on in compressive strength with every 50oC increase in temperature was recorded. At 21-day curing age, between 21oC and 800oC temperature range, the compressive strength values are 5.90 N/mm2 and 4.21 N/mm2 respectively. This gives a

10

reduction in strength values of 28.64%. An average of 3.18% reduc on in compressive strength with every 50oC increase in temperature was recorded. ( 3) At 90-day curing age a reduc on in strength value of 35.10% corresponding to an average loss in strength of 3.9% for every 50oC increase in temperature was observed. The investigation further showed that at 8000C/hour, in most specimens the periwinkle shells disintegrated considerably and had all broken into pieces. The rate of loss of strength by the test specimens was higher at the early stages of drying as the periwinkle shells tend to experience change in their structure due to temperature increase. This perceived structural change as a result of heat effect is responsible for rapid loss of compressive strength of the test specimens. As the temperature increased, the effect reached its peak, hence, the rate of influence on the compressive strength reduced. This trend in loss of compressive strength by test specimens with increase in temperature is also observed for all other mixes as indicated in Figs. 4.2.(ii), (iii) and (iv).In all cases, as the temperature increases, there is a gradual loss in strength of the specimens. At the temperature of 800oC/hr, heated specimens lost between 26% and 40% of initial strength values before the heating process commenced. ( 3) Also, the rate of loss in strength evaluated by the slope of Figs. 4.2(i), (ii), (iii) and (iv) curves tends to be higher in 1:2.5:2 mixes when compared to 1:2:2 mixes, irrespec ve of the water/cement ratio and the curing age. The compressive strengths of the test specimens were reasonably maintained up to 300oC, there after as temperature increases there is a severe and progressive decrease in strength. This is attributed to the formation of cracks in the specimens, coupled with poor bonding of the concrete matrix. The loss in strength is considerably lower before a ainment of 400oC temperature level, but at 600oC most of the periwinkle shells (aggregate) in the test specimens were fractured. This accounts for higher strength loss at higher temperatures.( 3)

11

Fig. 4.2.Variation of Strength with Temperature for Different Mix Ratios. (i) 1:2.5:2 mix with w/c ra o = 0.6, (ii) 1:2:2 mix with w/c= 0.6,

Fig.4.2. Variation of Strength with Temperature for Different Mix Ratios. (iii) 1:2.5:2 mix with w/c ra o = 0.8, (iv) 1:2:2 mix with w/c= 0.8. 12

5. Effect of reinforcement on behavior of lightweight concrete : 5.1. Compressive Strength Values of compressive strength for all mixes are shown in Table (5.1) and Figure (5.1) at 7 and 28 days, results demonstrated that in general, all concrete specimens exhibited an increase in compressive strength with increase the percent of steel fibers. The percent of increasing in compressive strength at 7 days about (27.18%, 43%, 30.32%, and 17.48%) for (1%, 0.75%, 0.5%, and 0.25%) steel fibers respec vely. While in 28 days, adding (1%, 0.75%, 0.5%, and 0.25%) steel fibers lead to increasing in compressive strength by about (30.33%, 51.73%, 33.79%, and 21.26%) respec vely. It can be seen that the increase in compressive strength of light weight steel fiber concrete at 28 days was greater than their corresponding compressive strength at 7 days. Such increase in compressive strength was attributed to the intensive product of hydration process around the steel fibers and in voids of concrete [5]. From Figure (5.1) it may also be concluded that the addi on of steel fibers up to 0.75% of concrete volume improved the compressive strength of light weight concrete due to the better mechanical bond strength between the fibers and the cement matrix which delays microcracks formation [6]. However, Adding more steel fibers up to 1% of concrete volume reduces the increasing in the compressive strength as compared with 0.75% but it remain higher than the reference mix and this is attributed to the voids introduction in the mix due to excessive fiber content that may lead to reduction in bonding and disintegration[7]. Table 5.1. Compressive Strength at 7 and 28 days Compressive strength

%Increase in compressive

Compressive strength

%Increase in compressive

MPa-7 days

Strength -7 days

MPa-28 days

Strength -28 days

A-0.00%S.F

22.66

……….

29.77

………..

B-1.00%S.F

28.82

27.18

38.8

30.33

C-0.75%S.F

32.41

43.00

45.17

51.73

D-0.50%S.F

29.53

30.32

39.83

33.79

E-0.25%S.F

26.32

17.48

36.1