REINFORCED CONCRETE SLAB DESIGN

INTRODUCTION

Reinforced concrete slabs are used in floors, roofs and walls of buildings and as the decks of bridges. The floor system of a structure can take many forms such as in situ solid slab, ribbed slab or pre-cast units. Slabs may span in one direction or in two directions and they may be supported on monolithic concrete beam, steel beams, walls or directly by the structure’s columns.

Continuous slab should in principle be designed to withstand the most unfavorable arrangements of loads, in the same manner as beams. Because there are greater opportunities for redistribution of loads in slabs, analysis may however often be simplified by the use of a single load case. Bending moment coefficient based on this simplified method are provided for slabs which span in one direction with approximately equal spans, and also for flat slabs.

The moments in slabs spanning in two directions can also be determined using coefficients tabulated in the code of practice, BS 8110. Slab which are not rectangular in plan or which support an irregular loading arrangement may be analyzed by techniques such as the yield line method or the Helliborg strip method.

Concrete slab behave primarily as flexural members and the design is similar to that for beams, although in general it is somewhat simpler because;

1.    the breadth of the slab is already fixed and a unit breadth of 1m is used in the calculations,

2.    the shear stress are usually low in a slab except when there are heavy concentrated loads, and

compression reinforcement is seldom required.

EFFECT OF DIFFERENT LIGHTWEIGHT AGGREGATES FOUND IN PAKISTAN ON PROPERTIES OF LIGHT WEIGHT CONCRETE

1. Introduction

Lightweight concrete has been successfully used now a days and it has gained its popularity due to its lower density and superior thermal insulation properties. As Compared with normal weight concrete, Lightweight concrete significantly reduce the dead load of structural elements, which makes it suitable in multistory buildings.

In practice, the density of normal weight concrete lies within the range of 2200 to 2600 kg/m³ (140 to 160 lb/ft³). Consequently, the self weight of concrete element is high and contributes a large proportion of the load on structure.

Source

Lightweight concrete has a density between 1350 to 1900 kg/m³ (85 to120l b/ft³). Thus using lower density concrete gives benefits in terms of small cross section of beams and a corresponding reduction in size of load bearing elements.

On the other hand, lightweight concrete has higher cement content than normal weight concrete. This represents additional cost, and so does the more expensive lightweight aggregate. A meaningful comparison of cost should be made on the bases of the design of the structure using lightweight concrete and it cannot be limited to the cost of material.

The density of Concrete can be reduced by replacing some of the solids material in the mix by air voids. There are three possible locations of the air: (1) in the aggregate particles, which are known as lightweight aggregates and resulting concrete is known as lightweight aggregate concrete, (2) in the cement paste, the resulting concrete being known as cellular concrete, (3) between the coarse aggregate particles, the fine particles being omitted. Such concrete is known as no-fines concrete.

The essential characteristics of lightweight aggregate are its high porosity, which results in a low specific gravity. I shall extend my research by using light weight aggregates from different sources in Pakistan to make light weight concrete.  

 2. Problem Statement

In concrete construction, self weight of concrete represents major proportion of load on structure, thus there are clearly considerable advantages in reducing weight i,e density of concrete for high rise buildings, long span bridges and for various types of structures.

Thus there is always realizing a need to study the various properties of lightweight aggregate concrete and developing co-relation for these properties. So, lightweight concrete can be used for structural applications with strength equivalent to normal weight concrete.

This research aims to study properties of light weight concrete using lightweight aggregates found in Pakistan.

3. Objectives

1.      Influence of different types of Lightweight aggregates on mechanical performance of Lightweight Aggregates Concrete (LWAC) will be studied.

2.      Quality of different light weight aggregates derived from different sources in Pakistan will be determined.

3.      Suitability of different types of light weight aggregates found in Pakistan with respect to different concrete properties will be determined.

4.      Cost comparison will be carried out for different types of light weight aggregate concretes cast in Pakistan.

4. Literature Review

1.  Muyasser M. Jomaa’h (Civil Engineering Department – University of Tikrit),           Hosam A. Daham (Civil Engineering Department – University of Tikrit), Saad M. Rao’of (Civil Engineering Department – University of Tikrit) in 2011 found  that  the elastic stage and beginning of first cracks stages of normal weight concrete beams was smaller than others stages. While the results of the lightweight concrete beams show this stage was longer than other stages. The steel for lightweight concrete beams yielded before the steel in normal weight concrete beams. Also one can conclude that the same behavior of cracks spread, appears of crush in concrete, and failure behavior for lightweight and normal weight of reinforced concrete beams but at different loads. 

2. Jan Lindgård and Tor Arne Hammer (research associates, SINTEF Civil and Environmental Engineering, Norway) concluded that LWAC structures have potentially higher fire resistance than NDC structures, due to the lower heat conductivity of the LWAC and thus a better insulation of the reinforcement cover. LWAC can contain more evaporable water than NDC due to absorbed water in the LWA particles. Thus the risk of spalling can be higher in a LWAC.

LWAC structures may have lower fire resistance than NDC structures when exposed to severe fire.

 3. Gunduz L (2008) made laboratory research showing that the cube compressive strength of LWC concrete, having a 1376 kg/m3 fresh density made with only pumice aggregate, rises up to 14 MPa at 28 days of curing time. A higher compressive

strength of LWC made with pumice is seemed to be impossible since the compressive strength of its uncrushed stone restraints it. However, by adding some mineral admixtures, using higher dosage of cement and superplasticizer and decreasing the water to cement ratio, make possible to produce somewhat high strength LWC from scoria aggregates

4. .I˙lker Bekir Topc-ua and Tayfun Uygunog lu (2007) Investigated physical and mechanical properties of LWC produced with diatomite and pumice lightweight aggregates after autoclave curing investigated.

5. Al-Jabri KS, Hago AW, Al -Nuaimi AS, Al -Saidy AH (2008) investigated that because of having large number of voids in the aggregate, LWC possesses a relatively higher thermal insulating efficiency than the normal weight concrete (NWC).

The compressive strength of LWC depends on a variety factors. These are the strength of

aggregate stone, type of coarse, medium, fine and very fine aggregates, concrete composition, mineral admixtures, cement quantity, water-cement ratio, curing conditions of hardening etc.

 5. References:

1.    Al-Jabri KS, Hago AW, Al -Nuaimi AS, Al -Saidy AH. “Concrete Blocks for Thermal

Insulation in Hot Climate”. Cem. Concr. Res. 2008, 35: 1472-1479.

2.    Gunduz L. “The Effects of Pumice Aggregate/Cement Ratios on The Low-Strength Concrete Properties”. Constr. Build. Mater. 2008, 22: 721-728.

3.    Al-Khaiat H, Haque MN. “Effect of Initial Curing on Early Strength and Physical Properties of Lightweight Concrete”. Cem. Concr. 1998, Res. 28: 859-866.

4.    I’lker Bekir Topc-ua, Tayfun Uygunog lu, “Properties of Autoclaved Lightweight Aggregate Concrete” Building and Environment, Vol. 42, 2007, pp. 4108–4116.

5.    ASTM C330– 00, “Standard Specification for Lightweight Aggregates for Structural Concrete"

6.    Neville, A.M. (1995). Properties of Concrete. 4th ed., Essex: Longman Group Limited. 844.

7.    Carryer & Associates LTD (1995) Pumice Resources of New Zealand.

8.    ACI (2003) ACI 213R-03, Guide for structural lightweight aggregate concrete., in ACI Manual of Concrete Practice, Part 1: Materials and General Properties of Concrete. American Concrete Institute: Farmington Hills, Michigan. p. 38.

9.    Babu G. K. and Babu D. S. Performance of fly ash concretes containing lightweight EPS aggregates, Cement and Concrete Composites, V. 26 (2004), 605-611.

10. International Building Code (IBC), International Code Council (ICC), Washington D.C., 2009. 

6. Methodology

1-    Literature survey will be continued and expanded to the latest research made on the topic in different parts of the world.

2-    Sources for different types of light weight aggregates like expanded shale, expanded clay, Pumice etc will be searched out in Pakistan and availability of materials will be made sure.

3-    Concrete mix. design will be carried out for normal strength concrete and normal weight aggregates.

4-    Physical properties tests will be carried on cement i.e., compressive strength, consistency, setting time, expansion, fineness etc.

5-    Sieve analysis will be carried out for sand, normal weight coarse aggregate and all types of light weight aggregates as per British Standard. Grading curves will be plotted and fineness modulus will be found.

6-    Aggregate tests like, Loss Angeles Abrasion Test, Bulk Density, Shape index, Acid reactivity etc will be carried out for both normal weight and all types of light weight coarse aggregates.

7-    The following tests will be carried out on normal weight concrete and all types of light weight concretes.

8-    If some tests are failed for some types of lightweight aggregate concrete, Mix. design will be revised after selecting suitable admixture or changing water cement ratio.

9-    Casting will be carried out again for that type of concrete only by using revised Mix. design. Tests will be revised and results confirmed.

10-  Graphs will be drawn and results will be analyzed.

11-  At the end, the whole research would be compiled into a report form.

7. Budget Description

Sr. No.	Description of items	Approximate Budget
1.	Purchase of material	50,000/-
2.	Experimental Expenditures	30,000/-
3.	Labor charges	20,000/-
	Total	100,000/-

8. Proposed Work Schedule

Project Start Date:   01-06-2012

01-06-2012 End of Literature review and Start of Practical work

01-09-2012 End of Casting of Cubes

01-010-2012 End of Curing Specimen

01-11-2012 End of Laboratory Testing of Specimens

01-12-2012 End of Comparison of Results

01-01-2013 End of Analysis, Conclusions and Recommendations

Project Completion Date: 01-01-2013

9. Capital Expenses

Experimental Equipment:

1. Compressive Test  Machine

2. Lab Testing

3. Miscellaneous

Total Amount   30000/= Rs

Material and Supplies:

1. Light weight Aggregates

2.  Fine Aggregates (Sand)

3.  Cement

Total Amount:    50000/=Rs

Human Resources

Investigators List                           Level of effort                    Total amount in

                                                                                                           Rupees

(Principal Investigator)

                                 

(Co-Investigator)

Lab Technicians                               3 Months PT                            10000

Labour                                              3 Months PT                            10000

                         Total Project Cost: 100,000/=Rs

Selection of Appropriate Coarse Aggregates from Local Crushing Plants of Taxila Region

Aggregates make up about 75% of the volume of concrete, so their properties have a large influence on the properties of the concrete. Aggregates are granular materials, most commonly natural gravels and sands or crushed stone, although occasionally synthetic materials such as slags or expanded clays or shales are used. Most aggregates have specific gravities in the range of 2.6 to 2.7, although both heavyweight and lightweight aggregates are sometimes used for special concretes. The role of the aggregate is to provide much better dimensional stability and wear resistance; without aggregates, large castings of neat cement paste would essentially self-destruct upon drying. Also, because they are less expensive than Portland cement, aggregates lead to the production of more economical concretes. In general, aggregates are much stronger than the cement paste, so their exact mechanical properties are not considered to be of much importance (except for very high-strength concretes). Similarly, they are also assumed to be completely inert in a cement matrix, although this is not always true. For ordinary concretes, the most important aggregate properties are the particle grading (or particle-size distribution), shape, and porosity, as well as possible reactivity with the cement. Of course, all aggregates should be clean—that is, free of impurities such as salt, clay, dirt, or foreign matter. As a matter of convenience, aggregates are generally divided into two size ranges: coarse aggregate, which is the fraction of material retained on a No. 4 (4.75-mm) sieve, and fine aggregate, which is the fraction passing the No. 4 sieve but retained on a No. 100 (0.15-mm) sieve.

Coarse aggregates occupy almost 50-60% volume of concrete. So the shape, texture and source of coarse aggregate greatly affect the properties of concrete.
See Also: Use of waste material as aggregates

Methodology:

Coarse aggregate samples will be taken from local crushing plants around Taxila. It will be tested by the procedures given by ASTM. The results will be compared with the standards provided by different institutes around the world. Also in the end,5  concrete cylinders of dia 6 inches and height 12 inches will be made from each sample of coarse aggregate and by keeping all other variables constant. The compressive strength will be noted after performing the compressive test on UTM. The results will be compared and evaluated.

References:

•  Concrete Technolog
  y by Shetty
• ASTM C39, 1996. “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens”. Annual Book of ASTM Standards, Vol. 04. 02, the American Society for Testing and Materials, Philadelphia, USA.
•  Concrete Construction Engineering Handbook by Edward G. Nawy

Use of waste material as aggregates

Use of waste material as aggregates in civil engineering applications is beneficial because it reduces the environmental impact and economic cost of quarrying operations, processing, and transport.

Reuse of construction and demolition waste is becoming increasingly desirable due to rising hauling costs and tipping fees for putting this material into landfills(Robinson, Menzie, & Hyun, 2004). In recent years, sustainable construction initiatives have also made reuse of construction and demolition debris (as aggregates and otherwise) an appealing option when considering design alternatives for many types of structures (Taylor & Morrissey, 2004).

Incorporating these aggregates into cementitious materials is practical, as cementitious materials are non-homogeneous composites that allow material of different sizes and compositions to be bound in a cementitious matrix. Typical structural clay products are building bricks, paving bricks, roofing tiles etc. These products are made from commonly occurring natural materials, which are mixed with water, formed into the desired shape, and fired in a kiln in order to give the clay mixture a permanent bond. With the large amount of brick masonry or ceramic waste produced in the country, this material may provide a significant source of aggregates that can be used to produce more sustainable concrete.

Benefits

In addition to reducing the amount of waste that is land filled (and become an environmental hazard), other benefits can be realized as well. Brick aggregates are lighter than normal weight aggregates, and would provide haul cost savings. Concrete that incorporates brick aggregates is also lighter than normal weight concrete, would also be cheaper to transport, and can significantly reduce the self weight of a structure. Brick aggregates also have the potential to enhance the fire performance of concrete due to their thermal expansion and conductivity properties.

Presentation on Use of waste material as aggregates.ppt

Use of Waste Materials As a replacement of Coarse Aggregate in Concrete Mix