Behavior of Short Reinforced Porcelinite Concrete Columns Under Concentric Loads

: This study presents on experimental method to investigate the behavior of reinforced porcelinite concrete columns subjected to concentric load. The experimental work includes investigation of ten lightweight concrete columns with square cross section 190 × 190mm and 1000mm in length subjected to concentric load to assess the effect of the amount of longitudinal steel reinforcement, lateral tie reinforcement and grade of concrete. From the experimental observation it was found that the porcelinite aggregate is a good structural material to produce lightweight concrete columns. It is noted from tested columns that keeping the amount of the longitudinal steel ratio constant, the increase in the amount of transverse steel to about four times (by decreasing the spacing to about a quarter according to the ACI-Code (318M-05)) gave a significant influence on column post-cracking behavior. The improvements in the column due to higher value of confinement include: (i) less brittle behavior; (ii) significantly less damage to the concrete core; (iii) 12% increase in column load carrying capacity. Also, A modified formula for column capacity is proposed to take into account the influence of spacing of ties (effect of confinement).

are key elements of all skeletal structures, and the study of their behavior is usually based on testing of concentrically loaded columns.Compression members, or columns, may be defined as members that carry axial compressive loads, and whose length is considerably greater than the cross-sectional dimensions.Lightweight concrete has successfully been used for many years for structural members and systems in buildings and bridges.In addition to its lighter weight, which permits saving in dead loads, and thus reducing the costs of both superstructure and foundation, it is more resistant to fire and provides better heat and sound insulation than concrete of normal density (1,2) .Lightweight concrete is considered as having a density not exceeding 1920 kg/m3, while normal density concrete is considered to have a usual density ranging between 2240 and 2480 kg/m 3 (3) .Also lightweight concrete with compressive strength ranging between 17 and 27 MPa is defined as lowstrength concrete.For compressive strength ranging from 27 to 41 MPa it is defined as medium-strength concrete.However, for compressive strength greater than 41 MPa, it is defined as high-strength concrete (1) .In Iraq, little work has been done on the use of lightweight concrete in structural members, such as slabs; beams, columns, foundations, etc.
Recent research (4,5,6,7) has shown that there is an abundant supply of lightweight rock that may be used to produce concrete of lower density than the density of normal concrete in the present practice in this country.The lightweight aggregate, which is used, is quarried from rocks discovered in the Iraqi Western Desert.It is called porcelinite (8) and Fig. (1) shows the location.Since the production of manufactured lightweight aggregate (from clay, shale, etc) is more costly, successful use of natural lightweight aggregate will be much more economic.The well-known advantages of using lightweight concrete worldwide are even more important in Iraq, since much of this country has low soil bearing capacity (9,10) .

Experimental Work
The tests were made on ten reinforced concrete columns, with a cross section of 190mm × 190mm, and a total length of 1000mm.The columns were loaded at the ends with concentric loads.The reinforcing bars were cut to the desired length.For all tested columns, 90degree hook was formed at the ends of each longitudinal bar.Lateral ties were made from 9.5mm diameter deformed bars.Additional ties were placed close to the ends of the specimens to prevent failure at these regions.Two mixes were used in the experimental work.Mix C1 was prepared without superplasticizer and used for groups G1 and G2.While mix C2 was prepared with superplasticizer and used for groups G3 and G4.Twelve 150 × 300 mm cylindrical control specimens were used for groups G1 and G2 and twelve specimens for groups G3 and G4 (three specimens to calculate the modulus of elasticity, three specimens to calculate splitting tensile strength and six specimens for calculating the compressive strength of concrete at age 7 days and at 28 days).An internal vibrator machine was used to compact the fresh concrete.After 24 hours the column and the control specimen molds were stripped and the specimens were kept moist cured in a water bath for a curing period of 28 days.The columns were divided into four groups (G1, G2, G3 and G4).For groups G1 and G3, three columns were tested for each group.The columns were designated as A1B1C1, A1B2C1, A1B3C1, A1B1C2, A1B2C2 and A1B3C2.For groups G2 and G4, two columns were tested for each group.The columns were designated as A2B1C1, A3B2C1, A2B1C2 and A3B1C2.The letter A refers to longitudinal steel ratio and the subscript number 1 refers to 4-Ø15.9mm, 2 refers to 4-Ø12.7mm and 3 refers to 8-Ø15.9mm.The letter B refers to transverse steel ratio, the subscript number 1 refers Ø9.5 @190mm, 2 refers to Ø9.5 @125mm and 3 refers to Ø9.5 @50mm.The letter C refers to compressive strength of concrete and the subscript number 1 refers to concrete without superplasticizer and the subscript number 2 refers to concrete with superplasticizer.Figs. ( 2) and ( 3) refer to the classification chart of column groups and column reinforcement before casting respectively.

Concrete Mixes
Concrete mixes containing porcelinite aggregate as lightweight aggregate should have an oven -dry density less than 2000 kg/m 3 , and a compressive strength greater than 15.0 MPa (11) .Mix C1 has (1:1.13:1.8)by volume with Ordinary Portland Cement of 550 kg/m 3 content, fine aggregate of 522 kg/m 3 content and LWA content of 518 kg/m 3 were used in this work.The w/c ratios were 0.43 for mix C1 and 0.35 for mix C2 to give a slump of 100±25 mm.The concrete contained 4% superplasticizer (SP1) by weight of cement.Table (1) shows the details of the mixes used throughout this investigation.

Materials 1-Cement
The type of cement used in this study was ordinary Portland cement (Type I).It was manufactured at Lebanon factories (cement Siblin).It was kept in a dried place to avoid atmospheric damage.The chemical and physical analyses of the used cement conformed to the Iraqi Standard Specification No.5/1984 (12) .

2-Fine Aggregate
Normal-weight natural sand obtained from Al-Akhaider region was used as fine aggregate.The sand was sieved on 4.75-mm sieve.The grading of the sand conformed to the requirements of BS 882-1992 (13)

3-Porcelinite Aggregate
Local naturally occurring lightweight aggregate of porcelinite stone was used as coarse aggregate.It was received in large lumps from the State Company of Geological Survey, Western Desert at Al-Anbar Governorate.The lumps were manually crushed into smaller sizes by means of a hammer in order to facilitate the insertion of lumps through the feeding openings of the crusher machine.The jam crusher was set up to give a finished product of about 12.5mm maximum aggregate size.The aggregate taken from the crusher was screened on a standard sieve series complying with ASTM C330 (14) .The individual size fraction for each batch was recombined in proper proportions to produce the desired grading.The required quantity of coarse lightweight aggregate for each batch was washed by water in order to remove the dust associated with crushing process of the coarse aggregate.A high proportion of dust leads to segregation and causes crazing of exposed concrete (15) .
The coarse aggregate was dripped off and spread inside the laboratory in order to bring the aggregate particles to saturated surface dry (SSD) condition.Due to the rapid water absorption of this aggregate type, the surface drying has been done by spreading the aggregate in the laboratory air for a suitable time.
Several physical and chemical properties were determined for coarse porcelinite aggregate.Some of these properties would be used as basis for mix proportioning purposes, such as PDF created with pdfFactory Pro trial version www.pdffactory.comdry rodded unit weight and absorption.Table (2) lists these properties and their corresponding proper specifications.

4-Admixture
The superplasticizer used in this work was RHEOBUILD (16) SP1 which is a liquid admixture for concrete, based on sulphonated naphthalene.

5-Water
Tap water of Baghdad was used for mixing concrete.The ideal water should be clean and clear.However, no test was carried out on the water to be used.

6-Reinforcing Steel
Reinforcing steel used as longitudinal bars and lateral ties in reinforced concrete columns met the ASTM A615 (17) requirements.All the longitudinal bars used were of the same type and had the same mechanical properties and rib geometry.
Longitudinal reinforcing steel bars, which were of Ukrainian origin, were purchased from a local market in Najaf.The bars have been tested at the Laboratory of Mechanical Engineering Department at Kufa University in Najaf.The longitudinal steel bars for all tested columns had diameter of 12.7 and 15.9 while 9.5mm bars were used as lateral ties.The main data obtained for testing bars are shown in Table (3).
Demec points were used to measure the surface strains in concrete.They were mounted along the concrete surface (or faces) within the effective span (measuring zone) of the tested samples.
Two demec gage points were mounted at spacing of 150mm at the column mid-height along the column vertical axis to measure the longitudinal compressive strains at two perpendicular faces of the column.Also additional two demecs were mounted horizontally to measure lateral strains at two perpendicular faces of the column.Details of demec gage distribution are shown in Figs.(4).

Results: 1-Compressive Strength
The strength values were obtained as the average of three cylinders for each test.It is noted that the use of superplasticizer (SP1) increases the cylinder compressive strength from 22MPa to 32MPa (+45%) and decreases the w/c ratio from 0.43 to 0.35 (-18.6%).

2-Spliting Tensile Strength
The values are obtained as the average of three cylinders at age of 28 days.It is noted that the splitting tensile strength increases by about (19.2%) when the compressive strength rises from 22MPa to 32MPa.

Photographs
of the tested concentrically loaded short columns are shown through Figs.(5) to (10).It is noticed from the tests that the appearance of vertical cracks in the concrete cover was always the first sign of failure of the tested columns.These cracks spread rapidly after spalling of concrete cover.At this stage the core of concrete carried the applied axial load because it is confined by the confining effect between the ties and the longitudinal bars.At the end of this stage the ties will slip (open) as the expansion of concrete core occurs.This was particularly evident for columns

4-Load versus Mid-Height Longitudinal Strain Results
Ten short reinforced lightweight concrete columns were cast and tested under concentric loads up to failure.Experimental behavior of load versus mid-height longitudinal compressive strain curves of all tested columns are presented in Figs.(11) and (12).
It may be noticed that the response by the load versus mid-height longitudinal and lateral strains can be divided into two distinct stages of behavior.
The first stage is characterized by an approximately linear relationship between the load and the longitudinal and lateral strains.This stage represents the response of columns before cracking of concrete cover.
The second stage represents the behavior beyond the initial cracking of concrete cover.At this stage the stiffness of the column is decreased as indicated by the reduced slope of the load versus longitudinal and lateral strain curves.During this stage, the cover starts to spall out.The end of this stage is distinguished when the longitudinal reinforcement starts to be dented and the core of concrete carries all the axial loading while the concrete cover is completely spalled out.Table (4) shows the ultimate experimental load capacities for all tested columns (18) .

5-Effect of Transverse Reinforcement Ratio
In order to examine the effect of using different amounts of transverse reinforcement in the lightweight concrete column on its axial behavior, Group G1 without superplasticizer and with transverse steel ratios of 0.48%, 0.73% and 1.83% respectively is considered.In this group the longitudinal steel ratio and concrete compressive strength were 2.19% and 22MPa respectively.Group G3 with superplasticizer (SP1) was selected and it consisted of columns A 1 B 1 C 2 , A 1 B 2 C 2 and A 1 B 3 C 2 with transverse steel ratios of 0.48%, 0.73% and 1.83% respectively.In this group the longitudinal steel ratio and concrete compressive strength were kept at 2.19% and 32MPa respectively.
It is noted that the variation of the transverse steel ratio slightly affects the post-cracking response (second stage) and the ultimate load.Also when the transverse steel ratio was increased from 0.48 to 0.73 percent (spacing between ties changed from 190mm to 125mm respectively) there was negligible effect on the postcracking behavior and the ultimate load.Fig. ( 13) shows the effect of transverse reinforcement on the load versus lateral strain.When the transverse steel ratio was increased from 0.48 to 0.73 percent the figure reveals that a slightly stiffer post cracking response is obtained.In contrast it is noted that when the transverse steel ratio was increased from 0.73 to 1.83 percent (spacing between ties was changed from 125mm and 50mm respectively) there was a significant effect on the postcracking behavior because of the better confinement of concrete core and less damage in the concrete core and longitudinal steel bars at failure.

6-Effect of Percentage of Longitudinal Steel Ratio
The longitudinal steel ratios, l ρ , which have been used were 1.40%, 2.19% and 4.38%.The transverse steel reinforcement and concrete compressive strength were 0.48% and 22MPa for group G2 respectively.While for group G4, the PDF created with pdfFactory Pro trial version www.pdffactory.comtransverse steel reinforcement and concrete compressive strength were 0.48% and 32MPa respectively.

7-Effect of Concrete Compressive Strength
The selected values of concrete compressive strength were 22 and 32MPa for columns A 1 B 1 C 1 and A 1 B 1 C 2 respectively.A relatively stiffer response in precracking stage was noted for the column with concrete compressive strength equal to 32MPa.The collapse load increased from 942 to 1220kN (+29.5%) when the compressive strength was increased from 22 to 32MPa (+45%).Therefore, it can be concluded that the concrete compressive strength significantly influences the postcracking stiffness and the collapse loads.

8-Nominal Axial Load Capacity
Based on the limited amount of experimental tests, the following expression of nominal load capacity for porcelinite lightweight concrete columns subjected to concentric load is proposed for concrete compressive strength ranging between 22MPa to 32MPa.It is noted from the tested columns that the porcelinite aggregate is a suitable structural material to produce lightweight concrete columns.

2-
It was observed that the use of superplasticizer in concrete mix at 4% (of cement weight) would increase the compressive strength of lightweight concrete from 22 to 32MPa and reduce concrete spalling at the outside cover.

3-
It is noted from tested columns that keeping the amount of the longitudinal steel ratio constant, the increase in the amount of transverse steel by about 50% (by decreasing spacing by 50%) from the ACI-Code (318M-05) minimum requirement has a negligible for effect on collapse load but it slightly improves post-cracking behavior.

4-
It is noted from tested columns that keeping the amount of the longitudinal steel ratio constant, the increase in the amount of transverse steel to about four times (by decreasing the spacing to about a quarter) according to minimum requirement of the ACI-Code (318M-05) has a significant influence on column postcracking behavior.The improvements in the column with higher value of confinement include: (i) greater ductility; (ii) significantly less damage to the concrete core; (iii) 12% increase in column load carrying capacity.

5-
It is noted from tested columns that keeping the amount of the transverse steel ratio constant, the increase in percentage of longitudinal reinforcement substantially affects the post-cracking behavior.It is noted that with the increase in longitudinal steel ratio from 1.4% to 4.38%, the collapse loads increased by about 41%.

Group and column classification
Total number of tested columns (10 samples)

s
= actual spacing between ties.

cd
= outside to outside dimension of tie dimension.

. Sarsam* Dr. Ihsan A. S. Al-Shaarbaf **& Dr. Mustafa S. Shuber*** Introduction Compression members
PDF created with pdfFactory Pro trial version www.pdffactory.com of crushed concrete being ejected from the core.Columns A 3 B 1 C 1 and A 3 B 1 C 2 have shown much more ductility of behavior after cover spalling, as compared with columns A 1 B 1 C 1 and A 1 B 1 C 2 .

Table ( 1) Details of Concrete Mixes
** Staining material test was done in the Building and Glass Research Center.+ Tests were carried out by the SCGSM.
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