LECA CONCRETE - LIGHT INSULATION AGGREGATE IN LIGHT WEIGHT CONCRETE This paper is prepared about Light weight concrete made from light expanded clay aggregate at Building & Housing Research Center, which is the research center for Ministry of Housing in Iran. Here is a summarized translation presented by Leca Co. Iran. One way for preparing building member in lightweight concrete is by using light expanded clay aggregate. For the first time, this kind of concrete was discovered in 1917. In different countries of the world, lightweight aggregate has produced and named differently and paid attention with increasing rate. In Iran, this aggregate is named LECA or manufactured aggregate. LECA is abbreviated form of light expanded clay aggregate. The purpose of this paper is to obtain highest possible compressive strength for Leca concrete while noting the advanced technology in producing lightweight concrete. This paper covers necessary mechanical properties for designing and manufacturing in following order: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Compressive strength Tensile strength Bond strength of reinforcement to concrete Durability and effect of freezing and thawing, fire and chemical attack Modulus of elasticity Creep Drying shrinkage Coefficient of thermal expansion Thermal properties Acoustical properties
Results obtained from this study shows that making building components from lightweight Leca concrete is possible. In fact it is more economical to use Leca concrete rather than ordinary concrete in constructing building since lighter weight of building also includes: Easy transportation, less reinforcement use and reduction in dimensions of foundation. Usually, with increasing density of Leca concrete it’s strength gets higher; this kind of concrete is used for slopping drainage and thermal resisting elements with low density (400-1000 Kg/m3), non structural elements with medium density (1000-1300 Kg/m3) and load resisting structural elements with high density (1300-1800 Kg/m3) *1. Leca concrete for structural elements must have design compressive strength of at least 150 Kg/cm2 and density of at most 1800 Kg/m3. Structural components made of Leca concrete have been used in America for the past fifty years. Leca concrete with compressive strength of higher than 70.3 Kg/cm2 is considered usable in building construction. *2 For Leca concrete with density of less than 1841 Kg/m3, compressive strength of 633 Kg/cm2 has been reported (2). Leca concrete, comparing with other lightweight concrete, possesses a very high strength to density ratio. Lightweight concrete can be used in making structural elements and save money, specially, in: high-rise building, building on a low strength soil and also where there is not sufficient coarse aggregate mines. It can be used also instead of ordinary concrete in making: 1. Casting concrete for making: bridges, buildings, roads and etc. 2. Precasting: joists, walls in different sizes, floor and roof panels and other structural components. 3. Production of: hollow and solid blocks of load bearing and non-load bearing units.
*1) Design compressive strength in about 95 percent of particular concrete mixture specimen must have compressive strength above that of the design compressive strength *2) The number in parentheses referees to the number of reference in table of references.
In addition to what has been mentioned, by using Leca aggregate one can get lightweight concrete with strength equal to that of ordinary concrete but with about 1/3 lower density. Federation international de la precontrainte (FIP) has reported annual production of Leca aggregate about 23073000 m3 in most countries of the world in about 1975.
Leca concrete like ordinary one can be prepared by mixing aggregate (Leca or Leca and Sand), cement and water. In Leca concrete instead of using ordinary aggregates one uses Leca aggregate or Leca and ordinary sand. Heating wetted and formed clay soil at 1300 degree centigrade in a kiln produces Leca aggregate. In heating process, the gasses are produced, condensed, escaped from aggregate and finally causes voids inside aggregates. Production of Leca aggregate is accomplished by different means. In Iran, they are produced by expansion of moist clay soil in a rotary kiln.
# AGGREGATE AND GRADATION In ordinary concrete different gradation of aggregates affects the required amount of water. Addition of some fine aggregate results an increase in required amount of water. This increase of water reduces concrete strength unless the amount of cement in the same time increases. Amount of coarse aggregate and its biggest size depends on the required workability of concrete mixture. Also in lightweight concrete, this relation exists among: the graduation, requested amount of water and obtained concrete strength, but there are other factors that must be paid attention to. In most lightweights aggregate as the size of aggregate increases the modulus of deformation, strength and bulk density of the aggregate decreases. Using very big size light weight aggregate with a lower strength results into a lower strength of the lightweight concrete; therefore, biggest size of lightweight aggregate must be limited to 25 millimeter at most. With using smaller size of lightweight aggregate and increasing the percent age of ordinary fine aggregate, although, increases the density of lightweight concrete, but its strength will be increased also. For example, if the biggest size of lightweight aggregate reduces from 25 to 16 millimeter the obtained Leca concrete strength will increase about 30 percent. Mostly, ordinary fine aggregate is added to coarse lightweight aggregate for: better workability, reduction of shrinkage of fresh concrete mixture, increasing concrete strength and, or better economy. For example, using ten volume percent of ordinary fine aggregate instead of fine light weight aggregate, bulk density, and increases about 100 Kg per cubic meter while the modulus of deformation increases about 20 percent. With this substitution, workability also gets better. Because of heavier weight of: Cement, water and ordinary fine aggregate mixture coarse lightweight aggregate may rise to the top and results in a nonhomogeneous concrete mixture. For avoiding this nonhomogeneity, it is necessary to control the required amount of water. Adding bentonite that is a kind of clay soil about one to two percent weight of cement used in concrete mixture results and increase of mortar viscosity, which causes a homogeneous lightweight concrete mixture. Air entrainment admixture also reduces the danger of nonhomogeneity. For preventing nonhomogeneity of concrete mixture, one can use suitable amount of fine aggregate smaller than 03.25 millimeter in size. It is shown in table 2 that for lightweight concrete, especially those with round and smooth surface of coarse aggregates, amount of required fine aggregate depends on the maximum size of coarse aggregate used. Table 2- Amount of required fine aggregate in respect to the max. size of coarse aggregate. Amount of fine aggregate in one Cubic meter of compacted concrete (Cement + fine aggregate smaller than 0.25 millimeter in size) Kilogram 525 - 180 Lit 450 - 150 420 - 140
Max. size of coarse aggregate in millimeter
8 16 25
Substituting ordinary sand for some light fine aggregate results to increase the strength of concrete obtained. This increase of strength is noticeable in two following reasons: 1.
Water absorption in light fine aggregates with angular and rough surface is more than in ordinary fine aggregate with round and smooth surface: Therefore, using all light fine aggregate increases the watercement ratio and decreases the strength.
2.
Ordinary sand-cement mortar is stronger than light sand-cement mortar. “When ordinary sand is used instead of light weight sand, for better workability, the maximum size of sand must be smaller than two millimeter, better workability results in using less sand (about 20 to 30 percent of total volume) and reduction of weight of concrete “. When using lightweight sand, especially for vertical component with exposed surface, more cement is required. While using ordinary river sand, less cement is required, it must be noticed that using low amount of cement mortar in concrete mixture makes the compaction difficult, with not enough compaction, too much pores is produced in concrete; and concrete with low strength will be obtained. On the other hand, using high amount of cement mortar, especially when there is too much water in the mixture, increases the risk of nonhomogeneity and raising the lightweight coarse aggregate up to the top surface of the mixture. In some cases (with suitable mixing and pouring concrete) one can use high amount of cement mortar with ordinary sand aggregate or air entrainment admixture. In making lightweight concrete, similar to ordinary concrete, one can use aggregate both uniform and nonuniform in gradation. Suitable gradation depends on required strength, weight workability, amount of available aggregate and facilities for storing lightweight aggregates in different sizes. Uniform gradation makes better homogeneous concrete mixture (4). For economic reasons, it is better to use heavier and stronger lightweight aggregate along with less cement for constant strength of concrete obtained. With heavier and stronger lightweight aggregate, the ratio of lightweight aggregate to ordinary sand increases in order to obtain stronger concrete with constant density. For getting lightweight concrete with higher strength and constant amount of cement, lightweight aggregate with higher density is required. In fact there is an optimum real density of aggregate for each specific concrete density, which gives the highest strength of concrete obtained. It must be noted again that higher amount of heavier light weight aggregate and less amount of ordinary sand used, results to constant density and higher strength of concrete. In this study Leca lightweight concrete specimens made from one type of aggregate (with specific real density) and two type of gradation of aggregates were used.
# AMOUNT OF CEMENT REQUIRMENT Amount of required cement for specific strength of concrete depends on different type of lightweight aggregate used. In fact amount of cement required for specific strength of concrete depends on strength and modulus of deformation of aggregate used and also on amount of free water and required workability. With increasing amount of cement, the strength of lightweight concrete increases. For making sure of: suitable workability, durability, protection of reinforcement from rusting and adequate bond strength between concrete and reinforcement one must use more than 300 kilogram cement per one cubic meter of concrete. On the other hand, increasing amount of cement in the mixture results to increase: the creep, shrinkage, heat of hydration of cement and the danger of cracking of concrete; therefore, less than 500 kilogram of cement per one cubic meter of concrete must be used. Because of high real density of cement (about 3100 kilogram per cubic meter) with increasing amount of cement, the
density of concrete increases. “ Adding 50 kilogram cement in one cubic meter of concrete results an increase in concrete density of about 30 kilogram per cubic meter.” With increasing the strength of cement, for specific strength of concrete, amount of required cement decreases. That makes a concrete with lower density.
# AMOUNT OF WATER REQUIRMENT The amount of water in lightweight concrete includes: free or effective water between aggregates and water in pores of aggregates. Effective water, which is the major part of water in the mixture, is in the cement paste, and this is the amount of water that determines the strength and workability. Amount of free water depends on; Max. size of aggregate, amount of slump for required workability, shape and texture of aggregate and with less percent on the amount of cement used in the mixture. Light weight Leca aggregate which is produced in rotary kiln, because of their round and smooth surface, their water absorption is much less than that of light weight aggregates with angular and rough surface. Amount of the slump of concrete mixture depends on: the type of structural component and the facilities and method for compacting the concrete mixture.
When the hand vibrator (which is dipped into the concrete mixture) is used, concrete mixture must be pastely. Concrete mixture with excessive water must be avoided. Water absorption of lightweight aggregate with too much pores is much more then that of ordinary aggregates (river aggregates). Determination of amount of water absorption in these kinds of aggregates, because of varying amount of absorbed water, is difficult. That makes the design of concrete mixture with lightweight aggregate with too much pores somewhat difficult. Leca aggregate produced in rotary kiln, because of their smooth surface, has water absorption nearly equal (somewhat more) to that of ordinary aggregate; therefore, design of light weight concrete mixture with Leca aggregate is as difficult as that of ordinary aggregate. For determining the amount of each ingredient in lightweight concrete mixture (along with the amount of absorbed water in light weight aggregates, specially those with too much pores with rough and angular surface, by making different mixtures) one can use the common design methods of ordinary concrete mixture. Since the complete explanation of these methods is beyond the scope of this task, only a brief explanation on this regard is given below: Amount of dried aggregate for one cubic meter of compacted concrete is about 1.15 to 1.22 cubic meter in bulk volume. But for spherical aggregate with complete uniform gradation is about 1.05 cubic meter in bulk volume. The total volume of fresh concrete mixture consists of the real volume of: cement, aggregate, water and total volume of pores including those, which have been made by air entrainment admixture (air entrainment admixture is added to concrete mixture to make concrete with higher resistance to intense freezing and thawing environment). One cubic meter of light weight concrete, like ordinary concrete, nearly consists of the sum of real volume of: 0.1 cubic meter of cement, 0.7 cubic meter of aggregate and 0.2 cubic meter of water and air. When estimated volumes of water, air and cement are subtracted from the total volume (one cubic meter) of compacted concrete, the remainder will be the volume of aggregate in (one cubic meter of) compacted concrete. With the use of suitable amount of aggregate and cement, one can prepare a trial mixture, and then add enough water to get concrete mixture with required workability. Calculation for specified volume of fresh concrete and amount of each ingredient in the mixture is performed. Amount of each ingredient in concrete mixture is calculated and their amount is adjusted. Making trial mixtures is continued until satisfactory amount of each ingredient is obtained. For trial mixtures it is necessary to determine: the density of aggregate, amount of moisture in aggregate, the optimum ratio of coarse aggregate to fine aggregate and amount of cement required for specified concrete strength.
# MAKING LECA CONCRETE SPECIMENTS AND DETERMINING THEIR STRENGTH By mixing: Leca aggregate and ordinary sand with two type of gradation, water and cement, Leca concrete mixtures are prepared and casted in two form of molds: one cubic (15 x 15 x 15 cm) and the other cylindrical (15 cm diameter by 30 cm height). 24 hours after casting, Leca concrete specimens were removed form the molds and conditioned in water tank with about 22 C temperature until the ages of 7 and 28 days. Then the specimens were tested for compression (cubic specimens) and indirect tensile splitting strength (cylindrical specimens). After testing the first group of specimens, it is determined that the optimum total water to cement ratio for heights strength of Leca concrete is about 0.62 for 300 kilograms of cement and 0.63 for 350 kilograms of cement in one cubic meter of compacted Leca concrete. Noting this optimum ratio of total water to cement, another groups of cubic and cylindrical specimens respectively for compression and indirect tensile splitting strength tests were made. Compacting specimens were accomplished by first rodding 20 times (a round rod with 1.5 centimeter diameter and 951 grams weight) and then compacting 20 times (a one end flat rod with rectangular 2.5x10 centimeter flatness and 823 grams weight) in three layers.
EXPLANATION: Compacting Leca concrete components in production line (here by rodding) can be accomplished by pressing and at the same time vibrating in such a way to obtain a homogeneous concrete mixture. Research that has been done in this regards in different countries of the world all agree on one thing that frequency of vibration for compacting light weight concrete with lightweight aggregate must be higher than that for compacting ordinary concrete. Although, they suggest different optimum frequency ranging from 12000 to 20000 cycles/min.
Amount by weight of Leca and sand aggregate for one cubic meter of compacted Leca concrete in two different gradations is given in table No.7 below. After testing the cubic and cylindrical specimens. The following results were obtained: Total of six group specimens were made and tested. The sixth group consists of only six cubic specimens, three of them 15x15x15 centimeter and other three-10x10x10 centimeter in dimension. Gradation of aggregate used in making these specimens is shown in table No. 13 and compaction was accomplished by three minutes vibration on vibrating table. Results from 28-day compression test were obtained as follow:
Table 13: The Ratio of Leca and sand Rerain of seive (weight) Seive No (mm) 9.5 4.9 2. 36 1.4 0.6 0.3 0.15 <0.15
Aggregate Leca Leca Sand Leca Leca Sand Sand Sand
+ Seive (kg) 114 160 361 52 108 152 76 76
By studying the results of all reports published in U.S.A, the relationship between amount of cement used and 28 day compressive strength of Leca concrete specimens have been obtained (7). This relationship is for Leca concrete specimens that have been conditioned in moist room and their slump mixtures are about 5 to 10 centimeter. This relation is linear and is shown in figure No. 3 below: C=3+
CU 1000
C = amount of cement in package for one cubic yard of Leca concrete Cu = 28 day compressive strength in Psi Or C = 196.2 + 0.9302 Cu C= amount of cement in Kilogram per cubic meter Cu= 28 day compressive strength in Kg/cm2
----------------------------------------------------------------------------------------------* For one package of cement equal to 50 Kilograms
Estimated amount of cement form figure 3, noting different kind of aggregate and required workability, can vary about 65.4 kilogram in cubic meter. As mentioned before with increasing the density of Leca aggregate, the strength And modulus of deformation of there aggregates increases. Although (with constant volume ratio of light weight aggregate to ordinary sand) the density of obtained Leca concrete increases, but the strength of Leca concrete with this heavier Leca aggregate increases also. Relation between different weight of Leca aggregate and 28 day compressive strength of Leca concrete cubic specimens made from these aggregate is shown in figure 4 below:
Compressive strength of ordinary concrete, especially with medium and low amount of cement, mostly depends on the strength of its mortar and less on the strength of ordinary aggregates used. On the other hand, compressive strength of Leca concrete mostly depends on the strength of its aggregates and less on the strength of its mortar. Effect of size and shape of Leca concrete specimens on its strength is like those of ordinary concrete specimens. The ratio of compressive strength of specimens mentioned below to the compressive strength of cubic specimen, 20x20x20 centimeter, with the same concrete is as follows:
SPECIMENTS Cylindrical, 15 centimeter Diameter by 30-centimeter height Cubic, 15x15x15 centimeter
COMPRESIVE STRENGTH RATIO 0.8 - 0.85 1.00 - 1.10
# BOND STRENGTH OF REINFORCEMENT TO LECA CONCRETE Bond strength between reinforcement and concrete determines the design of anchorage and effect on crack width and distance in reinforced concrete. This bond strength depends on the following factors:
1- Surface condition of reinforcement (plane or deformed reinforcement) 2- Concrete strength 3- Percent compaction of concrete next to reinforcement 4- Quality of mortar between aggregates 5- Strength of deformation of aggregate under concentrated load on surface of 6- Modulus of deformation of concrete
reinforcement
Lightweight aggregate compare with ordinary aggregate has lower strength under concentrated load. Therefore, lightweight concrete compare with ordinary concrete under lower bond stress on surface of deformed reinforcement is failed and crushed. Although the strength of mortar in both kind of concrete (Light and ordinary) be equal, but the bond strength of deformed reinforcement in light weight concrete is lower than that of ordinary concrete. In other words, if mortar strength in lightweight concrete were higher than that in ordinary concrete, the bond strength of deformed reinforcement in lightweight concrete can be higher than that of ordinary concrete. The strength of coarse aggregate has a small effect on bond strength of plane reinforcement to the concrete, and the most effect is from the mortar strength. Tests of pulling reinforcement from concrete specimens and beams held in building research site in England have shown that the bond strength of plane reinforcement in light weight concrete, usually, is lower than that of ordinary concrete. For Leca concrete this bond strength is nearly equal to that of ordinary concrete (11). Bond strength with adding ordinary sand to light weight concrete mixture gets better. As mentioned, bond strength of reinforcement in lightweight concrete is lower than that in ordinary concrete. According to ACI* code, the design of bond strength of deterred reinforcement to Light weight concrete is lower than that of ordinary concrete by a factor of 1.33.
# TENSILE STRENGTH OF LECA CONCRETE Some mechanical properties of reinforced concrete such as: shear strength, bond strength of reinforcement to concrete and resistance to cracking depends on tensile strength of concrete. Indirect tensile splitting test is used for estimating diagonal tensile strength of structural components. Tests have shown that there is a close relationship between indirect tensile splitting strength and diagonal tensile strength in structural components. Modulus of rupture* - Figure No. 5 below shows the results from study that has been done in America about relation between modulus of rupture and 28 day compressive strength in light weight concrete with different kind of light weight aggregates. Base on the results from this study, one can, on safe side, obtain the modulus of rupture from the relationship given below: M.R. â&#x2030;Ľ 0.06 cu 100 or CU = 28 day compressive strength in psi and for
M.R. â&#x2030;Ľ 0.06 cu 7.031 CU = 28 day compressive strength in Kg/cm2 and for
M.R.= Modulus of rupture for 28 day specimens in psi 28 day specimens in Kg/cm2
CU ≥
1000-psi
CU ≥ 70.3 Kg/cm2
----------------------------------------------------------------------------------------------*American Concrete Institution * Modulus of rupture = tensile strength in bending Deformation of Leca concrete - is done by following factors:
1- External loads (elastic and creep) 2- Hardening and changing the percent of moisture (shrinkage and expansion) 3- Changing temperature Elastic deformation accrues as soon as the external loads are applied. This deformation depends on momentarily deferability of hard cement paste and aggregates proportional with the percent of their volume acceptation in concrete and the magnitude of applied load. Result from uniaxial compression test shows that the variation of stress and strain in Leca concrete (specially with ordinary sand) is like that of ordinary concrete. Maximum compressive strain at the peak load of crushing in both ordinary −6
and Leca concrete is about 2000× 10 , 3500 × 10
−6
# MODULUS OF ELASTICITY OF LECA CONCRETE Modulus of elasticity of concrete depends on the modulus of elasticity of each ingredient in concrete, namely aggregates and hard mortar. Modulus of elasticity relates to the percent of volume acceptation of each ingredient that is aggregates and hard mortar. This relationship also depends on the type of Leca aggregate, its compressive strength and density. Modulus of elasticity of Leca concrete varies between 5000 to 24000 N/mm2 (8). Amount of modulus of elasticity of lightweight concrete is estimated by pauw (17) through the equation given below: Ecj (modulus of deformation) = 0.04
W 3 fcj
In this equation: Ecj is the secant modulus of light weight concrete in the age of j days in N/mm2, W is the bulk density of air dried 28 day specimen in Kg/m3 and fcj is the j day compressive strength of cubic specimen in N/mm2. Pauw’s relationship may give the modulus of elasticity of Leca concrete more or less than what relay is; specially when we design light weight concrete with steel and ordinary concrete as a composite design, since the modulus of elasticity of each composite material is important for determining stresses, the modulus of elasticity of light weight concrete must be determined by test correctly.
# CREEP Creep is the deformation of concrete under constant load as time passes. This deformation is imposed on concrete in addition to the deformation that takes place by: instantaneous deflection as soon as load is applied, shrinkage caused by drying and decreasing ambient temperature. Effect of creep in the case of reducing stresses caused by shrinkage can be useful. But usually, creep for increasing deflection of structural components is harmful and not suitable. In reference No. 8, medium range of creep of lightweight concrete in creep strain for unit of stress has determined equal −5
2
to (6.5 - 9 ý) x 10 per N/ mm . Since the creep deformation is the result of hard cement paste deflection under permanent load, amount of creep deformation increases as the amount of hard cement paste increases. Creep deformation depends on both the quantity and quality of cement paste. Creep deformation decreases as the void in cement paste decreases and strength of cement paste increases.
# SHRINKAGE Amount of shrinkage depends on the following factors:
1- Amount of cement paste in concrete (amount of cement in concrete) 2- Quality of cement paste 3- Kind of aggregate Aggregates prevent the shrinkage of hardened cement paste in different degree. The degree of prevention depends on the modulus of deformation of aggregates. Kind of aggregate also affects the required quality and quantity of cement paste for specific strength. Aggregate with low strength and modulus of deformation for specific strength of concrete needs more cement paste. Not adequate gradation and surface texture of aggregate because of the need of more water and cement paste for specific workability causes an increase in shrinkage of concrete. Shrinkage increases as the amount of cement increases. Final shrinkage for Leca concrete with 300kilogram cement in one cubic meter of concrete has been obtained about 0.49 - 0.65 mm/m (14). Concrete with higher amount of cement, about 650 Kilogram or more in one cubic meter of concrete has shown a final shrinkage about 1.2 mm/m (14). Although, shrinkage in light weight concrete is more than that of ordinary concrete, but because of lower modulus of elasticity of lightweight concrete, the stresses caused by constrained shrinkage in both light weight and ordinary concrete is nearly equal.
# COEFFICIENT OF THERMAL EXPANSION In concrete, thermal expansion mainly depends on: Modulus of deformation of aggregates, the volume ratio of aggregate to hard cement paste, percent moisture in concrete and temperature of concrete. Coefficient of thermal expansion of Leca aggregates is about 50 to 70 percent lower than that of ordinary coarse aggregate (about 12x 10
−6
/K) (8). −6
Coefficient of thermal expansion of lightweight concrete is about 6-11x 10 /K, and equivalent amounts for concrete with river coarse and fine ordinary aggregates is about 9-13x 10 −6
−6
/K, while for
concrete with limestone aggregates is about 6-9x 10 /K (19). Coefficient of thermal expansion is in lowest amount when the concrete is dry or very wet, while in moderate moisture content (about 5-10 volume percent) it is about 20-30 percent more (8).
# ACOUSTICAL PROPERTIES Amount of reduction of sound (in dB) after passing through the different materials depends on the density of each material. Studies of: French, German and British regarding acoustical proof shows that the lightweight concrete made of Leca aggregates with smooth surface performs better than the relation between increasing density and reduction of sound for other materials. For example, one Leca concrete wall with 200-millimeter thickness, which has a density of 1000 Kilogram per cubic meter, acoustically, is equivalent to an ordinary concrete wall with the same thickness and about twice weights, and is equivalent to a brick wall with 330-millimeter thickness and triple weight. Medium acoustical proof of all three kinds of walls mentioned above is about 52 dB (20).
# THERMAL PROPERTIES Thermal conductivity is the amount of heat that passes through the unit thickness of material. Generally, thermal conductivity of concrete depends on its bulk density and amount of moisture. But, relatively, it depends on the position of voids, chemical composition of the solid material (crystals, ceramics “amorphous”, glassy) and temperature. Thermal conductivity increases as the density, moisture and temperature increases. Crystalline material (quartz) conducts heat better than ceramic material. Modern studies emphasize that coefficient of thermal conductivity increases about 2-6 percent for one volume percent of increasing moisture. Amount of coefficient of thermal conductivity of Leca concrete that is obtained in practice is given in table No. 14 below (21).
Experiments in temperature ranging 20 to 60 degree centigrade has shown that with changing temperature, coefficient of thermal conductivity of concrete changes in a very low amount. In temperature higher than 100 degree centigrade, temperature has a high effect on coefficient of thermal conductivity of Leca concrete. For example, between 100 °C to 1100 °C degree centigrade, coefficient of thermal conductivity increases about 0.2
W for every 100 K increase. MK
Table 14: thermal conductivity of Leca with different density and Humidity
Kind of Aggregate
Volume weight Kg/ M 3
Humidity W.P
Coefficient of thermal conductivity
Light expanded clay Aggregate
900 1130 1250 1290 1300 1400 1500 1600 1300 1300
11.8 10.8 10.3 11.2 5.0 5.3 4.8 5.2 9.3 11.7
0.4 0.55 0.64 0.63 0.50 0.66 0.76 0.87 0.64 0.67
Light expanded clay Aggregate Light expanded clay Aggregate Light expanded clay Aggregate L.E.C.A + River sand L.E.C.A + River sand L.E.C.A + River sand L.E.C.A + River sand Light expanded clay Aggregate Light expanded clay Aggregate
# DURABILITY OF LIGHT WEIGHT CONCRETE Durability of lightweight concrete is defined by resisting against: atmosphere, fire, chemical attacks and mechanical deterioration. Light weight concrete of bending and horizontal components like: bridge deck, parking places and etc. that expose to severe climate must resist freezing, thawing and desalting. Vertical components like exposed walls and columns are not affected much by these factors. Freezing and thawing tests on light weight concrete with different lightweight aggregates (23, 24) has shown that using air entrainment admixture in concrete mixture increases durability. When dried aggregates is used in making light weight concrete, light weight concrete with light weight aggregates opposite to ordinary concrete even without using air entrainment admixture is durable in aggressive environment. This is because of the existing pores in lightweight aggregate, which act, similar to pores made by air entrainment admixture in concrete. This effect is because of lightweight aggregates are permeable, and if lightweight aggregates are saturated one cannot count on its better durability. Air entrainment admixture is suggested for lightweight concrete exposed to severe freezing and thawing.
# FIRE RESISTANCE Generally, fire resistance of concrete depends on the following factors:
1- Structural details 2- Heat conduction 3- Heat capacity 4- Concrete resistance to heat
Coefficient of thermal conductivity of lightweight concrete is less than that of ordinary concrete and shows better protection against increasing high temperature; therefore, for heat protection, less thickness of lightweight concrete on reinforcement is required. Experiments about fire resistance of concrete hold in England and Sweden shows that Leca concrete does not crack and peel while ordinary concrete made of quartz aggregates severely cracks and peels (25). Until how new, a reasonable explanation about this different resistance is not given. With strong supposition, peeling concrete from heat depends on the percent of moisture present in the concrete.
# CHEMICAL STABILITY Stability of all kind of concrete, light or ordinary, against chemical substances depends on nature and quality of cement. In aggressive environment, choosing right kind of cement and neutral aggregates is very important. When used aggregates consist of interconnected pores, resistance for absorption and penetration of aggressive material into the concrete decreases. Generally, resistance of both kind of concrete, ordinary and Leca, against aggressive material is nearly equal.