Behavior of High

The aim of this experimental study was to evaluate the effect of fire flame exposure with different intensities of firing on the mechanical properties of a high strength concrete (HSC). Many variables were encountered in this study. Two mixes of 80 MPa target compressive strength (1:1.22:2) with 3% Nano-Metakaolin and the other with 5% were used. Two types of coarse aggregate were used (natural crushed gravel and crushed dolomite rock with maximum size of 14mm). The specimens were moist cured for 28 days, air-dried in the laboratory to firing at ages 90 days at three temperature levels (400, 600, 800) o C and for three exposure periods (1, 1.5 and 2 hours). Different methods were used to cool specimens. The reduction values of compressive strength for all mixes ranged between (2-69.5)% at 400 o C, (8-72)% at 600 o C and (10-85)% at 800 o C , the reduction values of splitting tensile strength were (9-43.2)%, (17-50)% and (39-76)%, and the reduction values of flexural strength were (51.7-84.8)%, (52.8-87) %, (53.6-87.8)%, respectively


INTRODUCTION
he ability of concrete to withstand the damaging effects of the environment and of its service condition without deterioration for a long period of time is referred to as its "durability". Clearly the durability of concrete is of prime importance in engineering applications. (Phan and Carino, 1998). Fire is the remainder one of the serious potential risks to most buildings and structures. Since concrete is extensively used in construction, concrete structure behavior in fire conditions is governed by the properties of the constituent materials, concrete and steel at high temperatures. Both concrete and steel endure considerable change in physical properties; strength and stuffiness by the influence of heating, and some of these changes are not recoverable after subsequent cooling. (Ilker and Cenk, 2008).Thermal damage level depends on the spatial, the size of the structural member and temporal fire conditions such as maximum temperature, exposure time, heating rate and cooling type. (Lee et al., 2008). Concrete is well known for its capability to endure high temperatures owing to its high specific heat and low thermal conductivity. However, it does not mean that high temperature or fire, do not influence concrete at all. High temperature may cause changes of color along with significantly affecting the compressive strength of concrete, concrete density, modulus of elasticity and its appearance. Physical deterioration processes is one of the most important effects on the concrete durability structures by high temperatures.

Results and Discussion
This section includes the results and discussion of the hardened properties of HSC specimens before and after exposure to fire flame at 400, 600 and 800 o C, which include the values of compressive, flexural and splitting tensile strength.

Compressive Strength:
Compressive strength was prepared according to BS1881: part 108: 1983 and tested according to BS 1881: part 116: 1989. Standard cubs of (100)mm and (150)mm were used in this work. The variation in the residual compressive strength values for different mixes before and after exposed to fire flame temperature (400, 600 and 800) o C of cubes and cylinders specimens with different cooling are summarized in Table (

2-
The reduction of concrete specimens after exposed to fire flame for 2 hours was higher than that of (1, 1.5) hours by (25, 14)% respectively at all fire flame temperatures, the cause of that can be attributed to the higher rate of heating and intensity of firing, which formed relatively week hydration products.

3-
The reduction values of dolomite specimens was less than that of gravel concrete specimens at (400 to 600) o C, but these behaviors reflected at (600 to 800) o C. The reason for this behavior can be respected to fire flame in excess (660) o C, calcium and magnesium carbonate begin to break down to Cao, Mgo and CO2. This result in compatible with the study carried out by (John and Ban, 2003.) 4-The percentage residual in compressive strength of G5NMC80 mixes after exposure to fire flame at all temperatures (400, 600 and 800) o C and for all periods of burning (1, 1.5, 2) 862 hours duration, is higher than those of other concrete mixes. The results stated that the nanometakaolin improved the compressive strength under fire flame temperatures, because they are more dense and there is a decrease in the amount and extent of micro cracking in the transition zone.

5-
The decrease in compressive strength of concrete is attributed to the break-down of interfacial bond due to incompatible volume change between cement paste and aggregate during heating and cooling and dehydration of the calcium-silica hydrate in cement paste. This behavior was also confirmed by other (Moreley and Royels, 1983; Hinrichsmeyer, 1987; Umran, 2002; Awad, 2008)

Splitting Tensile Strength:
Splitting tensile strength was determined according to the procedure of (ASTM C496-2004), by two cylinders with dimensions of (100×200)mm. The residual values of splitting tensile strength of all concrete specimens before and after exposure to fire flame are abstracted in Table  (

4-
The reduction in the splitting tensile strength can be attributed to the contraction of the hardened cement paste upon cooling, which caused an increase in the amount and rate of crack formation.

5-
This trend is similar to that obtained by

Flexural Strength:
Flexural strength test (modulus of rupture) (MOR) was conducted according to (ASTM C-78-2002) by using concrete prisms of dimensions (100×100×500)mm. Modulus of rupture was conducted by using (100×100×500)mm prisms, each test was calculated by two prisms. It can be noticed from table (15)

2-
Sharply reduced flexural strength for all concrete mixes after exposed to fire flame higher that (600) o C was seen.

3-
The residual flexural strength of (G5NMC80) and (D5NMC80) concrete mixes is higher than that of other concrete mixes. Because it drastically transforms the microstructural characteristics of the transition zone between cement paste and aggregates. These transition zones are more compact than the relatively porous one usually obtained when containing nonmaterials (nano-metakaolin).

2-
Extra reduction in splitting tensile strength took place for all concrete specimens of HSC, particularly at 800 o C and cooled by water, the percentage of reduction ranged between (9-43.2)%, (17-50)% and (39-76)% for gravel and dolomite mixes at 400, 600 and 800 o C.

3-
The flexural strength were sensitive to fire flame temperatures and caused extra reduction in MoR for all concrete specimens, than other mechanical properties of HSC, cooled by water and 800 o C fire flame exposure temperature caused high rate of decrease in flexural strength.

4-
For all periods of exposure at (400 to 600) o C fire flame temperatures, high strength concrete which contains dolomite as a course aggregate gives more residual modulus of rupture than mixes contains gravel as coarse aggregate by about (5-10)%, but the reduction of dolomite concrete specimens are greater than gravel concrete specimens by about (10-20)% after exposure to fire flame temperature at (600 to 800) o C.

5-
Failure of concrete specimens during the firing process within the temperature range of 400 to 800 o C by addition of (0.5)% polypropylene fibers, splalling of concrete did not occur, however, surface cracking was still observed. While special type of failure occurred, some corners of the cube specimens cracked of after exposed to 800 o C fire flame temperature. 6-For all periods of exposure and 400 to 800 o C fire flame temperature, cooled by extinguisher powder gives higher residual values of mechanical properties, while using cooled by water causes further reduction of mechanical properties of all concrete mixes.