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Cracking and spalling of concrete were also observed inaddition to loss of adherence between the aggregates and cementpaste [12–17]. The factors that are thought to have an influenceon the severity of spalling in concrete [18] were mainly: com-pressive strength, moisture content, density, heating rate as wellas specimens dimensions and shapes. Spalling during a fire isconsidered very crucial in concrete structures as it exposes deeperlayers of concrete to fire temperatures and thereby increasing therate of transmission of heat to the inner layers of the structuralmembers and to the reinforcement. Furthermore, color-change wasalso reported in concrete structures exposed to high temperature,and was found to depend on the exposure temperature [7].The type of aggregate was also found to have a significantinfluence on the fire performance of concrete [19]. A lowerstrength reduction was found in concrete made with aggregatesthat do not contain silica, such as limestone, basic igneous rocks,crushed bricks, and blast furnace slab. On the other hand,dolomitic limestone is found to be beneficial in improving thefire performance of concrete because the calcinations processabsorbs heat, and the lower density ‘‘calcined’’ material providesa greater insulating effect [19].The changes in the mechanical properties of concrete withhigh temperature were also related to the rate of heating. Whenconcrete is heated between 100 and 200 1C, free water evaporatesslowly and no structural damage is observed. However, rapidheating rate results in higher vapor pressure and causes cracks inconcrete. Concrete starts to undergo loss in compressive strengthwhen heated between 200 and 250 1C [20]. At temperaturesbetween 300 and 500 1C, the compressive strength of concreteis reduced to about 15-to-70% of that of non-heated concrete [20].When heated, the water trapped in concrete starts to evaporate at300 1C, thus causing dehydration of the chemical compound CSH(calcium silicate hydrate) which is responsible for bonding thedifferent concrete constituents together [20]. The dehydration ofCSH crystals results in a decrease in strength of concrete, aprocess that is not reversible. At 530 1C, Ca(OH)2 turns into CaOresulting in a 33% shrinkage in volume [7].In summary, the changes in the mechanical properties ofordinary and high performance concrete with temperaturesdepend on several parameters such as the chemical and physicalproperties of the concrete constituents (cement, aggregate andadditive), the temperature to which the concrete structure isexposed to, the size of the concrete structure as well as theexternal applied loadings and cooling conditions to which thestructural member is subjected to. For this reason, it is verydifficult to quantify direct relationships between the temperatureincrease and decrease in the mechanical properties of concrete. Infact quantifying such relationships would be very useful forpracticing engineers in order to assess the strength of existingbuilding when exposed to fire.Malhotra [4] studied many different parameters on the com-pressive strength of concrete after different temperature expo-sure. It was concluded that, residual strength of the concrete athigh temperatures influenced by the aggregate/cement ratio.Although the lower cement content mixes have a lower intrinsicstrength than higher cement content mixes, they undergo asmaller proportional reduction in strength when heated to anygiven temperature. However, reduction in the compressivestrength of the concrete versus temperature relation is given onlydependent on aggregate type in both BS EN 1992-1-2:2004 [21]and ACI 216.1 standarts. None of the standard considers the effectof cement dosage on the reduction of compressive strength withtemperature. Previously found cement dosage–residual strengthrelations are not shown in the standards. On the other hand, it iswell known that the cement dosage, in a concrete mix, affects themechanical propertieties of hardened concrete and increases thedurability under ambient temperature. Therefore, one can expectthat the residual strength of the higher dosage concrete after hightemperature exposure is higher than lower cement as in the caseof Malhotra’s study. Omitting of this previous research results andcommon expectations bring out an important question. The mainobjective of this research is to clarify this gap with isolatingcement dosage and examining its influence on concrete after hightemperature. In this research, the results of total 432 compressiveand flexural tests were presented with the aim of contributing tothe experimental database and to better understand how thecement dosage in a concrete mix affects the relative residualstrength of concrete subjected to high tempreture. The concretemixes were grouped into two series in this experimental study:Series-I and Series-II with cement dosage of 250 and 350 kg/m3,respectively. The changes in the concrete mechanical propertiesof the two test series under elevated temperatures were analyzedby conducting velocity of ultrasonic transmission tests, as well ascompressive and flexural strength tests.2. Experimental studiesIn this research, two concrete test series; namely Series-I andSeries-II; were cast and prepared in the laboratory. Each testseries consisted of thirty six 150 150 150 mm cubes and thirtysix 100 100 300 mm prismatic beams. The concrete speci-mens were tested to failure to study the variation of the residualcompressive strength, residual flexural strength and velocity ofultrasonic transmission with temperatures and cement dosages.Thus, the main variables in this experimental investigation werethe cement dosages and the temperatures to which the specimenswere subjected to. For Series-I a cement dosage of 250 kg/m3wasused while for Series-II the cement dosage was set at 350 kg/m3.The designations, proportions, and some properties of the con-crete mixtures for the two test series are given in Table 1. Eachtest series consisted of six groups which subjected to six differenttemperatures of 20, 100, 200, 400, 600 and 800 1C. The samplestested at 20 1C were considered as control specimens.Commercial ordinary Portland cement type CEM I 42.5R,conforming to European Standards EN-197/1 [22], was used whenpreparing the concrete specimens. The cement used had a fine-ness of 4375 cm2/g. The targeted 28-days-compressive concretestrength was set at 48.5 MPa. Four different types of limestoneaggregates were used when casting each of the test specimens,namely: crushed stone III (12 to 22 mm), crushed stone II (6 to12 mm), crushed sand stone (0 to 6 mm) and river sand (0 to 4 mm).