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Effects of Fire on the Strength of Reinforced Concrete Structural Members

Lateef O. Onundi*, M. Ben Oumarou, Abba M. Alkali

Department of Civil and Water Resources Engineering, University of Maiduguri, Nigeria

Abstract: This multidisciplinary research focuses on the effect of fire on structures due to the Boko Haram insurgency in Maiduguri, Northern Nigeria. It is aimed at giving a further contribution to understand the effect of fire with respect to the local aggregates, quenching methods and proposing an assessment methodology based on a suitable analytical procedure applied to reinforced concrete subjected to sustained fire. Cement, river sand, Bama gravel and water in standard proportion in accordance to BS1881 Part 101 were mixed to produce beams, slabs and cubes of different sizes and cured for 28 days. The samples were burnt in a designed fire simulation furnace using sugarcane bagasse as fuel with varying air velocities for 2 hours. Cooling of samples was carried out using water splashing, CO₂ powder fire extinguisher and air cooling methods before the compressive strength tests using a Seidner Compressive Testing Machine and Non-destructive test with Rebound Hammer. The scanning electron microscopy tests were carried out using a Phenom ProX scanning electron microscope for imaging and analysis to physically examine and determine the elemental micro structural redistribution and influence on the tested samples. As the temperature within the experimental set-up reaches 900°C, the temperature around the vicinity of slab, beam or cube reached an average of 500°C. The results of the investigations show that different cooling methods induce varying levels of deteriorating and significant effects on the final internal micro-structures as well as physical, mechanical and thermal characteristics of concrete. Results from the destructive and non-destructive tests show that losses of strength vary from a minimum of 37.73% to a maximum of 86.67 % in the samples investigated. The local aggregates used namely the river sand and the Bama gravel surely influenced the medium grade reinforced concrete behaviour when subjected to fire.

Keywords: reinforced concrete, fire, structural members, Bama gravel, quenching method, spalling, strength loss

Received November 16, 2018; Revised January 10, 2019; Accepted January 19, 2019

Citation: Lateef O. Onundi, M. Ben Oumarou, Abba M. Alkali, Effects of Fire on the Strength of Reinforced Concrete Structural Members. American Journal of Civil Engineering and Architecture, 7 (1). 1-12. January 2019. doi:10.12691/ajcea-7-1-1

1. Introduction

Fire has been known since ancient times as one of mankind's greatest needs for cooking but could sometimes turnout to be our enemies in a number of adverse cases. During design, the specified minimum cover to structural reinforcements are sometimes inadequate to fully protect individual structural elements of the building exposed to long term high degree of temperature. This may cause the properties and the bearing capacity of materials to become significantly degraded. Therefore it is very important to get acquainted with the behaviour of the different materials used for construction when subjected to high temperature, since it may consequently lead to building collapse if not properly controlled. Fires can be of wild origin, accidental or criminal, set to destroy or cause damage, using various types of fuels ranging from dry bushes, gas, liquid fuels among others. Across the world; from Asia to Europe, Australia, Canada and the USA, records show that; wildfires starting from the 1825 Miramichi fire, new Brunswick which killed between 160 to 300 people to the 2018 California Camp Fire which destroyed more than 12, 786 structures and damaged more than 370 structures; fires have caused severe damages and losses [1].

Reports of collapse of a concrete structure or members of that structure during or after a fire are rare. Where parts of a building have failed structurally, the damage usually occur after a much longer period of exposure to fire than that for which the member was designed to resist. Reasons for the excellent performance record of concrete structures exposed to fire are complex. Consequently, the design of concrete structures for fire has been based on actual field performance, and on results of Standard Fire Tests of individual members of the structure. Both of these methods have shortcomings. Field performance depends upon the interpretation of behaviour by the observer and designer. Furthermore, actual behaviour of a concrete member in a building generally cannot be simulated in a standard fire test. These shortcomings have resulted in conservative designs with many redundant safety factors [2].

The June 14,2017, Grenfell Tower fire that broke out in the 24-storey block of flats in North Kensington, West London just before 1:00 am BST; caused 72 deaths, including those of two victims who later died in hospital. More than 70 others were injured and 223 people escaped. It was the deadliest structural fire in the United Kingdom since the 1988 Piper Alpha disaster and the worst UK residential fire since the Second World War. To date, the fire is still under public inquiry, police investigations and coroner's inquests [3].

The extensive use of concrete as a structural material as well as the resulting damages of the various fires has led to the need to fully understand the effect of fire on concrete. Generally, concrete has excellent properties with regards of fire resistance due to its inherent non-combustibility and poor thermal conductivity compared to other materials, and can be used to protect other structural materials such as steel [4]. During fire, the mechanical characteristics of the concrete are changed due mainly to the intermolecular redistributions, distortions and dislocations. During the cooling process concrete is not able to recover its original characteristics. The assessment of the residual load bearing capacity of reinforced concrete member after fire exposure is a complex task. It includes the estimate of the actual strength of both concrete and steel and the subsequent re-evaluation of the sections’ bearing capacity. However in most cases the knowledge of materials strength, even after accurate and extensive testing, is not sufficient for the purpose. In fact, in-situ testing cannot be extended over all the sections depth, while every single point of the sections contributes to the overall bearing capacity with a strength that depends on the local maximum temperature reached during the fire.

Wong et al., [5] using the Garley Building Fire of November 20, 1996-Hong Kong, presented a fire resistance of RC structure codified approach. Structure to possess an appropriate degree of resistance to: flame penetration (integrity performance), heat transmission (insulation performance), and collapse (integrity performance).

According to Zhaohui et al., [6], in the past two decades, a significant amount of research has been conducted into the performance of composite steel-framed structures in fire. However, the same level of development has not taken place for other forms of construction. In terms of reinforced concrete construction, design is still based on simplistic methods which have been developed from standard fire tests that do not necessarily represent real building behaviour. This makes it very difficult, if not impossible, to determine the level of safety achieved in real concrete structures, or whether an appropriate level of safety could be achieved more efficiently.

Adam [7] wanted to increase the awareness of the structural engineering field to the concepts behind structural design for fire safety. In the United States design for fire safety follows a prescriptive code-based approach. Building codes detail the types of construction materials, assemblies, and fire suppression systems that are required for various building types. This prescriptive method has prevented structural engineers from exposure to performance-based design approaches for fire safety. Extensive research has been published on the performance of structural steel in fire conditions, and simplified design tools already exist to describe its behaviour. Such tools do not exist for reinforced concrete structures. Research on concrete has been more focused on material properties rather than structural performance. The author presented a simplified design tool which assesses the fire performance of reinforced concrete. An Excel-based spread sheet application was developed for thermal analysis of concrete slabs. It accounts for different aggregate types, slab thicknesses, and fire exposures. Several analyses were performed with the spreadsheet application to examine the affect slab thickness and aggregate types have on the fire performance of concrete slabs in standard and natural fires.

Ian et al., [8] presented a “state of the art” review on current research into the effects of fire exposures upon concrete. The principal influences of high temperature in concrete are loss of compressive strength and spalling, the forcible ejection of material from a member. Though a lot of information has been gathered on both phenomena, there remains a need for a broader understanding of the response of concrete structures to different heating regimes and the performance of complete concrete structures subjected to realistic fire exposures. There is a lack of information derived from large-scale tests on concrete buildings in natural fires. Besides undertaking further fire tests, lessons can also be learnt from real fires and the University of Edinburgh has embarked upon detailed studies of the serious fire in the Windsor Tower, Madrid. In order to properly characterize the fire and the performance of the structure a data-gathering exercise has been undertaken and computer modelling tools are being applied in order to obtain better insights into the structural response. There remains some uncertainty about the precise mechanism of fire spread, but an external route is likely, facilitated to some degree by the glazed curtain walling construction; lack of fire protection on the steelwork was the major reason for the subsequent partial collapse of the upper floors and the localized failure of a concrete portal frame can be attributed to the same reason.

Khoury [9] studied the compressive strength as a property of concrete in fire. It varies not only from concrete to concrete depending on its constituents and other factors such as external loading, heating and moisture conditions. During heating concrete also experiences thermal strain, shrinkage, as well as load induced thermal strain (LITS). LITS comprises several components such as transient creep. LITS acts to relieve thermal and parasitic stresses. Rapid heating during fire could induce explosive spalling with serious consequences to structure and people. The two mechanisms of explosive spalling are thermal stress spalling and pore pressure spalling. Thermal stress spalling could be reduced by the use of thermal stable aggregates of low expansion, while pore pressure spalling could be reduced by the use of polypropylene fibres in the mix.

After the 9-11 attack on the World Trade Center New York- USA, interest in the design of structures for fire greatly increased. Some engineers have promoted the use of advanced analytical models to determine fire growth within a compartment and have used finite element models of structural components to determine temperatures within a component by heat transfer analysis. Following the calculation of temperatures, the mechanical properties at various times during the period of the fire must be determined. David and Mahmoud [10] provided structural engineers with a summary of the complex behavior of structures in fire and the simplified techniques which have been used successfully for many years to design concrete structures to resist the effects of severe fires.

Helder [11] is of the opinion that structural stability of reinforced concrete buildings exposed to fire is gaining a significant role in the design process, as users and authorities are increasingly demanding for fire safety solutions. And the application of advanced calculation methods constitutes the most accurate numerical procedure to trace the global behaviour of a reinforced concrete structure during the course of the fire, until collapse is achieved. In his work, the author considered a reinforced concrete frame subjected to different fire scenarios. He applied a Finite Element Code capable of simulating both materials strength deterioration due to temperature and fire induced effects. It is known that the application of advanced calculation methods demands a great deal of expertise, usually not part of structural engineers training. For this reason, fire safety assessment is often performed by means of simplified cross-sectional calculation methods based in single element verifications, where in accordance to the current version of Eurocode 2, it is possible to neglect fire induced effects.

Concrete has an excellent intrinsic behaviour when exposed to fire, especially when compared to other building materials [12]. However, its fire resistance should not be taken for granted and a proper structural fire design is certainly necessary. This design is based on the understanding of both the material and the structural behaviour of concrete exposed to fire. A number of complex physicochemical reactions occur when concrete is heated, causing mechanical properties as strength and stiffness to deteriorate. Furthermore, the phenomenon of spalling causes pieces of concrete to break off from the surface, reducing the cross-section of an element and possibly exposing the reinforcing to the high temperatures. Spalling can be highly dangerous and is most common in high strength concrete. However, its mechanism is still not fully understood. The Eurocode provides a number of procedures in order to design concrete structures for the fire situation, both prescriptive as performance based. However, of the latter, only the basic principles are given and several gaps still need to be filled through research. Thus in practical design, either tabulated data or a simplified calculation method is used. In many cases, these design methods fail to predict the true behaviour of concrete structures in real fires. Several elements of the behaviour of heated concrete still need to be researched [13,14,15,16,17]. A systematic study of the effects of realistic thermal exposures is needed and a lot more work is required in order to unravel the mystery of spalling. The study of the response of complete concrete structures presents another challenge, requiring large-scale fire tests.

Yakudima et al., [18] studied the impact of fire on steel reinforcement in reinforce concrete structures at elevated temperature that analyzed by means of a three dimensional (3D) nonlinear transient thermo-mechanical finite element (FE) analysis and validated with commercially software ANSYS and SAFIR. The behaviour of RC beam and column members at elevated temperatures are being studied experimentally and analytically widely. However, hardly any attention is given towards analyzing the behaviour of structures with SFRC, e.g. SFRC beams, columns, portal frame etc. exposed to fire. Hedayati et al., [19] reported the latest findings on fire spalling behaviour of concrete members and showed the deficiencies of the current experimental work and knowledge. Thermal instability “spalling” occurs when concrete is exposed to fire. This phenomenon, which happens early after a fire starts (about twenty minutes), is one of the most detrimental effects causing damage to concrete members. It can trigger an immediate degradation of concrete, exposure of steel reinforcements to high temperatures and then eventually can cause failure of the concrete members during or after a fire by decreasing the residual mechanical properties and durability of the structure. Kigha et al., [20] investigated reinforced concrete structures subjected to fire of various degrees of heat. After fire, if the structure does not collapse during the fire there is need for post-fire assessment of its structural integrity before the fate of the structure can be determined. With the knowledge of the temperature of the fire, thickness of concrete cover, residual strength of concrete and tensile strength of embedded reinforcement after fire exposure, we can predict the residual carrying capacity of the beams after fire. The experimental procedure involved some specimens of reinforcing steel bars (ϕ16mm) enclosed in varying concrete covers in concrete beams which were exposed to ISO 834 furnace temperatures for 2hrs. After fire, the steel bars were removed and tested for tensile strength characteristics and the reduction in strength trend compared with the current code predictions for stress strain relationship of hot-rolled reinforcing steels at elevated temperatures.

Rahul and Jayesh [21] studied the behaviour of concrete as an advanced mixture of various materials, and determined that its behaviour depends on its combine proportions and constituents when it's subjected to elevated temperatures. Principal effects of elevated temperatures are loss in compressive strength, loss in weight or mass, modification in color and spall of concrete. They presented the experimental results of traditional concrete and high strength concrete subjected elevated temperatures at 100°C, 300°C, 600°C, and 900°C and totally different cooling regimes viz. air cooling, water quenching on different grade of concrete.

Flavio et al., [22] assessed the decay in the mechanical characteristics of fire-exposed Reinforced Concrete (RC), it is crucial to reconstruct the temperature time history and the evolution of strain and stress fields. In this paper, the state of the art of assessment methods is presented and applied to a real structure damaged by fire. It is a prestressed RC industrial warehouse located in the outskirts of the city of Cagliari (Italy). The collected data of several assessment methods are presented in order to produce the flowchart of an integrated approach for post-fire investigation. Among the various techniques, the authors highlight a thorough laser scanner geometric survey and destructive and non-destructive testing. In addition, the temperature distribution and its time history has been reconstructed by means of optical and Scanning Electron Microscopy, X-ray diffractometry, thermogravimetric differential thermo-Analysis and calibrated colorimetry. Actually, refurbishment was needed, but the structure withstood the fire very well. Central columns displayed the most important damage, and several beams presented important deflections having lost the prestressing actions of the tendons.

During the recent Boko Haram insurgency in the North Eastern Nigeria, in Maiduguri, the state capital of Borno located on latitude 11.85° North and longitude 13.08° East, the veil was uncovered on the extent of disasters on constructions. It was noted the large scale of destruction resulting from fires which were either from direct Bomb blast, or through flammable materials and burning and flammable (e.g. gasoline) in the residential, institutional and industrial compounds. This multidisciplinary research paper investigates the effect of fire on structures, the impacts of sugarcane bagasse as a fuel, the effect of cooling/quenching method, mix proportions as well as curing time. It is aimed at giving a further contribution to recognize and understand better this problem with respect to the quenching method, proposing an assessment methodology based on a suitable analytical procedure which will be applied to a model reinforced concrete slab, beam concrete and cubes subjected to fire. The information obtained would be a useful reference in predicting the behaviour of concrete structures exposed to fire.

2. Materials and Methods

2.1. Preparation of Concrete Samples

Cement, Fine aggregates (river sand), Coarse Aggregate (Bama gravel) and Water with proportion 1:2:4 (Cement: Sand: Gravel) were mixed thoroughly on plane non porous surface, to produce beams, slabs and cubes.

The reinforced concrete (RC) beams were 8 samples (230 x 230 x 800 mm), 8 RC slabs (300 x 600 x100 mm thick) and 8 mass concrete (150 x 150 x 150mm) cubes cured for 28 days.

Coarse aggregates used in this work were Local natural gravel (Bama gravel), tested in accordance with the recommendations of BS 882, 1992. The coarse aggregates used constituted minimum aggregates in accordance with BS410. Fine aggregates were natural river sand determined in accordance with BS 812-103.1 [23]. Workability was controlled within the slump and compacting factor followed limits in accordance with BS1881 Part 101 [24].

As a result of the variability of concrete in production, it was necessary to design the mix to have a mean strength greater than the specified characteristic strength by an amount termed the margin. Thus:

[picture] | (1)

Where

fm = the target mean strength

fc= the specified characteristic strength

ks = the margin, which is the product of:

s = the standard deviation, and

k = 1.64 (a constant)

2.2. Fire Simulation Materials/Equipment

The materials used for the fire simulation are: Rice husk, groundnut shell and sugarcane bagasse, as fuel while the equipment used for these tests were: a blower [25], an existing incinerator, one (1) digital tachometer DT-2234 B photo type, 0.1 rpm-5-999 rpm, 1 rpm-1,000-99,999 rpm; one (1) Digital Anemometer CT LUTRON SP-8001, one (1) digital stop watch SUNWAY S1-1025, two (2) RKC Rex- C700 digital thermocouples with ranges of 0-1100°C. The blower has a 0.75 hp ATLAS motor mounted and being powered by a 950 Tiger generator. The Design Expert software version 7.0 was used to analyze and interpret the experimental results.

2.3. Fire Simulation Methods

Tests were carried out in the Faculty of Engineering, University of Maiduguri under actual working conditions of the blower according to a set up shown in Figure 1 [26]. A constant mass of 20kg of the agricultural wastes, each was measured for the variables and fed into the incinerator. The blower was operated with rotational speeds and air velocities of 3203 rpm, 3111rpm, 3078 rpm and 24.5 m/s, 23.8m/s, 21.3m/s using a tachometer and anemometer respectively. This speed variation is made with the aim of simulating the actual fire conditions. The ambient temperature was ranging from 29°C to 33°C while the temperatures at points 1 (i.e. below the grate) and 2 (i.e. above the grate) were recorded using thermocouples at intervals of 300 seconds using a digital stop watch. The fire was sustained for 2 hours without interruption. Note should be taken that the sugarcane bagasse was serving primarily as fuel and secondly as ash provider for an eventual replacement material for Portland cement in concretes.

The experimental factor considered in this work were agricultural feedstock at three levels (3) rice husk, sugarcane bagasse, and groundnut shell and operated at different blower speed with an air velocity varied at three levels each (3203 rpm, 3111 rpm, and 3078 rpm) and (24.5m/s, 23.8m/s and 21.3m/s) respectively. These variations were to check the influence of temperature on the ash quality of the agricultural wastes. A time interval of 300 seconds was considered while the experimental factor was varied at nine (9) levels. The obtained data were laid in Randomized Complete Design (RCD) and was replicated three times making a total of (3×3×3×9 = 243) experimental treatment [26].

Figure 1. Sectional view of the experimental fire simulation furnace with accessories

2.4. Cooling of Concrete Samples

The burning /cooling involved 8 reinforced concrete (RC) beams (230 x 230 x 800 mm) samples, 8 RC slabs (300 x 600 x100 mm) and 8 mass concrete (150 x 150 x 150mm) cubes out of which, two each were control specimen, two were air cooled, two were quenched with water and two cooled with Carbon Dioxide fire extinguishers found in households BS EN FIRE TEST RATING 55B, Model TCS- 5kg. The concrete class was designed as class 25 concrete reinforced with Y10 and Y8 steel bars in beams and slabs respectively. Six of the samples were subjected to fire in a special fire simulation furnace (Figure 1) with three different cooling methods whereas the remaining two were used as control.

2.5. Compressive Test of Concrete Samples

The compressive strength was tested at normal room temperature condition for each of the samples using a SEIDNER Compressive Testing Machine and Non-destructive test (rebound hammer test) then comparing these specimens which were cooled with the earlier mentioned different methods. The calculations for the Compressive and Flexural strengths were carried out using the appropriate formulae [27,28,29,30] for the cubes, slabs and beams respectively:

[picture] | (2)

[picture] | (3)

Where:

σ: is the Compressive Strength or Flexural Strength depending on the nature of the member.

P: point load, N

L: effective span of the beam, mm.

b: width of the beam, mm.

h: height of the beam, mm.

A: surface area of the cubic member, mm².

2.6. Scanning Electron Microscopy (SEM)

The slab sample was first stub and a double adhesive was placed on it, thereafter water was sprinkled on it, then taken to a sputter coated (Quorum-Q150R plus E) with 5mm of gold. The slab sample was then placed on a charge reduction sample holder and introduced into the column of the SEM machine (Phenom ProX) where it was viewed from a NavCam before it was sent to SEM mode. Different magnifications were stored in a USB stick after adjustment of brightness and contrast.

The PhenomProX scanning electron microscope (SEM) used for this work is based on the 5th generation Phenom desktop SEM platform and is a high-performance SEM for imaging and analysis, in ABU( Ahmadu Bello University), Zaria. With the PhenomProX desktop SEM, the slab sample (both control and burnt) structures were physically examined and their elemental composition determined. Besides point analysis, the optional Elemental Mapping and Line Scan software allowed further analysis of the distribution of elements.

3. Results and Discussion

Concrete when subjected to fire showed good behavior in general. The low thermal conductivity of the concrete associated with its great capacity of thermal insulation of the steel bars is responsible for this good behavior. However a phenomenon such as the concrete spalling and cracking may compromise the fire behavior of the elements.

Visual examination as the simplest and cheapest technique allowed the detection of cracks on both the burnt beams and slabs. These cracks detection are very interesting data regarding both the residual structural safety and mechanical characteristics after a fire. The visual examination provided a rapid survey of the damage caused by the fire as cracks, discoloration due to temperature variation, material compaction and spalling. The result of the investigation shows that the lighter or higher ash coloured appearance the higher the intensity of fire harassment on the concrete. This is the reason why higher degree of spalling and cracks are more visible within such zones. Whereas the areas with darker appearance suffered less fire intensity but they only accumulated flame and smoke deposition at the surface with relatively lesser degree of temperature penetration. In fire damaged structures assessing, these colours changes can help to trace or explain the sources of fire in a building. This proved more serious at generating spalling and cracks at the beam edges (Plate 1).

Plate 1. Discoloration, cracks and spalling on beam edges

Spalling can have insignificant or detrimental effects on the fire resistance of a reinforced concrete member depending on the type of spalling that occurs. Though most spalling that occurs from exposure to fire causes only superficial damage to a concrete member, the worst types of spalling cause the ejection of a large area of concrete from the exposed surface. This has the effect of reducing the protective cover or thickness of the reinforced concrete that was assumed in design calculations, which can lead to the premature insulation or structural failure of the member since the parameters of the reinforcement might be critically affected. The different types of spalling that have been observed during fire endurance testing have been grouped into categories based on the location, nature, and severity of the spalling [31].

i. Explosive spalling is the ejection of large pieces of concrete from the surface of a member due to high pore pressures caused by the production of steam within the concrete.

ii. Surface spalling includes pitting, blistering and local removal of surface material

iii. Aggregate splitting is failure of the aggregate near the surface and is often accompanied by surface splitting.

iv. Sloughing off occurs when the surface layer or corner of a concrete member is gradually eroded due to extended exposure to fire.

There are several factors that have been noted to influence the occurrence and scale of spalling. Moisture content of the concrete, Compressive stress caused by restraint of thermal expansion or external load, Aggregate type, Rapid temperature rise at exposed surface and concrete density and permeability, are known to induce spalling.

From these five factors it is generally accepted that the first three are the most influential to spalling, though there is a much higher occurrence of explosive spalling in high strength concrete containing silica fume. This is due to the extremely low permeability of concrete containing silica fume. To reduce the susceptibility of concrete to explosive spalling polypropylene fibres can be added to the concrete. Plates (1, 2, 3 and 4) show the discoloration as result to fire exposure, cracks due to high temperature and spalling caused by temperatures changes, aggregates types and different types of quenching/ cooling methods.

Plate 2. Discoloration, cracks and spalling on the edge of the beam

Plate 3. Full blown cracks on the Slab

Plate 4. Discoloration and spalling on cube as a result of exposure to fire

The rise in temperature affects the characteristics of both the concrete and the steel reinforcement. However, the rate at which the strength and modulus of elasticity decrease depends on the rate of increase in the temperature, duration of the fire and the insulating properties of concrete. Figure 2 and Figure 3 show the temperature variation within the sugarcane bagasse fired experimental set-up. As the velocity of the air increases, so does the temperature within the simulator. In real fire situations, fire is influenced by blowing winds, opening and closing of passages such as doors, windows and vents which supply the air necessary, and the increase or reduction in the fuel quantity sustaining the fire. The blowing winds are here represented by the air velocity changes from one value to the other while the fuel is represented by the content of the sugarcane bagasse. While Figure 2 shows the thermal changes at the fuel’s level, Figure 3 shows how the events at the fuel’s level impact the structures which are represented by the beams, slabs and cubes in this study.

One of the important requirements is that a structure should resist collapse during fire. As the temperature within the experimental set-up reaches 900°C (Figure 2), the temperature within the slab, beam or cube reaches the vicinity of 500°C (Figure 3). The strength – temperature characteristics of concrete will greatly influence its resistance to collapse. All concrete’s loose strength at elevated temperature, but the rate of reduction differs with the type of aggregate used. With the rise in temperature, the aggregates expand; the expansion of the matrix takes place. The resultant expansion differential causes internal cracking in the concrete and reduction in its stiffness. This phenomenon differs considerably with the types of aggregates. This phenomenon is most pronounced in case of concrete with siliceous aggregates, which at very high temperature also undergoes physical changes accompanied by a sudden expansion in volume, thus sometimes causing aggregates splitting and /or spalling.

Light-weight aggregates normally undergo heating process during manufacture and possess superior insulation properties. The physical compatibility between matrix and the aggregate with regard to deformability and expansion characteristics are also much better in light-weight concrete than in dense concrete and as a result much less damage and internal stresses are expected in light-weight concrete during heating. It can also withstand cooling shock much better than gravel concrete. Calcareous aggregates do not normally undergo physical changes during heating and are usually free from cracks and local damage. But at exceptionally high temperatures some chemical changes take place. During cooling, the expansion in volume causes cracks and damage [32,33].

Figure 2. Temperature, Time and Velocity relationship under the grate within the Fire simulator

Figure 3. Temperature, Time and Velocity relationship above the grate within the Fire simulator

Figure 4. Influences of Quenching and Destructive Methods on Concrete Beams and Slabs Subjected Fire with Temperature Variation of 450 to 900°C for 7200 Seconds

Figure 5. Influences of Quenching and Destructive Methods on Concrete Cubes Subjected Fire with Temperature Variation of 450 to 900°C for 7200 Seconds

These are seen in Figure 4 and Figure 5 where the cooling method; either by using the CO₂ fire extinguisher, water splashing or air cooling, has a serious effect on the final internal characteristics as well as physical and thermal characteristics. The destructive and non-destructive tests show the extent to which loss in strength occurs. These losses change from a minimum of 43.7% to 48.7% for beams, and a minimum of 85% to a maximum of 86.67 % in RC slabs. In cubes, the loss in strength varies from 37.73% to 56.04% for water cooling and air cooling respectively.

The local aggregates used namely the river sand and the Bama gravel influence the reinforced concrete behavior when in fire Plates (5, 6, 7 and 8). Several studies of concrete heated to high temperatures indicate that the type of aggregate employed is critical [34,35]. Nevertheless no standard specifications have been developed to define the aggregate properties desired for high temperature exposure. It seems obvious that aggregates with low coefficients of thermal expansion in the range of temperatures that the concrete is expected to experience would be preferable to those with high coefficients. A sizeable amount of expansion of the aggregates within the confines of a shrunken hardened cement paste would result in a disruption of the concrete mass. The conductivity of the aggregates employed will also be an important factor in determining the stability of the concrete. A comparison between Plate 6 (CO₂ cooling) and Plate 5 shows a clear difference. Some chemical components (Cement Slurry) of the slab materials are swollen or coagulated while non-uniformity in the blend is observed because of the fineness of the aggregates and the concrete behaving as a molded mortar in a 100 mm slab. This low slab thickness enhanced greater penetration of the fire intensity to expose the improper mix ability of the constituent materials during batching, hence leading to weak flexural strength of the concrete solid slab. The matrix is highly porous and contains a relatively large amount of free water unless artificially dried. When exposed to high temperatures, concrete undergoes changes in its chemical composition, physical structure and water content [36]. These changes occur primarily in the hardened cement paste in unsealed conditions.

Plate 5. SEM of Slab control at 50 µm Resolution

Plate 6. SEM of Slab cooled with CO₂fire extinguisher at 100 µm Resolution

Plate 7. SEM of Slab cooling with water splashing at 80 µm Resolution

Plate 8. SEM of Slab Cooling by Exposure to Air at 200 µm Resolution

Predominantly limestone (carbonate aggregate) provides higher fire resistance and better spalling resistance than that of siliceous aggregate predominantly quartz. This is mainly because carbonate aggregate possesses substantially higher heat capacity (specific heat), and this is beneficial for mitigating spalling and also increasing fire resistance. This increase in specific heat is caused by an endothermic reaction occurring around 600-750°C due to dissociation of dolomite in carbonate aggregate concrete. This endothermic reaction absorbs energy supplied by fire and enhances the specific heat of concrete in that temperature range. Generally speaking, aggregates that contain a comparatively high proportion of silica exhibit a higher coefficient of thermal expansion. Therefore they should be avoided in concrete which is to be exposed to high temperatures. Plate 7 (cooling by splashing water) shows a crystal-like structure caused without doubt by the sudden temperature change which can only be caused by water. This is probably explained by its glass like composition and high porosity. This type, therefore, retains a comparatively high amount of moisture, which is beneficial in dissipating heat within the concrete mass. Certainly, retention of water can be carried to an extreme wherein the generation of steam would be detrimental. The aggregate/ cement ratio appears to have an important effect on concrete strength exposed to high temperatures, the proportional reduction in strength being less for a lean mix than for a rich one. Concrete is known to be a composite material that consists mainly of mineral aggregates bound by a matrix of hydrated cement paste [37]. Such changes are reflected by changes in the physical and mechanical properties of concrete that are associated with temperature increase and sudden changes [38,39]. On the other hand, the sample being cooled by air (Plate 8) seems to be much more uniform, if not for the presence of visible cracks and pores which are visible on all SEM pictures but with different sizes.

4. Conclusion

1. The fact that structures are still standing because they have not burnt to ashes is not to say that they are structurally stable and safe.

2. The results from the destructive and non-destructive tests show that losses of concrete strength vary from a minimum of 37.73% to a maximum of 86.67 % in the samples investigated.

3. The local river sand and the Bama gravel aggregates significantly influenced the reinforced concrete behavior when in fire.

4. The quenching methods used in cooling the structure during and after the fire has a serious impact on the internal morphology of the medium grade concrete of the structural elements.

5. The results obtained from this study show that the aggregates gradation, mix proportion and the cooling methods are seen as good interacting parameters to consider in the prevention of the negative impacts of fire on a structure.

6. The information obtained from this experimental investigation would be useful reference in predicting the behaviour of concrete structures exposed to fire.

Acknowledgements

The authors wish to thank the Management of University of Maiduguri and TETFund for providing resources for funding this research work with grant reference No. DESS/UNIMAID/MAIDUGURI/RP/VOL.V dated February 13, 2017; the staff of the ABU, Zaria soil laboratories and the staff of Ramat Polytechnic, Maiduguri, for their support and cooperation while conducting this research work.

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