At extreme high temperatures which are generally experienced in fires, hydrated cement in concrete slowly gets dried out, returning to water (in reality, steam) and cement. This leads in a lessening of strength and modulus of flexibility (stiffness) of concrete. In a few fires, spalling of concrete happens – parts of concrete loosen up from whatever remains of the concrete, often bit violently. In this article, we address regarding the makers of precast concrete have with respect to imperviousness to fire and fire ratings of precast concrete items. Most fire resistance rating prerequisites are managed by construction standards, contingent upon the sort of building and its inhabitance.
The ratings are given in hours. For instance, the required fire resistance ratingss for segments in high rising hospitals and clinics are substantially more stringent than those for single-story structures utilized for capacity of noncombustible items or materials. In the high rise hospitals the segments may require a four-hour rating, while in the single story building outside dividers may require just a one-hour rating. In different cases, fire resistance ratings are indicated by proprietors.
For instance, a service organization may require that its vaults have two-hour rating despite the fact that the construction law might be quiet on necessities for such structures.
What products need fire ratings?
A fire rating or we can say “fire resistance rating” which is mostly used in the building codes, has a particular technical meaning. “Fire resistance” is a eloquent term which means the capacity to survive fire and give protection against different forms of fire.
Some specific kinds of precast concrete products like the wall panels, floor slabs and complete small building might need fire ratings based on the assortment of structures and their size, and on some occasions also their locations subjective to other different arrangements or property lines. Other different precast products like risers in game stadiums or section of prison structures might also require fire resistance ratings. Other different products like the manholes, drainages, center barriers, septic tanks and park furniture do not necessarily have to be fire resistant but they have to be noncombustible.
A perk of the concrete is that it is noncombustible. It does not burn and it also does not supports combustion, thus, its usage can be made in all places where noncombustible constructions are permitted.
What products do not need fire ratings but have to be fire resistive?
Precast concrete products which may have to be exposed to recurrent fires or to extreme hot unintentional fires have to be made as fire resistive as possible as it is. For instance, some cities give short manhole segments at beaches so fire used for cooking and camp fires could be made inside the manhole segments by visitors at the beach. Creation of such kind of manholes is high step towards fire security. However, unless they are properly manufactured and conditioned, there are good chances of spall violently when used by the novice users.
Fires also sometime occur in the electrical transformers. Such fires have extremely high temperature– often times they are so hot that they can melt siliceous aggregates. Precast transformer vaults have to be competent of enduring such kind of fires without chances of collapsing.
What does fire rating mean?
According to the 2000 edition of the International Building Code (IBC-2000), “fire resistance rating” refers to “the period of time a building or building component maintains the ability to confine a fire or continues to perform a given structural function or both, as determined by tests prescribed in Section 703.” For roofs, floors, walls, columns, and beams, tests adhered to are the standard fire test, ASTM E119, “Fire Tests of Building Construction Materials.” That standard needs that the sample to be tested be at least a given size, unless the real size is smaller than the minimum given.
In a fire test, the sample should basically support its highest superimposed load as the permission given by the national standards. The sample is then uncovered to a standard fire. Walls are uncovered to fire on one side, floor or roof samples from the underside, and beams also from the base and two sides, and columns from all 4 sides. The fire have to be of a given intensity as permitted by a time-temperature relationship – 1,000 degrees F at five minutes, 1,550 F at 30 minutes, 1,700 F at one hour, 1,850 at two hours and 2,000 F at four hours.
To adhere with ASTM E119, walls should be big enough so that minimum of 100 square feet is uncovered to fire with neither dimension smaller than 9 feet; minimum 180 square feet of a floor or roof have to be uncovered to fire and neither of the dimension have to be lesser than 12 feet. Columns need to have minimum 9 feet in height uncovered to the fire and beams 12 feet in length.
The experiment is carried on until the “end point” is achieved. A “structural” end point happens in the case if the sample collapses by some reasons. A “flame passage” end point happens in roofs, walls, or floors if a hole or crack happens that is substantial enough to permit hot gases from the fire to go to the other side. A “heat transmission” end point happens for walls, floors or roofs if the normal temperature of the surface not subjected to the fire amplifies by 250 F or if the temperature at a given point increases to 325 F. For concrete samples, this latter requirement mostly always governs; that is, the structural or flame passage end points very hardly occurs. There are other different requirements for end points, however, those are not significant for precast concrete products until and unless they are pre-stressed.
Standard fire tests are not very cost effective to execute, and few organizations and building structures are prepared to perform such tests. As a substitute to fire testing, IBC-2000 permits the usage of tabulated data incorporated in the code as explained later below.
What characteristics influence fire resistance?
Fire resistance of concrete is manipulated by aggregate variety, proportion of moisture, density, permeability and density. Different types of naturally found rocks such as limestone, dolomite and limerock are called “carbonate” aggregates due to the fact they have deposits of calcium or magnesium carbonate or blend of these two. When there is exposure to fire, these aggregates calcine – carbon dioxide is driven off and calcium (or magnesium) oxide remains still. Because calcining needs heat, the reactions soak up some of the fire’s heat. The reaction then starts at the fire-exposed surface and gradually moves toward the opposite side. The outcome is that carbonate aggregates’ reaction is improvised than that of the other normal-weight aggregates during a fire.
The proportion of the moisture has a multifaceted power on concrete’s reaction in fire. Concrete which has not been let to dry may spall, specifically if the concrete is greatly impervious, like the concretes made with silica fume or latex, or if it has an exceptionally low water-cement proportion. Concretes which are more porous will normally perform adequately, chiefly if they are incompletely dry.
Basically, the type of concretes with lower unit weights (densities) will react even better in fire; dried lightweight concrete also reacts better in fire than the normal-weight concrete. The thickness and the size of the concrete positively influence its performance during a fire.
How is fire ratings achieved?
As shown earlier, IBC-2000 lets assortments of techniques for attaining fire-resistance ratings. A fire test of a specific building section is an clear method. On the other hand, prescriptive designs as shown in the code might be used, or calculations are carried out in agreement with the measures given in the code are accepted. Even though the “calculations” segment in the code incorporates quite a few formulas, most of the data are listed in easy-to-use form and depends on outcomes of the standard (ASTM E119) fire tests.
For example, Table 1 shows the data from Table 7126.96.36.199 of IBC-2000 for the smallest amount of thickness of cast-in-place or precast walls for different fire resistance ratings. The data are indistinguishable to the minimum thickness of floor slabs shown in Table 7188.8.131.52 owing to the fact that the values are dependent on the heat transmission end requirement.
Table: Minimum Slab Thickness for Fire Resistance Rating:
|Concrete Type||1 hour||1.5 hours||2 hours||3 hours||4 hours|