To bridge something means to connect the two sides or parts. This can be done in building with a structure of wood, stone, block work, steel or concrete. The essential thing is that the door and window openings must be safely bridged so that the walls above or other members of the structure cannot collapse and damage the house or the people inside.
METHODS OF BRIDGING
There are various ways of bridging an opening. Which one of these is used depends on the distance to be bridged, the shape of the opening and the materials available.
One method which was common in former times was to make the openings so small that a single stone could be laid across them (Fig. 1). Openings which are low enough can also be bridged by inserting supporting blocks arranged like a "V" (Figs. 2, 3, & 4). This method is still common in the dry areas of Africa.
The wider an opening becomes, the more difficult it is to bridge. Builders from many parts of the world eventually learned that if the blockwork remains closed above an opening, both sides of the wall will support each other, maintaining stability. This knowledge was often applied not only to bridge openings but also to construct roofs. The technique was to let each course overlap the one below until the blockwork met at the top and the two sides bonded into each other (Fig. 5).
From this simple method, arches were the next development. They are much stronger and have a more attractive appearance (Fig. 6).
Lintels were introduced in order to produce a square opening into which a frame could be fitted.
BUILT-UP WOODEN LINTEL
A lintel, whatever material it is made from, is a horizontal member of the structure which bridges an opening (Fig. 1).
Its function is to distribute the weight of the blockwork above and any other loads to the supporting walls. With a lintel the opening of the door or window can be lower than with an arch, and it is also easier to fit a frame into the opening.
For short-span openings such as doors and smaller windows which have no additional loads above, the wooden built-up lintel can be employed.
This lintel consists of two or more hardwood boards which are nailed, bolted or screwed together (Fig. 1). in order to save materials when a wider lintel is required, the boards are sometimes cunnected with spacers between them (Fig. 2).
For long-span oponings and in situations where additional Was are present, arches or reinforced lintels are used.
REINFORCED CONCRETE LINTEL
Like the human bady which is strengthened by bones, reinforced concrete is made stronger by the steel bars or metal netting embedded in it.
It is known that concrete alone can withstand enormous pressures, but if it is exposed to tensile stresses it will break. Fig. 3 shows that a long board supported only on its ends will bend if weight is set on it. Similarly, a pure concrete lintel will try to bend under a heavy weight, but because the concrete is not flexible this will result in cracks forming across the soffit face, or even in collapse.
To prevent this, reinforcement bars are embedded in the concrete if the lintel is expected to receive tensile stresses. The combination of concrete and steel does the job where one of them alone would not work: the concrete resists all pressure while the embedded steel resists all stresses.
The reinforcement for a lintel consists of several members with different diameters, shapes and functions. The members are often assembled beforehand in the form of so-called cages (see Fig. 1, below).
A typical reinforcement cage is shown in Fig. 1. The lower bars are the main bars. They are normally 12 mm in diameter because they do the actual strengthening of the lintel. The upper bars are thinner and are called erection bars. Since their main function is to hold the cage together, their diameter is only 6 mm. The square-shaped pieces are called the stirrups. They hold the main bars and the erection bars in position. All the different members are bound together with binding wire.
All reinforcement bars have to be carefully measured and marked off before they are cut. A well equipped building site will have two different sized bolt cutters available. One is to cut the bars ranging from 4 to 10 mm in diameter and one is for bars up to 19 mm. If bolt cutters are not available, a hacksaw or a chisel may be used instead.
The bars are bent with the aid of the bending plate which is fixed on the work bench; and the bonding bars (Reference Book, page 23). Each diameter of bar has to be bent with a specific size of bending bar.
The bending is done according to a certain radius (this refers to the sharpness of the bend) in order to prevent overstraining and cracking of the bar (this will be explained in more detail later). The stirrups for the cages may be bent around a peg that has the same diameter as the main bars.
The members must be bound together in order to ensure that the bars remain in the correct positions while the concrete is being poured. Binding wire is bound around pieces at the connections (Figs. 2a & 2b). The wire should be stretched taut and then twisted tight with the pincers.
NOTE: Examine the bars before you use thorn to make sure that 'ftey are free from paint, grease, loose scale or mud. Slight rusting will do uo harm, but any loose rust should be removed.
For additional information on concrete and reinforcement steel, see the Reference Book, Materials and Products sections.
CASTING REINFORCED CONCRETE LINTELS
Concrete lintels are made in two ways. They are either cast-in-situ or they are precast.
The formwork for cast-in-situ lintels consists of two main parts: the shuttering and the strutting. Precast lintels need only shuttering because they are cast on the ground.
The form is simply a temporary box into which the freshly mixed concrete is cast and kept until it has hardened. The inside shape of the box will be the outside shape of the concrete member.
All parts and members of a formwork used for casting reinforced concrete lintels are shown in Fig. 1 below. The parts are nailed together in such a way that they can easily be taken apart after the concrete has hardened.
Fomwork is made of wood or metal and consists of two structural parts: the shuttering and the strutting.
The shuttering is the actual shaping part of the formwork which is in direct contact with the concrete. Usually Wawa boards are used for shuttering because they are soft and light-weight, thus easy to work with.
The strutting is the supporting and bracing part of the formwork. It keeps the shuttering in position and supports both the shuttering and the concrete inside it until the concrete has set hard. Odum boards are usually used for the strutting because they are harder and stronger than the Wawa.
Concrete cover, also called clear cover, is the thickness of concrete between the surface of the concrete and the nearest reinforcement bar enclosed in the concrete (Fig. 2).
When the formwork is ready, the reinforcement cage is set into it. Spacers are attached to the bottom side of the stirrups (Fig. 3). These ensure that the reinforcement bars are correctly positioned within the concrete. The spacers are made beforehand out of cement mortar to the dimensions specified for the concrete cover thic Jiess. A short piece of binding wire should be pressed into the fresh mortar of the spacer, so that it can be fixed properly on the rod.
There must also be spacers on the sides of the cage, to hold the stirrups away from the side boards of the shuttering.
NOTE: When casting concrete, take care that the cage remains in position and that the concrete is well compacted around the reinforcement bars.
The construction of the formwork is shown in Fig. 1, above. This illustrates a situation where the frame will be set later or where there is an opening without a frame.
Since in most cases the frames are set already, the formwork does not require special strutting (Fig. 1). The head of the frame then acts as part of the soffit of the shuttering. The side boards must be fixed so that the vertical sides of the concrete lintel will be flush to the surface of the blockwork (Fig. 2).
If the lintel will not be plastered over later, the inside of the shuttering boards which are in contact with the concrete should be planed until they are smooth. This makes the stripping easier and also gives the concrete surface a nicer appearance. The inside of the shuttering may be treated with a special formwork lubricant, if this is available. Old engine oil can serve the same purpose; it should be applied lightly.
In case the lintel will be plastered later, it is better to leave the contact faces of the shuttering rough-sawn. instead of oiling, wet the shuttering boards thoroughly with water, so that they don't absorb too much moisture from the freshly cast concrete.
Before casting the concrete make sure that the inside of the shuttering is clean; all joints, slits and gaps closed; and that the clear cover (the thickness of the concrete covering the reinforcement iron) is the correct thickness everywhere.
The reinforcement for a lintel consists of a combination of different iron rods assembled to form a so-called "reinforcement cage".
In Rural Building the most commonly used reinforcement iron is round and has a smooth surface. It is available in rods with diameters ranging from 6mm to 28 mm; but the most common sizes the Rural Builder uses are 6 mm (1/4") rods and 12 mm (1/2") rods.
If ribbed bars or tentor bars are available, these are preferred because the concrete grips better to a rough surface. Figs. 1a, b, c, and d show how the surfaces of some reinforcement bars look..
BENDING REINFORCEMENT BARS:
According to how they will be used, sometimes iron rods must be bent or shaped to a hook form at the ends so that they can be well anchored. This must be done according to the following rules:
All circular bars with smooth surfaces must have U-shaped end hooks (see Figs. 2a & 3a). For ribbed bars and tentor bars L-shaped hooks are sufficient (Figs. 2b & 3b).
The minimum length for the straight end part of the hooks is: 2 times the rod diameter for circular bars; and 5 times the rod diameter for ribbed bars and tentor bars (Figs. 2 & 3).
The smallest permissible bending diameter can be calculated according to the figures in the table below.
|DIAMETER OF ROD||CIRCULAR BARS||RIBBED BARS & TENTOR BARS|
|6 mm up to 20 mm||2,5 x rod diameter||5 x rod diameter|
|22 mm up to 28 mm||5 x rod diameter||7 x rod diameter|
|more than 28 mm||10 x rod diameter|
The smallest permissible bending diameter for the 6 mm circular bar in Fig. 3a will be 15 mm; or 2,5 x 6 mm. For the 6 mm ribbed bar in Fig. 3b, the bending diameter will be at least 30 mm, or 5 x 6 mm.
- NOTE: As a rule, main bars should never be extended. If this is necessary, the shortest overlap for bars under tension is 75 cm, and the ends of the bars must have U-shaped hooks (Fig. 4).
The construction of large reinforced concrete members usually requires complicated calcinations to determine the diameters, number, shape and arrangement of the rods. This job should be left to a qualified design engineer.
However small and simple members of the structure such as lintels, columns, short span beams, etc. can he constructed by the Rural Builder according to the sketches, particulars and hints given here.
To place the reinforcement rods correctly, it is essential to understand and be able to predict in which part of the concrete member tensile stress (tension) is likely to occur. This is where the member has to be reinforced by the main bars. Fig. 1a shows the bold outline of a lintel; the dotted line shows how it would tend to bend under pressure. The arrows show how the whole upper part is under pressure, while the lower surface is under tension.
Fig. 1b illustrates how the lintel can be reinforced against this tension. Note that the main bars are on the bottom, in the zone which is under tension; and that the ends of the main bars are anchored on top, within the zone that is under pressure.
Fig. 2 shows another situation, where a beam is supported from one side only. This is called a cantilever beam. The unsupported end will tend to bead as shown. Here the main bars are or top to counteract the tension in the top surface (Fig. 2b).
The longer reinforced beam or lintel shown here in Figs,1a and 1b (below)is reinforced with main bars on the top and bottom.
The cross section in Fig. 2 shows how the clear cover is measured from the outside of the stirrups to the outside face of the concrete. in other words, the clear cover or concrete cover is the distance between the concrete surface and the nearest reinforcement bar, regardless of the function of the bar.
The distance between the stirrups is never more than 20 cm. For safety, the Rural Builder is advised to put the stirrups no more than 15 cm apart. The stirrups closer to the supporting walls (50 cm away and closer) should be no more than 10 cm apart (see the illustrations here and below).
So-called "bent up" bars are sometimes used as part of the reinforcement system (Figs. 1 & 2) but these are not used in Rural Building because of the difficult calculations required to place them correctly, also they are not necessary for the small structural members which are required in Rural Building.
Below is a drawing of a reinforcement system for a continuous beam; this might occur for example in an eave beam supported by a reinforced pillar (Fig. 1).
This method is the most common one in Rural Building. The lintel is cast in situation; in the place where it is needed (Fig. 1, below). The advantage of this method is that no soffit board is needed for the form, because the head of the door frame acts as the bottom of the form, provided that the frame has been installed already. A further advantage is that any roof anchorage, if needed here, can easily be inserted into the reinforcement cage at the correct position.
A precast concrete lintel is a lintel which is made in advance. The formwork is on the ground and the concrete is cast there. When the construction reaches lintel height, the concrete lintel has set hard and can be set into position. The advantage of this method is that it saves time, since the wall above can be completed immediately after the lintel has been laid.
If several lintels have to be made, a form like the one shown in Fig. 2 can be used,
NOTE: Precast concrete lintels must be marked on their top face with the letter "T" to ensure that they are placed in the correct position and not upside-down.
PREPARATION OF THE FORMWORK:
No matter which of the above methods is used, the formwork has to be prepared before the concrete is cast. This involves cleaning dirt and dust from the surfaces which will be in contact with the concrete, and watering or#oiling them.
The formwork must be completely sealed so that no gaps remain for the cement paste to escape. This would result in voids and weak concrete.
The concrete is filled into the formwork in layers, and compacted by tamping witli an iron rod or the trowel. Tapping lightly on the formwork with a hammer also helps to consolidate the concrete.
When the concrete is starting to get hard, the lintel must be kept wet and covered. This process is called curing and must be continued until the concrete is completely set and the formwork can be removed.
Stripping refers to the removal of the formwork; this has to be done carefully to avoid causing shocks or vibrations.
After the formwork is removed, clean all the parts of it and remove the nails. Stack the different parts neatly to keep them from getting bent or warped.
This is the period between the casting of the concrete and the time the formwork can be stripped off. During this time the formwork containing the fresh concrete must not be disturbed, so that the concrete can set hard without any cracks forming in it.
Depending on the size, shape and position of the concrete member, the stripping time varies from 4 to 28 days.
CORROSION IN REINFORCED CONCRETE
It may seem strange, but the weakest part of a reinforced concrete structure is the hard iron, because it is easily broken down if it is not protected well. The structure remains stable only as long as the iron is protected against corrosion.
The main problem with reinforced concrete is the development of very small cracks in the concrete, which means that the iron is exposed to air and becomes rusty. On the concrete surface below the cracks, reddish brown rust stains appear. Once these are visible it is usually too late to save the member.
Unless immediate protective measures are taken, the rust develops more and pressure builds up between the iron and concrete, causing the concrete cover to break open; this speeds up the rusting process ai 1 eventually the reinforcement bars become too weak to carry loads or to withstand other forces. The system then collapses.
PREVENTION OF CORROSION:
Here we must include a strong warning against over-reliance on concrete strength. Not only laymen, but also many construction experts believe "the more cement, the harder and the better the concrete", and they tend to improve the mix proportions by using greater amounts of cement. This is wrong, because very hard concrete is not only expensive but it is also not necessarily the best protection against rust. Therefore, the Rural Builder is advised to follow the correct mix proportions.
If possible use high quality steel for the reinforcement as this does not rust as easily as common circular Lars.
The surface of the concrete should be protected against the weather by the application of a waterproof cement paint. However, such a paint coat is wasted if rust stains are already present. The paint does not seal the cracks and only makes the stains disappear for a short time, if at all.
NOTE: Never apply paint on concrete surfaces that have not set hard yet. Possible shrinkage of the concrete during the hardening process would cause tiny cracks in the paint coat, making the paint useless as protection against the penetration of water and air.
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