Concrete Mixers
Most people recognise a concrete mixer as the hand loaded, rotating drum type designed to produce both concrete and mortar without segregation of the mix. The normal process of concrete production is to add water to the rotating drum, followed by the cement and then finally the concrete aggregate material such as gravel. If the mix is for mortar then sand is substituted in place of the aggregate. The actual choice of mixer will depend on the nature of the project. The following factors need to be taken into consideration when making the selection:
The maximum output required
The total output required
The type of material being mixed
The methods of transporting the completed mix
The discharge height of the mixer
The location of the mixer relative to the place that the mix is needed
The sequence of work
The number of gangs requiring mixes from the mixer
Generally for hand fed batches from a mixer a rule of thumb is to allow 5 to 8 minutes per batch cycle this is approximately 10 to 12 batches per hour. The following questions must always be in the back of your mind
How much concrete is required?
How much concrete can be placed?
How much time is needed what mixing and what placing methods are to the used
Small batch mixers: these are the ubiquitous diesel and electric small output, 200 litres per batch variety, often used with wheel barrows for transportation and hand laid. Most of these varieties are either a tilting drum type or even a small electric mixer. They can cost as little as £200. Care is required with the mixes from this type of equipment to ensure good quality control of both single and successive mixes to ensure a consistent mix design.
Tractor Shovels
Combined loader/back actor: These are commonly known as the JCB, after the manufacturer - J C Bamford. They are actually manufactured by a number of companies, such as CASE.
Lightweight tractors fitted with wheels in place of tracks are used for comparatively light construction jobs. Equipped with a backhoe, which is an open scoop attached rigidly to a hinged boom, such a vehicle can dig shallow trenches; equipped with a front-end loader, a scoop shovel affixed to the front of the tractor, it can lift and carry gravel, stone, sand, and other construction materials.
Background: The primary earth-moving machine is the heavy-duty tractor which, when fitted with endless tracks to grip the ground and with a large, movable blade attached in front, is called a bulldozer. Bulldozers are used to clear brush or debris, remove boulders, and level ground. A scraper is a machine that may be pulled by a tractor or may be self-powered. It consists of a blade and a box or container. Dirt is scraped by the blade into the container; the dirt may then be released so as to form an even layer of a predetermined thickness, or be carried off for disposal elsewhere. Scrapers are used to level and contour land, as in road construction.
Somewhat similar to scrapers are graders, self-propelled, wheeled machines with a long, inclined, vertically adjustable steel blade. Graders are primarily finishing equipment; they level earth already moved into position by bulldozers and scrapers.
Draglines and power shovels are the primary forms of excavation equipment. A dragline is fitted with an open scoop supported from the end of a long boom by a wire cable. The scoop is dragged along the ground by the cable until it is filled with earth, which is then dumped elsewhere. Draglines are used primarily to excavate deep holes. Power shovels are fitted with specially designed buckets called clamshells, which dig into the earth and shovel it up. The bottom of the clamshell can then be opened to dump the dirt into a truck for removal.
Excavators
Excavating machines are one of the most fascinating pieces of modern equipment. They have had a fundamental impact on the way excavation work is carried out. They can have extremely high outputs capable of excavated in excess of 100 cubic metres per hour on bulk excavations. They are often able to excavate most types of soil, and can even remove rock using an hydraulic breaker fixed to the end of the excavator arm. There are a wide variety of designs and sizes but they can be loosely placed within one of tescalating machines are one of the most fascinating pieces of modern equipment. They have had a he following three broad categories.
Universal excavators: this is the common tracked excavator with a common power unit. The machine can slew or swing through 360 degrees. They are known as 360 machines. They are equipped with a hydraulic we operated ball to which are fitted a variety of sized buckets. They are very popular on virtually all sides and have an extremely good capacity for hard work.
Multi- purpose excavators: the best example of this type is the ubiquitous JCB's, a wonderfully adaptable machine with both front and rear lifting functions. They are a 'jack of all trades', being able to move bulk material using the front bucket, as well as in able to carry out small excavation operations using the back actor. Though they are of a lower output they are quick and efficient. These excavators are normally equipped with large rear wheels and smaller front wheels, all being driven under a four wheel drive arrangement. Within this category are also small tracked excavated which are now extremely popular called the mini-excavator : These are small tracked self propelled excavators which can swing through 360 degrees and have one hydraulic excavating arm. They are delivered to site on trailers and are very popular. Because of their small size, and ability to get anywhere they have pretty much replaced hand excavation. They are hard workers and are very economical to run. They are manufactured by JCB, Komatsu and many others.
Purpose designed excavations: these are machines which have been designed specifically to carry out one type of excavation, such as trench excavators. They are designed only for that purpose and will achieve quite good output figures, but only on this type of work. They are less common on building sites but are often used by specialist contractors who carry out cabling and other types of utility works.
Cranes
These are devices designed to lift materials by using a rope operation and move the load horizontally within the limitations of the machine. There are many types of crane available and the choice should be based on the load to be lifted, height and horizontal distance covered, time period for lifting and the degree of mobility that is required. Types of crane can vary from simple rope and pulley or gin wheel to the complex tower crane. The three most popular types of cranes are mobile, static and tower cranes.
Concrete Pumps
Concrete pumps are used for the placing of wet concrete in awkward places usually involving a large vertical distance or long horizontal difference from the point of delivery of the concrete. Concrete pumps or an integral part or piece of equipment, usually having a lorry mounted pumping system with a boom and a tubular feed pipe. The concrete is forced down the delivery pipe by the use of hydraulic rams, feeding in to a small diameter pipe usually approximately 100mm diameter. These pumps are usually limited to 85 metres vertically and approximately 200m horizontally. They are usually hired for a minimum period of four to eight hours. There is some preparatory work in which a cement and water grout is delivered down the tube prior to delivery of the concrete. When the concrete arrives it is discharged from the back of the lorry mixing drum into a hopper on the concrete pump lorry. From here the concrete is fed into the hydraulic rams. Following delivery of the concrete the system of tubes must be historic cleaned, to avoid the build up of cement grout within the tubes.
Cost versus hire options
Economic Considerations: Plant and equipment is usually hired on an hourly or daily basis. This can include the driver or operator as appropriate. The cost per hour of a machine is often an 'all in' rate, with driver, machine and fuel. The rate for a JCB is typically £16.00 per hour, a 360 excavator approx £24.00 to £26.00 per hour.
There will often be a minimum cost hire period of 4 to 8 hours; in addition there will be delivery charges, often called ON/OFF charges. The cost per hour can be converted to a cost per m3 if required as follows:
Rate per m3 = Rate per hour
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Output
Once an item of equipment has been acquired it must clearly be able to provide a sufficient rate of return on the investment and also recover the owning and operating costs.
An excavator is purchased new for £55000. Its estimated life is 10 years, with a resale value of £5000.
£
Capital cost 55000
Resale value 5000
Anticipated life 10 years
Insurance road tax and licences 1000
Maintenance 10% of capital cost
Consumables 500
Business overheads 5000 pa
Required rate of return 15% pa
Budgeted operating time 2000 hrs pa
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Hire charge calculation £ per year
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Depreciation (straight line) = 50000
-------- = 5000
10
Interest on finance, calculated using a capital recovery factor from interest tables
(CRF 0.199 at 15% for 10 yrs)
55000 x 0.199 x 10-50000
-------------------------------------- = 5445
10
Fixed overheads 5000
Insurance etc 1000
---------
OWNERSHIP (FIXED) COST 16445
Consumables 500
Maintenance = 10% of £55000 5500
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OPERATING COST 6000
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TOTAL COST £22445
Hire charge = 22445
--------- = £11.22 per hr
2000
Plant Costing:
For plant, running costs,
Plant and equipment is usually hired on an hourly or daily basis. This can include the driver or operator as appropriate. The cost per hour of a machine is often an 'all in' rate, with driver, machine and fuel. The rate for a JCB is typically £16.00 per hour, a 360 excavator approx £24.00 to £26.00 per hour.
There will often be a minimum cost hire period of 4 to 8 hours, in addition there will be delivery charges, often called ON/OFF charges. The cost per hour can be converted to a cost per m3 if required as follows:
Rate per m3 = Rate per hour
-------------------
Output
Maintenance Considerations
This important issue is often glossed over, the tendency is for the site maintenance and plant maintenance to be forgotten in relation to the capital cost and output obtained.
Planned maintenance is an ideal but very costly state of affairs. It enables a constant review of the mechanical efficiency of a piece of plant on the basis that prevention is better than cure.
It can only be related to the cost involved in respect of the company turnover. It follows that a large company, employing several hundred pieces of plant would require full time service engineers, with vans, to travel round from site to site to ensure that plant is being maintained and serviced regularly.
Some companies work on the basis that they pay the machine operators a machine bonus on the basis that if the machine is well maintained, lubricating oil and grease applied and when necessary, the overall cost is greatly reduced. An old adage is 'a pint of oil is the cheapest form of maintenance ever devised'.
It is the responsibility of all plant departments to ensure that when a fitter is sent to a particular site, his journey should be routed via other sites, where he can check over the condition of pieces of plant to ensure that maintenance and adjustments are being carried out. It must, however, be realised that the breakdown is top priority and men should be routed so that the breakdown is covered first and the preventative maintenance covered afterwards.
It must be remembered that every machine requires maintenance and this should be pre-planned. Prevention is better than cure. To bring about an efficient maintenance scheme, labour is required in the form of service engineers, vans and these, together with the material involved, must be allowed for when assessing the cost of a plant department in relation to the use of general and, especially, specialised equipment.
The time and labour savings
The type of plant and number required will be effected by
1. The sequence of the work dictated by the design of the building.
2. The methods of construction used by the contractor.
3. The working space available.
4. The quantity of work involved.
5. The time available to do the work.
This may involve the balancing of plant, for example and excavator can excavate 24 m3 per hour, which requires removal off site. The Lorries used are 8m3 capacity therefore 24/8 = 3 Lorries will be required. This is called plant balancing.
This balance will affect the numbers required as well as the duration of the work.
If the plants are not balanced efficiency on site will not be achieved.
Implications for programming methods
