Answers to Other Common Questions on Cement and Concrete
My local builders' hardware store has a variety of bagged cements. Which one should I purchase?
The most common bagged cements used for general household concreting projects are: Type GP (portland cement) and Type GB (blended
Manufacturers' brand names for each type of cement may vary. However, either Type GP or Type GB are suitable for most projects around the house.
How do I store cement? How long can I store it? How do I know it's OK?
Cement is a powder which relies on chemical reaction with water and therefore must be kept dry. It is essential that cement bags be stored, even for short periods, in a dry place and the bags stacked so that any moist or damp air is restricted from circulating too freely around them.
Ideally, cement bags should be stored in a dry, leak-free shed or garage stacked on dry boards standing on bricks or timber to keep the bags above
ground. The stacked bags should be kept away from open doors, windows or cracks which bring in moisture or damp air which will cause the cement
The stack should be clear of the walls of the shed or garage by at least 300 mm to allow some air circulation around the back and sides to prevent sweating.
Cement in bags may be kept for several weeks, but not indefinitely, under such conditions except in areas and/or periods of moist, humid weather.
On small jobs, where no shed or garage is available and where the cement bags will be stored for only a day or two at the most, bags may be stacked off the ground on a board platform and covered with a waterproof tarpaulin or other waterproof material.
The tarpaulin must be tucked in around the edge and weighted down at all times when cement is not being taken from the stack.
In all cases, it is desirable to order only the quantity of cement you will require for your project and have it delivered when you are ready to use it.
After completing the job, unused bags of cement should be stacked appropriately and marked with the date of delivery for future reference.
INSERT Table 5 Mortar for Masonry (based on Table 2:1 in AS 3700)
How do I make mortar for brickwork or masonry blockwork?
Firstly, you need to understand that mortar is a mixture of cement, fine aggregate (sand) and hydrated lime for clay brickwork. When ordering the
sand you should clearly state that sand must be suitable for use in mortar. The proportions (parts) of each material in the mortar mixture affects the strength and mortar mixes need to be selected for the appropriate use.
To comply with the Standards Australia's Masonry Code (AS 3700) the mortar mix must be one of the three listed in Table 5 appropriate to the type of masonry unit (clay, concrete, calcium silicate), the location of the work and the severity of its exposure to the elements.
Correct proportioning of the materials for mortar is very important. Each proportion (part) must be accurately measured by volume, using a suitable container. Materials should never be measured using a shovel; this leads to low cement contents, because sand will always heap on a shovel far more than cement. If you use a shovel, the result is likely to be lower strength, and softer mortars.
Problems may also be experienced through improper or inappropriate use of chemical admixtures to modify the workability or working properties of a mortar.
Admixtures must be used in the correct dosage usually a very small quantity. The wrong choice of admixture may well result in soft, weak mortar and inadequate bond strength. If you use an admixture, use only as directed, follow dosage recommendations, and ensure that it has been
proved by testing to comply with the requirements of AS 3700.
Using the Driveway
How soon can I drive the car on my new concrete driveway?
Concrete in domestic driveways, properly placed, compacted, finished and cured will gain full strength at 28 days. At 7 days, it will have normally
achieved one half to three quarters of its full strength.
Since the recommended curing time for concrete is 7 days, it would be best not to drive or park a motor car on a new concrete driveway within a week; but if necessary driveways may be used after 3 days.
Why has the surface of my concrete gone dusty? How do I fix it?
Concrete surface dusting is typically caused by working bleed water back into the concrete. This weakens the concrete surface, resulting in dusting
of the hardened concrete. Final finishing should not be undertaken until the concrete surface is free of all bleed water. Another cause is re-wetting the
concrete surface to try to improve the final finish.
If the problem is not severe, the surface can be repaired by applying a suitable clear sealant or paint coated finish to the concrete. If it is severe, the only method applicable is removal and replacement of the surface.
How does the weather affect concrete and concrete operations?
Weather conditions that give high rates of evaporation, such as warm, sunny days and windy days with low humidity, have a marked effect on
The rapid drying of the concrete surface that occurs due to these weather conditions can cause it to shrink before it has even set adequately. In this situation, it can only crack.
The remedy is to heed the prevailing weather conditions and always be ready to protect the concrete surface against high rates of evaporation.
There are some protective measures that can be employed.
Always pre-plan the job to avoid any unnecessary delays in placing, compacting and finishing the concrete, and ensure that finished concrete is properly cured using a recognised curing method.
A protective curing treatment may be used, available from major concrete admixture suppliers or through some pre-mixed concrete suppliers.
These materials are sprayed onto the concrete surface at the initial finishing stage, immediately after it is screeded. They will not affect final finishing and will protect the surface against rapid drying.
If concerned that the prevailing weather may inhibit the placing, finishing and curing of the concrete properly either postpone doing the work, or obtain advise from the pre-mixed concrete supplier to discuss delivery of an appropriate mix that will allow more time to complete the work satisfactorily and avoid premature shrinkage of the concrete surface.
Concrete Fish Ponds
I have built a concrete fish pond. Do I have to treat the concrete before the fish can be introduced?
Concrete fish ponds and ornamental pools are simple to construct but require initial treatment to stabilise them before the fish can be introduced.
Treatment can consist of seasoning the pool by filling and emptying it two or three times over a period of a month or six weeks, or by treating it with
Seasoning or treatment of the pond is necessary to prevent lime from the concrete killing the fish. If the water is fluoridated it should be conditioned with a water-ager prior to introducing any fish. A few tadpoles or small goldfish should be placed in the pond to ensure that treatment has been successful before more-valuable fish are introduced.
The pool may be treated by filling it with water containing alum (a salt comprising a double sulphate of aluminium and potassium) in the
proportion of 1 kg of alum to 500 litres of water. After one week it should be emptied and refilled with water containing 1 kg of Epsom salts per 500 litres and again allowed to stand for a week.
Finally, the pool should be emptied and refilled with clean water, then emptied again and refilled after one week. Fluoridated water should then be conditioned prior to stocking with fish.
Once the pool has been stocked and plant life established, it should not be disturbed for at least a year, except for occasional topping-up with fresh water.
Initially, the water may become cloudy, but this cloudiness should disappear again as the plant and fish life in the pond settle down.