1. What is the difference between release agent, form oils, form releaser, demoulding agent? (F1)
Release agent, form oils, form releaser and demoulding agent are materials for separating formwork from hardened concrete. Though they generally refer to the same meaning, there are slight differences among these terms.
Release agent: Materials that contain ingredients which are chemically combined with cement.
Form oils: Diesel oils or other oil types.
Form releaser
and demoulding agent: General terms to describe materials which perform separation of forms from concrete.
2. How can release agent help to separate formwork from concrete?
(F1)
There are generally two main types of form releaser: barrier type or chemically active type.
For barrier type (e.g. form oil), it creates a barrier between the form and the fresh concrete. However, the quick evaporation of diesel oils affects clean air.
For chemically active type (e.g. release agent), an active ingredient (e.g.
fatty acid) chemically combines with calcium (lime) in the fresh cement paste. This calcium/fatty acid product (grease or metallic soap) is stable and causes the formwork to release from the hardened concrete. It is this slippery, greasy, non-water soluble soap which allows the easy releasing of formwork from hardened concrete.
3. What are the potential problems of excessive application of form oils? (F1)
The problems of excessive application of form oils are:
(i) It stains the surface of hardened concrete.
(ii) Excess oils have nowhere to escape and find its way inside the cement paste and form holes subsequently. The oils bead up because of its incompatibility with water in chemical nature.
(iii) Higher cost is associated with increased usage of form oils.
(iv) In a relatively short time essentially most diesel oil evaporates so that it creates environmental problem.
3. How can permeable formwork improve the quality of concrete?
Permeable formwork serves as a filter that allows excess water and trapped air to escape from concrete surface. During compaction by vibrators, the fluid movement through permeable formwork drives out air and water, leaving behind a denser and stronger concrete. The movement of water results in a decrease in water in fresh concrete and fines cement particles from interior concrete shall be carried towards the formwork.
Hence, it lowers the water cement ratio at concrete surface and enhances a higher strength near concrete surface.
In traditional formwork, during concrete compaction process the vibration tends to force water to the surface of concrete mass owing to the rearrangement of solid particles in concrete. As such, the concrete in concrete/formwork interface possess more water than the interior concrete.
Consequently, the higher water cement ratio at concrete surface would lower the surface strength. Permeable formwork functions and solves this problem by allowing excess water to pass through.
4. What is the relation of pouring rate and temperature with concrete pressure on formwork? (F2)
Freshly placed concrete exerts pressure on formwork during the placing operation. It is influenced by the rate of placing and the air temperature.
For instance, if the concrete pouring rate is too slow, setting of concrete starts to take place. As a result, the concrete at the bottom of the formwork sets prior to the placing of fresh concrete at the top and the maximum pressure will be reduced.
Temperature affects the rate of hydration of concrete. The higher the air temperature is, the higher will be the rate of hydration reaction.
Consequently, fresh concrete tends to set at a faster rate. The pressure exerted on formwork decreases with an increase in temperature. For this
reason, formwork is subjected to a higher pressure exerted by fresh concrete in winter than in summer.
Fig. Diagram of design concrete pressure diagram on formwork.
5. Is late removal of formwork beneficial to cater for early thermal movement? (F3)
Let us take a circular column as an example to illustrate effect of internal restraint to thick sections.
When the temperature is rising, temperature in the core is higher than that at outer zone. The inner core will have a higher expansion and exert pressure to the outside. The induced compressive stress will result in the formation of radial cracks near the surface of concrete.
When the temperature drops, the concrete at the outside drops to surrounding temperature while the concrete at the central region continues to cool down. The contraction associated with inner concrete induces tensile strains and forms cracks tangential to the circular radius.
It is beneficial for thick sections (say >500mm) to have late removal of formwork to reduce early thermal cracking. This is to allow more time for the centre of concrete section to cool down gradually to reduce the risk of thermal cracking. This is effective in controlling the temperature differential across the cross section of the concrete structures and reducing the potential of internal cracking due to early thermal movement.
6. Comparing the rate of “Formwork exceeding 300mm wide, horizontal or at any inclination up to and including 5o to the horizontal” with the rate of “Formwork exceeding 300mm wide, at any
inclination more than 85o up to and including 90o to the horizontal”, which one is higher?
The item “Formwork exceeding 300mm wide, at any inclination more than 85o up to and including 90o to the horizontal” refers to formwork formed vertically and when compared with formwork erected in horizontal plane, the amount of falsework required is smaller.
The item “Formwork exceeding 300mm wide, horizontal or at any inclination up to and including 5o to the horizontal” refers to formwork to be erected in horizontal position and in general it requires much falsework to support this type of formwork. Therefore, the rate for this item is higher than the one mentioned in the above paragraph.
7. In erection of falsework, for a rectangular panel inside a falsework should it be braced along the two diagonals? (F4)
When a rectangular panel is subject to an eccentric load or a lateral load, it tends to deform into a parallelogram with one diagonal shortening and the other elongating. Theoretically, it is sufficient to brace along one of the diagonals (the one in tension). If one diagonal is only allowed to brace inside the rectangular panel, it should be not braced in the diagonal in compression because under severe lateral loading the diagonal may buckle leading to failure of structure.
However, in actual situation lateral loads may come from both sides of the panel and hence it should be braced in both diagonals.
8. For long slender structures like beams, propping is required after removal of formwork. Why? (F4)
After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.
Note: Propping refers to provision of falsework to support slabs and beams during their gain in concrete strength after concreting.
9. Is curing compound suitable for all concrete? (C1)
For concrete structures with low water-cement ratio (i.e. less than 0.4), it may not be suitable to use curing compound for curing. When hydration takes place, the relative humidity of interior concrete drops which leads to self-desiccation and drying-out. With no external supply of water, the cement paste can self-desiccate in such an extent that the hydration process stops. As such, curing compounds may not be sufficient to retain enough water in the concrete. In this case, wet curing is a better choice which serves to provide an external source of water.
10. What is the difference between curing compound and sealing compound? (C1)
Curing compound is primarily used for reducing the loss of moisture from freshly-placed concrete and it is applied once after concrete finishing is completed. Sealing compounds is adopted to retard the entrance of damaging materials into concrete and they are normally applied after the concrete is placed for 28 days. The harmful substances include water, deicing solutions and carbon dioxide which eventually cause freeze-thaw damage, steel corrosion and acid attack respectively.
11. Curing time in summer is less than that in winter. Why?
While concrete sets, it gains hardness and strength as the process of hydration slowly spreads the entire body of material. Curing should be allowed to continue for several days before subjecting the new concrete to significant stress. The period of curing depends on the temperature because the rate of all chemical reactions is dependent on temperature.
Therefore, in summer the rate of reaction (hydration) is faster so that a shorter curing time is required. On the contrary, in winter the rate of reaction (hydration) is slower so that a longer curing time is required.
12. Why does plastic sheet cause discolouration to freshly placed concrete? (C2)
Plastic sheets are commonly used in curing to prevent moisture loss from concrete surface. However, it is not uncommon that discolouration occurs on the concrete surface. When plastic sheeting is spread over concrete surface and in direct contact with concrete, it tends to leave colour streaks on concrete surface. The problem of discolouration becomes even worse
when calcium chloride is used in concrete mixes.
13. What are the disadvantages of curing by ponding and polythene sheets? (C2)
The purpose of curing is to reduce the rate of heat loss of freshly placed concrete to the atmosphere and to minimize the temperature gradient across concrete cross section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere.
Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing.
Polythene sheet: This method of curing is based on the principle that there is no flow of air over the concrete surface and thereby no evaporation can take place on top of the freshly concreted surface by provision of polythene sheets. However, it suffers from the demerit that polythene sheets can be easily blown off in windy condition and the performance of curing would be affected. Moreover, for water lost due to self-desiccation, this method cannot replenish these losses.