Writer: Smith Yong
Pile foundation is one of the important structural elements in a building construction. The primary function of pile foundation is to transfer the loads from the structure above to the more compact, solid and stiffer soil which have higher bearing capacity to withstand the structural load efficiently. Typically, reinforced concrete piles are widely use in Malaysia. Pile foundations system which commonly used in Malaysia market are reinforced concrete square piles, spun piles, bored piles, micropiles and etc.
Deep basement construction is getting common in Malaysia especially at the city centres due to rapid development of the city landscape coupled with scarcity of land for new development. When the structure extends far below grade, the challenges of waterproofing work increase exponentially as the hydrostatic pressure increases.
Without efficient basement waterproofing system, issues relating to water seepages could lead to serious structural damages in the long run due to corrosion of the steel reinforcement. Concrete element in building is reinforced with steel reinforcement or wire mesh. Failure of waterproofing system may lead to ingress of water to the concrete structures. Exposure to air and water leading to corrosion of the steel reinforcement and wire mesh which eventually will result in structural damage over time. Therefore, special care and attention are essential when selecting the appropriate waterproofing solution for basement structure.
Pile head waterproofing integrity is critical in deep basement construction and careful attention must be given to ensure no leakages occur. In comparison to basement slab, pile head waterproofing work is more difficult to treat as it involves lots of detailing work especially around the congested steel reinforcement area. Water can seep through the basement structures along the reinforcement steel if it is not treated properly. Selection of reprofiling material for pile head treatment is very important as the material must be able to provide not only waterproofing properties, but also able to withstand the heavy loads from the whole building. Therefore, the pile head waterproofing treatment system shall be integrated with the basement slab to form a watertight system.
At the construction site, pile head trimming is a process is to prepare the pile head to pile cut-off level and expose the reinforcement for incorporation into the pile cap. After the trimming process, the pile head will have an undulating surface as shown in the photo below. Thus, pile head reprofiling is required prior to the pile cap casting work. It is very common to see “non-shrink” cementitious grout being used as the material to reprofile the pile head. “Non-shrink” cementitious grout is chosen because it can be easily source in the market and have high compressive strength. It is considered as an economical way to grout the pile head. However, is “non-shrink” cementitious grout the right material for the work methods employed in the above application?
Photo 1: Uneven finishing of pile head after trimming process
“Non-shrink” cementitious grout is a single component material which contains cement, graded fillers and other additives. The cement acts as the binder and upon adding water, it forms a high strength cementitious material. Is “Non-shrink” cementitious grout really does not shrink? The market term “Non-shrink” grout is commonly referred to because they contain special additives which cause volume expansion either during the plastic stage and/or the hardening stage to counter the shrinkage during curing process. For precision grouting and formwork repair applications, water lost during the curing process is well-controlled as the material is applied into a confined space and the exposure area of the grout surface is small. This application allows the grout to have a proper curing process without significant change of volume and thus minimize the formation of shrinkage cracks while maintaining a good bond with the substrate.
However, if “Non-shrink” cementitious grout is applied on expose hot weather and low humidity environment, it is difficult to control the loss of moisture during curing and that leads to rapid hardening of the grout. Improper curing may result in permanent damaging effects to the cement based “Non-shrink” grout. This gives rise to shrinkage cracks, lower compressive strength, poor adhesion and etc. Furthermore, it is very challenging to ensure proper execution and protection work on site to avoid rapid drying issue. As with all cement based material, it does not have the “waterproof” property which this essential for pile head treatment in order to prevent the ingress of ground water from the piles. If there were any defects found on the pile head surface, rectification work is mandatory prior to basement slab casting work.
What’s next if “Non-shrink” cementitious grout is not suitable for pile head reprofiling work? It is perhaps time to look at epoxy grout. Epoxy grout is three components, high performance grout that derived from an epoxy based binder and a filler material. Epoxy grout use epoxy instead of cement as binder, thus making it to be waterproofed since epoxy is an impermeable material. Its zero water penetration properties provide a full waterproofing barrier to the pile head. In addition, epoxy grouts offer rapid curing, high early compressive strength and high ultimate compressive strength development. In terms compressive strength, epoxy grouts can achieve approximately 60MPa in one day and 90MPa in 7 days whereas “non-shrink” cementitious grout can only achieve approximately 30MPa of compressive strength in one day and 65MPa in 28 days.
At the job site, basement construction is always under the critical path of the construction schedule. Any delay along the critical path may affects the overall completion time of the project. With epoxy grout, the foundation contractor is able to shorten his hand over timeline to the main contractor for subsequent rebar work due to the shortened time to achieve the required compressive strength. “Non-shrink” cementitious grout takes longer time than epoxy grout to achieve its expected strength as per material data published by product manufacturer. “Non-shrink” cementitious grout takes 28 days to achieve full cure whereas epoxy grout only takes 7 days to achieve full cure condition. If the “Non-shrink” cementitious grout is not cured properly, it can lead to the formation of shrinkage cracks and thermal cracks. As a result, the durability and strength of the grout as well as the bonding to the piles will be compromised.
In contrast, epoxy grouts do not have shrinkage issue. This is one of the major benefits of epoxy grout over “Non-shrink” cementitious grout. When shrinkage and thermal cracks start to develop, coupled with high hydrostatic pressure from the ground water, water ingress from the ground will keep the basement structure wet over time. Injection grouting is the only way to solve this problem. It is impossible to hack off the existing pile head and redo the waterproofing work after the basement structure is completed.
The only reason why “Non-shrink” cementitious grout is widely used is due to the cost factor. The basement slab waterproofing system is also incomplete without considering the pile head treatment in relation to water ingress issues.
Perhaps we shall look at the overall cost instead of the upfront cost. When it comes to basement slab leaking repair work, the amount of time spend for investigation work and the remedial cost by the specialist contractor could be quite significant.
Pile head treatment is the final and most crucial application step for a successful watertight basement structure which is often overlooked. Every single pile has to be treated properly to prevent any leakage through the top of the pile. In conclusion, the differences between epoxy grout and “non-shrink” cementitious grout are summarised as table below:
Table 1: General Comparison of epoxy resin grout and “Non-shrink” cementitious grout.
Photo 2: Pile head treated with epoxy grout
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