Hi all,
Hope this is of assistance to someone out there at some point in time.
We write this knowing that all too often, concrete quality is overlooked in Bali, and unfortunately often such is exploited by those selling you the product.
Concrete as you probably know is a combination of cement, water and aggregates (sometimes an admixture is added and sometimes flyash is added - but not often in Indonesia). The aggregates are comprised of fine aggregates (sand) and coarse aggregates (batu koral, batu cor, I.e. A bitumen or gravel equivalents). The quality of the "mud" itself ("beton") is one of the most important factors in the integrity of your structure - and when combined with steel reinforcement, you have a monolithic compound (reinforced concrete), which is absolutely, without doubt, the single most important component of your structure. Bricks in Indonesia are mere wall fill, and though they have a certain amount of compressive strength (rather little), such is unreliable and inconsistent and in no circumstances should be counted as anything other than dead weight within the context of your engineer's structural calculations.
Concrete in Indonesia is available in many mix designs and qualities. The most common characterization of these designs is by the compressive strength they are supposed to yield. In Indonesia, the colloquial classification in represented as K###, where ### is equal to the amount of load in kilograms that a single centimeter (by surface area) should be able to be loaded to by a compressive force before failing. That means that a 10cm x 10cm concrete column, poured with K300 should be able to withstand a compressive force of 30 tons (10 x 10 x 300kg) without failing. To achieve these varying grades of mixes (K250, K300, K350, K400, etc) the concrete (or "beton") supplier will adjust their mix design, and their price per cubic meter. Generally speaking, higher grades of concrete have higher volumes of cement, which is the most expensive component of the mud in conventional concrete. Other factors will also affect the volume of cement required - such as aggregate size. The smaller the coarse aggregate for example, the more cement required, as the surface areas requiring bonding increase.
There are many factors that are considered in a mix design, enough to fill several semesters at a tertiary institution, but those details are not the intention of this post.
There are certain applications of concrete whereby K250 may suffice, there are many applications whereby K400 will be required. Most applications should be at least K300. In any case, what's fair is that you are receiving the product you paid for.
Always ensure you collect samples from the pour, on site - enough to test compressive strength at 3 days, 7 days, 14 days, 21 days (optional) and 28 days (essential). Any university with a Civil Engineering Department, including Univeritas Udayana can conduct these tests for you, and you would be surprised just how cheap they are.
Important tips
- Randomly collect these samples. Drivers talk to dispatch and if they see you're regimentally collecting samples on every second truck, you may have your mixes "selectively" dispatched
- Ensure your supplier knows that they'll incur heavy penalties for test results below ordered grade
- It's not a bad practice in Indonesia to "order one grade up" - a penalty for the supplier is nice, but at the end of the day, if the concrete has been poured, you probably have to live with it. Still, apply a penalty based on the ordered grade, not the minimum grade you need
- Always remember, if they think they can cheat you, they probably will
Other tips
- Workability on site is extremely important. A good test result means little if the correct application wasn't achievable. Always perform "slump tests", and be clear with your supplier of your slump requirements. Reject loads that don't fall within range
- Have ample vibration equipment on site - ramming bamboo or steel rebar up and down in the mud is a poor substitute, and on suspended pours, a piece of 22mm rebar rammed too far can take out a chunk of formwork (messy)
- Curing is as important as the application itself. A poorly cured pour will only strengthen to a fraction of its potential. Your lab tests may be fine, but if it wasn't cured properly on site, you have compromised the integrity of your structure. Ponding works well and 28 days is a good duration for suspended slabs
- Try to leave formwork on suspended slabs in place for as long as possible. 21 days is the minimum and 28 days is safe in most applications
There are many other very important aspects of ensuring a quality end product, but hopefully the few tips above may help.
All the best & happy pouring..
Populaire..
