It is possible air-dry cured concrete sample has the better durability resistance than full water cured concrete sample? In the case for lightweight concrete incorporation with lightweight coarse aggregate.
No, it is not. However, when you take sample out of water and test it directly, you may see that air dried concrete shows better performance. This is because some test methods are vulnerable to the degree of saturation. Since, the saturation degree is very low in air dried concrete, for example, you can observe higher compressive strength than water cured ones. The reason is the increase of internal pressure exerted by the available water in cappilaries. Therefore, I suggest you that before testing, apply a suitable drying process for both air cured and water cured concretes.
I agree with the previous answer. With air-cured concrete, it is most probable that the cement does not fully hydrate; therefore such concrete cannot reach its full design strength. However, if specimens are not allowed to dry properly between being removed from the curing tank and being tested for UCS/flexural strength/etc, it may appear that air-cured specimens are stronger than wet-cured, due to the effect that excess water in the pore system has with the specific tests commonly employed for testing strength properties.
The following is completely speculation, but since the question is quite interesting it got me thinking a little bit. One thing that may occur, specifically related to salt ion related durability of concrete, is that unhydrated cement may remove moisture from the pore system. This removal of moisture could inhibit the transport of salts within concrete, therefore increasing the amount of time required for chlorides to attack rebar or for salts to build up and cause crystallization damage. However, this effect would be confined to a specific amount of time and moisture; once the cement becomes hydrated enough, this effect would cease to operate. I re-iterate that this second paragraph is speculation on my part, but the person asking the question could attempt a literature search on the topic of moisture transport in concrete/porous materials and salts.
thanks Kasim and Sevasti for the explanation. How about for durability, such as water absorbtion test? According to the standard as precribed in BS or ASTM, the specimens is required to oven dried as specified temperature until constant mass is obtained and then left cooling to room temperature before testing. but what I have been observed is that the concrete surface of water cured sample formed some cracks while air cured sample was no after dryingt. The cracks become more visible when submerged in water more than 2 days. Is this phenomena caused by the expansion and contraction of concrete sample during drying process? will this lead to increase the water absorption capacity?
Savesti, I am very interest about the topic share by you now regards the remove of moisture inhibit the salt diffusion. Can I have some links about this issues that you have been read before? thanks
Have you experimented with different amounts of 'drying out' times before placing the water cured samples in the oven? Also, there are several aspects which can cause samples to crack such as contaminants in the curing tank water, the temperature of the water (it needs to be stable to enable reproducibility), and whether the samples are covered/sealed in their molds until the molds are removed. Are the 'air-cured' samples pulling away from the edges of their molds on their own, but the water cured samples not observed to do this?
Certainly, surface cracks could decrease the durability of a structure over long periods of time because they would decrease time required for chlorides to travel through the concrete to the rebar within.
As for references for the moisture inhibition, I do not have any readily available specifically dealing with salts in concrete, but the authors Hall and Hoff have published journal articles and text books regarding moisture transport in porous building materials generally. Their work deals also with salt transport in the context that salts are always transported by moisture.
Dear Thong, According to my experience, there are several aspects you have to consider. 1, What is the durability factor you are testing? 2, You are talking about light weight concrete, what aggregate do you use? 3, Aggregate phores are open, or closed? 4, Is it not possible, that the cement paste has collected on the surface due to compaction, and only that thin layer cracked due to shrinkage? Nevertheless I agree with previous notes :-).
No as curing is very important aspect of concreting due to heat of hydration,so I think
curing is a must.
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