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Trial Mix and Designed Mix

posted Jul 28, 2022, 6:12 AM by jeffery jim

Trial mix and designed mix are two of the most important essence in quality assurance (QA) for concrete which I have spoken many times previously. The gist to this exercise is to acquire statistical understanding of the concrete that shall be used for the construction in the sense of its physical properties and the narrative it provides during test which can be use during quality control (QC) during construction and/or execution. Among the important and essential information which can be use to overrule the standard specification (SP) is the concrete setting time. It takes precedence when determining the slump of concrete after 45 minutes or one hour as stipulated in the SP as well the allowed extension of time for the concrete to show its behaviour during stiffening and curing period.
2. The concrete usually will not start to cure after four hours and this can be determine based on time recorded during trial mix and temperature change aside from slump change. For the first four hours, the concrete will start to stiffen and it is a good practice to limit of time where additives can be applied to prolong the workability before stiffening which further reduce workability indicated by slump test. At this period of time, the concrete will gradually increase in temperature from room temperature to 40 up to 60⁰C, depending on other admixtures added (such as silica fumes and etc.) The increase of temperature above 40⁰C means that the concrete start to stiffen and hot joins are no longer allowed. Here, we can establish the relationship of workability and stiffening and time to prolonged the allowable hot joins for concreting work apart from dosing the plasticizer of the concrete as long as it does not increase to the limit of water/cement (W/C) ratio as indicated in normative references.
3. Trial mix and the approval of design mix also cover another QC issues which many have not taken into consideration, which is the phenomena of plastic shrinkage. After three to four hours into concrete pouring, concrete in a trial box, pushcart or wheelbarrow should be observed. This is an important stage of the QA works when conducting trial mix which many have not anticipate and most time, leftover concrete are discarded. The observation made on these samples will provide further information about the bleeding formation, expansion and evaporation rates. Capillary pressure can be monitored to understand the occurrence of plastic shrinkage where both are growing exponentially in a similar rate. Here, this observation will indicate the exact time when concrete curing via hessian method can be determine especially for structure member's with large surface exposed to element.
4. When another sample box which can be done to increase understanding of cracks developed by plastic shrinkage, reinforcement bars can be introduced to replicate the actual event at site when heat, wind, humidity are interacting with concrete and steel reinforcement bars' thermal expansion. Bleeding occurs very close to reinforcement bars and the outcome is usually crazing cracks which is almost parallel to reinforcement bars location.
5. I hope this brief write-up will provide you greenhorns a new perspective on increasing the QA Plan during the trial mix. Although the concrete may achieved target mean strength on the 28th day, it may need to revamp and redo to suit if plastic shrinkage is an issue. For most JKR contract, the typical requirement for W/C ratio will be under 0.50 and most time with the advances of admixtures, W/C ratio can further reduced to 0.40. Nevertheless, this does not mean lower W/C ratio provides durable concrete. It is commonly known lower W/C means less air voids and concrete will be robust against ingression of detrimental agents and elements. The formation of plastic shrinkage is a dire concern when compare to air voids as cracks are located very close if not provide direct access to the reinforcement bars. As a QAQC manager or engineer, you have to move along the limits or boundaries set in the literature to have a better concrete for construction purposes.

May be an image of text that says "Concrete surface Rebars Water channels Evaporation rate Bleeding water (a) Bleeding water Tensile forces F. (b) Plastic Shrinkage Cracks Dry Surface FF Wet Surface Tensile strength S. (c)"