Few days ago, I wrote about silica fume application for both Ordinary Portland Cement (OPC) concrete, and polymeric concrete or grout. Always be very careful when handling your design mix (not limited to the free water cement ratio only). Today when administering a grout trial mix, I experienced something I never knew although I have been doing concrete design mix and grout design mix close to ten years.

When it comes to concrete strength, everyone have great expectation with the grout compressive strength when following the prescribed mix as per Material Data Sheet provided by the manufacturer; somewhere at the region of 60-80 MPa. For self-subscribed mix by adding additional water and pebbles, we can make a calculated estimate that the strength would be somewhere 40 MPa, which is particularly useful when batching for almost homogeneous characteristic when apply to structure repair works which have characteristic strength of 30-35 MPa. This is to ensure that the infill will have almost similar physical characteristic, mainly the tensile modulus and several other relevant moduli. In actual truth, batching grout is a knowledge which is not only limited only to measurable/dimensional approach where dimensions such as weight and volume play important role in batching a good grout.

What I encountered today has opened a new perspective where cement also plays substantial role when it comes to workability and bleeding. For the purpose of compliance to JKR requirements (although it was not necessary based on British Standard, since the definition of the term “batch” is ambiguous in JKR standard), a trial mix should consist of 3 repetitions of batching works to be considered as one good batch. First two batches went well until I noticed heavy bleeding when sample on cube moulds start to set. It took 3 more batches to complete the third batch, 2 batches were rejected. The third batch can only achieved satisfactory efflux time after I instructed specialist to abandon the initial purchased cement bags for another bag, procured from another source and of the same brand. Unlike most grout which comes as pre-bagged and ready for action with the mix of water, the grout batched here is a combination of cement, water and admixture which works as a plasticizer which helps with the flow in confined space.

The batching was done accordingly, where the grout was stirred properly without lumps and yet, it failed to achieve the desired/recommended workability rate. So what went wrong? Cement is definitely the culprit and thus, some of the cement properties may have degraded or simply non-compliance. Visual check of the cement indicates it is still fresh without lumps and crumbs.

Although the cement supplier does have its supplies checked periodically and comply with BS EN 196, it is good to notice that most of the gypsum and clinker supplies are imported from various sources. This means, there are variables and impurities which play a role to the problem I faced today. After some readings, it could possibly be the content of anhydrite which substantiates the rate of the workability. For the rest of the chemical or molecular level of activities, it is negligible. At least, this problem is a new chapter for me to discover more about the molecular level of concrete reactions.