Sand can be classified into coarse sand and fine sand, based on particle size. The former is usually greater than 4.75 mm (retained in a No. 4 sieve), while the latter is less than 4.75 mm (passing through the No. 4 sieve) (1).The properties, including shape and texture, size gradation, moisture content, specific gravity, reactivity, soundness, and bulk unit weight, of different types of sand,influence concrete strength (2). The shape of sand particlescauses variations in the surface to volume ratio, and a larger surface to volume ratio improves bond characteristics but decreases concrete workability. The surface texture of sand also has an impact on concrete strength. Concrete workability can be improved by a smooth surface, while a rough surfaceincreases the strength of the bond between the paste and the aggregate and thus enhances the concrete strength. Experiment results clearly show that concrete made from sharp sand displays higher compressive strength (3).
Experimental procedures of making concrete
The experiment involves usage of hazardous materials. Wear protective equipment such as safety eyeglasses, dust mask, lab boots, lab suit and gloves. At the beginning, the amounts of cement, sand and aggregates should be calculated at a ratio of Cement: Sand: Aggregate = 1:2:3. For example, if the total weight of the concrete mixture is 3.6 Kg, the weight of cement, sand and aggregates should be 0.6, 1.2 and 1.8 Kg, respectively.Water is then added to the concrete mixture to reach a water to cement ratio of 0.4:1 (w/w).
Follow the procedures listed below:
- Weigh out the calculated amounts of cement (600g), sand (1200g) and aggregates (1800g) using an electronic scale. Make sure that the weight of the measuring container is subtracted from the total weight obtained.
- Measure 200ml water using a measuring cylinder. Make sure that the reading is taken from the bottom of the meniscus. More water could be needed during the mixing stages due to evaporation and/or leakages in the apparatus.
- Slowly add sand and cement into a mixing bucket to avoid creating excessive dust. Slowly add aggregates and water into the same bucket. Mix the ingredients until a uniform paste forms.
- Apply a thin layer of a release agent to the inner surface of the mould container by using a brush, and then pour the concrete into the mould.
- Compact the concrete by using a compacting rod. Try to avoid air bubbles and make sure that the concrete is compacted to the most.
- Scrape off any residual mixture from the mould. Make the surface of the specimen as flat as possible to avoid point loading during the test.
- Label the samples with group letter and sample type (C1 and C2).
The principles of concrete curing
Hydration of cement is a chemical reaction between cement and water to form a hydration product, such as, cement gel, which can be laid down only in water-filled space. Curing is a procedure used to facilitate the hydration of the cement in a newly placed concrete. It generally involves control of moisture loss throughtemperature control. ‘Hydration can proceed until all the cement reaches its maximum degree of hydration, or until all the space available for the hydration product is filled by cement gel, whichever limit is reached first’ (4).
Samples were de-moulded after 24 hours. During the de-moulding, care was taken, in order not to damage the edges of the samples. The two samples were then cured for 28 days under two different conditions. One sample (C1) was exposed to air, and the other (C2) was cured in a water tank,in order to test which sample would possess better compressive strength. Previous compressive test results show that the curing in water increases concrete strength and abrasion resistanceand decreases the chance of concrete scaling, dusting and cracking.
Experimental procedures of curing specimens after de-moulding
Before testing for compressive strength, the two samples were weighed, and the volumes of the samples were calculated, in order to determine the density of the samples. The cross sectional areas of the loading faces were also calculated. To avoid point loads by creating a smooth flat surface, the two samples were capped with sulphur on both the loading faces. To let the surface of the sample dry and to produce enough friction to hold the sulphur cap in place, the sample cured in water needs to be taken out 24 hours prior to the sulphur capping.
Follow the procedures listed below:
- Clean the bearings on the testing machine and wipe out the surface of the two samples, before placing them in the testing machine.
- Follow the guidelines and align the sample cured in air carefully.
- Apply a constant compressive force at the rate of 7 KN/s, until the peak load is obtained. After the peak load is reached, continueincreasing the forcefor a further 30% of the failure load. At this stage,sample deformations could be observed, characterized by surface cracks and sometimes absolute crumbling.
- The quality of the sample could be determined by how much more load it could take after the 30% advance. Then the peak load of a sample was recorded, in order to calculate the compressive strength of the sample.
- The procedure was repeated for the sample cured in water.
The experiment was carried out while the following parameters were kept constant among different samples in order for a better comparison.
- The same testing machine, in which the samples were tested.
- Concrete mixture ratio of the samples.
- Rate at which the compression force was applied.
The experimental results should only be valid if the cracks of the samples are observed, as shown in Figure 1.
Figure 1: All four exposed faces are cracked approximately equally, generally with little damage to faces in contact with the platens.
Figure 2: Graph plotted for mean compressive strength against sample density. (Air-cured)
Figure 3: Graph plotted for mean compressive strength against sample density. (Water-cured)
Figure 4: Graph plotted for mean compressive strength against the mixture used.
Figure 5: Graph plotted for mean compressive strength against the curing environment.
Figure 2 and Figure 3 shows that the mean compressive strength is the least affected by the density of the samples. This was due to the constant volume of the samples.The mass of the samples, however,varied slightlywith the type of starting materials used. For example, the concrete product made from sharp sand and 20mm aggregates had the largest mass, as these particles were bigger in size compared to fine sharp sand and 10mm aggregates.
Figure 5 shows a clear relationship between the mean compressive strength and the curing environment.Samples cured in water displayed stronger compressive strength than those cured in air.
In conclusion, fine sharp sand should be used to make concrete with high workability and low compressive strength, which are ideal for masonry applications, such as wall plastering. Sharp sand should be used to make concrete with high compressive strength, such as those used to make architecture columns and beams.
Errors could occur in this experiment.For example, the peak load recorded as 158.9 KN for the sample cured in air of group E could be an error.