Experimental study of the effect of limestone grading on some mechanical properties of concrete

This study reports the experimental work undertaken to investigate the optimum grading of limestone to be used in concrete mixes. 36 concrete cubes were prepared for testing. Four different Fineness Moduli and grading were tested 2.4, 2.6, 2.92 and 3. For all tests, the cubes were left in curing until testing at the age of 3, 7 and 28 days respectively. Samples were loaded to failure and the average compressive strength was used for comparison purposes. Flow table test has been performed on the fresh concrete directly after mixing to measure workability and the average of the maximum concrete spread parallel to the two edges of the table was recorded. Results revealed that the optimum Fineness Modulus for the limestone to be used as fine aggregate in the concrete mix to get maximum compressive strength is 2.78. The flow table tests revealed an increment in the workability of fresh concrete with higher Fineness Modulus of limestone used in the concrete mix as fine aggregate. The workability of the optimum Fineness Modulus, 2.78, was found to be 412 mm which is a mix with considerable workability. An ideal grading has been recommended in this research study and checked to match the ASTM grading requirements for fine aggregate.


Introduction
Concrete is the most commonly used material in construction industry all over the world. The conventional concrete is produced using natural sand from marine, river or dune as fine aggregate. Digging sand from river bed or marine in excess quantity is hazardous to environment. The cheapest and the easiest way of getting substitute for natural sand is obtained from manufactured sand from aggregate quarries known as limestone. Concrete made with limestone filler as replacement of natural sand in concrete was investigated by Omar M. Omar et al. [1] measuring its compressive strength, tensile strength, permeability and modulus of rupture. Concrete using various combinations of manufactured sand and quarry dust as complete or partial replacement for conventional river sand was investigated ASCMCES-17 by a group of researchers such as Joseph O. Ukpata et al. [2], Rana Burhan Abdurrahman Alshahwany [3] and A. Jayaraman and V. Senthil kumar [4].
UAE is located in the middle of the gulf desert, and with the majority of its cities located close to the sea, there are two main possible sources of sand that can economically be used in the construction work, which are the desert or the sea. Desert sand is very fine which adversely affects the different properties of the produced concrete mixes in terms of compressive strength, workability and bond. On the other hand, the salt in the marine sand would affect the durability of reinforced concrete structures.
The sand in UAE is excavated from the bottom of the sea or ocean for different purposes including the excavation done for different offshore construction work. This sand is then transported to different local suppliers. The suppliers have the responsibility of washing the sand from salt and sieving to remove the large sized sea shells. Even though the marine sand, which is the source of sand used in UAE, is washed before distribution, it is not possible to get rid of all salts adhered to sand particles. Anyway, the marine sand is fine graded and cannot used lonely as fine aggregate in the concrete mixes.
Limestone, which is considered as aggregate quarry waste, could add the required properties to the concrete mix as fine aggregate in terms of compressive strength, workability, durability and sustainability. This study reports the experimental work undertaken to investigate the optimum grading of limestone to be used. The effects of limestone as fine aggregate on fresh and hardened properties of concrete had been investigated. The investigation included testing of compressive strength and workability of different concrete mixes.
While concrete made with limestone filler as replacement of natural sand in concrete was widely investigated for the best percentage of limestone to be used in the mix, no significant research work has been done to examine the effect of the Fineness Modulus and grading of limestone used as fine aggregate in concrete mixes and its effect on the different properties of concrete.

Materials Used in Experiments
2.1.1Cement: Ordinary Portland Cement (OPC) locally available in UAE was used in the experiment.

Water:
Portable water available in the Civil and Infrastructure Engineering laboratory was used. Limestone with different Fineness modulus was used for each concrete mix. The required fineness modulus was achieved by sieving the limestone using automatic sieve shaker for a specified enough time to maintain the required quantity of retained limestone size on each standard sieve. Fineness modulus of 2.4, 2.6, 2.92 and 3 were prepared for the different concrete mixes. Tables (2) through (5) show the different grading of limestone used to achieve the different Fineness modulus used in each concrete mix. All used limestone grading matches the ASTM grading requirements of fine aggregate as mentioned in Neville and Brooks [5]. Table (1) below shows the ASTM standard grading requirements.

Comparing the Limestone Grading Used in the Tests with ASTM Requirements
The grading of the limestone filler with different grading and fineness moduli 2.4, 2.6, 2.92 and 3 are all compared to the grading of fine aggregate required by ASTM. Tables (2) through (5) shows that all limestone grading used in the tests met the ASTM fine aggregate grading requirements.

Fineness Modulus (F.M) Calculations
The Fineness modulus (F.M) is calculated for each grading by finding the sum of the cumulative percentages retained on the sieves of the standard series, divided by 100.The standard series consists of sieves, each twice the size of the preceding one, 150µm, 300µm, 600µm, 1.18mm, 2.36mm, 5mm and 10mm. Higher value for Fineness Modulus indicates a coarser grading.

Mixes Used in Experiments
A fixed concrete mix proportion with effective water cement ratio 0.64 was used for all mixes with cement-fine aggregate-coarse aggregate mix proportions of 1: 2.4: 1.8 and the materials mixed thoroughly using pan mixer.

Results and Discussions
The optimum Fineness Modulus and grading of limestone used as fine aggregate has been examined to improve the compressive strength and workability of concrete mixes. Table (6) and Figure (2) shows the average compressive strength for each Fineness Modulus of limestone used as fine aggregate to prepare standard specimens of the concrete mixes tested at the age of 3, 7 and 28 days.  Solving this derivation, the optimum fineness modulus for the limestone to be used as fine aggregate in the concrete mix to get maximum compressive strength is found to be 2.78   As shown in Figure (3) above, a polynomial interpolation to the third degree is used in order to determine the expected workability of the optimum Fineness Modulus for the limestone to be used as fine aggregate in the concrete mixes which is found previously as 2.78. Applying the Fineness Modulus value of 2.78 in the cubic equation, the expected flow table record would be 412 mm and this is a mix with considerable workability.
The recommended grading then can be found for the optimum Fineness Modulus for the limestone as shown in Table (8) below. Also, the recommended grading was checked and compared with the ASTM requirements for the fine aggregate in Table (8) below.