Concrete

Concrete comprises of cement, sand, aggregate and water mixed together in specified proportions. In case steel reinforcement is incorporated to increase the tensile strength of concrete, the same is referred to as reinforced cement concrete (RCC). Concrete without reinforcement bars is referred to as plain cement concrete (PCC).

In RCC, since reinforcement steel needs to be protected from harmful effects corrosion causing agents present in the environment, I.S 456-2000 recommends a minimum grade of M20 concrete (volumetric mix proportions - 1:1.5:3 with maximum water content is 27.5 liters per bag i.e. 50 kg of cement) for structures exposed to mild environment.

Suggestions for making good concrete

Concrete strength and durability depends upon the quality of its ingredients like cement, sand, aggregate and water. Usually, the ingredients are volumetrically proportioned relative to one cement bag (50kgs by weight or 35 lit by volume). In order to correctly proportion the volume of the sand & aggregates, measuring boxes, having dimensions 35x25x40cms should be used.

The shuttering should be sufficiently strong to withstand the weight of concrete, reinforcement steel, weight of persons (involved in placing and compacting concrete) and impact loads(due to concrete placement and vibrations of vibrator).

For footings, columns, beams, slabs etc., the minimum grade of concrete, exposed to mild environment, as per IS 456-2000, is M20 for which the volumetric mix proportion is 1:1.5:3.

Concrete strength and durability depends mainly on the water-cement ratio used in the concrete mix. It is recommended that the mixing water should be properly measured while preparing the concrete mix. The maximum water that can be added per bag (50 kg) of cement is restricted to 27.5 liters.

Only that quantity of concrete mix should be prepared that can be consumed within 30-40 minutes. If concrete is not deposited & compacted in forms within the stipulated time, the initial setting of the cement takes place and results in reduced workability of concrete.

Good concrete is homogeneous and is prepared by intimate & thorough mixing of all ingredients. To ensure homogeneity of concrete mix, it should be prepared in mechanical mixers and mixed for at least 2.5 minutes. Under inevitable circumstances, if the concrete is to be made by hand mixing, it should be done on a clean & water tight platform. Cement content of concrete should be increased by 10%, whenever hand mixing is done.

The concrete mix should be placed in the forms gently and ensuring that it is not dropped from a height of more than 1.5m.

Freshly prepared concrete contains entrapped air, and the same should be removed by compaction either by vibration (with help of needle or plate vibrators) or rodding (with help of a 16mm bullet nosed rod of 2' length). Proper compaction of concrete is extremely important because for every 1% voids in concrete, its strength reduces by 5%.

Ready Mix Concrete Advantages & Limitations

Ready mix concrete is a tailor-made concrete which improves durability and sustainability. Instead of purchasing the raw materials by individuals and experimenting every time with handling and proportioning, it would be far better idea to entrust all these activities to some expert supplier who is having a professional acumen.

Advantages of Ready Mixed Concrete:

• Quality assured concrete:- Concrete is produced under controlled conditions using consistent quality of raw material.
• High speed of construction- Speed of construction can be vary fast in case RMC is used.
• Reduction in cement consumption by 10 – 12 % due to better handling and proper mixing. Further reduction is possible if mineral admixtures or cementitious materials are used.
• Versatility in uses and methods of placing: The mix design of the concrete can be tailor made to suit the placing methods of the contractor.
• Since ready mixed concrete (RMC) uses bulk cement instead of bagged cement, dust pollution will be reduced and cement will be saved.
• Conservation of energy and resources because of saving of cement.
• Environment pollution is reduced due to less production of cement.
• With better durability of structure, their overall service life increase and there is saving in life-cycle cost.
• Eliminating or minimizing human error and reduction in dependency on labour.
• Timely deliveries in large as well as small pours.
• No need for space for storing the materials like coarse and fine aggregate, cement, water and admixtures.
• No delay due to site based batching plant erection/ dismantling; no equipment to hire; no depreciation of costs.
• Reduced noise and air pollution; less consumption of petrol and diesel and less time loss to business.

Limitations of Ready Mix Concrete:

• As the Ready Mixed Concrete is not available for placement immediately after preparation of concrete mix, loss of workability occurs. In addition, there are chances of setting of concrete if transit time involved is more. Therefore, generally admixture like plasticisers/ super plasticisers and retarders are used. Addition of retarders may delay the setting time substantially which may cause placement problems. In addition, it may also affect the strength of concrete. Therefore, it is necessary that the admixtures i.e. plasticisers and super plasticisers/ retarders used in Ready Mixed Concrete are properly tested for their suitability with the concrete. In case loss of strength is observed, the characteristic strength may have to be enhanced so that after loss of strength, required characteristic strength is available.

Because of large quantity of concrete available in short span, special placing and form work arrangement are required to be made in advance.

Unit Weight of Materials Used at Construction Site

Following table shows unit weight of materials used at construction site. Please note this is for reference purpose only and may vary from place and type of material.

We are thankful to Engineer Ravi Verma for submitting this very useful information to us. If you also have any other formula and shortcuts which can be used on construction site do submit it to us using the submit form here

S.No	Material	Theoretical Weight in(KG/M³)	Approx Weight at Site in		Remarks
			Kg	Per
1	Cement	1440	50	Bag
2	Steel	7850	d²/162		d -dia in mm
3	Sand-
	Dry	1600	50 to 55	farma	1 farma=1.25cft
	River	1840	57 to 63	farma	1 farma=1.25cft
4	Stone(basalt)	2850 to 2960	48 to 52	farma	metal 12mm to 20mm
5	Water	1000	1	liter
6	PCC	2240	8.24 to 8.5	Cube mould	cube mould size=15x15x15cm
7	RCC 2% Steel	2420
8	Bricks	1600 to 1920	1.9 to 2	no	9x4x2 3/4″
			4.8 to 4.9	no	9x6x3 3/4”
9	Brick Masonry	1920
10	Soil(damp)	1760	50 to 55	cft	Black cotton
11	Cement concrete block(solid)	1800	18 to 20	cft	30x15x20 cm
			10 to 11	no	30x10x20 cm
12	Cement Mortar	2080	57 to 62	cft
13	Lime Mortar	1760	48 to 52	cft
14	Lime	640	30	bag
15	Glass	2530	0.9 to 0.95	sft	4mm tk plain
16	Teak Wood	670 to 830	18 to 20	cft
17	Sal Wood	990	22 to 24	cft
18	Marble mosaic tile		2.8 to 3.2	no	25x25x22mm
			4.8 to 5.2	no	30x30x25mm
19	Chequered tile		2.5 to 2.8	no	25x25x22mm
20	Glazed tile15x15cm		0.20 to 0.25	no	5mm tk
21	Marble Stone	2620	5.1	sft	3/4″tk
22	Granite Stone	2460-2800	5.35	sft	3/4″tk
23	Coddappa	2720	6.4	sft	1 1/4″tk
24	A.C.sheet corrugated	16	1.2	sft
25	Bitumen	1040	220	Drum	200liter drum
26	Window frame (simple design)		1.9 to2.1	sft
27	Door Frame
	a)3’00×7’0		25 to 27	no	section 4″x2 1/2″
	b)2’6″x7’0		24 to 26	no	section 4″x2 1/2″

Ready Mix Concrete (RMC) Customer Check List

1	Raw Material	Test Certificates Available
2	Job Mix Formula	Meets requirement in terms of ·         Strength ·         Workability ·         Cohesiveness ·         Place ability
3	Batching Plant	·         Calibration done ·         Yield checked ·         Minimum 30 mixing time ·         Batch sheet furnished
4	Transit mixers	·         Reach site agitation mode ·         Drum rotated at full speed for 20 before discharging ·         No site addition of water (Re-tempering) into drum ·         Does chemical admixture (re-dosing) and mix at full speed if workability is low ·         Trip sheet furnished and observations recorded
5	Concrete pump	·         Sufficient pipeline to place at required area ·         Located at a point accessible to transit mixer ·         No addition of water or slurry into the pump hopper (Re-tempering) once concrete pumping starts ·         Agitator in working condition
6	Sampling	·         Concrete taken from same vehicle and mixed well ·         Workability in terms of slump or flow checked ·         Cube moulds are at right angles and accurate ·         Cube moulds cast at location mutually agreed upon ·         Required cube moulds filled simultaneously in layers and compacted
7	Testing	·         Slump or flow test done randomly for field control ·         Cubes tested for compressive strength at laboratory ·         Cube testing machine is calibrated and in good condition ·         Test report furnished with slump observed in field and ultimate compressive strength at 7 & 28 days
8	Acceptability	·         Intimated about precaution to avoid plastic shrinkage cracks (Delayed finishing / Early curing) ·         Cement content and type in accordance with job mix formula ·         Workability as agreed upon mutually ·         Strength >value agreed upon or characteristic strength, as appropriate
Formwork, Rebar, Cover, Embedment, Pipeline Support, Compaction, Finishing, Curing & Allied Works My Responsibility

Quantity of concrete materials

Quantity of materials such as cement, sand, coarse aggregates and water required per cubic meter of concrete and mortar varies with the mix design of the concrete and mortar respectively. Following table gives the estimated quantity of materials required per cubic meter of mortar and concrete for various nominal mixes.

Nominal Mix			Water Cement Ratio	Water Bag per 50kg Bag of Cement	Cement		Sand (Cum)	Crushed Stones (Cum)
Cement	F A	C A			Weight (Kg)	Number of Bag
1	1	–	0.25	12.5	1015	20.3	0.710	–
1	1.5	–	0.28	14	815	16.3	0.855	–
1	2	–	0.3	15	687	13.74	0.963	–
1	2.5	–	0.35	17.5	585	11.7	1.023
1	3	–	0.4	20	505	10.1	1.06	–
1	4	–	0.53	26.5	395	7.9	1.106	–
1	6	–	0.7	35	285	5.7	1.197	–
1	8	–	0.9	45	220	4.4	1.232	–
1	1	2	0.3	15	560	11.2	0.392	0.784
1	2	2	0.42	21	430	8.6	0.602	0.602
1	1.5	3	0.42	21	395	7.9	0.414	0.828
1	1.66	3.33	0.48	24	363	7.26	0.419	0.838
1	2	3	0.5	25	385	7.7	0.539	0.808
1	2	3.5	0.53	26.5	330	6.6	0.462	0.808
1	2	4	0.55	27.5	310	6.2	0.434	0.868
1	2.5	3.5	0.57	28.5	305	6.1	0.534	0.748
1	2.5	4	0.6	30	285	5.7	0.499	0.798
1	3	4	0.65	32.5	265	5.3	0.556	0.742
1	2.5	5	0.65	32.5	255	5.1	0.446	0.892
1	3	5	0.69	34.5	240	4.8	0.504	0.84
1	3	6	0.75	37.5	215	4.3	0.452	0.904
1	4	8	0.95	47.5	165	3.3	0.462	0.924

Notes:

1. F.A.= Fine Aggregates, C.A.= Coarse Aggregates
2. The table is based on assumption that the voids in sand and crushed stone are 40 and 45 percent respectively.
3. Air content of 1 percent has been assumed.
4. For gravel aggregates decrease cement by 5 percent, increase sand by 2 percent and coarse aggregate in proportion to fine aggregate in mix.
5. No allowance has been made in the table for bulking of sand and wastage. />