Concrete Strength: Effects of Water/Cement Ratio on Workability and Compressive Stress

Running head: CONCRETE STRENGTH
CONCRETE STRENGTH
Name
Course
Professor
University
City/state
Date

CONCRETE STRENGTH 2
Concrete Strength
Introduction
Concrete is a combination of cement, coarse aggregates, sand or fine aggregates, water and air.
Concrete mix design is the process of selecting the aforementioned elements and determining
their proportions that will achieve the desired properties of concrete so that it performs the
intended function effectively. Concrete mix design requires the following information: target
minimum compressive strength after 28 days of curing, type of cement shape and nominal size of
coarse aggregates, degree of workability needed during placing, degree of quality control during
placing of the concrete, and the kind of conditions that the concrete structure will be exposed to.
The focus of this lab is on water/cement ratio. This is the most essential element that has
significant effects on various properties of concrete including workability, compressive strength
and durability of concrete (Jankovic, Nikolic, Bojovic, Loncar, & Romakov, 2011).
Generally, the compressive stress of concrete with the same water/cement ratio increases with
the increasing number of days of curing up to 28 days when the concrete is expected to attain the
maximum strength. An increase in water/cement ratio also results to a decrease in compressive
stress of concrete, and vice versa (Apebo, Shiwua, Agbo, Ezeokonkwo, & Adeke, 2013).
Last but not least, the presence of entrained air in concrete reduces its compressive stress.
Aim
The main aim of this lab is to compare the workability and comprehensive strength of three
water/cement ratio mixes at 7, 14 and 28 days. This will help in determining the effect of
water/cement ratio and curing conditions on the workability and comprehensive strength of
concrete.

CONCRETE STRENGTH 3
Method
The concrete mix design for this lab was M45. This means that the minimum compressive
strength of the concrete after 28 days of curing was expected to be 45 N/mm2. The design of this
concrete mix was in accordance with the British Method. The nominal size of coarse aggregates
used was 14mm and both the coarse and fine aggregates used were air dried. The concrete mixes
were made with three different free water/cement ratios of approximately 0.4, 0.5 and 0.6. The
mixing of concrete, casting of the concrete test cylinders and pouring of these cylinders was done
by following the standards procedures. Some test cylinders were cured in water while others
were air cured.
Testing & Results
The compressive stress of each concrete mix was determined by subjecting a cylindrical
specimen of diameter 100mm and height 200mm of the respective mix longitudinally to uniaxial
compressive loading until it failed. The load at which the specimen failed was recorded as
breaking load.
The summary of breaking load for the different concrete cylinders of different concrete mixes is
provided in Table 2 below
Table 2: Breaking load of different mix designs
Water: Cement 0.4 0.5 0.5 air 0.6
Curing Days Breaking Load (kN)
7 days 341 (S1) 271 (S3) 255 (S7) 220 (S5)
348 (S2) 259 (S4) 243 (S6)
14 days 371 (S1) 305 (S3) 282 (S5) 280 (S6)
357 (S2) 292 (S4) 263 (S7)
21 days 391 (S1) 310 (S2) 271 (S5) 291 (S3)
28 days 369 (S1) 330 (S3) 266 (S7) 285 (S5)
392 (S2) 338 (S4) 245 (S6)